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A large brain causes the innate immune system to totter

09.03.2016: A big brain might make the aquarium fish guppy smarter, but maybe also more prone to diseases. A team of researchers from the Konrad Lorenz Institute of Ethology at the University of Veterinary Medicine, Vienna found out that fish with larger brains are less resistant than the ones with smaller brains. The higher energy demand of larger brains seems to lead to a lack of energy in the innate immune system for defence responses. A study on this topic was published in the journal “Proceedings of the Royal Society B”.

The researchers compared the efficiency of the immune system in guppies with larger and smaller brains. They transplanted scales between pairs of the popular aquarium fish together with the attached mucous layer and pigment cells and observed the rejection reactions. Both the brain and the immune system are energetically demanding. “The high energy consumption of a relatively large brain could put the immune system on the back burner,” the researchers from the Konrad Lorenz Institute of Ethology at the Vetmeduni Vienna explained.

Small brain, strong reaction

The innate immune system already reacts at the first contact with invaders and foreign bodies such as transplants. In small-brained guppies, the rejection reaction of the innate immune system after the scale transplantation was stronger than in large-brained fish. “In the rejection process, the tissue around the transplanted scales first swelled slightly, then the mucous layer there became cloudy. In the end the foreign pigment cells were degraded by the recipient’s immune system,” Kotrschal and Penn explained. After little more than a week, all foreign material except for the scale itself was degraded and overgrown with autologous material. Just the transplanted scale remained a permanent part of the recipient.

Adaptive immune system not influenced

After three weeks, the researchers repeated the scale transplantation in the same animals. Thus, they could observe the response of the specific – adaptive – immune system. The adaptive immune system remembers foreign bodies and is consequently able to combat them quicker and more effectively at the next contact. “We had also expected reactions of varying severity in fish with smaller and larger brains. But this was not the case,” Kotrschal stated.

Thus, the results show that the development of a larger brain is at the expense of the innate immune system, but not of the adaptive one. “Smart fish pay for their intelligence by a lower immune response at the first contact with foreign bodies,” Kotrschal and Penn concluded.

The article "Selection for brain size impairs innate, but not adaptive immune responses", by Alexander Kotrschal, Niclas Kolm and Dustin Penn was published in the Journal Proceedings of the Royal Society B.