Guppy. Photo: Paul Bentzen
Guppy. Photo: Paul Bentzen

“Females with larger brains did better than those with small brains. It may be that those with larger brains have better cognitive abilities. They are smarter. The results suggest that there are important advantages associated with developing a larger brain. The risk of being killed and eaten may thus be a key factor that affected animals during evolution and may have led to the unusually large brains in humans. The early humans lived in an environment full of predators”, says Niclas Kolm, a researcher at the Department of Zoology, Stockholm University, one of those who made the study.

Huge Trinidad stream aquariums

Niclas Kolm and his colleagues conducted the study of nearly 5000 guppy fish with known brain size from their previous selection experiments on brain size of these guppies. The researchers bred fish with different size of the brain and marked all individuals with a special tattoo technique to distinguish between fish with large and small brains. The fish were then kept in a huge aquarium at the Konrad Lorenz Institute in Vienna. The aquarium was decorated to resemble six smaller streams in Trinidad, which is the natural environment for the guppy. Besides guppies, also pike cichlids, a fish that eats guppies in the wild, were also released into the semi-natural streams. A weekly census where all fish were collected and counted then continued for five months. After five months, the females with large brains had a > 13 percent higher survival rate than females with small brains. However, researchers found no difference among males.

Males eaten whatever brain size

“The results show that there is clearly an advantage for females to have larger brains in terms of threats from predators. As for the males, we could not see the same benefit. They were eaten about the same regardless of brain size. This can be explained by that males with large brains in these selection lines also have been more colorful patterns, which may reduce the potential cognitive benefits as they are more visible for a visual predator like the pike cichlid. The next step is to identify the mechanism/s that generated these differences. Are females with big brains better at spotting predators and/or do they have a better arsenal of behavior when they have discovered a predator?” says Niclas Kolm.