Do we have large brains “because we are social”?

Why do we apes have such relatively enormous brains? Surprisingly, tiny insects may hold clues to the answer.

Social behaviour may drive evolution of big brains. Photo:

Currently, the trendy explanation hinges on our complex social lives (the “social intelligence hypothesis”). Keeping track of friends and enemies takes a lot of brainpower. This new paper out this week shows that even computerized virtual animals, when programmed to cooperate, evolve bigger virtual “brains”.

But this is not the whole story. For example, large brains also help us deal with a complex environment, with seasonal food and prey. Famously, the hippocampus (the bit of the brain that remembers directions and maps) grows much larger in London cabbies than in the rest of us.

Scrub jay caching food.

Over evolutionary time, the same is true. In Clark’s nutcrackers, birds that store a lot of food over the winter: the hippocampus is larger than related scrub jays, which don’t store so much.

But scrub jays’ food often goes rotten in sunny California, and so these birds are especially good at also remembering when they hid the food, rather than just the locations of many items, and they can even plan when to hoard food so it will be fresh when they recover it.

All this stacks up to the idea that specific parts of the brain evolve in response to specific needs: social cooperation may indeed be one such need, but others can be just as important.

What light can insects shed on this question with their tiny brains?

Locust brains
Brains of solitary (left) and gregarious (right) locusts. Mushroom bodies are in yellow. From Ott & Rogers (2004).

Two papers caught my attention recently. This 2004 paper shows that locusts have a fascinating Jekyll-and-Hyde brain that helps their fascinating Jekyll-and-Hyde life.  Some locusts are born loners, grumpy, aggressive and solitary. Others are naturally gregarious, seeking out other locusts and forming giant “marching bands” (eventually plague swarms).

The authors dissected out the brains of solitary and gregarious locusts and found that certain bits were enlarged and reduced: solitary locusts had enlarged basic sense organs, and gregarious locusts had bigger “mushroom bodies” – structures for more complex information processing. Thus, group-related behaviour may have selected for expansion of the higher processing bits of the brain.

Potter wasp
Potter wasp adding caterpillar prey to her nest. Photo: Allesh Sinu

Second, though, this fascinating recent paper weighs up the evidence for “social intelligence” in social bees, ants and wasps.  They have relatively huge mushroom bodies, which have been linked to social behaviour. But the researchers found that the big mushroom bodies came first, with social behaviour evolving later. Before they were social, wasps were parasitoids (i.e. bursting out of caterpillars like the alien in Alien).  The authors argue that this lifestyle needs a big brain to hunt down caterpillars while remembering where you left your nest so that you can drag your prey back to it.  We are back to the “environmental brain” again.

I am interested in these questions because my current study species (Acacia thrips) may be able to shed some light on the issue.  The species are cooperative to different extents, but they all live in the same kinds of nests.  I am considering dissecting out their brains to see whether increased cooperation leads to increased brain size even in similar environments.  But given that a thrips measures ~2mm at most, this may be a challenging task!