Researchers from the Universitat Autònoma de Barcelona (UAB), together with scientists from the universities of Stirling and Bristol (United Kingdom), have for the first time observed an increase in body temperature of between two and four degrees in zebrafish, when these are subjected to stressful situations. This phenomenon is known as emotional fever, as it is related to the emotions that animals feel in the face of an external stimulus and it has even been linked, not without some controversy, with their consciousness.

Until now emotional fever had been observed in mammals, birds and certain reptiles, but never in fish. For this reason fish have been regarded as animals without emotions or consciousness. The experiment, with 72 zebrafish, has brought this view into question.

The researchers divided the fish into two groups of 36 and they were placed in a large tank with different interconnected compartments with temperatures ranging from 18ºC to 35ºC. The fish in one of these groups - the control group - were left undisturbed in the area where the temperature was at the level they prefer: 28ºC. The other group was subjected to a stressful situation: they were confined in a net inside the tank at 27ºC for 15 minutes. After this period the group was released. While the control fish mainly stayed in the compartments at around 28ºC, the fish subjected to stress tended to move towards the compartments with a higher temperature, increasing their body temperature by two to four degrees. The researchers point to this as proof that these fish were displaying emotional fever.

Scientists differ on the degree to which fish can have consciousness. Some researchers argue that they cannot have consciousness as their brain is simple, lacking a cerebral cortex, and they have little capacity for learning and memory, a very simple behavioural repertoire and no ability to experience suffering. Others contest this view, pointing out that, despite the small size of the fish brain, detailed morphological and behavioural analyses have highlighted homologies between some of their brain structures and those seen in other vertebrates, such as the hippocampus (linked to learning and spatial memory) and the amygdala (linked to emotions) of mammals.

In the words of Sonia Rey, of the Institute of Aquaculture, University of Stirling and the UAB's Institute of Biomedicine and Biotechnology (IBB), "these findings are very interesting: expressing emotional fever suggests for the first time that fish have some degree of consciousness".

The research was published recently in Proceedings of the Royal Society of London, Biological Sciences. It began three years ago at the UAB's Institute of Biomedicine and Biotechnology and was concluded at Stirling University by the same researchers, Sonia Rey, Simon Mackenzie, Reynaldo Vargas and Sebastian Boltaña, in collaboration with Felicity Huntingford, of Stirling and Toby Knowles, of Bristol University, who helped with the statistical analysis of the data.