By Christie Wilcox
There’s a lot to be said for smarts—at least we humans, with some of the biggest brains in relation to our bodies in the animal kingdom, certainly seem to think so. The size of animal brains is extravagantly well-studied, as scientists have long sought to understand why our ancestors developed such complex and energetically costly neural circuitry.
First, the team selected for larger and smaller brains from the natural variation in guppies. They successfully created smarty-pants guppies that had brains about 9% larger than their counterparts through artificial selection. Then, they put them to the test. While the males seemed to gain no benefits from possessing larger noggins, the females with bigger brains were significantly better at the task.
But what was really remarkable was the cost of these larger brains. Gut size was 20% smaller in large-brained males and 8% smaller in large-brained females. The shrunken digestive system seemed to have serious consequences reproductively, as the smarter fish produced 19% fewer offspring in their first clutch, even though they started breeding at the same age as their dumber counterparts. And, the authors noted, this was in an idealized tank setting with an plenty of food—what about in the wild, where resources are harder to come by? How much of a cost does a reduced gut have when meals aren’t guaranteed?
“Because cognitive abilities are important to facilitate behaviors such as ﬁnding food, avoiding predation, and obtaining a mate, individuals with increased cognitive abilities are likely to have higher reproductive success in the wild,” explain the authors. These benefits, though, don’t come cheap. “Our demonstration of a reduction in gut size and offspring number in the experimental populations selected for larger relative brain size provides compelling experimental evidence for the cost of increased brain size.”
There are still many questions to be answered. For example, the authors aren’t entirely sure why females were the only ones to show cognitive improvement with larger brains. They suggest that, perhaps, the researcher’s measure of intelligence (the numerical task presented to the guppies) may be be geared toward female behaviors. “In the guppy, females are more active and innovative while foraging,” they explain. “Because females feed more, they may thus have had more time to associate the cue with food in our experimental design.”
The clear trade-off between brains and guts, though, is an important finding. By providing empirical evidence for the physiological costs of brains, this study provides the ﬁrst direct support for the expensive-tissue hypothesis, and can provide us with insights into how our own big brains evolved. One of the prevailing hypothesis for our own brain growth is that the incorporation of more animal products into our diets, through hunting or cooking or however, allowed us to obtain more energy from less food, thus offsetting the cost of a reduced gut. The less food we needed to eat for the same amount of energy, the more our brains could grow even if our guts suffered for it. The debate, however, is far from over. Comparative analyses in primates don’t support a gut-brain tradeoff, and there are certainly plenty of other hypothesis as to how and why we developed our massive lobes, and what prices our bodies paid for them.
Citation: Kotrschal et al. (2013) “Artificial selection on relative brain size in the guppy reveals costs and benefits of evolving a larger brain.” Current Biology 23, 1–4. DOI: 10.1016/j.cub.2012.11.058