Fish have long been known to follow their instincts when it comes to managing their survival, but according to a new study, these animals also experience a form of emotional fever in which their body temperature slightly increases when they are exposed to stressful situations.

In a study featured in the journal Proceedings of the Royal Society of London, Biological Sciences, scientists from the University of Bristol and University of Stirling in the United Kingdom and the Universitat Autònoma de Barcelona (UAB) in Spain observed that when zebrafish (Danio rerio) experience stress, the temperature of their body rises by around two to four degrees.

Despite having been seen in birds, mammals and certain species of reptiles, this emotional fever has never been observed in fish before. The researchers believe the experience is linked to what the aquatic animals feel in relation to external stimuli.

Emotional Fever in Fish

To find out how this phenomenon occurs in fish, UAB researcher Sonia Rey and her colleagues collected 72 individual zebrafish and arranged them into two groups consisting of 36 fish each.

They then placed the animals in a large tank filled with water and fitted with interconnecting compartments that had varying temperatures, from 18 degrees Celsius (32.4 degrees Fahrenheit) to 35 degrees Celsius (63 degrees Fahrenheit).

Fish that were part of the control group were not subjected to any changes in temperatures and were left to stay in waters at 28 degrees Celsius (50.4 degrees Fahrenheit).

The rest of the animals, on the other hand, were placed in a net found inside the fish tank with the temperature set at 27 degrees Celsius (48.6 degrees Fahrenheit). This was done in order to simulate a stressful situation for the fish. They were later released after 15 minutes.

While the animals in the control group preferred to stay in compartments with the temperature at 28 degrees Celsius (50.4 degrees Fahrenheit), those exposed to the stressful situation often transferred to areas of the fish tank with high temperatures, causing their body temperature to increase by around two degrees to four degrees.

According to the researchers, this is an indication that the zebrafish were experiencing a form of emotional fever related to their situation.

"These findings are very interesting - expressing emotional fever suggests for the first time that fish have some degree of consciousness," Rey explained.

Animal scientists, however, have different theories regarding the level of consciousness that fish can manifest.

Some experts point out that animals are not capable of having consciousness because of the simple structures of their brains, which do not carry a cerebral cortex. Fish have a limited capacity for memory and learning, as well as an inability to feel suffering.

Other researchers argue that even though fish only have a relatively small brain, they have observed homologies between some structures of the animals' brains and those in other vertebrates. These include the amygdala, which is associated with emotions, and the hippocampus, which is associated with spatial memory and learning in mammals.

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