3.6 Life History Theory

Life History Theory is the framework that helps biologists and anthropologists explain how individual animals (and species) navigate development, reproduction, and death. In general, we can apply life history theory to a species to understand the typical behaviors of that species. However, we must also keep in mind that individuals also vary in their experiences and the trade-offs made.

Metabolic Costs

Every organism exchanges chemicals to produce energy for the body. This process – called metabolism – is important to keep in balance throughout one’s life. Failing to do so could result in starvation, excess fat storage, and reproductive issues.

At minimum, animals need sufficient calories to for the processes of life – respiration, brain activity, etc. This basal metabolic rate must be maintained before growth or reproduction can occur. However, as soon as the animal begins to move, hunt, forage, think, or do anything else, they need additional calories to support those activities. This active metabolism requires an input of calories equivalent to those expended. This means that there are days an animal may need many extra calories, and there are days they do not need as much. For example, a cross-country runner may “carb load” the day before a race to store easily accessible calories. After the raise, they may take a day off – focusing on simple to digest fruits and veggies. Accordingly, active metabolism also helps repair injuries, whether severe (e.g., a broken limb) or minor (e.g., microtears in muscles from running or strength building).

Yet, there is more. During growth and development, additional calories are required to expand muscle, build bone, and keep the nervous system healthy. This growth rate must also be accounted for in the caloric budget of bodies. This is particularly noticeable during the transition from adolescents to young adulthood. As adrenarche spurs development of secondary sex characteristics, growth spurts, and key shifts in the brain, the body may go through huge expenditures of calories. This is one of the reasons teenagers can eat an egregious number of calories without gaining weight – that energy is often going to major changes in the body.

During puberty, one more important form of metabolic investment begins – reproductive effort. For female bodies, menarche triggers monthly cycling and a range of hormonal adjustments. For male bodies, increases in testosterone result in increased muscle and production of spermatocytes. This is the beginning of fecundity – the ability to produce new offspring.

For male bodies, the costs of reproductive effort are relatively low. For female bodies, the stakes are much higher – especially in mammals. Pregnancy, parturition, and infant care such as breastfeeding need a significant number of calories. So much so that females who are not receiving enough calories for active maintenance may cease menstruation. This is also why teenage pregnancy can be risky for mother and child. Both the female body and the developing fetus need a large number of calories to support growth, and in many instances, one goes without the total number needed.

r/K Selection

Because of these metabolic conflicts, many species, including humans, have evolved different strategies for development and parental investment. Reproductive strategies can range from rapid maturity, short lifespans, and numerous offspring with little to no parental care (r selected) to slow maturation, long life spans, and fewer offspring with significant parental care (K selected). Balancing metabolic costs with reproduction and one’s own survival is known as a life history trade off. When we view reproductive fitness as “garnering enough resources to survive and reproduce” then consider that reproductive fitness is necessary to remain part of a species evolution, it becomes clear how important these processes are.

Humans may be an extreme example of K selection. In a natural fertility environment, a woman may only give birth to four or five children over her lifetime. However, she will invest a significant amount of calories, energy, time, and care into each child. So much so, that many human females require the help of others in the family to properly care for each newborn.