Today’s dialogue on sexual dimorphism and human evolution, hosted by BU’s Anthropology department and featuring Michael Plavcan and Phil Reno as speakers, was great. But it was not great because it came to any grand resolution on questions of the importance of sexual dimorphism throughout human evolution, what its implications are for our understanding of the human fossil record, or even how best to assess it in limited samples. It was great because it made clear how slippery a concept sexual dimorphism can be.
Put simply, sexual dimorphism is the characteristic differences in form between males and females. This is clear and not, at least superficially, complex, and it is something pretty much everyone on the planet is familiar with. But in its details, particularly when applied to fossil samples and when address at questions of the evolution of the world’s strangest primate, Homo sapiens, it becomes something like a theoretical fog.
On the technical side, sample size is a big issue, especially when overall levels of dimorphism skew towards the low end (as they do with contemporary humans). In these cases, the degree of overlap between males and females within a species make the assessment of patterns of dimorphism in fossils difficult. Additionally, sexual dimorphism varies across traits measured, and the correlation between these traits (e.g. skeletal size dimorphism, body mass dimorphism and canine dimorphism) is not always clear. Finally, in samples of potentially mixed conspecifics, sorting out intraspecific variation (between males and females) and interspecific variation (between species 1 and species 2) becomes incredibly difficult.
An additional challenge is figuring out what a given measure of sexual dimorphism, once agreed upon, means for a given fossil sample. I tend to think that in the case of human evolution, most estimates of sexual dimorphism are telling us very little about reproductive ecology, but perhaps do relate to ecological and energetic factors. This is an area that could use a lot more work and empirical data.
But the bigger, or perhaps biggest, theoretical issue is that in examining questions of sexual dimorphism in humans we are typically taking a comparative approach to try to explain an outlying variable. Human sexual dimorphism is weird. In most primates, the bigger you get the more dimorphism you have. In most Old World primates, large canines, frequently dimorphic, are the norm. In most primates, ovulation is visible and sexual receptivity is constrained to specific moments of time. None of this is true for humans. In addition, and this was an issue raised effectively several times by Michael Plavcan in today’s event, humans are outliers in how the magnitude of dimorphism is spread out across post-cranial skeletal size, neurocranial size, facial size, lean body mass, total body mass and canine size. Humans have a unique pattern of sexual dimorphism that is derived, at some point, from a more common primate pattern. Understanding how this happened, what evolutionary processes shaped the pattern we see in humans, is an important question. It is, however, a theoretically challenging task to explain via comparative analogs how something goes from expected to unexpected and it is a technically challenging task to develop an empirical understanding of this process from the human fossil record.
