Got Milk?
By Lindsey Konkel

There is a hollowed-out, fallen log on the SOC campus that every student of black bear ecology visits. One winter, a female black bear and her cubs hibernated here.

The students are always surprised to hear that female black bears give birth in the middle of winter, while hibernating. I too was pretty amazed when I first found this out. Lactation can be quite a costly proposition! How does the mother bear survive through the winter nursing her cubs while she is not eating herself?

I grew up in Wisconsin, where the license plates proclaim America’s Dairyland. In my mind, images of milk are often associated with dairy products, but in the realm of animal nutrition, there is more to milk than Holsteins and milking machines.

Milk comes from cows, yes, but also from other animals – black bears, dolphins, horses, lions, and bats to name a few. In fact, all mammal mothers lactate – lactation, or milk production is a defining characteristic of mammals.

Lactation, which is unique to mammals, provides a method of transfer of nutrients from mothers to their youngsters. When mammals are born, they are quite dependent on their mothers, having not yet developed the structures necessary, such as teeth, to forage or hunt like adults of their species. Milk, composed of sugar, fat, protein, and water, gives the infant energy and helps it grow. Transfer of nutrients via milk allows for a delay in maturity, a longer growth period in which the infant stays with and learns from the mother.

Though mammals are united by the act of lactation, not all milk is the same; the composition of milk, including the amount of sugar, fat, protein, and water can vary greatly. Throughout the course of mammalian evolution, nursing strategies and milk composition have evolved to match the life-history of different mammalian species.

What does this have to do with our black bear mother and her cubs? A female black bear with nursing cubs rarely leaves the den to forage over the winter. She relies instead on stores of fat and energy that she has accumulated over the previous seasons. This poses a conflict. The female must provide enough energy-rich milk for her cubs to survive over the winter while at the same time retaining enough energy for herself, to keep her own organ systems working properly. Organs such as the brain require simple sugars like glucose to function.

Evolution’s solution is simple yet elegant. Mother bears produce high-fat milks that are low in sugar. This enables the female to retain stores of readily available simple sugars while at the same providing her cubs with energy-rich milk, enabling them to pack on the pounds. Weighing only a half a pound to a pound when they are born, bear cubs need to grow quickly in order to survive the winter. Milk is an evolved food, a compromise between what the mother can give and what the infant needs.

There are many other mammals besides bears living in the forest. Each species produces a slightly different milk that reflects its limitations and lifestyle. For example, there are many small rodents such as mice or voles living in the forest. Like black bears, these mammals tend to produce very dense, energy-rich milks, but for a different reason. Small species are limited in how much they can consume by the size of their stomachs; high-fat milk allows for the transfer of a large amount of energy in a small amount of milk.

Deer tend to fall on the opposite end of the spectrum, producing milk that is very low in fat. A fawn is rarely seen far from its mother’s side and is allowed to nurse several times a day. Because fawns grow slowly and nurse often, there is no rush for the mother to transfer large amounts of fat to the infant; the milk is dilute, containing mostly water and sugars.

Though New Jersey’s black bear and deer populations are doing quite well respectively, perhaps too well some would suggest, the study of milk composition has become an important component of many wildlife conservation projects. If the concern for a particular species is poor reproductive success or high infant mortality, then looking at milk is an obvious place to start.