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But why? What is it about the primate lifestyle that, over the last 60 million years or more, has resulted in them growing larger brains than other, similarly sized, mammals? This is obviously a significant question, since it relates to what is undeniably the key defining feature of our own species and might explain why our planet is inhabited by sapient urbanised monkeys rather than, say, sapient city-building cats.
There have, as you might expect, been a number of different explanations offered. Given that we can't simply rewind time and watch, perhaps the best we can do to decide between these explanations is to see how well they fit with the various living species of primate. Not all primates have brains of the same relative size, so is there something about some species that explains why they have larger brains than others?
One popular explanation is that large brains initially evolved to help us deal with complex social lives. If this is so, we would expect that, on average, primate species that live in large groups where they have to keep track of, and interact appropriately with, many other individuals, will tend to have larger brains than those which are more solitary.
A study in 2007 compared brain sizes across several different species of mammal, including bats, carnivores, and ruminants as well as primates. This found that species that form stable pair-bonds with their partners tend to have larger brains than those which live alone. Furthermore, in some primates, but not in, say, lions or herd-dwelling antelopes, the larger the group they habitually lived in, the larger the brain. This, the authors argued, shows that larger brain size may initially have evolved to facilitate pair-bonding, but, in some primates, the same changes became co-opted to allow more complex social interactions with individuals outside the pair-bond, allowing the formation of what we could loosely term 'societies'.
That seems plausible, but there is a crucial caveat: I said 'some' primates for a reason. That's because the rule apparently held for monkeys and apes, but failed when they tried to look at other primates. So there's clearly something else going on, and it's probably this other thing that explains why primates in general - and not just monkeys in particular - have larger brains than (most) other mammals.
That something may well be diet.
Specifically, it seems that whether or not a given species of monkey eats a large quantity of fruit is a better predictor of its brain size than the complexity of its social life alone - although both are relevant. Fruit-eating is not unique to primates, but it is especially common among them, especially when we look at animals where fruit forms the great bulk of their diet, rather than just a side-dish when it happens to be available. One possibility here is that finding suitably ripe fruit among trees is a cognitively demanding task that requires a detailed understanding of the environment and an ability to map the locations of a sparsely distributed resource. It may also be that the high calorie count of fruit means that more energy is available for growing brains, allowing the extra brain power to be co-opted for other purposes.
If this is so, we might expect some co-evolution between the size of the brain and the shape of the teeth, since the latter are obviously also strongly affected by diet. Back in the 1970s, a popular theory was that teeth - especially the premolars and molars - shrank as brains grew. This would be because primate diets were becoming softer and more nutritious and would help explain why human teeth are unusually small compared with those of chimps and the like. More recent analysis, however, has shown that it really isn't this simple, not least because, at least in our own immediate ancestors, brains grew much faster than teeth shrank and the two processes do not seem to be directly linked.
The studies that have looked into this have, not unreasonably, tended to focus on monkeys and apes, since those are the animals most closely related to us. But monkeys and apes are by no means the only primates. An entirely separate branch of the primate family tree arose around 60 million years ago. Most members of this group suddenly died out in an extinction event about 30 million years ago but not before some had made it across to Madagascar where the majority of their descendants live today.
This is the group technically known as the "strepsirrhine" or "wet-nosed" primates, of which the most famous examples are probably the lemurs. Not all strepsirrhines live in Madagascar since a few held on in Africa and there are some in Asia, too, but it remains the centre of their diversity. With little direct competition on the island, they expanded to fill several different niches, and consequently have a wide range of diets; some eat fruit, others feed mainly on leaves, and a fair proportion have switched to eating insects.
They tend to have smaller brains, relative to their body size, than monkeys do, but these are still larger than those of most other mammals. So they may provide some insights into the early evolution of large brain size in primates, and the group is diverse enough that we can see whether or not this might be linked to their diet.
A survey published in 2009 seemed to show that there was a link. While group size and pair-bonding apparently have no relationship to brain size among strepsirrhine primates, fruit-eating species do have larger brains than those that feed primarily on leaves. There also seems to be a correlation with activity pattern, whereby animals with more flexible habits - that is, equally likely to be active day or night, depending on the circumstance rather than being truly nocturnal or diurnal - also have larger brains on average.
However, there could be other factors involved here, since brain size does not simply scale with the size of the body. Larger animals actually have proportionately smaller brains than small animals do, presumably because there's a certain minimum size that a brain needs in order to be able to carry out its tasks. Adjusting for this and other factors, a study published earlier this year found no correlation between brain size and diet in the group. But that isn't to say that it didn't find anything.
The researchers had been comparing details of the brains, teeth, and diets of twenty different species of strepsirrhine, covering all seven widely recognised families within the group. To their surprise, they found that, while the size of the brain doesn't seem to be related to diet, its shape does. Furthermore, in this group, unlike monkeys, tooth size does seem to be at least partially related to brain size, perhaps because of differences in the quality of the diet, if not its fine detail. So it's not as if there's no correlation there at all.
By comparing the variation in brain and tooth shape across the twenty species with the details of their ancestral family tree, they also reported that fruit-eating strepsirrhines have evolved changes to these features more slowly than those which rely on leaves or insects for the bulk of their diet. That would support existing theories that the earliest lemur-like animals were fruit-eaters so that it's those that switched to a different food source that had to make the most physical adjustments.
There was one species, however, that was radically different from all the others, showing that it must have undergone a remarkably rapid evolution after last separating from its closest relatives. The researchers found this surprising, although it seems less so to me, because the species in question is known for being downright weird. It's the aye-aye (Daubentonia madagascarensis) an animal that, among other things, is an example of parallel evolution with squirrels, while still absolutely being a strepsirrhine primate (despite what a previous commenter on this blog has tried to claim).
But then, part of the reason for its strangeness is its reliance on an unusual and specialised diet, even for insectivorous primates. If anything's going to buck a trend, it's probably the aye-aye.
[Photo by Charles J. Sharp, from Wikimedia Commons. Cladogram adapted from Pozzi et al. 2014.]
I think I was Richard Dawkins who had in one of his books an anecdote about taking a primatology class where "except Daubentonia" was a constant refrain from the lecturer.
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