Researchers in Spain and the U.K. have made the first observations infish of an increase in body temperature of 2–4 ºC when zebrafish were subjected to a stressful situation (they were confined in a net inside the tank at an uncomforable 27ºC for 15 minutes).*

This phenomenon is called “emotional fever” because it’s related to the emotions that animals feel in the face of an external stimulus, which been linked, controversially, with their consciousness. Until now, emotional fever had been observed in mammals, birds and certain reptiles, but never in fish, which is why fish have been regarded as animals without emotions or consciousness.

Does consciousness require a cerebral cortex?

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 behavioral 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 behavioral analyses have highlighted similarities between some fish 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.

The research was published in an open-access paper recently in Proceedings of the Royal Society of London, Biological Sciences. It began three years ago at the Universitat Autònoma de Barcelona. Scientists from Stirling and Bristol universities helped with statistical analysis of the data.

* The researchers divided 72 zebrafish into two groups of 36 and placed them 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.

Abstract of Fish can show emotional fever: stress-induced hyperthermia in zebrafish

Whether fishes are sentient beings remains an unresolved and controversial question. Among characteristics thought to reflect a low level of sentience in fishes is an inability to show stress-induced hyperthermia (SIH), a transient rise in body temperature shown in response to a variety of stressors. This is a real fever response, so is often referred to as ‘emotional fever’. It has been suggested that the capacity for emotional fever evolved only in amniotes (mammals, birds and reptiles), in association with the evolution of consciousness in these groups. According to this view, lack of emotional fever in fishes reflects a lack of consciousness. We report here on a study in which six zebrafish groups with access to a temperature gradient were either left as undisturbed controls or subjected to a short period of confinement. The results were striking: compared to controls, stressed zebrafish spent significantly more time at higher temperatures, achieving an estimated rise in body temperature of about 2–4°C. Thus, zebrafish clearly have the capacity to show emotional fever. While the link between emotion and consciousness is still debated, this finding removes a key argument for lack of consciousness in fishes.