Contributed blog: density dependence and human body size

Greetings human macroecologists,

I asked Rob Walker, the lead author on one of our papers for next week ("Growth rates and life histories in 22 small-scale societies"), to blog about some of his research and his thoughts on what determines adult body size in humans. Some of the exciting and relevant features of Rob’s research include his argument that there are two pathways to small body size. One driven by resource limitation that causes smaller growth, and the other by high mortality. I am biased to favor this view because Rob is an old friend and collaborator and more importantly, his view agrees with the Charnovian perspective we covered in class (remember the equation with growth over mortality? – Rob’s model shows the same predictions as that model we went over). Think about how these processes of body size change might relate to the patterns demonstrated in Ruff and the potential implications for H. floresienses (to the degree that insular dwarfism may be a possible explanation for this dwarfed hominid).

Rob has written a slew of papers on human life history and behavioral ecology. To check out his other papers go to his homepage.

Here’s Rob’s blog:

I have some general interests in the evolution of the human life course and especially factors that influence patterns of growth and development. To prepare the paper on ’22 small-scale societies’ that you are reading, I started putting together data from as many societies as I could that had life-history information (e.g., growth rates, age at menarche, first reproduction, and age-specific mortality). I wanted to get a better feel for the overall variation in these traits across human populations. We were able to show that in general larger, better-nourished societies have faster/earlier growth and development (we call this the conventional model). However, while the sample is rather small, there is also some indication that societies that suffer from high mortality also grow relatively fast (relatively given their adult body size, a proxy for energetic intake). This may support the general life-history model where higher mortality prompts faster growth and development in order to get past the high mortality juvenile stage (or high mortality at small body size).

The results from this paper led to some more thinking about how mortality, resource limitation, and growth related to patterns in size and density dependence seen among contemporary hunter gatherer populations. This led to the development of a life history model for human body sizes – a manuscript presenting the model can be found here.

Here's a non-technical summary of that paper:

Average body size ranges immensely across human populations. Many of the world’s smallest populations, like the Agta hunter-gatherers in the Philippines, live on islands at relatively high population densities. We show that population density has a negative effect on adult body mass across hunter-gatherers. Humans slow down growth and development and demonstrate smaller adult body sizes in high population density contexts presumably because of less food and more disease. In addition, there is evidence of selection for relatively faster/earlier development in societies that suffer from high mortality. We interpret this finding as natural selection for earlier reproductive maturity (menarche and first birth), and consequent smaller adult body size, in high mortality regimes in order to more quickly pass into the safer adult stage. In sum, comparative results support density-dependent effects on body size that act through two pathways (energetic constraints and juvenile mortality) at varying intensities in different societies contributing to a nearly two-fold range in body size across human populations.

These ‘two pathways’ to smaller adult size are summarized in the following figure:

Cheers,

RW