Opossum seen grooming Deer for Ticks in Vermont.

The Vermont Wildlife Coalition has released a camera-trap photograph in which as Virginia Opossum, Didelphis virginiana, can be seen removing Ticks from the face of a Deer, which has apparently approached the Marsupial for this service. Virginia Opossums are known to be major consumers of Ticks, and therefore thought to be a significant control of Tick-borne diseases such as Lyme disease, which they usually acquire by visiting Tick-infested areas of woodland and then picking off any Ticks that attempt to attach to them, but this is thought to be the first time another animal has been seen approaching an Opossum for grooming, a form of behaviour more associated with marine organisms such as Cleaner Fish.

A Virginia Opossum, Didelphis virginiana, grooming a Deer in a forest in Vermont. Vermont Wildlife Coalition.

Virginia Opossums are North America's only native Marsupial, and, unlike many Marsupials, are extremely adaptable in their habits, rapidly taking to new environments and food sources when these become available. They were confined to the southwestern United States, Mexico, and Central America as far as Costa Rica, until the early twentieth century, since when they have colonised much of the rest of the United Stares and southern Canada, spreading through man-made habitats, such as farms and gardens, into areas such as the forests of New England where they were not previously found.

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Unexpected social behaviour in South American Opossums.

Many species of Placental Mammals are highly gregarious, often exhibiting complex social structures. Such behaviour is much more unusual among Marsupials, with some Australian species living in small colonies with limited social interaction, and occasionally forming larger ‘mobs’ under certain conditions, and almost no social interaction known among American Marsupials. Most American Opossums, Didelphidae, live more-or-less totally solitary lives as adults and under most circumstances are hostile towards co-specifics (members of the same species). Opossums can learn to tolerate one-another in captivity (though they usually do this by ignoring one-another), but examples of social interaction in the wild are very rare. Juveniles, as with all Mammals, live with their mothers until close to maturity, and in some species sub-adult siblings have been shown to continue to live together for short periods after the mother has left the communal nest. Some species also show extended mating periods, with males and females remaining together for several days prior to mating (though interactions can be fairly aggressive during this period). However the fossil Dideplphid Pucadelphys andinus, which lived in the Palaeocene, appears to have been far more social, and to have lived in larger groups, suggesting that Opossums have not always been solitary animals, and that such behaviour could be occurring unrecorded in modern members of the group.

In a paper published in the journal Biology Letters on 17 June 2015, Diego Astúa, Rafael Carvalho and Paula Maia of the Laboratório de Mastozoologia at the Universidade Federal de Pernambuco, Arthur Magalhães of the Laboratório de Mamíferos at the Universidade Federal da Paraíba and Diogo Loretto of the Laboratório de Biologia e Parasitologia de Mamíferos Silvestres Reservatórios describe the results of a study of the utilization of artificial nest boxes by Opossums in Brazilian Atlantic forests and urban sites on the campus of the Universidade Federal de Pernambuco which revealed several unexpected examples of social behaviour.

Astúa et al. came across ten examples of cohabitation, or close association, among five species of Opossums in Atlantic forest close to Rio de Janeiro. In the first a pair of Brazilian Gracile Opossums, Gracilinanus microtarsus, comprising an adult male and a subadult female were found cohabiting a nest box at the start of the mating season. In the second a pair of Tate's Woolly Mouse Opossums, Marmosa paraguayana, again comprising an adult male and a subadult female, were also found cohabiting in a nest box at the start of the mating season. In the third three juvenile Bare-tailed Woolly Opossums, Caluromys philander, two females and a male were found in the same nest box at the end of the breeding season; DNA testing revealed these to be siblings. In the fourth two juvenile female Big-eared Opossums, Didelphis aurita, were also found cohabiting a nest box at the end of the breeding season; these were not DNA tested, but are thought likely to have been siblings. In the fifth instance two subadult female Gray Slender Opossums, Marmosops incanus, were found sharing a nest box at the end of the breeding season. The six the instance also involved a pair of subadult Gray Slender Opossums cohabiting at the end of the breeding season, though in this case they were a male and a female. The seventh example a pair of adult Gray Slender Opossums were found cohabiting in a nest box at the end of the mating season, again a male and a female. The eighth case involved a pair of adult Bare-tailed Woolly Opossums, a male and a female, found cohabiting at the end of the mating season, in this case the female had four newborn infants in her pouch. The ninth example comprised two adult Bare-tailed Woolly Opossums of unknown sex found together in a nest box at the end of the mating season. The tenth case involved two adult female Opossums of different species, one Tate's Woolly Mouse Opossum and one Bare-tailed Woolly Opossum found nesting in close association (nests about 2.5 m apart within the same tree) during the mid-breeding season.

However the most remarkable discovery came at the Universidade Federal de Pernambuco campus, not within a nest box, but in a nest constructed within an electrical connection box, to which the team were called by university maintenance staff. Here a total of 13 White-eared Opossums, Didelphis albiventris, were found cohabiting within a den. All the animals were awake when investigated, and showed no sign of hostility towards one-another. The group comprised at least three adults, plus several sub-adults and juveniles. One of the adults sought to protect the juveniles and subadults during removal.

Specimens of Didelphis albiventris found in a communal den at Universidade Federal de Pernambuco campus, Recife, Brazil. At least nine specimens (out of a total of 13) of different sizes can be seen on the photo. Astúa et al. (2015).

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Hunting the lost Opossum.

The One-striped Opossum, Monodelphis unistriata, was described as a species by JA Wagner from a skin and skull of an adult male animal, brought back from Brazil by the explorer Johann Natterer in 1821, which...

A 'South American' Marsupial from the early Eocene of Australia.

Modern Australian Marsupials are generally held to be a distinct evolutionary lineage, distinct from South...

New species of Ground Opossum from eastern Bolivia.

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Hunting the lost Opossum.

The One-striped Opossum, Monodelphis unistriata, was described as a species by JA Wagner from a skin and skull of an adult male animal, brought back from Brazil by the explorer Johann Natterer in 1821, which had apparently been obtained near Itararé in São Paulo State, Brazil. The material was deposited in the Vienna Museum, and is now in the collection of the Naturhistorisches Museum Wien, though at some point the skull has been lost. In a purported second specimen of Monodelphis unistriata is in the collection of the Museo Argentino de Ciencias Naturales in  Buenos Aires, Argentina. This was captured by the explorer Luis Boccard in April 1899, somewhere in what is now northeastern Argentina, and also comprises a skin and skull (which in this case has not been lost) of an adult male Opossum.

In a paper published in the journal Zootaxa on 18 April 2013, Ronald Pine of the Biodiversity Institute at the University of Kansas and the Division of Mammals at the Field Museum in Chicago, David Flores of the Museo Argentino de Ciencias Naturales and Kurt Bauer of the Naturhistorisches Museum Wien, redescribe the two specimens, and discuss the results of an DNA analysis carried out in order to determine that the two specimens were in fact the same species.

The One-striped Opossum, Monodelphis unistriata; (A) Austrian specimen and (B) Argentinean specimen. Scale bares are 20 mm. Pine et al. (2013). 

The DNA analysis confirmed that the two specimens are members of the same species, and do not belong to the same species as any other known specimen. The distribution and habitat of the One Striped Opossum remains a mystery, and it is unclear if the species still exists. The study suggests that its closest relative is Ihering's Three-striped Opossum, which is known from the southeast of Brazil.

Lateral (above, left), ventral (above, right), and dorsal (below, left) views of incomplete cranium, and lateral (below, right) view of incomplete right dentary of the only known extant skull of Monodelphis unistriata. A distinctive combination of characters is shown, i.e., first upper premolar separated from canine and P2; infraorbital foramen placed at level of P3; posterior tips of nasals projecting between frontals, being not manifestly widest at frontomaxillary suture; and anterior tip of lacrimal at level of P3. Scale bar is 5 mm. Pine et al. (2013). 

The approximate locations of the two sites where Monodelphis unistriata specimens were collected. ICUN Red List.

See also A 'South American' Marsupial from the early Eocene of Australia, New species of Ground Opossum from eastern Bolivia, What killed the Australian Thylacine and Walking with Giant Wombats.

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New species of Ground Opossum from eastern Bolivia.

Ground Opossums of the genus Monodelphis are common on the grasslands of South America, with at least twenty known species, most of which are widespread in distribution. In June 1990 a joint expedition by the American Museum of Natural History and the Museum of Southwestern Biology at the University of New Mexico, was temporarily stranded at Santa Rosa de la Roca in eastern Bolivia, by a vehicle breakdown. This was not the area they intended to sample, but some traps were put down opportunistically on the border  between a wet savanna and a forrest. These yielded three common Rodents and a single specimen of an unknown Opossum. No similar Opossum has been found in the intervening twenty years, making this specimen somewhat of a mystery.

In a paper published in the American Museum Novitates on 18 April 2012, Robert Voss of the Division of Vertebrate Zoology (Mammalogy) at the American Museum of Natural History, Ronald Pine of the Natural History Museum and Department of Ecology and Evolutionary Biology at the University of Kansas, and Sergio Solari of the Instituto de Biología at the Universidad de Antioquia in Medellín, Colombia, formally describe the Santa Rosa de la Roca Opossum for the first time; the specimen had been mentioned in a number of previous studies, but never previously given a full taxonomic description.

The Santa Rosa de la Roca Opossum. Voss et al. (2012).

The specimen is named as Monodelphis sanctaerosae, the Santa Rosa de la Roca Opossum. It is a 108 mm long grey Opossum with reddish-brown fur on the sides of its head and around its ears. It is considerably smaller than its closest relatives, Monodelphis domestica and Monodelphis glirina.

The conservation status of Monodelphis sanctaerosae is unclear; only a single specimen has ever been captured, from a location which has not been well sampled by biologists. It is unclear how abundant the species is at the type location, or if the species is found anywhere else.

Map of Bolivia showing the known distributions of Monodelphis domestica, Monodelphis glirina and Monodelphis sanctaerosea. The numbers provide a key to museum specimens. Voss et al. (2012).

See also New Caviomorph Rodents from the Early Oligocene Tinguiririca Fauna of the Andean Main Range of central Chile, A new Ground Sloth from the Late Miocene of Argentina, The origin of domestic dogs, Three new species of Tapir from the Early Eocene of Pakistan and Mammals on Sciency Thoughts YouTube.

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