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Fish respond to stress with an "emotional fever" comparable to that found in mammals -- including humans -- according to a new study published in the Proceedings of the Royal Society B.

An emotional fever is a temporary one or two degrees C increase in body temperature that appears in response to stress, such as that induced by an unfamiliar environment. The phenomenon of stress-induced hyperthermia has been observed in mammals, birds and some reptiles.


The paper states that "emotional fever is a true fever, and the same internal pathways that activate behavioural fever are activated, but the trigger in this case is not an exogenous or endogenous pyrogen" such as bacterial or viral infection, and it contributes to effective fight or flight responses.

Emotional fever states are regarded to be a hallmark of sentience and consciousness, indicating the a create has an emotional response to potentially threatening situations. Based on earlier tests involving goldfish and toads, it was previously thought that only amniotes -- vertebrates that lay eggs on land or retain the fertilised egg within the mother -- could experience emotional fever states.

The fact that fish and amphibians apparently did not exhibit emotional fever was taken as evidence that they lacked even basic sentience.

However, when given access to a tank containing a range of water temperatures which they could freely move between, stressed zebrafish consistently moved to a warmer area than their relaxed counterparts, increasing their body temperature to between two and four degrees C above that of unstressed fish, effectively demonstrating an emotional fever state.

Zebrafish normally prefer a water temperature of around 28 degrees C, but after experiencing stress as a result of being netted and restrained, the fish consistently spent more time --- up to eight hours -- in warmer water.