Photo: Paul Bentzen
Photo: Paul Bentzen

The study has measured how the immune system of fish responded to transplanted scales and found that individuals with small brains have stronger immune rejection responses than those with large brains. Developing and maintaining a larger brain surely requires more energy and resources, which may explain the negative effects on immune function.

“The results suggest that apart from the obvious advantage of making you cleverer, a larger brain can also come at a cost – an impaired immune system,” says Alexander Kotrschal, the lead author of the study.

The researchers transplanted scales of large-brained fish onto small-brained fish and vice versa and observed how those alien scales were rejected. What they found was that large-brained individuals showed a weaker rejection reaction than those with small brains. This showed that the innate immune system is impaired in large-brained individuals.

Three weeks later scales where transplanted between the same pairs of fish to test the adaptive immune system which develops specific antibodies, for example in response to vaccinations. This test showed no difference between large- and small-brained fish, but males showed a stronger rejection response than females. The results indicate that male guppies have stronger acquired immunity than females.

“In most species, males have weaker immune responses than females and increased susceptibility to infectious diseases. It is thought that males can benefit by investing more resources into mating effort, even at the expense of reduced immune responses. However, we are unaware of any theory to explain our findings and the increasing number of other cases that provide exceptions to this general rule”, says Dustin Penn, the senior author on the study.

The next step is to identify the mechanisms that generate these sex differences in immunity, and to explain how genes that influence brain size also control innate immune responses.


The article “Selection for brain size impairs innate, but not adaptive immune responses” by Alexander Kotrschal and Niclas Kolm at the Department of Zoology, Stockholm University and Dustin Penn at the Konrad Lorenz Institute of Ethology of the University of Veterinary Medicine, Vienna was published in the journal Proceedings of the Royal Society – B.

The article:

Contact information

Alexander Kotrschal, Department of Zoology, Stockholm University, phone +46 (0) 8 16 40 46, cell 0046-76 575 16 61, e-mail