A team of researchers has found the main downside to having a large brain—an increased need for energy, usually in the form of calories or increased oxygen demands. The team used large-brained fish species to study their theory on the connection between increased energy demands and enlarged brains. The findings provide new insights into how primates, including humans, evolved such large brains in comparison to other animals. The details are in a paper that was just published in the journal Proceedings of the Royal Society B.
A large brain offers many benefits yet human-sized brains are uncommon in the animal kingdom. This is mostly because brains are energetically expensive–brain tissue requires a ton of energy to operate properly. Evolving a large brain takes an increase in energy consumed (from calories or better nutrition, for example) or a decrease in energy required elsewhere (such as more efficient organs). Although these theories are not new, there has been little hard evidence to back them up.
Researchers studied mormyrid fish, strange animals that are also called elephantfish because of their long mouth extensions that resemble elephant trunks. These odd fish can generate electric fields and use electricity to navigate, communicate, and locate prey. They are also some of the most intelligent fish and have huge brains in proportion to their body sizes. Since there are many species in the Mormyridae family, the research team compared them to study the evolution of large brains.
The team studied more than 30 separate species of mormyrid fish, comparing brain size with factors such as oxygen consumption. The researchers found that the fish with larger brains also needed more oxygen and had less tolerance of low-oxygen environments. In other words, a large brain demands extra oxygen—an energetic trade-off that may be worth it when a big brain helps an animal survive and reproduce.
The team’s findings show that large brains demand more energy but this was only true up to a point. The researchers found that this relationship explained moderate brain size increases but not necessarily the extremely large brains of some species, such as humans. Further research is needed to fully understand how humans evolved such large and complex brains.
Sukhum et al. The costs of a big brain: extreme encephalization results in higher energetic demand and reduced hypoxia tolerance in weakly electric African fishes. Proceedings of the Royal Society B (2016).