At Language Log, Mark Liberman has an excellent discussion of a new paper that reports a correlation between voice pitch and number of children for men in a hunter-gatherer population. Men with deeper voices had more children. This portion of Liberman’s post surprised me:
This particular form of sexual dimorphism is apparently not shared with our relatives the chimps and gorillas, so it must have evolved during the same period that human speech and language did. Therefore, starting at some point during the last five million years or so, there must have been a selective advantage for male hominins with lower voices. And according to the featured study (C.L. Apicella, D.R. Feinberg, F.W. Marlowe, “Voice pitch predicts reproductive success in male hunter-gatherers”, Biology Letters, published online 9/25/2007), evidence of this selective advantage can still be found today.
I agree with all of this. The puzzle is that the effect remains. Five million years is a long time; shouldn’t the dimorphism have gotten larger and larger until an equilibrium was reached, and then stayed at that equilibrium? Once equilibrium is reached it will be the average voice pitch that is most successful.
I can think of several possible answers.
1. The correlation is due to random variation. Because lots of surveys have shown that women prefer men with deeper voices, this is less plausible.
2. Evolution is still happening on this dimension. That is, equilibrium hasn’t yet been reached.
3. This particular tribe was pushed away from equilibrium for an extended time — that is, for a long time higher-pitched men’s voices were more advantageous than usual. Whatever caused that has disappeared so this group is moving back toward equilibrium.
4. It’s about signalling. The voice-pitch variation observed in populations is mostly due not to genetic variation but to early environment (say, testosterone in the womb) and is correlated with something less visible that makes a difference in the reproductive success of one’s children.
I imagine the authors of the paper favor #4. When the full text is available for free, I’ll find out and post again.
I have two thoughts about this selective advantage…first, it could still be that the relationship is non-causal, in the sense that gene G being selected for causes phenotypical effects P1 and P2, where P1 is being selected for and P2 is lower masculine voice. I don’t see that this possibility has been eliminated by the discussion I see so far.
Second, I would point to a zoologist’s book about communication with dogs:
It seems likely to me that the selective advantage was already present among our shared ancestors with chimps and gorillas, but became more important simply because human signaling became more verbal. (But it’s still a Just-So story; I don’t have further testing in mind at the moment.)
How about #5) Chicks dig deep voices.
Or was that included in #2>
“Chicks dig deep voices” is a different category of explanation. Such explanations are called “proximate” while evolutionary explanations are called “ultimate”. Weird terminology, but you get the point.