When faced with stressful situations mammals, birds, and certain reptiles exhibit emotional fever, or a slight rise in body temperature in response to external stimuli. For the first time, researchers from the Universitat Autònoma de Barcelona have documented similar behaviors in fish.  

For their study, researchers placed two groups of 36 zebrafish in a large tank with different interconnected compartments that had temperatures ranging from 18 degrees Celsius to 35 degrees Celsius, according to a news release. While one of the fish groups was kept in an undisturbed area where the temperature was set to 28 degrees Celsius – their optimumal environment – the other group was subjected to a stressful situation: confined to a net inside the tank set to 27 degrees Celsius for 15 minutes.

When released, the control fish stayed in their designated compartments, but the distressed fish quickly swam towards warmer temperatures, increasing their body temperature by two or four degrees. 

Fish have often been regarded as having a low level of consciousness, or sentience, simply because they lack a cerebral cortex and –presumably – the ability to experience suffering. In addition, they posses little capacity for learning and memory. However, this recent study suggests fish respond to hypothermically stressful situations.

"These findings are very interesting: expressing emotional fever suggests for the first time that fish have some degree of consciousness," Sonia Rey, from UAB's Institute of Biomedicine and Biotechnology, said in the university's release.

Zebrafish are small tropical freshwater fish that are part of the minnow family. While they are native to the Himalayan Region and have a broad omnivorous diet, ranging from mud and sand to insects and arachnids, plants, algae, fish scales and even rubbish. 

The study was recently published in the Proceedings of the Royal Society of London, Biological Sciences.

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