Captive tilapia study shows cold-blooded animals using behaviorally-induced fever to battle infection

A team of life scientists at East China Normal University has found that at least one type of cold-blooded animal uses behaviorally-induced fever to battle infections. In their paper published in Proceedings of the National Academy of Sciences, the group describes how they put Nile tilapia, a type of fish, in tanks with a heat gradient, infected them with a bacterium and then watched how they behaved—and what they learned by doing so.

Warm-blooded animals use fever to help rid themselves of infections. Raising the core body temperature not only makes life more difficult for infectors but also boosts the immune system. In this new effort, the research team wondered if cold-blooded creatures induce fevers to help them ward off infections.

Since cold-blooded creatures cannot warm their core body temperatures internally, their only choice is to do it externally—by lying in the sun, for example, or moving to warmer water in the case of fish. To find out if that might be the case, the researchers collected a large number of Nile tilapia and brought them back to their lab for study.

Back in their lab, the researchers exposed the fish to Edwardsiella piscicida, a type of bacteria that can lead to epidemics in fish, particularly on fish farms. Each fish was then placed in a special type of tank that allowed for a temperature gradient, ranging from 28°C to 34°C.

At first, before the fish felt sick, they swam in the cooler end of the tanks, the researchers found. After they became visibly ill, however, they moved to the warmer end of the tank, suggesting the use of a behaviorally-induced fever to fight off their infections.

In taking a closer look, the research team found that, after infection, the fish demonstrated a loss of appetite, which in turn set off a moderate degree of autophagy, where the process of cleaning up damaged cells was stepped up—one sign of a boosted immune response, and more evidence that the fish were boosting their immune system through behavioral activity. The researchers also found evidence of behaviorally-induced fever boosting the expression of genes related to T cell longevity, which suggests a longer-term immune response.

The researchers conclude by suggesting their experiments show that at least one type of cold-blooded creature uses behaviorally-induced fever to battle infections.

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