Being a Little Bit Pregnant

Sun bear

In my ongoing series on the members of the weasel family, I have, on a number of occasions, referred to the concept of delayed implantation. A number of members of the weasel family exhibit this phenomenon, and they are far from alone among mammals.

To begin with, the egg cell starts to divide immediately after fertilisation. In most mammals, it then rapidly forms into a hollow ball of cells that move down through the reproductive tract into the womb. During this time, it really doesn't change much in size; the ball of cells, known as a blastocyst, is not much larger than the unfertilised egg, and the individual cells within it are much smaller.

Once it reaches the womb, the blastocyst attaches itself to the uterine lining and begins to send out tiny tendrils of cells that penetrate the wall in search of blood vessels. Once it finds them, the embryo can obtain nutrients from the mother, and starts to grow and develop. The side of the structure in contact with the uterine lining develops into the placenta, while the remainder forms the embryo proper, and the amniotic membranes surrounding it. This even occurs in marsupials, although, in their case, the placenta is primitive and short-lived, and, for that matter, there's something similar in some reptiles, such as sea snakes.

The Mother Elk's Dilemma

A male elk

Giving birth and looking after newborn young are particularly dangerous times for the large hoofed herbivores. That isn't to say that it isn't difficult for other mammals, too, or, indeed, other animals in general, but the larger herbivores are especially vulnerable. Smaller animals can hide their young in burrows, or other secluded dens, until they come of age, while the larger carnivores don't generally have to fear being eaten. Large herbivores do not have such a luxury.

However, hiding from predators while the young are still small and vulnerable is not the only concern that mothers have to face. Giving birth is costly in terms of resources, and after that, the young must be fed milk until it is large enough to search for food on its own. The mother, therefore, needs to keep herself well fed, and needs a good supply of food to keep both herself and her infant healthy. While this is obviously true at any time of the year, during the birthing season it becomes particularly important for the mother to find an area that is both free of predators, and has a ready source of available food.

That can be a problem if the areas that have the best food are also the ones that have the most predators.