Gracilinanus microtarsus, Wagner, 1842

Russell A. Mittermeier & Don E. Wilson, 2015, Didelphidae, Handbook of the Mammals of the World – Volume 5 Monotremes and Marsupials, Barcelona: Lynx Edicions, pp. 129-186 : 172-173

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Gracilinanus microtarsus


75. View Plate 8: Didelphidae

Brazilian Gracile Opossum

Gracilinanus microtarsus

French: Opossum du Brésil / German: Kleinful3-Zwergbeutelratte / Spanish: Marmosa gracil de pies pequenos

Other common names: Brazilian Gracile Mouse Opossum

Taxonomy. Didelphys microtarsus Wagner, 1842 ,

“ Ypanema ,” Sao Paulo, Brazil.

There is strong geographical variation along its distribution, with three clear phenotypes, and geographically structured populations with high genetic divergence, but combined analyses of morphological and genetic characters are inconclusive in recognizing these three phenotypes as distinct taxa, and they are thus currently treated as a single species. Additionally, the presence of G. microtarsusin Lagoa Santa, Minas Gerais, the type locality of G. agilis , raises the possibility that the type specimen of G. agilis could prove to be a specimen of G. microtarsus , as currently understood. Monotypic.

Distribution. E & SE Brazil (from C Bahia S to N Rio Grande do Sul) and NE Argentina (Misiones). View Figure

Descriptive notes. Head—body 6.7-18.5 cm,tail 7-177 cm; weight 12-58 g. The Brazilian Gracile Opossum has a uniformly reddish-brown dorsal coat of gray-based hairs with orange to reddish-brown tips. Head is colored as dorsum, with no mid-rostral stripe. Black, large, prominent eye-rings surround black eyes, running from nose to bases of ears (although, in some populations, they project to but do not reach bases of ears); cheeks are same color as rostrum. Transition from darker crown fur and clearly paler mid-rostral fur is marked. Tail length is ¢.135% of head-body length, and tail is brown and usually bicolored (darker dorsally than ventrally), although uncolored tails are present in some populations. Ventral fur is usually gray-based cream (varying from whitish to yellow-gold to pale brown) from chin to anus, and tinged orange on neck and chest; some populations exhibit gray-based hairs only from lower pectoral area to anus. Throat gland is present. Fur is long (c¢.10 mm on dorsum) and soft. Feet are creamy-whitish, and lateral carpal tubercles are present in old males. Ventral fur of forelegsis gray-based and yellow-tipped. Ears are large (more than 21 mm), translucent, tinged orange at bases and brown toward tips. Females lack a pouch and have 13 mammae, six on each side and a medial mamma. The Brazilian Gracile Opossum has a 2n = 14, FN = 24 karyotype, with all biarmed autosomes, and with a small biarmed or acrocentric X-chromosome and a small acrocentric Y-chromosome. A FN = 20 has also been reported. Skull size is sexually dimorphic.

Habitat. Atlantic Forest biome from sea level to 1800 m and also cerrado formations bordering the Atlantic Forest. The Brazilian Gracile Opossum has been captured in secondary forests, forest fragments, and Eucalyptus plantations.

Food and Feeding. Analysis of stomach contents of five Brazilian Gracile Opossums from an Atlantic Forest reserve indicated consumption of Coleoptera, Hemiptera, Hymenoptera (including but not restricted to ants), Isoptera, Lepidoptera (adults and larvae), Orthoptera, Arachnida, and snails. Seeds of Anthurium ( Araceae ) were found in feces of Brazilian Gracile Opossums in another Atlantic Forest site, and seeds of Clidemia, Miconia ( Melastomataceae ), Myrtaceae , Santalaceae , and Smilacaeae in a cerrado area. It can be an important disperser of seeds of several riparian forest species in cerrado and aroids ( Araceae ) in the Atlantic Forest.

Breeding. Female Brazilian Gracile Opossums make nests with dead leaves in tree hollows, and one was found using an abandoned nest of a mottled piculet (Picumnus nebulosus) in a tree hollow that had an internal chamber ¢.5 cm wide and ¢.10 cm high. Although it used the bird’s nest, it added other leaves that were not initially used by the bird. They also built nests in bamboo nest boxes set on trees in a secondary Atlantic Forest fragment in south-eastern Brazil, creating nests with a central chamber where the female rested. Nests are built early in the rainy season (September— November) using dry and fresh (possibly actively collected) leaves and apparently are used only for breeding, because only females with young were found using the artificial nests. The breeding season has been estimated to occur from September to December. During three years, females only used nest boxes, indicating that breeding is seasonal and coincides with onset of the rainy season. Young are therefore weaned at periods of maximal rainfall (and thus increased resource availability). In another five-year population study using artificial nest boxes in an Atlantic Forest reserve in south-eastern Brazil, male Brazilian Gracile Opossums were recorded in nest boxes on several occasions. At this site, only one of nine females using nest boxes had a litter, with four young. In an Atlantic Forest site in south-eastern Brazil, litter sizes varied from four young in onesite to 9-12 young in another. Depending on the study site, the breeding season has been estimated in September-December, December— February, or October-March, during the wet season, all in Atlantic Forest sites. In all these cases, breeding roughly coincided with onset of the rainy season or its peak. An additional two-year population study in another Atlantic Forest site recorded juveniles only in January—May (thus resulting from mating in the previous year or early that year), corresponding to end of the wet season and onset of the dry season. In this study, no individuals captured one year were recaptured the following year, suggesting a pattern of semelparity, although number of captures was insufficient to adequately quantify survival rates.

Activity patterns. As with many other species of opossums, the Brazilian Gracile Opossums is frequently reported to be nocturnal, mostly because most trapping protocols use traps set overnight. The only reported data on the activity pattern of the Brazilian Gracile Opossum in south-eastern Brazil confirmed a nocturnal habit with two peaks of activity.

Movements, Home range and Social organization. Based on frequency of captures in the arboreal stratum, particularly in those studies that sample both understory and canopy, the Brazilian Gracile Opossum is considered predominantly arboreal, being captured almost exclusively in traps set on trees and predominantly in those set in the canopy, although it does occasionally forage on the ground. It also used artificial nest boxes set 3 m high. Home ranges of the Brazilian Gracile Opossum in an Atlantic Forest area were (0-4—0-5 ha. A population from a cerrado area in south-eastern Brazil had home ranges of 0-:05-0-63 ha, and home ranges were directly influenced by body size (although this population was not studied during a breeding season, and therefore influence of nutritional requirements due to reproduction was not assessed). Brazilian Gracile Opossums are able to move along branches, and they move relatively faster than larger species of opossums on experimental horizontal supports. They are also able to jump from one branch to another to move across gaps. Females occupied artificial nest boxes as close as 70 m and as far as 350 m from each other. Average distance moved between two successive captures was 65-2 m for males and 38-8 m for females; average maximal distances moved were 84:6 m by males and 59-9 m by females. Density, estimated based on nest occupancy, was 500 ind/km?, although this is clearly an underestimate because it only included nesting females during the reproductive season. Brazilian Gracile Opossums also seem to be strictly solitary because only a single female with herlitter was found in any of the artificial boxes.

Status and Conservation. Classified as Least Concern on The IUCN Red List. The Brazilian Gracile Opossums is tolerant of habitat modification, presumably has a large overall population, and occurs in several protected areas. There can be significant deforestation in parts ofits distribution, but in Atlantic Forest fragments interconnected with narrow vegetation corridors, Brazilian Gracile Opossums were only recorded in corridors and the cultivated matrix between fragments and not in fragments themselves, suggesting some level of tolerance to edge effects.

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Gracilinanus microtarsus

Russell A. Mittermeier & Don E. Wilson 2015

Didelphys microtarsus

Wagner 1842