EXCLUSIVE: Increased brain size comes at a cost, reproductively speaking

Researchers from Uppsala University say that big-brained guppies are costly, evolutionarily speaking.

January 04, 2013

Researchers from Uppsala University say that big-brained guppies are expensive, evolutionarily speaking. Researchers contend that their findings confirm the idea that larger brains and increased cognitive ability are linked. This idea, researchers say, is the subject of much debate in the scientific community.

We took some time to ask Professor Niclas Kolm of Uppsala University in Sweden a few questions about his study and here is what he had to say:

Science Recorder (SR): The press release on the study notes that the guppies were given a test of numerical learning. Could you explain a little more about how this test was given and scored?

Niclas Kolm (NK): For this test we had 48 individuals of both sexes divided across the different treatments (large and small brains) and replicates (each treatment was replicated three times, i.e. we had three replicate groups that were up-selected and three that were down-selected in terms of relative brain size). Each individual was presented with two stimuli cards, one on each side of an experimental aquarium. On one card, there were two black objects, on the other there were four. We fed the experimental fish in front of the card with four objects twice per day. Every fourth day, we released the fish in the middle of the tank without providing food and checked to which side they swam. In total we provided 48 training sessions and 8 test sessions without food reward. Already at the first test session, large-brained females swam more often than small-brained females up to the stimulus card we had trained them on, the one with 4 objects. And after all 8 test sessions they were much better at swimming up to the stimulus with the number of objects they had been trained on. This means that large-brained females learned to remember that a certain number of objects was associated with a food-reward while small-brained females could not learn this. Interestingly, we found no difference for males of different brain sizes. We think this is because food is a much better reward for females than for males since females are much larger than males in the guppy and growth is much more strongly linked to fitness in females. We are undertaking similar experiments for males now but with a different reward: the opportunity to team up with some females. We believe this will be a more efficient training reward for guppy males.

SR: What sort of implications does this study have for humans?

NK: It suggests that increased brain size carries costs of reproduction but cognitive benefits. Hence, the low fecundity we find in primates (including humans) and for instance dolphins, could have generated the opportunity to increase brain size in these groups.

SR: If you were to place the guppies in a more competitive, semi-natural environment with limited resources and fewer predators, what do you expect will happen?

NK: If you mean placing our guppies with large- and small brains into such an environment, I suspect the large brained fish will do better the tougher the selective pressure on cognitive ability from competition and predation. Given the difference in fecundity between them, I suspect large-brained individuals with fewer, but perhaps higher quality offspring will do better where there is strong selective pressure on cognitive ability but that small-brained individuals might do better when only offspring number is important. This is the scenario we believe might explain why there is such enormous variation in brain size at all taxonomic levels and this is also exactly the experiment we plan to run with our brain size selected guppies shortly.

SR: What was your motivation for this study?

NK: To provide an experimental test of the costs and benefits of evolving a larger brain in a vertebrate. The story published in Current Biology is only the first set of results of a series of assays that we have run/are running on our selection lines with large and small brains

SR: Are you conducting any additional research on this topic?

NK: Indeed, as mentioned this is the first step. We have already undertaken several other assays on other aspects of behavioral differences between the large- and small-brained lines. For instance on personality differences and differences in sexual traits in males.

The findings lead Kolm and his team to conclude that the relatively small family sizes of humans and other primates might have helped to make our big brains possible.

The study’s findings were recently published in the journal Current Biology.

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