Scientists are still trying to figure out whether fishes are sentient beings or not. 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.
While the other group points 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.
One of the characteristics which is thought to reflect a low level of sentience in fishes is their inability to show stress-induced hyperthermia (SIH), a transient rise in body temperature shown in response to a variety of stress factors. This is a real fever response, so is often referred to as ‘emotional fever’. Until now emotional fever had been observed in mammals, birds and certain reptiles, but never in fish.
For the first time, scientists have observed this phenomenon of ’emotional fever’- an increase in body temperature when subjected to stress in a new research published in Proceedings of the Royal Society of London, Biological Sciences– which has been controversially linked to consciousness.
Researchers from the Universitat Autònoma de Barcelona, together with scientists from the universities of Stirling and Bristol (United Kingdom), observed an increase in body temperature of between two and four degrees in zebrafish, when they were subjected to stressful situations.
The researchers experimented with 72 zebrafishes, 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.
Though the link between emotion and consciousness is still debated, this finding removes a key argument for lack of consciousness in fishes, say the researchers.
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”.