Monodelphis dimidiata (Wagner, 1847)

Astúa, D., 2015, Didelphidae, Handbook of the Mammals of the World – Volume 5 Monotremes and Marsupials, Barcelona: Lynx Edicions, pp. 129-186 : 154

46.

Orange-sided Opossum

Monodelphis dimidiata

French: Opossum a flancs jaunes / German: Orange-Flanken-Spitzmausbeutelratte / Spanish: Colicorto de la Pampa

Other common names: Eastern Short-tailed Opossum, Red-sided Short-tailed Opossum, Southern Short-tailed Opossum, Shrewish Short-tailed Opossum, Yellow-sided Opossum

Taxonomy. Didelphys dimidiata Wagner, 1847 ,

“ Maldonado am la Plata ,” Maldonado, Uruguay.

Based on recent phylogenetic analyses, this species includes M. sorexand M. henseli as synonyms. Monotypic.

Distribution. SE Brazil, S Paraguay, N & E Argentina, and Uruguay.

Descriptive notes. Head-body 9.5-15.4 cm (males) and 8:8—-13 cm (females), tail 3.7-7.6 cm (males) and 3.8-5.7 cm (females); weight 40-84 g. Sides of the body of the Orange-sided Opossum are orangish, and dorsal fur is typically ash-colored to olive brown, with no stripes. Dorsal color extends to legs and sides of head; head lacks any mid-rostral stripe or eye-rings. Specimens assigned in the past to the form sorex have gray dorsal fur thatis finely grizzled tawny-yellowish on forequarters, neck, and head. Rump, body sides, sides of neck, and cheeks are dull rusty to reddish. Tail length is ¢.40-45% of head-body length (c.60% in specimens identified as sorex), and tail is naked. Ventral fur is pale to bright orange-tan, gray-based. Fur is short and smooth. Females lack a pouch. Number of mammae in typical Orange-sided Opossums is not reported, but a litter of 16 young was observed. In specimens traditionally identified as sorex, however, 27 mammae are present, with 13 on each side and the usual medial mamma. This is the largest number of mammae found in any species of opossum. The Orange-sided Opossum has a 2n = 18, FN = 30 karyotype, with small acrocentric X-chromosome and Y-chromosome. A FN = 32, with a biarmed X-chromosome and minute dot-like Y-chromosome, has also been reported. In Argentina, molt of a subadult began mid-September (end of winter/ beginning of spring), starting on neck, and then progressing to shoulder blades, sides back, and hindquarters, and on head up to eyes; molt was finished by mid-November; and new coat was duskier than pre-molt coat. Orange-sided Opossum is characterized by extreme sexual dimorphism. Adult females weigh only ¢.40% as much as males, although they can reach 85% of males’ lengths. Canine teeth and heads of males are also disproportionately larger than females.

Habitat. Pastures, wetlands, marshes, pampas grasslands, and riparian areas next to waterways. Orange-sided Opossum seems to prefer areas with dense native grasses, which is where highest densities are usually found. It occurs also in creek edges and rocky areas. Specimens usually identified as sorex occur in Atlantic Forest habitats, apparently tolerating some degree of disturbance because they can be found in cultivated areas close to forests and in ecotone vegetation.

Food and Feeding. The Orange-sided Opossum feeds mostly on insects; 100% of stomachs analyzed in one study contained insects. Vertebrate remains, mostly mammals, were found in 33% of stomach samples, and 9% contained plant matter. Insect groups consumed included Hemiptera, Hymenoptera (ants), Lepidoptera larvae, and Arachnida. Small rodents consumed included the Little Laucha (Calomys laucha), the Dark-Furred Akodont (Necromys obscurus), the Flavescent Colilargo (Oligoryzomys flavescens), and Robert's Hocicudo (Oxymycterus roberti). The holotype had a stomach full of insects, mostly ants and hemipterans. Captive Orange-sided Opossums consumed mollusks (snails), leeches, earthworms, and isopods when they were offered; they also ate fresh meat. When offered live House Mice (Mus musculus), some female Orangesided Opossums hesitated to attack them, and if they did, they mostly attacked young mice. Males and some females were more aggressive and did not hesitate to attack and kill adult and young mice. Orange-sided Opossums assume a typical semi-erect posture when feeding and carefully manipulate foods items; sometimes prey items are caught with the paw, sometimes with the mouth, and sometimes with both. Careful manipulation includes rolling or scratching hairy moth caterpillars to remove all hairs before eating them. Dietary studies on specimens identified as sorex have found invertebrates in almost 95% of the stomachs analyzed. Specimens of Orange-sided Opossums from southern Brazil fed on Coleoptera, Hymenoptera, Blattodea, Orthoptera, Decapoda, Opiliones, and Acari. Mammalian remains were found in about one-third of the samples. Other items occasionally found in feces of Orange-sided Opossums included crustaceans, birds, and seeds, mostly of Cecropia sp. (Urticaceae) and Rubus sp. (Rosaceae) . Seeds were found in less than 10% of samples, but level of damage to seeds indicated that fruit was consumed and Orange-sided Opossums may act as dispersers of these seeds. Captive specimens accepted and fed on crickets, grasshoppers, small frogs, raw chicken meat, and fruits. They manipulated food with their hands to bring it to the mouth, biting and chewing one bite at a time, usually positioning items to be bitten by the last molars.

Breeding. To make nests, captive female Orange-sided Opossums carried leaves with theirtails, after gathering them with forefeet and pushing them below their bellies— a behavior similar to that of other species of opossums. Reported litter sizes varied from eight to 14 young, with a maximum of 16 young. Neonates weighed 0-08-0-11 g. Two different studies showed that Orange-sided Opossums are semelparous, meaning that both sexes live only one year at most and breed only once during their lifetime. Monthly distribution of body sizes, showing that large (mature) individuals disappear from the population right after the breeding season, and actual disappearance of mature individuals in capture-mark-recapture studies, support this hypothesis. Onset of the breeding season in Argentina occurs in spring, with a sudden increase in body weight in both sexes, but particularly in males,just prior to breeding. By autumn, mature Orange-sided Opossums disappeared; mature males were captured only in spring and mature females from mid-spring to late summer). Near Buenos Aires, Argentina, the whole population simultaneously reaches sexual maturity in December, and the breeding season is in December—January.

Activity patterns. In Argentina, Orange-sided Opossums were active only during the day from late morning to sunset, with most activity during late afternoon. Specimens have been captured at 13:00 h, 15:00 h, 17:30 h, and 19:00 h. In high-elevation grassland in southern Brazil, Orange-sided Opossums are diurnal, having been captured after sunset only occasionally, but a captive specimen was active night and day.

Movements, Home range and Social organization. Reported densities of Orange-sided Opossums are less than 200 ind/km?. Although they are considered primarily ground dwelling, captive individuals can climb well and can jump, and although their tails are short, they are used when climbing up or down.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Orange-sided Opossums has a wide distribution, occurs in several protected areas, and appears to have some tolerance of habitat modification. Nevertheless, it has been reported that it might be in decline. Although no major conservation threats appear to face the Orange-sided Opossum, numerous populations are threatened by progressive habitat conversion to agriculture or urbanization. Other biological features obtained in detail more recently, such as low densities and possible dependence on native grasslands, coupled with its peculiar reproductive seasonality, point to a probable higher vulnerability than previously supposed. In particular, growing expansion of agriculture in the Pampean region can isolate populations of Orange-sided Opossums remaining near pristine grassland fragments.

Bibliography. Abdala et al. (2006), Baladrén et al. (2012), Blanco et al. (2013), Busch & Kravetz (1991), Caceres (2005), Carvalho, Oliveira, Langguth et al. (2011), Carvalho, Oliveira, Nunes & Mattevi (2002), Casella & Caceres (2006), Chemisquy & Prevosti (2014), Emmons & Feer (1997), Gardner (2005), Gonzalez & Claramunt (2000), Mares et al. (1996), Melo & Sponchiado (2012), Pavan et al. (2014), Pine & Handley (2007), Pine, Dalby & Matson (1985), Pine, Flores & Bauer (2013), Redford & Eisenberg (1992), Reig et al. (1977), Smith (2008f), Solari (2010), Svartman (2009), Vieira & Paise (2011), Vilela et al. (2010), Voss, R.S. & Jansa (2009), Voss, R.S., Myers et al. (2009), Voss, W.A. (1975).

Figures

65. Pygmy Opossum (Chacodelphys formosa), 66. Agricola’s Opossum (Cryptonanus agricola), 67. Chacoan Mouse Opossum (Cryptonanus chacoensis), 68. Guaiba Mouse Opossum (Cryptonanus guahybae), 69. Unduavi Mouse Opossum (Cryptonanus unduaviensis), 70. . Emilie’s. 9 Opossum.. Gracilinanus. (null), 71. Aceramarca Opossum (Gracilinanus aceramarcae), 72. Agile Opossum (Gracilinanus agilis), 73. Wood Sprite Opossum (Gracilinanus dryas), 74. Northern Gracile Opossum (Gracilinanus marica), 75. Brazilian Gracile Opossum (Gracilinanus microtarsus)

Distribution. SE Brazil, S Paraguay, N & E Argentina, and Uruguay.