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.
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.
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.
See also...
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