Are Humans Naturally Polygamists?

There are morphological indications that we're somewhere between a species meant to pair bond and our polygamist evolutionary relatives.
  • Transcript


Question:  From an evolutionary perspective, are humans naturally polygamists?

Laurie Santos: The human reproductive system is one that we don’t really have a great grasp on because in some sense we pair bond, for the most part, but across all human culture, there’s really a push to polygamy. So, in most human culture there are at least some males who take on multiple mates and have, you know, multiple mating partners.  But not nearly to the degree that you see in chimpanzees.  They way you can tell this morphologically is by the size of the testes relative to body size. So a chimpanzee's testicles relative to body size are just like enormous. You would blush to see the size of these things.  Not nearly the same ratio as you see in humans.  However, humans have a larger testicle to body size ratio than you might see in other primates where we know that the females don’t kind of sleep around as the case of gorillas. 
So, the human mating system is kind of somewhere in between. We’re sort of pair bonded. There’s this push to polygamy, there’s a push of males taking on multiple female partners, but there also seems to be a push toward polyandry. In other words, females taking on multiple male partners, or else why would males kind of grow these big testicles to kind of compete at the level of sperm.  So, we're in this funny puzzle, in terms of why humans might pair bond.  One of the pushes towards pair bonding in the animal kingdom has to do with the kind of size and cumbersomeness of your off spring.  So, the taxo where you see the most pair bonding is in birds.  I mean you can see this in kind of the standard, sort of “March of the Penguins,” where the two parents you know, very cutely take care of the kids. But it’s true in birds because the offspring actually require a lot of work. There’s this extremely fragile egg, you know, that’s very tasty and you have to defend it from predators and so on.  And they it actually requires both parents to actually incubate the egg, you know, protect it and so on. 
The idea as they say might be true of humans.  You know, human infants are born incredibly precocial. So human infants are born incredibly precocial, much more so than you know, other close primate relatives, you know, they’re pretty fragile.  You know, if you just left a human baby newborn there for a long time, you know, it wouldn’t do so well.  The thought is maybe this human pair bonding actually came as a result of the fragileness of human infants.  You know, in that it might require two parents to actually take care of these offspring. 
But again, these are... there are a lot of just so stories out there, you know, it really hard to figure out exactly why we have the reproductive system we have and it’s a complicated one that we can’t pin down.  The sad thing that I mentioned in my course is that we know much more about the reproductive systems of pipe fish and swans and lions, then we do about our own species.  Which is kind of pathetic.

Recorded May 21, 2010
Interviewed by Andrew Dermont