Reproductive biology

Information on reproduction is available for only a few species. Some species show one defined reproductive event during the year that coincides with the greatest seasonal abundance of food. Other species, particularly those inhabiting

A southern opossum (Didelphis marsupialis) with its young. (Photo by Laura Riley. Bruce Coleman, Inc. Reproduced by permission.)

more tropical climates, may have two reproductive periods every year or even have an indistinct pattern in which reproductive females can be found in every month of the year.

New World opossums are polygamous. Like other marsupials, have a very short gestation period, followed by a long developmental period. After a gestation between 12 and 15 days, the embryo-like newborn, naked, with eyes and ears closed but strong arms and well-developed claws, crawls along a path between the cloaca and the marsupium (or mammary region in those species with no marsupium) that has been licked by the female. Once in the mammary region, each individual attaches to a nipple, where it will remain for four to eight weeks, depending on the species. Offspring can then detach from the nipples for the first time. When they are too large to fit in the female's pouch, they crawl on her back or are simply dragged behind her, while still attached to the mammae. Young remain dependent on their mothers until they are two to four months old, after which they are weaned and proceed to disperse.

Litter size varies greatly in different genera. Five to 12 and up to 16 offspring are born in Monodelphis, two to five in Caluromys, four to 12 in Marmosa, two to five in Chironectes, six to 15 in Didelphis, and one to nine in Philander. Likely, many more young are born than those found by scientists attached to the mammae, but they die before they can attach themselves to a nipple.

Sexual maturity is attained at three to nine months in different genera. Many species construct nests inside rotten trees both standing and fallen, while others have nests on the ground or, in the case of the water opossum, in tunnels excavated in stream banks. Some species of mouse opossum utilize hummingbird nests as their own resting places. New World opossums use primarily plant matter to construct the spherical nests. These materials are transported in the partially rolled-up tail while the animal moves to the nest.

Many species are semelparous with males dying shortly after mating and females after weaning their first and only lit-

A Virginia opossum (Didelphis virginiana) uses its tail as an aid in climbing down. (Photo by Animals Animals ┬ęTed Levin. Reproduced by permission.)

ter produced. Females of some species, however, can breed twice in the same year with only a few months apart between births, and individuals of a few species can have two years of reproductive activity.

Essentials of Human Physiology

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