New Insights into Emotions in FIsh

In December Proceedings of the Royal Society B: Biological Sciences published an article called “Fish can show emotional fever: stress-induced hyperthermia in zebrafish“. We asked one of the authors, Dr. Sonia Rey Planellas from the University of Stirling, to comment on this study.

The Study

Whether fish are sentient beings remains an unresolved and controversial question. Among characteristics thought to reflect a low level of sentience in fish is an apparent inability to show stress-induced hyperthermia (SIH). This is a transient rise in body temperature shown in response to a variety of stressors. SIH is commonly used as a Stress Indicator (SI) in mammals and birds in which it is detected by Infra Red Thermal cameras, internal thermal sensors or thermometry. The increase in temperature is driven by stressful situations (novel environment, confinement, fighting, etc) and the pathways activated are to those activated by infectious fever however the trigger in this case being not a viral or bacterial infection but an emotion (fear, anxiety, etc).

Elephants born in stressful conditions might age faster than others The speed of reproductive ageing of female elephants seems to depend on the season in which they were born

This is a real fever response, so it is often referred to as “emotional fever”. In a series of influential papers, 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, the apparent the lack of emotional fever in fish 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 2-4ºC. Thus zebrafish clearly have the capacity to show emotional fever, increasing their internal body temperature behaviorally, by moving to hotter waters. While the link between emotion and consciousness is still debated, this finding removes a key argument for lack of consciousness in fish.

Background

The importance of emotional fever in the debate about fish sentience and consciousness lies in the fact that earlier studies failed to identify SIH in fish (goldfish)and amphibians (toads) even although these animals show well-developed physiological responses to the stress. On the basis of these findings, it was concluded that consciousness, and with it the ability to experience emotional fever, evolved exclusively in the amniotes lineage (mammals, birds and reptiles) and is absent in fish and amphibians. These views are still influential in the scientific community.

We provided fish with the opportunity to move freely within a temperature gradient centered on the species’ preferred temperature as opposed to offering a choice between two relatively high temperatures as in previous studies, and this seems to be critical for demonstrating emotional fever. In the present experiment fish were free to make fine-scale behavioral adjustments and such adjustments take the form of emotional fever. This is supported by an unexpected finding in a previous study describing behavioral fever induced by a viral infection in zebrafish. Although not designed for this purpose the previous study indicated that zebrafish that had been stressed (by handling and control saline injection) spent significantly more time in the warmer chambers than unstressed controls. Thus by quantifying the movement of zebrafish within a thermal gradient we were able to demonstrate the occurrence of SIH, or emotional fever in fish.

Future directions

Whatever the underlying mechanisms and associated benefits of the phenomenon and notwithstanding the complex relationship between emotion and consciousness, the fact that the fish used in this study are capable of SIH, or emotional fever, means that the absence of this ability cannot be used to argue for a lack of consciousness in this taxonomic group. In fact a growing body of information from other sources indicates that at least some of the brain mechanisms involved with feeling and emotion in mammals are conserved vertebrate features, that the responses of fish to noxious stimuli are complex and include a motivational/attentional component and that fish have well developed learning capacities and show complex behavior. Our results add to the emerging picture of fish, as behaviorally complex animals that may well be to an extent sentient and conscious. This has important implications both for how the welfare of fish is protected and for our understanding of the evolution of emotions and consciousness in vertebrates. ‬‬‬