Sibling rivalry in wild Chacma Baboons.

Jealousy occurs in Humans when we perceive our relationship with a person important to us is threatened by the actions of someone else. This is a complex emotion, which often manifests in attempts to disrupt social interactions that we find threatening. Whether jealousy is a uniquely Human emotion is less clear, as it can be hard to judge the genuine emotional state of non-Human Animals, at least in part due to the dangers of anthropomorphising behaviours that resemble Human emotional traits.

Non-Human Primates would appear to be a good starting point for those wishing to study jealousy outside our species, but very few studies appear to have been done in this area, and all of those on sexual jealousy in captive Primates held in unnatural conditions.

In Humans, one area which has been extensively studied is sibling rivalry among children. Human mothers are unusual in that they often care for multiple offspring of different ages at the same time, leading to forms of competition for parental attention and resources which do not occur in most Animals. Intense rivalries can develop between child siblings when they perceive that they are being treated differently, sometimes leading to conflicts which extend into adulthood. Such rivalries are most likely to develop when children are the same sex and close in age.

Such rivalries between siblings of different ages have not been studied in non-Human Animals. Instead, research has concentrated on competition between siblings produced in large clutches or broods, where they are part of a group reliant on parental provisioning until reaching independence. There have also been some studies concerning mother-child competition for resources in monotocous species (species that have one young at a time), plus a few studies looking at species in which older siblings cooperate in the rearing of young. 

Monotocous Animals with long child-rearing periods, including many Primates, form an intermediate group between Humans and polytocous species (species that have produce many offspring in a single clutch or litter). In such species different aged siblings interact with one-another and their parent(s) on a daily basis, and competition between siblings has the potential to have a significant impact upon fitness. Siblings will compete with one another in ways which they do not compete with non-siblings, requiring maternal care and attention rather than just foraging competitively. 

In a paper published in the journal Proceedings of the Royal Society B on 11 February 2026, Axelle Delaunay of the Institute of Evolutionary Science of Montpellier, and the Tsaobis Baboon Project in the Tsaobis Nature Park in Namibia, Vittoria Roatti, also of the Tsaobis Baboon Project, and of the Department of Anthropology at University College London, Rose Ellis and Punaete Kandjii, again of the Tsaobis Baboon Project, Alecia Carter, again of the Tsaobis Baboon Project and the Department of Anthropology at University College London, and of the Gobabeb Research Institute, Guy Cowlishaw of the Tsaobis Baboon Project, the Gobabeb Research Institute, and the Institute of Zoology, Marie Charpentier, also of the Institute of Evolutionary Science of Montpellier, and Élise Huchard, once again of the Institute of Evolutionary Science of Montpellier, the Tsaobis Baboon Project, and the Gobabeb Research Institute, present the results of a study into the role of jealousy in driving sibling competition in wild Chacma Baboons, Papio ursinus, a monotocous, social Primate.

Chacma Baboons live in matrilineal societies in which males disperse to new groups around puberty, while females remain within their birth groups, inheriting their mother's dominance rank. On average, females produce one young every two years, with growing Baboons are weened after about two years, but have a long developmental period and have long-lasting strong bonds to their mother, expressed through grooming relationships in which preference of grooming partner is an important signifier of relationship status.

In order to do this, they looked at instances in which young Baboons interrupted their mother when she was grooming a sibling. They did this to test three hypotheses, that young Baboons interrupted on such occasions because they were jealous, because they wished for attention from their mother themselves, or because they wanted to interact with their sibling. Theoretically, a jealous Baboon would interrupt even if it was not likely to gain any reward, a Baboon wanting attention from its mother would only interrupt if this was likely to result in it getting such attention, and a Baboon wishing to interact with a sibling would only do so if it was likely to gain such interaction. 

Theoretically, a Baboon which was jealous or wanted to play with a sibling would be more likely to approach its mother when she was grooming that sibling, while a Baboon simply wanting its mothers attention would be most likely to approach her when she was unoccupied. Furthermore, the jealousy hypothesis suggests that a Baboon would be more likely to interrupt its mother if she was grooming a sibling which tended to monopolise her attention, or otherwise appeared to be a favourite. This would fit with the patterns observed in Human children. 

It was also predicted that, in Baboons, jealousy would be more prevalent among same-sex siblings, and in particular between sisters, since younger sisters can come to outrank older sisters with maternal support. Since males leave the troop at puberty and do not rely on their mothers for social status, they were predicted to be less prone to jealousy. 

Furthermore, it was predicted that Baboons seeking maternal care would be more likely to interrupt when their mother was grooming a younger sibling, as these tend to be easier to displace, and that females may be more inclined to interrupt when their mother was grooming a male sibling, as mother-daughter social bonds are stronger than mother-son bonds.

If Baboons interrupt grooming because they want to play with a sibling, then males interrupting when other males were being groomed would be predicted to be the more frequent occurrence, as young males play with other young males more frequently than females play with females, or young Baboons play with members of the opposite sex. This would also lead to more interruptions when the Baboon being groomed was close in age to the Baboon interrupting. 

Dalauney et al. studied Chacma Baboons from two well habituated troops (L and J) in the Tsaobis Nature Park, which lies on the edge of the Namib Desert in Namibia, between August and December 2021. The Baboons were followed from dawn to dusk each day by trained observers who were able to identify all Baboons in the troops, including infants. Every family group which included at least a mother and two offspring was included in the study. This comprised eight families in each troop, with between two and five offspring, with a total of 28 female and 21 male young Baboons, ranging in age from six days to 8.9 years. This included adult females, but not adult and subadult males still living in the maternal group, due to the limited amount of interaction these males have with their mothers. 

Female Baboons were observed for five minute intervals when they were either grooming one of their offspring, or resting, and all interruptions were recorded, as well as whether this was be a sibling or non-sibling of the Baboon being groomed. Interruptions could be aggressive, such as attacking, chasing, biting, pushing, slaping, threatening, displacing, or supplanting; affiliative behaviours such as body contact, come-here faces, grunting, jumping on one groomer, lipsmacking, playing, presenting, or touching; tantrum behaviours such as gecks, complaint grunts, or other screams; maternal care solicitations, such as soliciting access to the nipple, soliciting grooming, suckling, or starting a triadic grooming session with the groomers; or simply approaching within a metre of a grooming pair. Under this analysis, a very broad range of behaviours were treated as 'interruptions', the object being to understand the motives of young Baboons in approaching their mothers. As such, focusing only on agonistic behaviours or effective disruption would only provide a partial picture of sibling interference, and might not pick up on some relationships, for example if younger siblings were afraid to behave aggressively towards older siblings being groomed by their mothers, but still found other ways to gain her attention. 

Having come up with a definition of 'interruptions', Dalauney et al. then looked at the outcome of these interruptions, grouping them into three basic categories; instances where the grooming continues, instances where the grooming stops, and instances in which the interrupter replaces one of the Baboons in the grooming relationship. In the final case, they recorded which Baboons were in the new grooming relationship (i.e. mother and interrupting youngster, or interrupting youngster and previously involved youngster). Finally, Dalauney et al. recorded every youngster within 10 m of their mother at the start of a five minute session. If the mother broke off from her original activity (grooming or resting) to engage in some other activity during the five minutes of the observation, that observation was abandoned. Each family group was monitored for no more than five minutes each hour. 

A juvenile male just approached and initiated body contact with his younger sister being groomed by their mother. This interference did not interrupt the ongoing grooming interaction, nor allow him to groom with his mother or his sibling. Axelle Delaunay in Dalauney et al. (2026).

Dalauney et al. found that young Baboons were significantly more likely to interrupt their mother when she was grooming a sibling than when she was resting, which predicted by the theories that the young Baboon was jealous or wanted the attention of its sibling, but not by the theory that the primary motivator was gaining the attention of the mother. This was unaffected by the presence of other siblings within 10 m (which was predicted to make interruptions less likely if a young Baboon was looking for a sibling to play with), nor was it affected by the sex of the youngsters, nor the social status of those involved. Baboons did become less likely to interrupt their mother grooming a sibling as they got older.

Younger Baboons were more likely to interrupt their mother when she was grooming a sibling, but this was apparently unaffected by the age-difference between them and the sibling being groomed. However, older siblings were twice as likely to interrupt their mother when she was grooming a younger sibling, and all Baboons were more likely to interrupt their mother when she was grooming a sibling of the same sex, with males more likely to interrupt when a brother was being groomed than females were to interrupt when a sister was being groomed. If one of the mother's offspring was perceived as a favourite, and received more grooming attention than its siblings, then those siblings were more likely to interrupt when it was being groomed. The age of this favourite did not appear to matter. This again supports the theory that interrupting Baboons were driven primarily by jealousy. 

Of 501 instances of Baboons interrupting their mother while she was grooming a sibling, 95 instances (19%) resulted in grooming being broken off (the predicted desired outcome of the jealousy model), while in 44 cases (9%) resulted in the interrupter entering into a grooming session with the mother (the predicted desired outcome of the maternal attention-seeking model), and only 12 (2%) resulted in the interrupter entering into a grooming session with the sibling (the predicted desired outcome of the sibling attention-seeking model). Entering into a grooming relationship with the mother or a sibling did not necessarily disrupt the original grooming relationship, as Baboons can form grooming triads. Data was not initially collected on interrupters entering into play relationships with the sibling being groomed (another possible outcome of the sibling attention-seeking model), but only 11 instances of this were observed, limiting support for this hypothesis. Thus, while most attempts at interrupting a mother grooming a sibling failed to disrupt that activity, they were significantly more likely to cause that grooming session to break off than to allow the interrupting Baboon to enter into a grooming relationship with one of the originally involved Baboons, supporting the hypothesis that the main reason for such interruptions was jealousy.

Interference are rarely successful, but more often disrupt the ongoing grooming than they grant access to maternal or sibling grooming. Proportion of successful interference as defined under the jealousy hypothesis (i.e. the interference effectively disrupts the ongoing mother–sibling grooming, in pink), the care-seeking hypothesis (i.e. the interferer gains access to maternal grooming, in orange), and the sibling-seeking hypothesis (i.e. the interferer gains access to sibling grooming, in yellow). Note that interferer do not need to disrupt the ongoing grooming to gain access to maternal or sibling grooming, as interference leading to triadic grooming interactions was also considered successful. The blue bar represents all the interferences that were not successful under any hypothesis, i.e. when the grooming kept going and the interferer did not get access to either of the groomers. Dalauney et al. (2026).

Dalauney et al.'s study lends to support to the idea that young Baboons interrupt their mothers when they are grooming siblings out of jealousy. They were more likely to interrupt their mothers when they were grooming siblings than when she was apparently available. They were also more likely to interrupt when the sibling being groomed was younger than them, of the same sex, or perceived as their mothers favourite. This interference appeared to be largely aimed at the mothers rather than their siblings. In Baboon societies, mothers are a more useful social connection than siblings, as they provide support to female offspring throughout their lives, and to male offspring for as long as they remain within their maternal group, whereas brothers will leave the group at some point, and sisters will shift the focus of their support to their own offspring when they start to have them.

Younger siblings were preferentially targeted over older siblings. This may be because younger Baboons tend to monopolise their mothers attention, or because they are less likely to respond aggressively to such interruptions. There is a lack of comparative studies of this behaviour in Humans, where only the jealousy of older siblings towards younger siblings has been explored, and then only in the context of age difference between siblings in modern Western societies.

What has been recorded in both modern Western societies and non-Human Primates is that behaviour predicted to be caused by jealousy is more commonly directed at younger siblings, and siblings of the same sex (which implies that in both cases offspring of different sexes are competing for slightly different maternal resources, and that siblings of the same sex are therefore a greater threat). In Baboons and Humans, young males engage in more rough-and-tumble play than young females, which might lead to a desire to play with siblings becoming a reason to disrupt interactions between those siblings and their mothers. Dalauney et al. believe that by looking at a wide range of interrupting actions and their outcomes, that their study has screened for this possibility

Dalauney et al. believe that the tendency of Baboons to interrupt when their mothers are grooming a sibling perceived as a favourite is important. This matches the finding in Humans that children who feel disfavoured by their parents report higher levels of conflict with both parents and siblings, often with lifelong consequences. In Humans, siblings are less likely to be jealous if they perceive that siblings receiving different treatment are doing so because they have different needs. Non-Human Primates are known to be able to track relationships between their close family and social partners, as well as those between third parties. Dalauney et al.'s findings suggest that Chacma Baboons are able to judge the strength of their maternal bond compared to that of their siblings, although the cognitive ability needed to do this and the perception of fairness in Baboons will need to be the subject of further studies. Negative responses to uneven food rewards have previously been recorded in several Primate species, as well as Domestic Dogs and Corvids. Whether Primates can assess maternal care in the same way is still unclear, but the ability to do so would clearly have implications for family dynamics.

One of the most important questions that arises from Dalauney et al.'s study is whether there is any benefit to a young Baboon in simply disrupting its mother grooming a sibling, and whether there is any more general advantage to jealousy. Interrupting activity was shown to be twice as likely to result in the mother abandoning grooming of a sibling as it was to result in the interrupter entering into a grooming session with the mother, and entering into such a grooming bout does not actually require the mother to abandon the other youngster, as Baboons can form grooming triads. Notably, the most common outcome was that nothing changed, i.e. the mother continued to groom the sibling she had been grooming when interrupted. This suggests that any benefits associated with jealousy may be complex and time-delayed, particularly if young Baboons are able to manage their emotional state sufficiently to refrain from an activity unlikely to pay off. Any adaptive benefits of jealous behaviour are likely to change over a lifetime, which should also alter their expression. There is less advantage to a Baboon monopolising its mothers time and attention as it becomes older and more independent, able to find its own resources and make its own social connections. Under these circumstances, sibling connections might become more important than maternal connections. In strictly evolutionary terms (i.e. the passing on of genes), it may be advantageous to an older juvenile Baboon to stifle jealous instincts in order to let a younger sibling have more time with its mother.

Ultimately, whether we are Human or Baboon, our emotions are a private matter, confined within our heads, which others can only judge by our actions. It is therefore possible that, when judging the emotions of another species, that we anthropomorphise their motivations, and provide emotional explanations for actions which have been determined by entirely different biological pathways. However, there is a growing body of evidence coming from a wide range of taxa, including Primates, Ungulates, and Corvids, that individuals will try to interfere with a close partners interactions with others, apparently in order to protect a valuable social relationship. Most studies have not attempted to evaluate the emotional underpinning of such actions, but these responses clearly correspond to what we would call jealousy in Humans. Dalauney et al. suggest that the role of emotions in managing social relationships in non-Human Animals has been underestimated, despite the fact that the primary function of emotions is to provide a way for individuals to act upon external stimuli. Jealousy is a social emotion, something which helps us to navigate complex social environments, and comparisons of Human and non-Human responses to similar social stimuli can potentially unlock ways to understand the emotional lives of Animals. 

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Lethal conflict during the fission of a Chimpanzee group in Kibale National Park, Uganda.

Warfare, and other forms of collective violence, are one of the more distinctive, if less pleasant, forms of Human behaviour. In these acts, Humans are able to sort themselves into groups defined by traits other than kinship, such as ethnicity, religion, language, or other cultural traits, in order to engage in conflict against other groups of Humans. It has been proposed that this is a result of our ability to use cultural traits to define cohesive social groups, giving us a distinct sense of who is 'us' and who is 'them', enabling closer cooperation with members of our group, while at the same time maintaining a hostility to outsiders. 

However, this cannot explain how conflict can erupt within formerly cohesive groups, leading to rebellions and civil wars. This has been explained by suggesting that shifting interpersonal ties and rivalries can sometimes reach a point where internal hostilities overcome the cohesion of the group, regardless of the cultural connections which have been built up. This hypothesis has some evidential support, with observations suggesting that Humans can rapidly come to regard former members of an in-group as members of an out-group for the most arbitrary of reasons. 

Non-Human Animals also engage in territorial aggression, and sometimes lethal conflict with cospecifics, despite not having the religious, cultural, or political ideologies seen in Humans. This has been observed in a variety of Animals, including Banded Mongooses, Lions, Wolves, and Primates. Non-Human Animals living in social groups also have episodes of fission, in which one group splits permanently into two or more, something which is often explained in terms of feeding competition and social pressures, although most Animal groups do not engage in lethal combat during such fission episodes.

Chimpanzees, Pan troglodytes, are among our closest relatives, and in some ways can be seen as a bridge over the gap between Human and non-Human behaviour. Male Chimpanzees stay within the group they were born in, and engage in cooperative defence of the group's territory, as well as raids on the territories of neighbouring groups. Lethal violence between males of different groups is quite common during both of these activities. This can be explained in terms of hostility towards outsiders, while remaining loyal to members of the kin group. Chimpanzee groups are known to occasionally split, a process which is thought to involve lethal violence between males familiar with one-another.

In the 1970s, a group of Chimpanzees living in the Gombe National Park in Tanzania was thought to split in two, with the males of one of the new groups subsequently killing one of the adult females and all six adult males in the other group over a period of about four years. However, this group was not under constant observation, so that much of this activity is inferred rather than having been directly witnessed, and no subsequent observations of similar splits in Chimpanzee groups were recorded in the following decades. Furthermore, a study published in 2014 which looked at the genetic structure of Chimpanzee groups suggested that such ruptures were extremely uncommon, with groups splitting on average once every 500 years.

In a paper published in the journal Science on 9 April 2026, a group of scientists led by Aaron Sandel of the Department of Anthropology at the University of Texas at Austin, present a detailed and extensively documented study of a fission event which split a group of Chimpanzees living in the Kibale National Park in Uganda, and the lethal violence associated with this split.

Sandel et al. analysed 24 years of data on the social interactions of the group, ten years of GPS data, and 30 years of demographic data on the Ngogo Chimpanzee Group, which demonstrated a three step process, in which a formerly cohesive group polarised into two clusters with little social interaction between these clusters, these clusters then actively avoided one-another for two years, before engaging in a series of lethal aggressive actions. Sandel et al. take this as evidence that Chimpanzee groups can fracture and engage in collective violence against members of the same community without any of the cultural markers seen in Humans.

The Ngogo Chimpanzees have been the subject of a continuous research project since 1995. For the first two decades of the study, they remained a single group, although they did go through a regular fission-fusion dynamic in which the larger group split into parties which changed throughout the day, though individuals frequently moved between parties and all remained within the overall territory; this is something commonly seen in Chimpanzee groups. Females typically migrated from (or to) the group at adolescence, while males born into the group remained there for life. The males formed a strict dominance hierarchy, associated in mixed-sex parties, hunted together, and cooperated in territorial patrols.

Each adult male in the group was followed for 2-3 months each year between 1998 and 2024 to see which other individuals they associated with, stayed close to, and engaged in grooming with. Despite being a single group, the Ngogo Chimpanzees typically split into two-to-four clusters over the course of a year, with two persistent and long-lived clusters, the Western and Central clusters. Membership of these clusters was fluid, with 29% of Chimpanzees switching cluster each year, and extensive ties maintained between Chimpanzees in different clusters. 

As well as clusters, Sandel et al. identified a number of 'cliques' of males that consistently stayed together, even when switching clusters. One of these cliques comprised a group of three males that would go on to form the core of the post-fission Western Group. These three males remained together consistently, even when forming clusters with males that would go on to be in the post-fission Central Group. A cluster comprising exclusively males that would go on to form the post-fission Western Group first appeared in 2014.

In 2015, Ngogo Chimpanzees at the edge of their territory hear calls from neighbouring Chimpanzees; adult males embrace each other in an act of reassurance before moving toward the calls and engaging in an intergroup encounter. 'C' and 'W' designations refer to males who would later become members of the Central and Western groups, respectively. Sandal et al. (2026).

Despite the different social clusters, all of the Ngogo Chimpanzees, including all of the males, had overlapping space use patterns, and all males shared the same set of reproductive partners. All Chimpanzees born within the Ngogo Group between 2004 and 2014 that it was possible to genetically sample had both parents from within the group, though 44% had parents from two different clusters. 

The first sign of a split between the clusters was observed on 24 June 2015, when members of the Western and Central clusters were seen to approach one-another near the centre of the territory. Unexpectedly, rather than the two groups merging as usually happened on such occasions, the Chimpanzees of the Western Cluster ran away, with the Chimpanzees of the Central Cluster chasing them. The two clusters then avoided one-another for six weeks, something which had never been observed before.

The events of 2015 precipitated the greatest change of social change seen throughout the decades-long study of the group. What had been a single large group of Chimpanzees split into a number of smaller units, stabilising as two new groups by 2018.

The first patrol by one group against another happened in 2016, when males of the Western Group, accompanied by two of the Central Group males, staged a patrol against the Central Group. All subsequent patrols by the Western males contained only members of that group. In 2017, the Central Group males staged their first patrol against the Western Group, with aggressive interactions between the two groups escalating rapidly from that point. During one encounter in 2017 the males of the Western Cluster attacked the alpha male of the Central Cluster (who had been part of the Western Cluster before 2014), severely injuring him. Both groups subsequently increased the number of patrols against the other group.

By 2017 the two groups were using largely distinct territories, with the overlap between the two groups being similar in size to that seen between unrelated groups. The centre of the shared territory had become a border. This was accompanied by reproductive isolation between the two emerging groups, with the last infant with parents from different groups being conceived in March 2015. All subsequent births had both parents from the same new group.

By 2018, the original group appeared to have split into two new groups, entirely separated from one another. At this time the Western Group included 10 males and 22 females over the age of 12, and the Central Group comprised 30 males and 39 females over the age of 12. Until 2018, a few of the females and infants from the Central Group would occasionally join the Western Group when foraging from Fig trees. After 2018 all such activity ceased.

Following this complete split, the Western Group initiated a series of lethal attacks against members of the Central Group. On all occasions members of the Central Group were attacked by multiple members of the Western Group, during a patrol by members of the Western Group into the territory of the Central Group. Six lethal attacks on males belonging to the Central Group were observed between 2018 and 2024, with a seventh attributed with a high degree of confidence. From 2021 these attacks were also carried out on infants from the Central group, with fourteen infanticides observed, and another three inferred. 

Sandal et al. note that this is a conservative estimate. Between 2021 and 2024 another 14 adult and adolescent males from the Central Group disappeared, and are thought likely to have been killed. None of these individuals showed signs of any illness when they were last observed.

In 2019, the Western Chimpanzees, including W1, W2, and W3, attack and kill the adult male Central Chimpanzee C1, who they had associated with before the fission, marking a notable shift in relationships. Sandal et al. (2026).

By using decades of data gathered on the Ngogo Chimpanzees in Kibale National Park, Sandel et al. have been able to demonstrate the occurrence of a fission event within a wild group of Chimpanzees, something predicted to happen only once every 500 years. This was followed by a series of lethal aggressive interactions, with targeted violence continuing years after the split, something not observed in any other non-Human Primate. The rate at which killings occurred was far higher than that seen in small Human societies. This demonstrates that Chimpanzees can develop new group boundaries and defend them violently, despite not having any concept of ethnicity, religion, or political ideology.

Sandal et al. speculate that a number of factors might have contributed towards the division of the Ngogo Chimpanzee Group. Firstly, the group was unusually large, with almost 200 individuals, and almost 30 males, much larger than other known Chimpanzee groups, which may have strained the males ability to maintain good relations with all other members of the group. Secondly, feeding competition has been shown to play a role in group fission in other Primate species. The area occupied by the Ngogo Group had abundant food, but the large size of the group may have caused strain at some times of the year. Thirdly, the two groups became reproductively isolated before finally separating into two groups, something which may have increased male-male aggression as they had to compete for a smaller number of mates.

Other factors which may have played a role are also observed by Sandal et al.. Firstly, six adult members of the group, five males and a female, died in 2014. The cause of these deaths is unknown, but two showed signs of illness before their deaths, making it possible that the group was hit by a disease. The loss of more than 10% of the male Chimpanzees in the group may have weakened the groups network of social relationships, leading to the group beginning to break up in 2015. 

Next, the dominant male in the group changed in 2015, immediately before the fission of the group. Such changeovers are known to raise tensions among male Chimpanzees, leading to increases in behaviours such as aggression and avoidance. The former dominant male belonged to the Central Group, whereas the new dominant male came from the Western Group, but moved to the Central Group when he ascended to the top of the hierarchy, which Sandal et al. suspect may have increased tensions between the two groups.

Finally, in January 2017, the group was hit by a respiratory epidemic which killed 25 Chimpanzees, including four adult males and ten adult females. Two of the males that died were from the Western Group, including one of the last males in that group to be maintaining relations with the Central Group. Thus, even though this event happened after the groups had started to split, it may have contributed to the final breakdown in relations between the two. 

Chimpanzees are known to have a strong sense of who is in their group, and who is not. Female Chimpanzees leave the group they were born into as adolescents and look for a new group to join, but males remain in the group they were born into their entire lives. Thus any unfamiliar male Chimpanzee is treated as a stranger, with no recorded instances of wild male Chimpanzees forming relationships or cooperating with Chimpanzees in other groups. Under these circumstances, intense hostility towards outside males appears to be an adaptive trait in male Chimpanzees, leading to potential territorial extensions for the group, which in turn leads to more food and other resources, increased female fertility, and a greater survival rate among juveniles. 

This cannot, however, explain the lethal aggression sometimes displayed to members, or former members, of the same group. During the fission of the Ngogo Group, males which had lived, fed, groomed, and patrolled together for years became targets of lethal attacks on the basis of their new group membership. This leads Sandal et al. to conclude that Chimpanzees have a sense of who belongs to their group which is based upon more than simple familiarity, and which can be updated to reflect changes in circumstances.

These results challenge previous assumptions about intergroup conflict in Chimpanzees in a number of ways. All observed attacks were initiated by the numerically smaller Western Group, which contradicts the assumption that larger groups have an advantage in such conflicts. The emergence of greater social cohesion in the smaller group appears to have given them an advantage in conflict which more than made up for the greater numbers of their opponents. These closer bonds appear to have developed before the onset of aggressive activity, suggesting that an external threat is not needed to forge these bonds. Conversely, the original Ngogo Group underwent a territorial expansion in 2009, which appears to have reduced the threat that it faced from rival groups, something which may have contributed to the long-term decline in social cohesion within the group. If this is the case, than an external threat may not be needed to forge close bonds, but it may be needed to maintain them.

Sandal et al.'s findings also have implications for Human behaviour. If Chimpanzee groups can split in this way without any of the cultural markers associated with Human conflict, then these may be less important to Humans than we generally assume, masking the actual roots of aggression, which may have more to do with interpersonal relationships. Humans tend to attribute the conflict to ethnic, religious, or political divisions, but this may be misleading, covering the actual causes of conflict. If this is the case, then  it may be in the small, daily acts of reconciliation and reunion between individuals that we find opportunities for peace.

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Tapanuli Orangutans brought closer to extinction by Cyclone Senyar.

The Tapanuli Orangutan, Pongo tapanuliensis, is thought to have moved significantly closer to extinction after Cyclone Senyar passed across Sumatra on 25 November 2025. The species, which was only discovered in 2017, is considered to be the world's rarest Ape, with a population of about 800 all living within an area of less than 1000 km² in North Sumatra. It is considered to be Critically Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species.

A male Tapanuli Orangutan, Pongo tapanuliensis. Maxime Aliaga/Sumatran Orangutan Conservation Program.

Cyclone Senyar caused more than 1000 mm of rain to fall in a day in parts of North Sumatra, triggering a series of catastrophic floods and landslides which killed over a thousand people on the island. The three districts where the species is found, North, Central, and South Tapanuli, have been particularly badly hit, with satellite images showing that between 48 and 72 km² of the forest inhabited by Tapanuli Orangutans has been destroyed, according to Erik Meijaard of Borneo Futures, who has been studying the storm's impact on the Apes. This could potentially equate to between 33 and 55 Orangutans, or between 6.2% and 10.5% of the total population.

So far, only a single dead Orangutan has been found, leading to the possibility that the Apes may have been able to escape the area before the worst of the event, although this is not typical Orangutan behaviour; they usually seek the nearest shelter and attempt to wait out large storms. Even should this have been the case, the storm appears to have destroyed large areas of their native habitat, including food sources, which makes it likely that more Orangutans will be lost in the near future. 

A dead Tapanuli Orangutan found amid storm debris in the village of Pulo Pakkat, North Sumatra, following the passage of Cyclone Senyar. Decky Chandra/The Guardian.

The population was already considered to be threatened by the expansion of mining, hydropower projects and palm oil plantations, within their habitat, as well as a rapidly changing environment, with global warning already having led to an increase in rainfall of between 28 and 160% across Sumatra. One possible benefit to the Orangutans is that the Indonesian government has ordered a halt to all new development projects in the Tapanuli area, pending a survey of the region, which may lead to their gaining some extra long-term protection. The government has also indicated its support for forest restoration projects in the region, which could lead to an increase in the available habitat for the Apes.

The storm is also reported to have completely destroyed the Ketambe Research Station, within the Gunung Leuser National Park in Aceh Province, which was the first specialist Orangutan research centre in the world, opened in 1971 by Dutch primatologist Herman Rijksen, and a leading centre for research into the Sumatran Orangutan, Pongo abelii, which is also considered to be Critically Endangered.

Damage caused to the Ketambe Research Station, within the Gunung Leuser National Park by Cyclone Senyar. American Association of Zookeepers. 

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The origin of kissing.

Kissing, if interpreted as mouth-to-mouth contact between members of the same species with no food being transferred, is widespread in the Animal Kingdom, as well as in most (but not all) Human societies. Variants on this have been seen in Mammals, Birds, Fish and even Insects. If this process is narrowed to oral-oral contact with some movement of the lips, then it is still widespread in Old World Monkeys and Apes. 

The advantages of kissing, viewed in a strictly evolutionary sense, are hard to define. Kissing does not appear to aid survival, or enhance reproductive success, and presents an opportunity for the transfer of infections. It has been suggested that kissing may enhance mate selection, enabling one kisser to evaluate the odour, health and/or social skills of the other, although there is no real way to prove this. Another suggestion is that kissing enhances arousal, thereby increasing the likelihood of mating success, though again, this is unprovable. Neither of these explanations can account for kissing between individuals who do not seem to be intent on mating, which is just as widespread as mating-related kissing. One theory which could account for this is that kissing is a social gesture which displays trust, coming intentionally close to the (potentially dangerous) mouth and teeth of a social peer or potential mate, while another is that the practice might facilitate the transfer of potentially beneficial microbes, though again, these hypotheses are impossible to prove one way or the other.

The Kiss, by Gustav Klimt. Österreichische Galerie Belvedere/Google Arts & Culture/Wikimedia Commons.

The meaning of kissing is subject to cultural interpretation in Human societies. Some societies do not kiss at all. In others kissing is an everyday, yet meaningful, event. It can also be a highly symbolic activity. Given this cultural variation in Humans, it is quite possible that kissing is also cultural in non-Human Primates, and may be present in some populations and not others. 

While a number of researchers have come up with different possible explanations for the purpose of kissing, the origin of the practice does not appear to have been examined to date, despite the insight that understanding this could provide on the purpose of the practice. The presence of kissing in different, but closely related, species of Primates suggests that it has not appeared separately in each of these species, but that it first appeared in a common ancestral species, and was then adapted by each evolving lineage to meet their needs.

In a paper published in the journal Evolution and Human Behaviour on 19 November 2025, Matilda Brindle of the Department of Biology at the University of Oxford and the Department of Genetics, Evolution and Environment at University College London, Catherine Talbot of the School of Psychology at the Florida Institute of Technology, and Stuart West, also of the Department of Biology at the University of Oxford, present the results of a study in which they developed a comparative framework for the practice of kissing across different species of Primate, which was then used to examine the evolutionary history of the behaviour within the group.

Top panel: kissing across the Animal Kingdom (clockwise): Rhesus Macaques, Macaca mulatta; Galapagos Albatross, Phoebastria irrorata; Polar Bears, Ursus maritimus; Wolves, Canis lupus; Prairie Dogs, Cynomys ludovicianus. Bottom panel: non-kissing mouth-to-mouth behaviours (left to right): premastication in Orangutans, Pongo sp.; trophallaxis in Indian Black Ants, Camponotus compressus; and Kiss-fighting in French Grunts, Haemulon flavolineatum. Brindle et al. (2025).

A full systematic review of kissing in Primates in academic literature proved to be impractical. The lack of clear scientific terminology around kissing, combined with a general reticence to discuss such matters in scientific studies, and a very large body of work on kissing in fields such as literature, rendered specialist search engines such as Web of Science unable to produce useful results. Consequently, Brindle et al. settled on a non-systematic review of the literature, combined with searching platforms such as YouTube for evidence of kissing behaviour, with the intent of establishing evidence of kissing in a species, not any data about this. They note that absence of such data does not mean the behaviour does not exist.

In doing this, Brindle et al. set out to answer seven questions, namely: (i) Which primates have been observed kissing? (ii) Does kissing show a phylogenetic signal? (iii) When did kissing first evolve in this group? (iv) How many times did kissing evolve? (v) Has kissing been lost across the course of evolution in any lineages? (vi) Are Neanderthals likely to have kissed? And (vii) how well do different life history variables correlate with the occurrence of kissing?

For the purpose of their study, Brindle et al. defined kissing as 'non-agonistic interactions involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer'. This definition still included behaviours seen in Animals such as Ants, Birds, and Polar Bears, but was much more common and widespread in Primates. Brindle et al. further restricted their study to Old World Monkeys and Apes, groups where there was sufficient data for useful comparisons to be made. 

Non-sexual kissing in Orangutans, a mother kissing her child. Sumatran Orangutan Society.

Brindle et al. make it clear that they were looking for the ultimate cause of kissing rather than the proximal one, which is to say, the reason why Primates first started kissing rather than the reasons for this occurring in any group of Primates who already had the behaviour.

They further note that kissing is likely to have begun as a modification of another behaviour. They note that a recent suggestion has been made that kissing began as a part of a oral grooming behaviour, but doubt the veracity of this, noting that this is not usually how Primates groom, and that no evidence was offered to support the hypothesis. They suggest instead that kissing might be a modification of oral-to-oral food transfer, a common phenomenon in Primates.

Because kissing has been speculatively linked to mating success, Brindle et al. compared the presence of kissing in a Primate species to the mating system used by that species. They also compared the presence of kissing to the diet, food sharing and premastication (chewing food before giving it to another individual, typically an infant) habits of each species, since these behaviours might give a species a preadaptation towards kissing. They note that species with frugivorous or omnivorous diets are more likely to share foods with infants, due to the patchy distribution of foods within the environment. Fruits and meats are also the foodstuffs most commonly premasticated before giving them to infants. Reliable data on premastication was only available for the Great Apes. Data on diets from Neanderthals and Modern Humans was included in the dataset.

Kissing the War Goodbye by Victor Jorgensen. US National Archives and Records Administration/Wikimedia Commons.

Brindle et al. used a phylogenetic tree constructed using the 10kTrees Project V3.0 resource, which was cut to include only Old World Monkeys and Apes, including Humans and Neanderthals. Neanderthals were included not just because they nested within the available data, but because it has been suggested that there is evidence from the oral microbiome that Modern Humans and Neanderthals may have exchanged microbial species via kissing some time after the two species split.

The phylogenetic tree was constructed using a Bayesian Markov chain Monte Carlo framework with the practice of kissing identified as either present or unknown (since it is impossible to assume that a species does not kiss simply because we have not observed it). Neanderthals were also classified as unknown. 

Kissing was found in all species of Great Ape except the Eastern Gorilla, Gorilla beringei. It was also observed in a variety of Old World Monkeys, although chiefly within the Papionini (Macaques and Baboons). Brindle et al. note that much of the data on the Papionini relates to same-sex kissing, at least in part because that was the subject of the studies which had looked at the behaviour in this group.

Brindle et al. conclude that kissing appeared in the common ancestor of all extant Great Apes some time after the split with the common ancestor of all extant Lesser Apes (Gibbons), i.e. between 21.5 and 16.9 million years ago. The trait has subsequently been retained, with the one possible loss in Eastern Gorillas. Kissing was found in eight species of Papionini, but it was impossible to determine that the trait was present in the common ancestor of the group; it has either evolved or been lost multiple times (possibly both). Brindle et al. also conclude that kissing was almost certainly present in Neanderthals.

Phylogeny illustrating the reconstructed evolutionary history of kissing within the Apes (Hominoidea). The occurrence of kissing and other life history variables is displayed at the tips of the tree. Left to right: kissing (observed/not observed); mating system (single/multi-male); diet (folivorous, frugivorous, omnivorous); food sharing (present/absent); and premastication (present/absent). At the tips and nodes of the tree, black circles indicate a trait has been reported or reconstructed as ‘present’; white circles indicate kissing has not been reported or was reconstructed as ‘absent’; grey circles indicate equivocal reconstructions (mean probability less than 0.65); circles are not present where data are missing. The Neanderthal tip represents the mean probability that kissing was ‘present’, based on Bayesian estimation. Maximum Clade Credibility tree created from a sample of 10,000 molecular phylogenies from the 10kTrees project. Brindle et al. (2025).

Brindle et al. also note that there seems to be a strong association between kissing and multi-male mating systems (i.e., systems in which multiple males may compete for the right to mate with a female), with the kissing existing alongside long bonding of females to a single male only in Western Gorillas and some Human societies. Premastication was found in every species in which kissing has been observed, but there was generally an absence of data for species where kissing had not been observed, leading Brindle et al. to refrain from making a judgement on the relevance of this trait.

Brindle et al. conclude that kissing is present in all Great Apes except Eastern Gorillas, and that there is strong evidence that the trait appeared once in the group, between 21.5 and 16.9 million years ago. The data is less clear for Old World Monkeys, where the trait may have appeared once and subsequently been lost in many lineages, or appeared several times in different lineages.

The retention of kissing as a behaviour in Great Apes strongly suggests that it provides an evolutionary advantage, which Brindle strongly suspect is associated with sexual selection, although they do not believe this is sufficient to assert that this was the reason it originally evolved. They note that the limitations of the data they used leave plenty of opportunity for further research on the subject.

See also...

Nasalis larvatus: Estimating the conservation status of the Proboscis Monkey on the Klias Peninsula, Borneo.

Almost half of all non-Human Primates are considered to be threatened by habitat loss due to Human activities, principally the clearing of forests for urban expansion, agriculture and agroforestry, or simply timber. Primates are thought to be particularly vulnerable to these pressures due to their long life-cycles and slow breeding rates, requiring access to a range of forest resources, which in turn makes it hard for them to adapt to changes to their environments, particularly those driven by Humans. Because of this, conservation efforts directed at Primates need reliable information on the distribution of Primates and the resources upon which they rely.

The forests of Southeast Asia are considered to be of international importance by conservations, with a number of biodiversity hotspots. These forests have come under considerable pressure in recent years, with huge areas cleared for agriculture, principally the cultivation of Oil Palms, Elaeis guineensis. Particularly threatened by this expansion are the forests of Borneo, an island with a high level of endemism (i.e. species not found anywhere else) where large areas of forest have been cleared to make way for Oil Palm plantations, and where, in addition to the direct loss of forests, many wildlife populations are additionally threatened by hunters making use of the road networks put in to support the Oil Palm industry.

Proboscis Monkeys, Nasalis larvatus, are large, sexually dimorphic Monkeys endemic to Borneo, which are considered to be Endangered under the terms of the International Union for the Conservation of Nature's Red List of Threatened Species. They get their common name from the prominent noses of the adults, these being more distinctive in the larger males than the smaller females. These Monkeys typically live in groups with a single male, plus several females and their young, although groups of young males are also found. The species is found in lowland forests close to water, such as riverine forests, mangroves and peat swamps, environments which are increasingly threatened by the expansion of Oil Palm cultivation, which is typically carried out on lowland floodplains. This expansion is known to have caused problems for Proboscis Monkeys, with their habitat increasingly being fragmented into smaller areas.

 

A male Proboscis Monkey, Nasalis larvatus, in Borneo. Mark Louis Benedict/Rainforest Rescue.

In 2004 a survey of the Klias Peninsula on the eastern tip of the Malaysian state of Sabah, found 569 individual Proboscis Monkeys living in groups, while a second survey carried out in 2005, which surveyed forests along the major rivers and tributaries of Sabah State by boat, found 818 Proboscis Monkeys on the Klias Peninsula, living in 75 groups. Both surveys found that the Klias Peninsula population was the largest surviving population in the state, and probably the only population large enough to have long-term survival prospects. This reliable presence of Proboscis Monkeys has turned the Klias Peninsula into somewhat of a tourist centre, with a number of companies offering opportunities to view the Monkeys.

In a paper published in the journal Raffles Bulletin of Zoology on 9 June 2021, Henry Bernard of the Unit for Primate Studies-Borneo at the Universiti Malaysia Sabah, Nicola Abram of Forever Sabah, Menaga Kulanthavelu and Felicity Oram, also of the Unit for Primate Studies-Borneo at the Universiti Malaysia Sabah, and Ikki Matsuda, again of the Unit for Primate Studies-Borneo at the Universiti Malaysia Sabah, and of the Academy of Emerging Sciences at Chubu University, the Wildlife Research Center of Kyoto University, and the Japan Monkey Centre, present the results of a previously umpublished survey of the Proboscis Monkeys of the Klias Peninsula, undertaken in 2014, combined with a comparison of this survey to the 2004 survey, undertaken with a view to understaning any changes in the population over a ten year period.

The Klias Peninsula has an area of about 1300 km², and is genrally flat, with a maximum elevation of about 50 m above sealevel. Historically, the peninsula was covered by complex mixture of Mangrove, Nipah Palm forest, freshwater swamp forest, and peat swamp forest, interspersed with open areas and extensive wet grasslands, but much of this has been cleared to make way for small scale Human settlements and farming, as well as larger Rubber and Oil Palm plantations. Bernard et al. carried out surveys by boat, along the Padas, Padas-Damit, Klias, and Bukau rivers, concentrating on areas where Proboscis Monkey populations had been detected by the 2004 and 2005 surveys. These included the Padang Teratak Bird Sanctuary, the Padas Damit Forest Reserve, the Menumbok Forest Reserve, the Binsulok Forest Reserve, the Klias Forest Reserve, the Kg. Hindian Forest Reserve, and the Nabahan Forest Reserve.

 

The Klias Peninsula region in western Sabah, in the northern part of Borneo (inset), Malaysia, and the research sampling sites in riverine, Mangrove, and mixed Mangrove-riverine forests along rivers in Padang Teratak Bird Sanctuary, Padas Damit Forest Reserve, Menumbok Forest Reserve, Binsulok Forest Reserve, Klias Forest Reserve, Kg. Hindian Forest Reserve, and Nabahan Forest Reserve, where the river surveys of the sleeping sites of Proboscis Monkeys were conducted. Bernard et al. (2021).

Proboscis Monkeys are almost always found close to water, favouring flooded forest environments, so boat surveys are generally the best way to survey them. These Primates return to a communal roost in a tree by a riverbank each evening, which makes surveying them relatively easy, particularly if this is done first thing in the morning, before they leave the communal roost, or late in the afternoon, after they have returned to it. Therefore, Bernard et al. carried out surveys of Proboscis Monkeys between 5.30 and 9.30 am and between 4.00 and 6.30 pm, between January and March 2014. All of the main rivers and tributaries in the study area were surveyed once only, to prevent double counting. Where waterways were close together they were surveyed on the same, or at least consecutive, days, for the same reason. A total of 106 km of waterways was surveyed, in sections ranging from 6.5 to 21.7 km. The locations of trees with Monkeys in them were recorded accurately with a GPS receiver, as were the number of Monkeys in each tree, and the type of group present (single male with females or all male group), and the type of forest the location was in (Mangrove, riverine, or mixed Mangrove-riverine). The extent of each habitat type was also recorded by length along the riverbanks, and the preference of the Monkeys for each habitat calculated from this.

Bernard et al. also carried out a number of vegetation surveys. There were carried out on 40 m circular plots, with 4-6 at each site, for a total of 34 plots. Within these plots all trees with a trunk-thickness greater than 10 cm at breast height were assessed, in order to work out the total area of all the tree stems within each plot, the largest tree within each plot, and the species richness at each plot.

To assess the amount of Human disturbance at each site, Bernard et al. used two measures; the distance from the centre of each site to the boundaries of the three closest agricultural sites, and the distance from the centre of each site to the boundaries of the three closest Human settlements.

Bernard et al. estimated the abundance of Proboscis Monkeys at each site by dividing the number of Monkeys by the area of the site, then used this measure to compare to the total area of all the tree stems within each plot, the species richness within the plot, and the size of the largest tree, as well as the distance to agricultural land and Human settlements.

Bernard et al. attempted to esitmate the loss of available habitat to the Proboscis Monkeys by calculating the potential range as being all the land 1 km inland of all the surveyed rivers, calculating the land cover throughout this area for both 2004 and 2014 using Landsat and Google Earth imagery, then calculating the change from suitable to unsuitable for Monkey habitation, based upon the parameters established by the ground surveys.

Finally, Bernard et al. calculated the amount of Monkey habitat currently within protected areas, using data from the Sabah Forestry Department, as well as the extent of the habitat outside these protected areas upon which titles have been granted for large scale plantations or other agribusiness enterprises (typically Oil Palm growth), what proportion is under Native Titile (reserved for the use of indigenous people), and what proportion is under, or available for County Lease (available for development for any purpose, and therefore again likely to be converted to Oil Palm planting), as well as land that was either no form of title, or for which the title was impossible to determine.

During 35 days of fieldwork on the Klias Peninsula, Bernard et al. carried out 42 surveys at seven study sites. They found 679 Proboscis Monkeys living in 75 groups; 44 groups comprising a males and a group of females plus their young, 16 groups of young males, and 15 groups whose composition could not be determined. The Monkeys were at their most abundant in the Padas Damit Forest Reserve in the central part of the peninsula, where there were 200 Monkeys living in 21 groups. Proboscis Monkeys were only found in Mangrove and riverine forests, giving them a total available riverbank habitat of 105 km (65 km of Mangrove forest, 35 km of riverine forest, and 5 km of mixed Mangrove and riverine forest). More Monkeys were found in Mangroves (412) than riverine forest (267), although once the greater availability of Mangroves was taken into account, this indicated a preference for riverine forests (where there were an average of 7.6 Monkeys per km) over Mangroves (where there were an average of 6.3 Monkeys per km).

Within these three forest types there was a significant variation in the vegetation, which also appeared to influence the abundance of Proboscis Monkeys. The species richness of the forests did not appear to be important to the Monkeys, but the total trunk basal area (a measure of tree maturity) was important, with more Monkeys being found where this was highest, possibly because mature trees are important for roosting sites. The closeness of Human habitation or agricultural land did not appear to be a problem for the Monkeys.

Between 2004 and 2014 the Klias Peninsula lost 11 520 m² of intact forest, most of it riverine forest (11 450 m²). In addition, 11 960 m² of degraded forest (forest which still existed as woodland but had lost much of its original biodiversity) and swampland were lost. During the same time the area of Oil Palm plantations within Proboscis Monkey habitats grew by 23 210 m², although 21 860 m² of this (94%) was later judged to be economically unprofitable due to flooding and swampy conditions.

Within the total potential range of the Proboscis Monkeys in 2014, 49% was covered by Mangrove forests, 11% by 'other forest types' (presumably riverine forest), and 18% by degraded areas and swamps, suggesting that 78% of the species total range was covered by suitable habitat and 18% by marginal habitat. The remaining parts of the range were covered by Oil Palm plantations (14%), community agriculture (7%) and aquaculture (1%), all unsuitable habitats for Monkeys.

Much of the remaining potential habitat is not protected, with only 20% of riverine forests and 59% of Mangroves having protected status (i.e. 52% of the remaining suitable environment). In addition 6% of degraded forests and swamplands are protected. This means that 80% of riverine forests, 41% of Mangroves, and 94% of degraded forests and swamplands are at risk of being lost.

Of the remaining forests, 14% (19% of the riverine forest and 13% of the Mangroves) was in Production Forest Reserves, 16 920 m² (all of it Mangroves) in Class V Mangrove Forest Reserves and 29 360 m² (11 820 m² of riverine forest and 17 540 m² of Mangroves) was in Class IV Amenity Forest Reserves, where some commercial exploitation is allowed.

About 27% of the remaining forests (including intact and degraded forests and swamps) is on land which has been granted for development. This toral includes 43% of remaining riverine forests, about 13% of Mangroves, and around 35% of degraded forests and swamps. Furthermore another 16-17% of each forest type is located on State Land, for which permits for use could potentially be issued.

 

Land use and land cover data for 2014/2015 within the 1-km buffer distance from surveyed rivers, overlaid with Proboscis Monkey sightings from the 2004/2005 and 2014 surveys, Protected Areas, and Production Forest Reserve boundaries. Bernard et al. (2021).

Bernard et al. discovered populations of Proboscis Monkeys at all of the surveys locations on the Klias Peninsula, with the largest population on the central part of the peninsula. This suggests that the area can be seen as a population stronghold for the species. The distribution of the Monkeys was similar to that observed in 2004 and 2005. The 2004 and 2005 surveys found 569 individuals in 65 groups, and 818 individuals in 75 groups, respectively, to which the 679 individuals in 75 groups found by Bernard et al. compares favourably. It is likely that the differences in Monkey numbers between the counts relate to the erratic nature of Monkey behaviour and the difficulties of surveying these primates in tropical wetland environments rather than actual fluctuations in the population, and, therefore, that the population remained fairly constant between 2004 and 2014). This would appear to correlate with the limited loss of environment suffered by the Monkeys between 2004 and 2014, during with time only 2% of riverine forests disappeared, and no Mangroves.

The Monkeys were not evenly distributed throughout their environment, apparently prefering riverine forests to Mangroves, and Mangrove forests to all other environments, including mixed riverine/Mangrove forests (although the avoidance of this later environment might be an artefact, as onlu 5 km of this environment was surveyed during the study). Studies in other areas have suggested Probiscis Monkeys prefer riverine forests to either Mangroves or mixed riverine/Mangrove forests, probably due to a higher plant diversity and greater variety of food in this habitat, although Mangroves are still clearly also an important habitat.

The presence of large trees appeared to be particularly important to these Monkeys, probably because of their role as roosting sites. Larger trees both provide a greater number and variety of nesting sites, but also provide a greater distance between these sites and the ground, making it harder for predators to reach the resting Monkeys undetected.

The study did not find any direct impact on Proboscis Monkey populations made by the proximity of Human settlements of agriculture, although this may have been due to the small size of the survey. The largest threat to wildlife populations on Borneo is currently considered to be the expansion of Oil Palm plantations, and, as with other Primate species, Proboscis Monkey's tend to avoid these environments, which are comprised of extensive monocultural stands of an unfamiliar fruit tree. Other than this however, Proboscis Monkeys seem to be relatively tolerant of the presence of Humans, and will even roost close to Human settlements if that is where suitably large trees are to be found. 

Little forest was lost on the Klias Peninsula between 2004 and 2014, but there is clearly the potential for much more to go, with the 28% of riverine forest currently designated for development being of particular concern. Grants of land made under such schemes in Sabah come with time limits; i.e. if the holder does not use the land within a certain period of time, then they lose the title, which can be granted to another user. These schemes currently make no provision for the retention of intact forest, clearly creating an incentive for developers to fell such forests to retain control of the land. As such Bernard et al. strongly recommend that the law be changed to allow landowners to leave areas of forest intact in order to protect the Proboscis Monkeys.

Also of concern are the large areas of forest designated as Production Forest Reserves, particularly the area within the Padas Damit Class IV Amenity Forest Reserve, which is the area with the greatest population of Proboscis Monkeys. These areas were originally intended to be areas of forest which were left largely intact, but where a range of leisure activities were permitted, however a follow up study in 2017-18 found that large areas under this designation had been converted to Oil Palm cultivation.

Proboscis Monkeys require forests along river margins to survive. As such the fragmentation of such environments not only lowers the amount of available habitat, but also removes their ability to move from one area to another, as their preferred environment is essentially linear. Thus protecting the species in future will require not just careful monitoring of their population, but careful management of the areas where they live, and in particular careful monitoring of land use changes likely to have an adverse effect.

Bernard et al. feel it would be particularly useful to grant full protection to Proboscis Monkeys living on government land, as this would have the issuing of land grants within areas of Monkey habitat, and allow the designation of protected areas to protect the Monkeys.

The Klias Peninsula is a popular tourist destination, and viewing Proboscis Monkeys is a popular activity with tourists. Thus good management of the Monkey population is likely to be of economic as well as conservational benefit, drawing tourists to Sabah, and generating income for local communities.

See also...

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