I think it was Andrea Kuszewski who alerted me to this paper. And it definitely pricked up my ears. Is sleepin’ around good for big brains? Does promiscuity make for additional smarts? Does getting more tail make your brain extra hale?
Well…sort of. But “promiscuity” is probably the wrong word. Take out promiscuity…and put in the word “social”.
Kingsbury et al. ” Monogamous and Promiscuous Rodent Species Exhibit Discrete Variation in the Size of the Medial Prefrontal Cortex” Brain, Behavior and Evolution, 2012.
What is it that drives certain brain areas to increase in size? Well…use it or lose it. And if you do use it, and use it a lot, you might get a corresponding increase in size. For example, animals that rely a great deal on spatial memory and learning tend to have bigger hippocampi than those who don’t, those who rely heavily on olfactory input have bigger olfactory bulbs than those who don’t, birds who have complex song patterns have larger vocal brain areas than those who don’t, etc, etc.
And what about we social animals? Well, social animals get larger brains, too.
These findings gave rise to the
social brain hypothesis, which posits that species living
within complex social groups have evolved larger brains
to deal with the increased cognitive demands that arise
from monitoring social relationships and responding ap-
propriately to individuals within the group [Dunbar and
Shultz, 2007; Shultz and Dunbar, 2007].
But it’s not the overall brain that necessarily increases in size, it’s generally specific areas, such as the medial prefrontal cortex (mPFC). This area has a lot to do with learning and memory, object recognition, etc, but can also be pretty essential to social structure. After all, you need to remember and identify the people you meet.
And what about people you “meet” in the biblical sense? The authors of this study wanted to look at whether mating systems had effects on mPFC size, as well as the retrosplenial cortex (RS), an area associated specifically with spatial memory. In order to do this, they turned to the vole.
Ah, the prairie vole, symbol of happily monogamous mating systems everywhere (seriously, someone should start selling vole couple t shirts and vole pair jewelery). The prairie vole is known for its amazingly uxorious mating habits. This behavior is mediated in part by oxytocin, and studies in voles were the first to bring oxytocin in to the public eye. Prairie voles are monogamous and mate in single pairs for a very long time, including sharing parental duties.
Compare this idyllic 1950’s suburban existence to that of the prairie vole’s very close cousin, the meadow vole. These meadow voles are promiscuous on both ends, with males and females gettin’ it on with everybody. In this case, the females end up taking on the entirety of the parenting duties, but it works out all right for them.
The authors of this study looked at the brains of the prairie vole vs the meadow vole, and compared them to the brains of two mouse species, the monogamous California mouse, and the promiscuous white-footed mouse. They compared the mPFC size and RS size as a function of mating strategy.
What you can see above are the data for the mPFC, divided by sex and species. You can see that overall, the promiscuous species had larger mPFC volume than the monogamous species (the RS had no difference, and no other brain areas were compared).
So is it all that tail that makes those brains so big? The authors don’t think so, and neither do I. Instead, the authors hypothesize that what we’re seeing here is convergent evolution (two different species ending up at the same result independently) as a result of social interaction, not necessarily just the sleepin’ around.
There are several reasons why social interaction seems like the good choice here. The promiscuous species have larger ranges than the monogamous species, meaning they will need to process more spatial information. The promiscuous species also meet (and mate with) far more individuals than the monogamous species, and they need to remember who and where those individuals are. In sum, the promiscuous animals are dealing with a more complex habitat and more behavioral flexibility, not just getting more action.
This makes me sad, though, that the authors never tested the spatial memory abilities of the two groups. Bigger mPFC volume is fine, but what about its function? It would be interesting to see if the promiscuous animals display increased performance in complicated spatial memory tasks, and if this goes along with increased performance in things like social recognition (where you expose an animal to a new animal, and one it hasn’t seen before. If it’s good at social recognition, it will want to check out the new guy).
I also wish they’d taken the chance to look at hippocampal volume in these animals. If mPFC has something to do with spatial memory, it is nothing without the hippocampus, which plays a huge role in learning and memory. And I wonder if the increased social interaction of the promiscuous animals impacts their hippocampus and its function as well.
But in the end, it appears that promiscuity does result in bigger brain areas, at least in mice and voles. But it’s not just about getting some action, it’s about being social.
Kingsbury MA, Gleason ED, Ophir AG, Phelps SM, Young LJ, & Marler CA (2012). Monogamous and Promiscuous Rodent Species Exhibit Discrete Variation in the Size of the Medial Prefrontal Cortex. Brain, behavior and evolution PMID: 22759599