Humans might be the only species to sniffle and giggle through a romantic comedy, but there's good reason to think many other animals also have some capacity to share emotions with their kindred.
A new study led by researchers from the Instituto Gulbenkian de Ciência in Portugal has provided evidence of the chemical mechanisms behind the spreading of fear among zebrafish, hinting that human empathy could have originated in our aquatic ancestors hundreds of millions of years ago.
Seeing other members of a social group react in fear is handy for anticipating danger. With numerous eyes keeping watch, you have a better chance of getting out alive if you can detect another's panic at a glance.
Fish are no exception. Previous studies on zebrafish (Danio rerio) provided insights into their transmission of alarm responses, presented as erratic and freezing behavior in individuals, and the elevation of cortisol levels in nearby observers.
Perhaps surprisingly, the degree of fear observers experience depends on whether their buddies look distressed or a complete stranger. The greater the familiarity between the fish, the greater the stress response in the bystander watching the distress unfold.
Central to emotional signaling in animals such as ourselves is the peptide oxytocin. Though commonly referred to as the love hormone, thanks to how it promotes social engagement, its rule over animal biology is far more complicated than making us crave hugs.
To better understand the role the hormone could play in the transmission of fear in zebrafish, the researchers used mutant varieties that had broken versions of the chemical and two of its receptors.
Since zebrafish – like others in its order – release a chemical cue from their skin when injured that also provokes a response in nearby individuals, the team placed unfamiliar fish into separate tanks where they could still see others swim. This allowed them to tweak the controls, adding or withholding chemical cues to the water at will.
Watching a distressed shoal from afar, fish with functional oxytocin hormones and receptors all froze as expected. Mutated versions of the zebrafish, however, swam on like silvery little psychopaths, barely caring one whit for their terrorized neighbors.
Adding oxytocin to the water or injecting it straight into select mutants changed their tune, further demonstrating the social hormone was responsible for their sensing of fear in others.
Follow-up investigations on the fish using neuronal activity markers tracked responses deep inside their brain, finding parallels between the areas responsible for their reactions and those responsible for emotional contagions in rodents.
Based on these findings, it could be concluded that the biology of experiencing stress after witnessing fear responses in another is similar enough in all vertebrates to have evolved in a shared ancestor.
In its simplest form, the release of oxytocin could also simply trigger a mirroring of behaviors in the fish. Seeing distress in a fellow swimmer may just create the same internal response in their peers, for example.
To see if this was what was happening, the researchers showed individual fish video clips of others in distress or neutral states before placing them together, along with some other fish, after a short amount of time had passed.
Interestingly, the observer preferred to cuddle up to the individuals they'd previously seen distressed than to the neutral-acting fish, suggesting something more had to be going on.
Just how deeply we might read into the experiment is hard to say. It's easy for us to see our own behaviors in the fish – we've had millions of years of evolution setting our empathy systems to hyperdrive, after all.
It's tempting to think the biochemical machinery responsible for our own social behaviors are built on foundations very similar to the mechanisms of fear contagion in fish.
"However, to what extent the social contagion of fear observed in zebrafish and in mammals is homologous, or represents a case of convergent evolution, remains an open question," the authors admit.
Perhaps a rom-com for zebrafish is overdue for release?
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