The pouch: what a very useful structure!
By definition a “pouch” is a small sac-like container to store or keep things in. Biologically speaking, it is a pocket-like space in or on the body and the ‘pouches’ in our cheeks, for example, come to mind. They are useful for sucking up liquids (without these pouches as in the case of many carnivores like dogs and cats, liquids would have to be leapt up).
The cheek-folds of a hamster also come to mind, and they are really big pouches. But bags under the eyes of an elderly person or a fat tummy may jokingly be referred to as “pouches” as well and containers known as pouches can contain a handkerchief or small change, tobacco and lipstick, food items, useless stuff or nothing at all. Most famous amongst animals with pouches are the marsupials, in other words, kangaroos and kin.
I shall return to the kangaroos further below, but first wish to talk about some egg-laying pouched animal, namely the echidna, also known as ‘spiny anteater’ (Tachyglossus aculeatus). Together with the platypus (Ornithorhynchus anatinus), the echidna belongs to the Monotremata, i.e., milk-secreting animals that lay eggs and, like birds and reptiles have a cloaca and not separate genital and anal openings. The echidna, which adorns the 5-cent coin of Australia, has a pouch in which its single, rubbery egg of approximately 15 mm in diameter, is kept. After ten days the little only 1 cm long echidna baby frees itself from the egg with an egg-tooth and then attaches itself to one of the teats still inside the echidna mother’s pouch. For some 200 days the little one gets milk, but as the spines on the baby’s back sprout at about 9-10 weeks, carrying such a prickly ball in its pouch becomes increasingly unpleasant for the mother. Ultimately she forces the young one out, hides in the bush, and returns only periodically to nurse her offspring.
An analogous pouch is the defining character of the marsupials, a group of mammals, traditionally seen as being closely linked with Australia and containing kangaroos, the wombat, the koala and many more. But South America also had (and still has) a number of marsupials, including the opossum, which is now common also in North America. In all of these marsupials the young, born tiny and blind, climb into a pouch and become firmly attached to a teat to continue their development on a diet of milk. Kangaroos may have one fertilized egg cell in their body, one minute baby kangaroo attached to a teat, and one older, half-grown sibling that can still hide and seek refuge in its mother’s pouch. There are grass-eating marsupials, there are diggers and climbers, carnivorous marsupials, tiny flower-visiting marsupials, marsupials that glide and marsupials that lead a subterranean existence, but what Australia does not have is an aquatic marsupial.
Well, a pouch under water would drown the babies, one might think. However, South America has an aquatic marsupial: the yapok or water opossum Chironectes minimus. This ca. 30 cm long animal with an equally long tail and webbed hindfeet carries 1-5 young in a watertight baggy pouch for up to 50 days. The ventrally-located pouch opens rearward (not upwards as in kangaroos) and, moreover can close tightly due to a special muscle, keeping the babies dry when their mother is diving. Another feature of this strange animal is that similar to the extinct Tasmanian wolf, the males also possess a pouch, but not to carry offspring in it: the male can tuck its genitals away in them during dives. This could render the male animal more streamlined or serve to protect its most important parts. So, whether an egg container, a baby hoarder, a hiding place, or a protector of genitals, what a useful thing: the pouch!
Just one thought: won’t these animals’ pouches need cleaning once in a while after dirt and perhaps parasites have accumulated in them? I bet they do, but whether and how the pouch owners clean them is something to look into.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2019.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.