By Jacalyn Giacalone, Ph.D.

The pygmy opossum is the smallest marsupial found on BCI. It is easily mistaken for a mouse, but several characteristics distinguish these petite opossums from rodents. Every paw has a thumb which is opposable with the other digits, an adaptation for grasping thin vines. The tail is prehensile (grasping), mostly hairless, scaly, and thick where it joins the body. Unlike rodents, opossums will defend themselves by opening the mouth, hissing and snapping. The jaws gape open much more widely than in rodents, and the teeth are very sharply pointed and numerous. Dark hair forms a ring around the large eyes, like a mask, and the body is cinnamon brown on top with a creamy yellow belly. Pygmy opossums have a combined head and body length of 5 to 8 inches, with a tail somewhat longer. Females weigh up to 60 grams and males 110 g.

Pygmy opossums are nocturnal and arboreal, and usually travel alone. Their greatest activity is on moonlit nights, according to Dr. Robert Enders, who studied many mammal species on BCI in the 1930s and later. This finding is unexpected because they are preyed upon by owls, who find their prey easily in moonlight. They are not found in permanent nests and use whatever shelter is available at daybreak, sometimes an abandoned bird nest or tree hollow. Females build temporary nests for their young. They can be found in secondary forest, at forest edges, in tree fall gaps, and in other disturbed areas.

The population density of Marmosa on BCI tends to be highest at the end of the wet season and during the dry season, and varies greatly from year to year. They are difficult to see in the wild because of their size, their strictly nocturnal activity, and their preference for the cover of thick vine tangles. They have been caught in squirrel traps with a bait of fresh fruit such as bananas, papayas, and mangoes. The natural diet consists of insects and fruit.

Pygmy opossums appear to live an average of only one year in the wild, and during that brief life they breed just once, between April and September. They have a reproductive strategy that is termed semelparous- that is, they have a short life with one breeding which coincides with favorable ecological conditions. This strategy is also used by some of the tiny Australian marsupials of the genus Antechinus , as well as some of the Madagascan insectivores, the tenrecs. In the case of Antechinus, the strategy is carried to an extreme: males are disposable, meaning that they quickly age and die soon after one mating. Such is not the case in Marmosa, who at least have a chance to live out the season and mate several times.

Unlike many other marsupials, this species has no pouch. After a gestation period of about 14 days, up to 13 young are born. The newborn, which are naked, blind and helpless, attach firmly to the teats for 30 days. Although the mother has no protecting pouch for her babies, she carries them everywhere until they have grown a little. She walks with her hip region raised high to keep the young from dragging and being injured. When the young start to detach from the teats, the mother places them in a nest for about 30 days.

An unexpected adaptation found in Marmosa robinsoni is a very large kidney that has a specialized structure usually found only in desert mammals. If the kidneys provide pygmy opossums with an effective water conservation system, then perhaps this tells us that the stresses of dry season in a rainforest are quite severe for small mammals.