In her lab, Jessica Nowicki can often be spotted pinching and plucking a small female poison dart frog in the leg. She then returns the frog to her terrarium home alongside her male partner and waits for a sign that he is sharing her pain – a wince, a jolt, a small leap towards his hurting partner, perhaps.
While Nowicki, a neuroethologist at Stanford University, has yet to see such an outright display of concern, she has discovered something similar. The male frog experiences a small spike in stress hormones once he is reunited with his stressed counterparts: inside his body, he matches his partner’s emotional state.
This suggests frogs are capable of the most ancestral form of empathy, according to Nowicki’s new study published in the journal Royal Society Open Science – a finding that could upend how we understand animal feelings, despite the obvious difficulties we face when attempting to test them.
“The first step is to stop assuming empathy is not there,” says Nowicki. “And the second is to be more holistic in how we can measure it.”
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Figuring out whether non-human animals feel empathy is extremely challenging for scientists: animals don’t clearly, unequivocally communicate with us, and they cannot self-report their feelings in a test.
To a certain extent, it’s also impossible to confirm whether the happiness one human says they’re feeling is the same happiness another human knows to exist in their body.
“That certainly doesn't mean that emotions don't exist,” says Nowicki. “It just means that empirically they're impossible to prove.” But emotions also have biological markers – they are related to certain chemicals in the bloodstream, and certain signals in the brain. These elements can be empirically tested.
Several studies have already tried to spot these markers of empathy in animals. In 2016, scientists found that prairie voles match the stress hormones of their partners and also console them by grooming them more if they notice they’re stressed.
Birds have been suggested to change their song melodies and match those of partners in stress, and fish can tense up by simply watching other members of their group becoming agitated.
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But seldom do scientists pose these queries to reptiles and amphibians. So Nowicki turned to poison dart frogs (Ranitomeya imitator). These frogs are monogamous: males and females rear their offspring together, communicating and helping each other out along the way, so Nowicki figured it would be easier to spot an emotional connection. She used the same experimental setup as the 2016 prairie vole study.
Surely enough, when her team stressed out a random female frog and placed her in a terrarium with a male, nothing much happened. But when they stressed out a female frog and reunited her with her mate, the male frog’s levels of corticosterone, a physiological biomarker of stress similar to cortisol, matched those of his partner.
“It was like, ‘wow!’” says Nowicki. This is evidence of a frog rubbing off its emotional stress on another frog, and is therefore a form of empathy, she says.
The fact the frog only reacts to his partner’s feelings, not simply any other frog, means the transfer of distress is not just automated contagion – like the spread of alarm, a danger signal preparing the frog to deal with a potential threat, says Dr Inbal Ben Ami Bartal, a researcher of pro-social behaviours in animals at Tel-Aviv University.
These findings are “a good example showing that the basic building blocks of empathy are shared across species,” she says.
Though the frog didn’t change his behaviour in the way Nowicki initially hoped, it doesn’t discount the possibility that emotional contagion was there.
The types of responses in amphibians could be very different than those of mammals. “I think we’d need to be open-minded about how to research this question in amphibians,” says Ben Ami Bartal.
Animal welfare researcher Dr Helen Lambert agrees. “Empathy is a subjective experience. It may manifest in physical ways but is still personal and unique to the individual,” she says.
These new findings “may certainly be evidence of something more complex,” but we just need to work out how best to study this in amphibians. “Further research like this is desperately needed,” she says.
However, this type of approach might not be the right one, though. That's according to Jessie Adriaense, a comparative psychologist at the University of Zurich, who in 2020 wrote a paper on the challenges of measuring empathy across the animal kingdom.
Why? She doesn’t think the frog study is measuring what it claims to be. Pinching and poking the female frog in the first place didn’t induce a grand amount of stress in her, according to the findings. So the male frog is matching a stable emotional state, but for there to be evidence of empathy, there always needs to be a change in emotions in the first place, she says.
And the correlation between the female’s and male’s corticosterone levels isn’t very strong at all either.
“I do not think this can confidently say anything about emotion contagion in poison frogs,” says Adriaense.
But it’s still absolutely crucial to continue looking for answers to these questions if we want to truly know if empathy is unique to humans.
About our experts
Jessica Nowicki is a research scientist based at Stanford University's Laboratory of Organismal Biology where she studies pro-social behaviour in early vertebrates.
Her work has been published in the Journal of Experimental Marine Biology and Ecology, Philosophical Transactions of the Royal Society and Comparative Biochemistry and Physiology.
Dr Inbal Ben Ami Bartel is a researcher based at Tel-Aviv University’s Psychology Department and School of Neuroscience where she studies social neuroscience, pro-social behaviour and empathy.
Her work has been published in the journals Elife, Frontiers in Psychology and Proceedings of the National Academy of Sciences.
Dr Helen Lambert is an animal welfare scientist and head of Animal Welfare Consultancy.
Her work has appeared in the journals Applied Animal Behaviour Science, Animal Welfare and Animals.
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