Well, we have all heard the age-old adage of bigger brains being the better and how humans have the largest brain-to-body ratio among all animals. So, scientists have been asking this question: What would happen if the size of brains continue to increase? Would the organism become more intelligent? Would there be any cost?
Evolutionary biologists in 1990’s proposed the The Expensive Tissue Hypothesis to account for the costs and benefits of brain size. Brains are highly useful organs; more amount of brain cells would allow for more flexibility in doing different behaviours, better control of larger bodies, and, obviously intelligence. But if bigger brains were always better, then every animal would have them. So, the biologists reasoned, there has to be a downside of such increased brain size. Hence, the hypothesis suggests that while having larger brains are awesome, however their extremely high energetic cost limits their size and tempers their growth.
For example in humans, our brains take up just 2% of our bodies, but they take up a whopping 20% of our energy requirements. And one has to wonder: if our brains use up that much energy, which body parts have paid the price? The hypothesis suggested our guts have suffered, but the extra intelligence gained by having more brain cells made up for more efficient foraging and hunting, hence overcoming the obstacle. Despite over a century of research on the evolution of brain size, empirical support for the trade-off between cognitive ability and energetic costs is based exclusively on correlative evidence and the theory remains controversial.
What they did?
A study published in Current Biology this month, led by Niclas Kolm and others have attempted to solve this question by conducting an empirical study on guppies (Poecilia reticulata, pictured above). They used artificial selection on relative brain size in the guppy, to provide a direct test of the prediction that increased brain size is genetically associated with increased cognitive ability but that a large brain is also traded off against gut size and results in reduced reproductive performance. Breaking it down, they had four steps:
- To test the evolutionary response to divergent selection on relative brain size.
- The cognitive ability of large and small-brained individuals was tested using an associative learning assay designed to investigate numerical quantification, a relatively advanced form of cognition.
- The correlated evolutionary response of gut size in response to direct selection on brain size was also tested.
- Lastly, it was tested whether the important proxies of reproductive fitness (offspring number, offspring size, age at first reproduction) are anyhow affected by brain size evolution.
What they found?
First, the team selected for larger and smaller brains from the available natural variation in guppies. They then successfully created smart 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 the larger brains, the females with bigger brains were significantly better at the tasks.What they found was that the evolution of relative brain size in guppies can be a fast process when under strong directional selection as in the study. Also, the relative brain size was found to be highly heritable in both sexes.
What was really remarkable was the cost of these larger brains. Gut size was found to be 20% smaller in large-brained males and 8% smaller in large-brained females. The reduced digestive system seemed to have serious reproductive consequences, as the smarter fish produced 19% fewer offspring in their first clutch, even though they started breeding at the same age as their lesser bright counterparts. One thing to keep in mind here which the authors also noted, was that this experiment was conducted in an idealized tank setting with all the guppies receiving plenty of food—So what about the wild, where resources are harder to come by? How much of a cost does a reduced gut have when resources are scarce?
Though, 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 measure of intelligence used (the numerical task presented to the guppies) may be favourable toward female behaviors. As is known from literature, in the guppy, females are more active and innovative while foraging. As females feed more, so they may have had more time to associate the cue with food in the experimental design.
The clear trade-off which the authors see between brains and guts, is an important finding. By providing an 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.