Do non-human animals feel emotion?

The question of whether or not non-human animals feel emotion, and if so, which — and which emotions — been debated for centuries. Charles Darwin wrote in his 1871 book The Descent of Man that "the lower animals, like man, manifestly feel pleasure and pain, happiness and misery." By the middle of the 20th century, though, behaviourists were arguing that it was not possible to study an animal's inner state, and so no one should try. More recently, however, psychologists have started to venture into other animals' minds, revealing insights into the emotional experiences of all kinds of animals, from chimpanzees to crabs.  

What researchers believe

Given this shift in approach, Matthew Zipple at Cornell University and colleagues wondered what current animal behaviour specialists think about the emotional capacities of animals. For their paper, published last year in Royal Society Open Science, they surveyed 100 researchers in a range of relevant disciplines, including evolutionary biology and neuroscience, as well as cognitive and biological psychology.

The results were striking. The 100 respondents ascribed emotions to "most" or "all or nearly all" non-human primates (98% agreed with this); other mammals (89%), birds (78%); octopuses, squid and cuttlefish (73%) and fish (52%). Most even ascribed emotions to some insects (67%) and other invertebrates (71%).

Also, while just under half felt that anthropomorphism – projecting human experiences onto animals — is problematic for research in animal behaviour, a far greater proportion (89%) felt that anthropodenial — willfully ignoring the existence of any human characteristics in animals — is a problem. "This distribution of thought likely represents a dramatic shift from the twentieth century, when an aversion to anthropomorphism was dominant among animal behaviour researchers," the team writes.

One challenge for work in this field is the lack of consensus on the definition of 'emotion'. However, as Zipple and his colleagues note, many researchers do agree on a few defining factors: emotions happen in response to a stimulus; they guide decision-making and behaviour; and they involve a subjective experience.

That said, given that even bacteria will move away from a harmful stimulus, the question of what the animal actually feels is one of the thorniest. Many studies looking into this question depend on observations (and human interpretations) of a species' general behaviour, which can provide unreliable conclusions. Others make use of things like facial expressions, indicators of physiological arousal (such as stress hormone levels), and vocalisations to gather clues as to what the internal experiences of non-human animals might be. While these approaches all have their pros and cons, many have given us valuable insights.

So what do non-human animals feel?

Perhaps unsurprisingly, most of the research concerning emotion in non-human animals has been on our closest relatives, such as chimps and bonobos, as well as species that we spend most time with — such as dogs, horses and farm animals.

The work on chimps has identified facial expressions that consistently appear in fearful, for example, or playful situations. Some studies also suggest that chimps are capable of empathy. A study led by psychologist Christine E. Webb at Emory University and colleagues even concluded that a group of chimpanzees housed at the Yerkes National Primate Research Center showed relatively stable individual levels of empathic concern. This finding was based on the team's analyses of interactions after conflicts, when they looked to see which chimps tended to try to console those who were left distressed after a fight.

But evidence of empathy isn't limited to chimps, the researchers note. Research on prairie voles, for example, has found that when member of a monogamous pair becomes stressed, its partner develops the same raised stress hormone levels — just as would be expected in a person feeling empathy for an upset partner. They then groom the stressed animal, whose own stress hormone levels start to fall as a result.

As any dog owner will attest, dogs also experience distress, as well as other emotions. A 2024 paper in Scientific Reports, in which 60 shelter dogs were put through six different ten-minute scenarios, added more emotions to this repertoire. Measuring a series of physiological and behavioural measures throughout each of the trials, which were designed to elicit positive or negative reactions, the team were able to identify several markers that appear to reliably indicate fear, anxiety, calmness, and happiness in dogs.

Work published in Psychological Science has also found that dogs display signs of jealousy — an emotion that some researchers have previously suggested is unique to humans — when their owner appeared to be striking a realistic fake dog. Do dogs feel jealousy in the same way that we do? The research can't answer that. But, the researchers note, dogs do seem to be one of the few species that responds pretty similarly to a child whose parent is showering attention on a sibling.

Cries of emotion

Of course, dogs — again, like kids — whine when they're put in an undesirable situation. Such vocalisations have not gone unnoticed, or unprobed, by the scientific community. In fact, research into interpreting the emotional content of animals' whines, grunts, and cheeps is being increasingly used as a non-invasive alternative to taking physiological measures (such as blood samples to analyse for levels of stress hormones) for getting at what they might be feeling.

Some researchers are keen to better identify and quantify animals' suffering — or, indeed, positive emotions — are using AI to analyse their vocalisations. In February, for example, a paper in iScience reported using machine learning to analyse animal calls and distinguish between positive and negative emotions in seven species, including cows, pigs, and wild boars. This particular study found that changes in duration and sound qualities could, in fact, predict whether a cry was associated with a positive or negative state with an accuracy of around 90% — a finding they hope may find some application in automated wellbeing monitoring tools.

Last year, a study published in Applied Animal Behaviour Science also categorised different levels of distress that can be heard in the peeps of domestic chicks. Psychologist Kenneth Sufka at the University of Mississippi, who was involved in this chick research, also notes that some research on new anti-anxiety and anti-depressant drugs involves generating 'anxiety-like' and 'depression-like' behaviour in chicks, as well as mice and rats, and then testing the effects of the drugs on them. The fact that these animals are used a models for these human emotion-related disorders, and that sometimes the drugs do seem to work, makes a very strong argument that these animals indeed experience these negative emotions, he argues, adding that this has obvious implications for animal welfare.

Unfamiliar creatures, uncertain experiences

For animals that are less closely related to us than other mammals or birds, and that have very different brains, digging into their emotional lives poses more problems. Though it is widely accepted that vertebrates can feel pain, whether some invertebrates might do, too, has been hotly contested. Recent evidence, however, has spurred legislators into assuming that they may. As demonstrated by Robyn Crook in 2021, octopuses will avoid locations where they have experienced pain, for example, and research shows that a local anaesthetic stops an octopus from grooming the injection site of an irritant. As a result, octopuses, crabs, and lobsters were recognised in the UK under the Animal Welfare (Sentience) Act in 2022, giving them protections against cruelty and harm.

Do octopuses, or lobsters, feel other emotions that many psychologists are now happy to ascribe to dogs or chimps — such as fear? The jury is still out on that one, as it still is on some other key questions about animals' emotional lives. Still, the vast majority of the 100 researchers surveyed for the Royal Society Open Science paper said they were optimistic that the tools to get at these answers either currently exist, and so should shortly bear more fruit, or will exist in the future. What's needed now, they add, is greater encouragement of research on non-human animal emotions.