Tuesday, June 21st 2011
This is the first in a new series at Context and Variation where I will attempt to be more concise. As I continue along the Summer of the Pill series I want to make sure I still put some attention on other topics within biological anthropology. C&V shorts allows me to share something I find cool, in half the words I usually do.
What would you do if you knew you had eighty years to live? What if you knew you only had forty?
Life history theory is the idea that the timing of major life events is adaptive. That is, when to be born, when to wean, when to grow, and when to reproduce are dependent on selection pressures in the environment. The most important concept within life history theory, then, is that of tradeoffs, because when you time these events is based off how you want to allocate your resources. In your environment, would it be best to grow right now? Should you grow under the care of your mother, or should you be independent? Is it time to have a child? How about your second, or third, or fourth child?
For this reason, many people study life history transitions, which means the critical yet variable period when people move from one state to another: from growing to reproductive cycling, from cycling to gestating, gestating to lactating, even lactating back to cycling. And much of what governs these transitions has to do with energy, because energy is finite: energy you use towards one purpose, like growing, cannot be used for another, like reproducing. This is especially true in humans because we permanently transition from allocating to growth to reproduction at puberty, unlike other species that keep growing throughout their reproductive years.
But energy isn’t the only factor that enters into our physiological decision-making: time is also important. And as I hinted in the first paragraph, if you have some sense that your time on this earth will be short or long, you might make different decisions about when to do what.
|Found here. Perhaps part of a
modern menarche ceremony?
A few years ago, Walker et al (2006) looked at all the available data on growth and development in small-scale societies – that means foragers and agrarian populations. They found that girls with a later age at menarche – that is the first menstrual period – are shorter in stature. If we consider only energy, this makes no sense! The later you wait to start reproducing should mean you had more time to grow, so why did Walker et al (2006) find the opposite?
The answer is timing. It turns out that mortality rates tell us something about growth and development: the higher the mortality in a population, the earlier their age at menarche and age at first reproduction. So, the higher your chances of being offed at any given moment, the more likely you are to favor reproduction over growth so you can move ahead with the whole reproductive success thing. So, constraints on time and energy affect our physiology differently, and mean we may have to make different predictions about life history transitions that are dependent on human societies. Predation or access to health care impact mortality, but so do homicide or war.
In 2011, McIntyre and Kacerosky performed a similar analysis, only this time they compared small-scale societies with industrialized ones – industrialized societies are those that are more urban, technology-driven, with greater access to modern health care, like the USA. Their analysis of small-scale societies confirmed Walker et al’s (2006) results.
But McIntyre and Kacerosky (2011) found the opposite relationship in industrialized societies: there, the later you hit menarche, the taller you were. And this makes sense if we think we can assume most industrialized populations have lower mortality than the foragers: within the industrialized pops, those who had the time and energy to grow big by holding off on menarche, did.
McIntyre and Kacerosky (2011) are hesitant to be full adaptationists in their paper, which I appreciate. So, they offer two hypotheses and suggestions for future testing. The first hypothesis falls in line with the life history theory described in this post, though their focus is more on parental investment than mortality. But still, environment, and access to time and energy, set life history trajectories for different populations. However they are also careful to point out a nonadaptive hypothesis: it could be that variability in stature is decreasing as heritability is increasing, meaning we are hitting up against biological constraints for size.
Genes and environment interact to produce phenotype, and this is something most people remember from high school biology. But sometimes it’s nice to peek under the hood and learn a little something about the life history mechanisms that are set into motion by this interaction. Early life events, perhaps even life events of our mothers and grandmothers, start our life history trajectories. Then tradeoffs at certain important transition periods nudge us a little further one way or another for the rest of our lives.
McIntyre MH, & Kacerosky PM (2011). Age and size at maturity in women: a norm of reaction? American journal of human biology : the official journal of the Human Biology Council, 23 (3), 305-12 PMID: 21484909
Walker, R., Gurven, M., Hill, K., Migliano, A., Chagnon, N., De Souza, R., Djurovic, G., Hames, R., Hurtado, A., Kaplan, H., Kramer, K., Oliver, W., Valeggia, C., & Yamauchi, T. (2006). Growth rates and life histories in twenty-two small-scale societies American Journal of Human Biology, 18 (3), 295-311 DOI: 10.1002/ajhb.20510