The bigger an animal’s brain, the less equipped it is to battle illness, a new study suggests.
An evolutionary trade-off between brain size and immune systems has been highlighted for the first time, supporting the theory that animals have a limited amount of energy to put into our biggest organs – and a big brain may lead to compromises elsewhere.
The study, led by Alexander Kotrschal, a zoologist at Stockholm University in Sweden, and published in the Proceedings of the Royal Society B, tested the immune responses of populations of Trindadian guppies (Poecilia reticulata), bred by scientists who’ve selected for small and large brain sizes.
These brain-varied guppies were used in a previous study to show that animals with larger brains also evolved smaller guts.
Similarly, in the case of primates, the size of our brains may have robbed us of a larger digestive system – our gut is relatively small, and this is likely because we invest energy into powering our sizable brains instead.
Guppies were again used in this study to test for immunity. But because they’re are so small, blood and tissue tests aren’t very effective at picking out their immune cells.
Instead, the researchers used a common process known as scale allograft – that is, grafting a guppy’s scale onto another individual to test its rejection response. Signs of rejection reflect a good immune system.
The procedure involved sedating a pair of guppies, one with a small brain, one with a large brain, and lying them side by side on gauze. The researchers then removed one scale from each fish and grafted it onto the other.
In the eight days following the procedure, researchers watched for signs of healing, swelling or inflammation, and rated each guppy’s response.
By the sixth day, the large-brained fish were showing a lower rate of rejection than the smaller-brained fish, suggesting a lower innate immune response.
According to the researchers, the weaker immune response of big-brained animals could also relate to changes in other areas, such as the size of the gut, or differences in hormone responses.
It’s not all bad news for us big brainers, though: according to the paper, larger-brained animals have other weapons to protect them against infection and illness.
Big brains mean higher cognitive function, and an earlier study into guppies, again, backed this theory.
There’s also the added bonus of behavioural defences in smarter big-brained animals, such as taking measures to avoid sickness, or using immunity-boosters to help the body fight back.