13.4: Adaptations For Survival

Survival of any species depends on surviving to the reproductive period. We act instinctively, and don’t die easily of cold exposure or heat prostration. When cold, we’re more apt to feel like eating, and are virtually compelled to exercise by shivering and perhaps stomping our feet or jumping up and down. When hot, we don’t feel like exercising and aren’t as hungry. This is good, since we get warm from exercise and eating.

Our ability to maintain body temperature in extreme climates has literally allowed us to inhabit the ends of the earth. Note that people from extremely hot climates (e.g., Africans) tend to be tall and lean (more surface area and less fat- insulation), whereas those from extremely cold climates (e.g., Eskimos) tend to be shorter and have more body fat.

An immediate threat to survival anywhere is starvation. So the body minimizes calorie use when food is scarce and efficiently stores calories when food is abundant. In a famine, basal metabolism falls, and we feel weak and minimize our physical activity—including sex (pregnancy uses a lot of calories, and there’s even less food with children to feed). If a woman’s fat stores become extremely low—whether because she’s starving, a marathon runner, a ballet dancer, or has anorexia nervosa—she may become less fertile and stop menstruating.

For many women in low-income countries, fat stores are crucial in providing the energy needed to carry a pregnancy to term. During pregnancy, they also need to store fat to provide energy needed for breastfeeding. Too often, this isn’t possible, and they can’t make much milk.

A dilemma for nutritionists working for world-health agencies is how to distribute limited food aid. Should they give more food for the mother, so she can breastfeed her baby, or more food for the baby directly?

Even today in some parts of the world, obesity in young women is encouraged for childbearing reasons. In the Sahara where famine is still a threat, obesity signifies wealth and is considered sexually attractive in women. Some brides-to-be have been forced to drink several quarts of camel’s milk daily until they become obese. During a famine, their large store of body fat helps provide the calories needed for a successful pregnancy.