Having a bigger brain may not be better. If you are a guppy, that is. A recent study from Stockholm University found that having a smaller brain provides immune benefits for guppies.
Developing and maintaining a large brain requires more energy and resources, however, large-brained animals are often thought to be smarter than their small-brained counterparts. In fact, researchers the University of Wyoming confirmed this theory, showing that species with larger brains relative to their body size are better at solving problems.
However, the latest study from Stockholm University focused on guppies, measuring their immune system and how it responded to transplanted scales.
"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," said Alexander Kotrschal, the lead author of the study.
Researchers transplanted scales of large-brained fish onto small-brained fish and vice versa. This revealed that guppies with smaller brains have stronger immune rejection responses than guppies with larger brains. In other words, their immune system was better prepared to defend against the alien scales. This suggests that the innate immune system is impaired in large-brained individuals. It is believed that the additional energy and resources required to power a larger brain might be responsible for the negative effects on one's immune function.
To test the animals' adaptive immune system, researchers repeated the experiment three weeks later. With an adaptive immune system, the guppies would have developed antibodies.
However, their subsequent experiments revealed no difference between large- and small-brained fish but did show that males have a stronger rejection response than females. This suggests that male guppies have a 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," added Dustin Penn, senior author on the study. "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."
Next, researchers hope to further investigate the mechanisms that generate these sex differences in immunity. The hope, they said, is to explain how genes that influence brain size also control innate immune responses.
Their study was recently published in the journal Proceedings of the Royal Society B.
