Feeding ecology and diet

New World opossums are generalist omnivores. Food items include insects, insect larvae, worms and other invertebrates, bird's eggs and nestlings, fruit, carrion, reptiles, amphibians, birds, and small mammals. Almost all species are omnivorous with varying degrees of frugivorous or carnivorous tendencies. Species in the genera Caluromys and Caluromysiops are considered primarily frugivorous, although they do include animal matter in their diet. Smaller species such as those in the genera Marmosa, Marmosops, Gracilinanus, etc., tend to be primarily insectivorous with some eggs, fruit, and meat also included. Mouse opossums kept in captivity will not hesitate to attack large moths or grasshoppers, grasping them with their hands and quickly administering killing bites all over the body of the insect. They normally discard the wings and legs, and other chitinous, undigestible pieces. Caterpillars are rapidly rolled and rubbed against substrates to remove stinging hairs. Fruit can be plucked off tree branches directly or can be eaten after the fruit has fallen to the ground. Sweet, juicy fruits are preferred, such as zapotes, blackberries, guavas, chirimoyas, etc., although other drier fruits such as wild figs and wild cacao, as well as introduced, human-grown varieties like bananas, figs, citrus fruits, apples, and cherries, are also consumed. Common and four-eyed opossums take large amounts of fruit from secondary forest species such as Cecropia. Common opossums (Didelphis) seem to be better seed dispersers and to invest more in seeking fruits of Cecropia than gray four-eyed opossums (Philander).

The water opossum (Chironectes minimus) is particularly interesting in that it seems to be the only New World opossum species that is completely carnivorous. This species feeds mainly on fish, crustaceans, mollusks, and frogs. Its long fingers and bulbous fingertips are held outstretched while the animal swims. Under water, their hands are used to feel under rocks and logs for potential prey. They coexist and overlap locally with the common Neotropical otter (Lontra longicaudis). In the Old World, in Africa and Asia, common otters coexist with clawless or small-clawed otters (genera Aonyx and Amblonyx), which have similar habits and hand morphology to the water opossum. This allows them to coexist by partitioning food resources. The water opossum and the otter may be in the same situation, with the otter feeding primarily on fish and less on crustaceans and mollusks, whereas the opossum probably takes more invertebrates than fish.

In Mediterranean habitats of South America, Thylamys el-egans has been shown to regulate the frequency of torpor periods, and therefore energy expenditure, by the availability of food. When food is plentiful, animals never enter torpor, and frequency of torpor periods increases with decreasing food availability.

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