Elephants, great apes, dogs, and rats consistently rank among the animals with the most well-documented emotional lives, based on decades of behavioral experiments, brain imaging, and field observation. But the science of animal emotion has expanded far beyond mammals: octopuses show signs of affective pain experience, crows hold what look like vigils over their dead, and bumblebees display something resembling optimism. The real surprise is not that a handful of charismatic species feel things, but how deep the evidence runs across the animal kingdom and how many of our assumptions about which creatures “count” as emotional have turned out to be wrong.
How Scientists Actually Detect Emotion in Animals
Animals cannot fill out mood questionnaires, so researchers rely on a combination of behavior, physiology, and brain activity. One of the most widely used methods is the judgment bias test, which works on the same principle as the “glass half full or half empty” idea. An animal is trained to associate one cue with a reward and another cue with no reward. Then it is presented with an ambiguous cue, somewhere between the two. If the animal treats the ambiguous cue as though it predicts something good, that is interpreted as an optimistic emotional state; if the animal treats it as predicting something bad, it suggests a more negative state. This approach has been used across a wide range of species and contexts, and an animal’s bias can often be linked directly to its welfare conditions.
Physiological tools add another layer. Infrared thermography, for example, measures changes in skin temperature that track with emotional arousal. The nasal region appears to be especially sensitive in primates. Studies on common marmosets have validated the technique as a way to quantify emotional reactions without any physical contact or restraint, making it useful for studying both captive and wild populations.1PubMed. The use of infrared thermography to investigate emotions in common marmosets The same method has been applied to wild crested macaques in their natural habitat, confirming that it works beyond the lab.2PubMed. The use of infra-red thermography to study emotions of wild crested macaques (Macaca nigra) in their natural habitat
Brain imaging has also entered the picture. In one study, twelve dogs were trained to remain still, awake, and unrestrained inside an MRI scanner while they were presented with different scents. The dogs’ caudate nucleus, a brain region associated with positive expectations, responded most strongly to the scent of a familiar human, even when that person was not present. The response was not just about recognizing a scent; it suggested a positive emotional association with it.3PubMed. Scent of the familiar: an fMRI study of canine brain responses to familiar and unfamiliar human and dog odors These converging methods, behavioral tests, body-temperature tracking, and neuroimaging, mean that claims about animal emotion are no longer just interpretations of cute behavior. They rest on measurable, reproducible data.
Elephants and the Rituals of Grief
Elephants are probably the most frequently cited example of animal grief, and the evidence supports the reputation. A review of the scientific literature found that elephants are one of only a few cognitively advanced groups, alongside primates and cetaceans, that stand out for the range and duration of their behaviors around dead members of their species. Elephants approach, touch, and investigate carcasses at every stage of decay, from freshly dead bodies to sun-bleached bones scattered across the landscape. Researchers have also observed elephants visiting and revisiting carcasses over time, showing extensive investigative behavior, prolonged stillness near the body, self-directed behavior, and streaming from the temporal glands, a physiological response associated with heightened emotional states.4PubMed. Elephant behavior toward the dead: A review and insights from field observations
Elephants’ interest in death is not limited to their own relatives. They investigate carcasses of unrelated elephants as well. But there is evidence that the loss of a herd member can affect individuals on a deeper level. Research has examined whether elephants change their personality and behavior following the death of a companion, moving beyond the question of whether they simply “notice” death toward whether it alters them in lasting ways.5PubMed. Personality and behavioral changes in Asian elephants (Elephas maximus) following the death of herd members
Perhaps the most striking recent finding involves Asian elephants in North Bengal who have been documented burying deceased calves. In fragmented forest corridors running through tea estates, herds were observed placing dead calves in shallow irrigation drains in a legs-upright position, then dragging the bodies collectively before burial. Five calves that died from different causes, including fungal infections and respiratory failure, were all buried in a similar manner. Contusion marks on the bodies indicated the herd had transported them some distance. Researchers linked this behavior to the especially strong mother-calf bond seen in elephants and interpreted it as a form of collective mourning.6Animal Behaviour and Welfare Cases. Digging Deeper: Understanding the Calf Burials Behaviour among Asian Elephants in North Bengal
Great Apes and the Roots of Empathy
If elephants are the iconic grievers of the animal world, great apes are its empaths. Consolation, the act of approaching someone in distress and offering comforting contact, is well documented in both bonobos and chimpanzees. After social conflicts, bystander bonobos approach the victim of aggression and provide stress-alleviating behaviors including embracing, touching, and mounting. This consolation behavior occurs across all age and sex classes, from juveniles to adults.7PubMed Central. Bonobos respond to distress in others: consolation across the age spectrum Consolation requires sensitivity to another individual’s emotional state, the consoler has to recognize distress in someone else and then act to reduce it, which puts it a step beyond simple social grooming or proximity-seeking.
Bonobos and chimpanzees show similar overall consolation tendencies, though within-species variation, differences between individual animals, actually eclipses the differences between the two species. In other words, personality and social context matter more than which species of great ape you are studying.8Evolution and Human Behavior. Within-species variation eclipses between-species differences in Pan consolation This is a useful reminder that when we talk about “the most emotional animals,” the category is fuzzy. Even within a single species, some individuals are far more emotionally expressive or responsive than others.
Capuchin monkeys reveal another dimension of primate emotional life: a sense of fairness. When adult monkeys were asked to exchange tokens for food rewards, they reacted negatively to receiving a less-preferred reward when they could see a partner getting a better one. Researchers ruled out simpler explanations like mere frustration at not getting a treat or the visibility of better food; the negative reaction was specifically triggered by seeing their partner receive a superior reward for the same task.9PubMed Central. Inequity responses of monkeys modified by effort That kind of response, an emotional protest against perceived unfairness, looks a lot like what drives cooperation and punishment in human societies.
Dogs and the Chemistry of Cross-Species Bonding
Dogs occupy a unique niche in emotional research because their emotional lives are so intertwined with ours. The hormonal basis for this connection has been mapped in detail. When dogs gaze at their owners, the owners’ urinary oxytocin levels rise, which in turn increases the owner’s affiliative behavior toward the dog, which then raises the dog’s own oxytocin levels. This oxytocin-gaze loop does not occur between wolves and their human handlers, suggesting it is a product of domestication rather than a general canine trait.10PubMed. Social evolution. Oxytocin-gaze positive loop and the coevolution of human-dog bonds Dogs have essentially co-opted the same bonding chemistry that evolved to attach human parents to their infants.
Beyond attachment, dogs also display jealousy, one of the more socially complex emotions. Dogs showed significantly more jealous behaviors, including snapping, pushing, and physically inserting themselves between their owner and the object of attention, when owners displayed affection toward what appeared to be another dog compared to nonsocial objects.11PubMed Central. Jealousy in dogs Follow-up work found that dogs mentally represent jealousy-inducing situations even when the rival interaction happens out of sight. The jealous response emerged only for a perceived social rival, not simply from the presence of another dog, and it occurred even when the dog could not directly see its owner interacting with the rival.12PubMed. Dogs Mentally Represent Jealousy-Inducing Social Interactions That level of inference, constructing a mental image of an unseen social threat, suggests dogs are not just reacting to stimuli but imagining scenarios that provoke emotional responses.
Rats, Laughter, and Helping Strangers
Rats tend to surprise people. They are among the best-studied emotional animals in neuroscience, and the findings push against the common image of them as simple or unfeeling. When tickled by a human hand, rats emit ultrasonic vocalizations at around 50 kHz, calls that researchers have labeled “rat laughter.” These calls are associated with positive emotional states and are thought to be functionally similar to human laughter during play.13PubMed Central. Who’s laughing? Play, tickling and ultrasonic vocalizations in rats Rats that produce more of these vocalizations during tickling also show more optimistic responses on judgment bias tests, treating ambiguous cues as though they predict something good.14PLoS ONE. Laughing Rats Are Optimistic The chirping rate varies with social housing conditions and genetics, reinforcing the idea that it reflects a genuine internal state rather than a mechanical reflex.15PubMed. 50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables
Rats also show empathy-driven helping. In a well-known experiment, a free rat was placed in an arena with a cagemate trapped inside a small restrainer. Over repeated sessions, the free rat learned to intentionally open the restrainer and liberate its companion. The rats did this consistently and quickly, even when a separate container of chocolate chips was available as an alternative. They freed their cagemate first, then shared the chocolate. The researchers described this as strong evidence for biologically rooted, empathically motivated helping behavior.16PubMed Central. Empathy and pro-social behavior in rats
Pigs, Crows, and Other Unexpected Candidates
Pigs are increasingly recognized as emotionally sophisticated. Research on emotional contagion in pigs found that a companion pig, even one that had not directly witnessed a frightening event, could “catch” the fearful emotional state of a pig that had. The companion displayed similar fearful behaviors and also provided social support to the distressed animal afterward.17PubMed Central. Emotional Contagion and Social Support in Pigs with the Negative Stimulus Emotional contagion, the ability to absorb another individual’s emotional state, is considered a building block of empathy, and the fact that pigs display it clearly has implications for how we think about welfare in farming.
Crows and their relatives show their own kind of emotional complexity. When western scrub jays encountered a dead jay near a feeding station, they produced loud calls that attracted neighboring jays, creating what researchers described as “cacophonous aggregations.” This response was specific: a dead jay lying in a natural position always triggered the gatherings, while a stuffed, upright jay produced milder reactions. American crows responded much more strongly to a dead crow than to a dead pigeon, and their foraging near the corpse was disrupted, suggesting genuine alarm or disturbance rather than simple curiosity.18iScience. Comparative thanatology – Section: Do birds hold ‘funerals’? Whether corvids actually “grieve” is debatable, but their behavior around death is markedly different from most birds. Pigeons, for example, showed little interest in dead conspecifics.
Octopuses, Bumblebees, and the Frontiers of Feeling
The most boundary-pushing research on animal emotion has moved beyond vertebrates entirely. Octopuses, with nervous systems organized completely differently from ours, show behavioral and physiological signs of affective pain experience. In conditioned place preference experiments, octopuses that received a mild acid injection into one arm subsequently avoided the chamber where the injection happened and preferred a location where they experienced pain relief. All octopuses injected with acid groomed the injection site with their beak for the full twenty-minute observation period, but this wound-directed behavior was absent or brief in control animals. Local anesthesia abolished the grooming behavior, indicating it was not simply a reflexive response to tissue damage but something more like a felt experience of pain.19PubMed Central. Behavioral and neurophysiological evidence suggests affective pain experience in octopus
Even insects have entered the conversation. Bumblebees trained to associate one cue with a sugar reward and another with no reward showed something that researchers described as dopamine-dependent positive emotion-like states. After receiving an unexpected sugar treat, the bees responded more optimistically to ambiguous cues, treating them as though they predicted something good. The effect was mediated by dopamine, the same neurotransmitter involved in reward processing across the animal kingdom.20PubMed. Unexpected rewards induce dopamine-dependent positive emotion-like state changes in bumblebees Nobody is claiming bumblebees feel joy the way you do, but their behavior fits the operational definition of a positive emotional state in ways that cannot be easily dismissed.
When Animal Emotions Break Down
One of the more sobering lines of evidence for animal emotion comes from studying what happens when emotions go wrong. Wild animals exposed to predator cues can develop lasting changes in brain and behavior that parallel post-traumatic stress disorder in humans. In one study, wild animals exposed to predator sounds showed heightened fear responses that endured for at least seven days. Their sensitivity was generalized: they were more reactive to entirely new predator cues, not just the specific ones they had experienced, a hallmark of the hypervigilance seen in human PTSD. The researchers confirmed enduring changes in the amygdala and hippocampus, key regions in the brain’s fear circuitry.21PubMed Central. Predator-induced fear causes PTSD-like changes in the brains and behaviour of wild animals
Wild rodents show a similar pattern. When exposed to owl territorial calls, even rodents that had never encountered owls in nature developed PTSD-like behavioral responses. In some individuals, the effects extended to enduring physiological and structural brain changes, including blunted stress hormone responses, lower levels of the neuropeptide orexin-A in the hypothalamus, and reduced dendritic growth in the hippocampus, all of which parallel findings in traumatized laboratory animals and humans.22PubMed Central. Sounds of danger and post-traumatic stress responses in wild rodents: ecological validity of a translational model of post-traumatic stress disorder The fact that PTSD-like states arise in wild settings, not just in laboratory stress paradigms, strengthens the case that these are genuine emotional disorders rather than artifacts of captivity.
Why Emotional Contagion Evolved in the First Place
The capacity for emotion is not distributed randomly across the animal kingdom. It tends to be strongest in species that live in social groups, and theoretical models help explain why. Emotional contagion, the ability to rapidly copy another individual’s emotional state, carries both costs and benefits. Picking up a groupmate’s fear response could help you escape a predator you had not yet noticed. But it could also trigger mass panic when no real threat exists. Modeling work shows that as group size increases, individuals with the ability for emotional contagion tend to evolve weaker sensitivity to direct environmental cues, essentially outsourcing some of their threat detection to the group’s collective emotional state.23PubMed Central. Evolution of emotional contagion in group-living animals This means the most emotionally contagious species are not simply the “smartest” ones; they are the ones whose social structures have made shared emotional states a net survival advantage over millions of years.
How Sentience Science Is Changing Law and Policy
The accumulating evidence for animal emotion has not stayed inside laboratories. Animal sentience is now recognized in legislation across the European Union, the United Kingdom, New Zealand, and parts of Australia, with discussions underway in other countries.24PubMed Central. The sentience shift in animal research These laws do not simply ban cruelty in the traditional sense. They treat the capacity to experience positive and negative emotional states as a fact that must be accounted for in welfare regulations, farming standards, and research ethics. The shift matters practically because it changes what counts as acceptable treatment. When a pig is recognized as an animal that can catch fear from a companion, the design of slaughterhouses becomes a welfare question in a way it was not before. When an octopus is shown to experience something like pain, the question of whether invertebrates deserve legal protection stops being philosophical and starts being legislative. The science of animal emotion, in other words, has moved well beyond the question of which animals “feel the most” and into the much harder question of what we are obligated to do about it.