An evolutionary tradeoff exists between brain size and immunity, according to new research.
Bigger brained animals may be more vulnerable to a barrage of illnesses than species whose evolution has selected for immunity over braininess.
"Organisms have to deal with the limited energy they have available -- they cannot have it all," Alexander Kotrschal of Stockholm University's Department of Biology told Discovery News. Kotrschal and colleagues Niclas Kolm and Dustin Penn conducted the research, which is published in the journal Proceedings of the Royal Society B.
Kotrschal explained that "investing more in one costly organ, such as the brain, will deduct energy away from other costly organs, like gut, muscle and fat. Humans have quite small guts compared to other primates. In fact, across primates: the larger the brain, the smaller the gut."
The researchers decided to investigate whether larger brains also could lead to reduced immune responses. They decided upon guppies as a model animal, since these little fish have a fast average time between two generations and have been well studied. Guppies also share many critical molecular pathways with humans, other mammals and numerous additional animals.
The scientists examined the relationship between brain size and immune response to scale tissue grafting in lab-grown Trinidadian guppies that were artificially selected for large or small relative brain size.
As predicted, the smaller-brained individuals of both sexes mounted a much stronger immune response than did those with bigger brains. Having a big brain was not all bad, though. Earlier research found that female guppies with more brainpower had a cognitive advantage over others.
Kotrschal explained: "They were better at learning a numerical learning task than small-brained females."
As the studies indicate, brain size can differ not only among species, but also among individuals of the same species. Relative brain size, or how the brain measures when compared to the rest of the entire body, is the most important factor -- including very tiny creatures, such as ants and honeybees, which are considered to be quite brainy and intelligent.
"In primates, males and females generally have similar brain sizes when controlled for body size," Kotrschal said. "This is also true for humans."
On the other hand, he added that there are a few animals, such as sticklebacks (another type of fish) where one sex always seems to have a larger brain. In the case of sticklebacks, males are nearly always brainier than females. Researchers think that is because these fish must work overtime caring for, and protecting, their young.
As for when the tradeoff between brain size and immunity or other traits happens, Kotrschal believes it occurs during the very earliest stages of development.
"For instance, (it may happen) when the embryo ‘decides' how many stem cells are attributed to neural development versus the immune system," he said.
Hans Hofmann is a professor of integrative biology at the University of Texas at Austin, where he's the director of the Center for Computational Biology and Bioinformatics.
Hofmann said it would be important to repeat the experiments in order to better determine how the tradeoff affects both innate and acquired immune function.
"It is also interesting to ask what the underlying mechanisms are that give rise to this apparent tradeoff," he said.
It should also be noted that intelligence can help an individual to prevent some illneses, so there is a connection between brainpower and health. Two other studies published this week in the journal Biology Letters focus on how ants, honeybees and other very social insects prevent illness while living in some of the most crowded conditions imaginable.
Such insects have an arsenal of prevention techniques that include personal fastidiousness, cleaning others and reducing their contact with contagious individuals. Ants have zero tolerance for a sick ant that shows up to work. If efforts to disinfect or otherwise cure the ill ant fail, healthy ants will throw the sick individual out of the nest.