WHAT IS THE OPOSSUM POUCH AND WHAT IS IT FOR? - BIOLOGY - 2025

The opossum marsupium is a specialized bag of the females of these marsupials that has the function of carrying, protecting and feeding the young of this species. When they are born, the young go out to the pouch, where they feed on the mother's nipples. Up to 13-14 young can get hold of the teats, leaving the pouch between 70-125 days after birth.

Opossums, well known in Mexico and Central America and called opossums in the rest of the Spanish-speaking countries, are omnivorous mammals that usually inhabit trees and burrows.

Open marsupio, species Didelphis Virginiana. Image via Konrad Wothel / Minden Pictures

They are characterized by the marsupium, a kind of bag or fold of the skin, within which the development period of the young ends. These are born before having completed all the phases of it.

This characteristic is present in other animals of the same subclass, such as kangaroos, koalas, etc., although there are smaller genera, such as the mouse opossum or Mexican marmosa, which lacks a marsupial bag.

The development and definition of the pouch is also a variable characteristic between the different genders, since it can range from a very rudimentary bag to a fully formed one that is visible to the naked eye.

The gestation form of the opossum or opossum does not include a placenta and lasts only 12 and a half days inside the uterus while the rest of the development process takes place in the pouch. It is the shortest period of calf formation among all mammals.

This fact is an advantage for the study of the species and for the advancement of biology as a science, since it allows us to directly observe the growth of newborns and their behavior within the pouch. In fact, much of the existing information about the opossum is due to this type of development and morphology studies.

It may also interest you: how do opossums take care of their young?

What is the opossum pouch? Physical characteristics

This fold of the skin of female opossums is located on the mammary glands, although in the case of the Didelphidae category, it does not cover them in their entirety, from where the young will feed.

It is usually better developed in tree-dwelling species than in terrestrial ones and may have the opening in the front or back.

In the photo below you can see an open pouch, which although it belongs to the species Didelphis Virginiana (also known as the American opossum) is very similar to the opossum pouch.

The hair that is observed around the marsupium of the female that is feeding her young, usually turns an amber color, product of secretions of sweat glands located within the marsupium.

There are three differentiable regions in the opossum marsupium, one dorsal, one located between the thorax and the abdomen and the one closest to the genital tract, called the pars pudenda.

Females can open or close the pouch voluntarily, since it is provided with a sphincter or set of musculo-skeletal fibers that, when contracted, make this function possible, which is necessary before delivery.

It is a way of regulating the temperature and preparing the internal environment of the bag. It is also useful at the time of delivery, as it allows the pouch to relax so that the embryos can reach it more easily.

The function of the pouch: development and protection

The pouch basically acts as an incubator, since newborn marsupials are so small that their weight is never greater than 1 gram, even a complete litter does not exceed 1% of the mother's body weight.

Despite the fact that it is a species that is born practically in an embryonic state due to the immaturity of its body, the forelimbs and muscles are sufficiently developed and they already have small claws on the front legs.

In this way, they are able to crawl from the vagina (when they are born) using the mother's fur, to the pouch, to begin the lactation period by adhering to the mother's breasts.

How long the hatchlings will last inside the pouch is variable. It will depend on factors such as the body mass of the mother, the size of the litter and the number of litters per year, but it is known that the lactation period lasts approximately two months.

Gradually, after this time, the young will feed less on the mother's milk until they leave the pouch. However, they will return to cover and suck the milk intermittently, although in some cases, calves have been observed suckling outside the pouch.

Since the litter will spend considerable time inside the pouch, the pouch is expandable enough for all the hatchlings to settle into it.

They can even move as they grow, since the mother's nipple gradually lengthens to compensate for the growth of the young. This expansion of the skin is only temporary.

After weaning of the pups has occurred, the pouch is reduced in size although it never returns to the same dimensions as it was before pregnancy.

In addition to providing the appropriate environment to complete the last phase of development of newborns, the pouch fulfills a function of protection from possible threats from the outside world.

Additionally, it allows the young to maintain a suitable temperature, since they are not yet able to control their own body temperature.

Of course, the absence or presence of the pouch indicates whether it is a male or female opossum specimen.

By way of interesting data, it has been observed in experiments conducted by biologists that female opossums and, in general, those belonging to the Didelphia category accept young from other litters that are placed in their pouch.

In short, this non-placental type of gestation has always been a subject of study for researchers and biologists, with the pouch being the point of interest due to what happens inside it, its functions and the accessibility it offers to carry out observations of specimens.

References

1. Feldhammer et al. (2015). Mammalogy: Adaptation, Diversity, Ecology. Baltimore, Johns Hopkins University Press.
2. Feldhammer et al (2003). Wild Mammals of North America: Biology, Management, and Conservation. Baltimore, Johns Hopkins University Press.
3. Hunsaker, D. (1977). The Biology of Marsupials. London, Academic Press Inc.
4. Jones et al. (2003). Predators with Pouches: The Biology of Carnivorous Marsupials. Australia, Csiro Publishing.
5. Krause, W, and Krause W. (2006). The Opossum: Its Amazing Story. Columbia, Department of Pathology and Anatomical Sciences.