Posted by: Chris | May 4, 2010

Big Breasts and Bipedalism

Via Andrew, I came across an interesting article on the evolution of human female breasts, which are larger compared to other great apes.  I had always suspected that it was an effect of some combination of drift and sexual selection: with males (using mammary size as an indicator of fertility and one’s capacity to raise children) mating with larger breasted females at an increased rate and small effective population sizes of hominids dramatizing the effects, increasing breast sizes seem quite plausible. 

However, the article proposes another explanation that takes better account of the variation between different ape species.  In addition to sexual selection caused by an evolved attraction to mammaries as indicators of good mates, the author references a number of articles that suggest that large breasts take advantage of inherent sensory biases in males.  In most ape species, females indicate that they are in estrous and ready to mate through swellings on the buttocks, a signalling system that males have a coevolved attraction to (and interlocking sexual selection has likely increased the severity of both).  However, the entirely independent shift towards bipedalism amongst human populations would have simultaneously made buttocks swelling disadvantageous, as it prevents swift locomotion on two feet (and because of the not-exactly-related switch away from an estrous cycle), and made the front torso much more visible, as the females would be obviously walking upright instead of on all fours.  The innate attraction that males had for two swollen mounds was transferred to females with larger breasts, with flat-chested females without any comprehensible method of displaying their sexual availability losing out.  This secondary hypothesis is lended even more credence, as the article notes, by the fact that certain groups of bonobos who display a degree of bipedalism also have reduced buttock swellings and larger breasts than the average bonobo chimp.

Thus the transition toward bipedalism helped catalyze the more intuitive sexual selective process outlined in paragraph one.  Indeed it seems to me that the functional correlates outlined in the original hypothesis are necessary of the second to succeed (one would imagine a sensory bias causing attraction to two malignant facial tumors being a bit of a dead-end).  It also seems, if we take the observed correlation in the bonobos between the upright stance and breast size as causative, that the linkage between the two is strong and perhaps even predictable, for two ape species independently converged on larger breasts sizes shortly after adapting bipedalism.

Finally, to take this train of thought in a completely different direction, the acceptance of the hypothesis would lend further support to the veracity of the Patterson-Gimlin film.  One of the most effective criticisms I had seen of the footage notes that the creature’s oversized breasts are not reflective of the anatomy of most female apes (and darkly implies that hoaxers, thinking that humans and apes were all the same, strapped boobies on Pattie for faux-authenticity).  But, if bipedalism catalyzes sexual selection towards breast enhancement, one would expect that the two-legged Bigfoot would have fatty mammaries in females, busting this criticism and offering a subtle lift to the titillating idea that the film contains a real unidentified ape.

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