Dolphin brains and the Aquatic Ape Hypothesis: a dubious link

by • April 29, 2013 • Dolphin News, Dolphin Science, Personal MusingsComments Off6809

Did dolphins evolve large brains because they ate seafood? This was a suggestion put forth by a proponent of the Aquatic Ape Hypothesis (AAH) in a recent article in the Guardian. The AAH attempts to explain the origins of unique human characteristics (e.g., hairlessness) by suggesting that we evolved in an aquatic environment. The recent Guardian article garnered a lot of negative attention from AAH detractors (i.e., nearly the entire scientific community) soon after it was published. For those of us interested in dolphin science, there is a fantastic – if bizarre – reference to dolphins in the closing paragraphs:

“Docosahexaenoic acid (DHA) is an omega-3 fatty acid that is found in large amounts in seafood,” said Dr Michael Crawford, of Imperial College London.

“It boosts brain growth in mammals. That is why a dolphin has a much bigger brain than a zebra, though they have roughly the same body sizes. The dolphin has a diet rich in DHA. The crucial point is that without a high DHA diet from seafood we could not have developed our big brains. We got smart from eating fish and living in water. (SOURCE)

The idea that omega-3 fatty acid is “why a dolphin has a much bigger brain than a zebra” is not supported by anything from the scientific literature on dolphin brains as it pertains to cognition and behavior. As it stands, the leading hypothesis as to why dolphins evolved large brains is that they required the complex cognition generated by a large cortex in order to navigate their complex social worlds (i.e., the social brain hypothesis). Past hypotheses have involved the need to process complex auditory information (from their echolocation), or that large brains helped dolphins to generate heat. But nowhere in the scientific literature on dolphin brain evolution (as it pertains to behavior/cognition) is there any mention of “fish oil” being a continuing factor.

Of course I am not all that familiar with brain evolution as it pertains to fatty acids. So just to be sure I wasn’t missing something from the literature, I did a quick Google Scholar search for Docosahexaenoic acid and dolphin brains. I found an article from 1987 co-authored by (as far as I can tell) the same Dr. Michael Crawford quoted above wherein he states THE EXACT OPPOSITE of what he said in the Guardian:

The data demonstrate that, despite the high proportion of n-3 fatty acids in the marine environment, the free-living dolphins have remarkably high concentrations of arachidonic and other n-6 fatty acids in their tissue or membrane phosphoglycerides. In this respect, they are similar to land mammals. (Page 679)

In other words, dolphins don’t seems to have a diet rich in omega-3 fatty acids (or at least they don’t store much of it in their tissues) but instead have the same kind of boring-old n-6 fatty acid concentrations we find in small-brained zebras. So it follows that omega-3 can’t then be a contributing factor in hypotheses involving dolphin brain evolution (one would think). Maybe recent literature has overturned this finding (and I am sure this line of research is more complex than I understand it to be at first glance), but even if there is a strong link between ingesting DHA and brain size, we’re left with a much more fundamental problem: eating fish oil doesn’t cause brains to evolve to be larger in some weird Lamarckian way. To explain the evolution of large brains, we need to determine what selective pressures made having a large brain (and the complex cognition that goes with it) an advantage to dolphins’ ancestors. You can’t just say “fish oil did it” until you have an adaptive explanation to go along with it.  Here is how the fish oil explanation (as portrayed in the Guardian) looks alongside the current leading hypotheses as to why dolphins evolved large brains:

–> Because of the need to navigate a complex social system, dolphins evolved lager brains

–> Because of the need to process echolocation signals, dolphins evolved larger brains

–> Because of the need to fish oil, dolphins evolved larger brains

One of these things is not like the other.

Is the suggestion really that eating seafood leads to the evolution of larger brains? This can’t be the whole argument. If so, how do we explain large brains not having evolved in sea lions, seals, or other fish-eating marine mammals? Or for sharks? Or for fish-eating birds? Or for fish that eat other fish? Obviously, simply ingesting fatty-acids is not enough. I suppose that one can make the case that a combination is required; selective pressure driving the evolution of large (intelligent) brains and the presence of fish oil in the diet. This explanation does exist to explain the evolution of the human brain. But this then does not explain the evolution of large brain size (relative to body size) in other intelligent non-fish-eating-primates like Rhesus monkeys or chimpanzees. Or vegetarian elephants. Or dolphin species that don’t eat fish (e.g., mammal-eating killer whales). Nor does it explain the fact that we see intelligent behavior in non-fish-eating-small-brained species like scrub jays. The whole idea sounds like a just-so story that just-ain’t-so, and isn’t really helping the Aquatic Ape Hypothesis gain much traction with skeptical scientists.

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