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Research from France suggests that horses can develop something akin to depression in response to social or physical discomfort.

For a study conducted at University of Rennes, researchers selected 12 horses who were considered “withdrawn” and 15 who behaved normally. “Withdrawn horses are rather easy to identify as they have long bouts of immobility with ears, neck and head fixed, wide open eyes with unusually long intervals in eye blinking, leading to an impression of total fixity,” explains Céline Rochais, PhD.

To gauge how the horses responded to stimuli, the researchers subjected them to five unusual sounds: baboon, goose and whale calls; the call of a horse not known to the study horses; and piano music. Each day each horse heard one of the sounds—played for three seconds from a speaker next to his stall—and his reaction was videotaped. The sounds were rotated daily until all the horses had heard all of the sounds.

The data showed that, on the first day of the study, the withdrawn horses were significantly less likely to pay attention to the noises. Only 50 percent of those horses reacted to the noise—responding by pricking the ears, lifting the head or some other sign of attention—as opposed to 90 percent of the normal horses. Over the five-day course of the study, the control horses showed habituation to the unusual noises, but the reactions of the withdrawn horses did not change significantly.

These findings, the researchers say, indicate that the withdrawn horses had undergone a cognitive shift: They were so physically or psychologically stressed that they had tuned out their surroundings.

“In humans and animals, being attentive is one aspect of subject cognitive abilities/capacities,” says Rochais. “The delay in responding showed that withdrawn horses had ‘switched off’ from their environment and showed sensory inattention. Such lapses of attention are likely to be associated with the chronic effect of stressors, which might be expected to induce a lowered state of arousal.” She adds, “In humans, for example, pain or strong discomfort ‘captures’ the attention of the subject. Here we do not know precisely how the [horses] feel but they might have chronic disorders, which are one hypothesis for an ‘inward-oriented attention.’”

She emphasizes that the absence of reaction did not indicate calm. “Some horses showed reaction times longer than others, potentially suggesting they were calmer than others,” she says. “But the withdrawn horses did not react at all on the first day.”

This study adds to a growing body of research that suggests horses can enter a depression-like state as a reaction to their environment or chronic pain, says Rochais: “A series of studies performed by our group has shown that these withdrawn horses are in a ‘pathological’ and depressed state, as they have an abnormally low level of cortisol [hence a physiological ‘depression’] and express anhedonia [less pleasure to feed on appetent substances]. Hence, taken together this suggests strongly that the lack of reaction is more related to an abnormal state.”

Rochais suggests taking a closer look at horses who may be withdrawn. “Such horses should be examined for any health problems and the life conditions should be questioned, especially if other horses in the stables also show this syndrome, are aggressive or stereotypic,” she says. “Food, space and social conditions should be examined, as well as riding techniques that can lead to chronic back disorders.

Reference: “Investigating attentional processes in depressive-like domestic horses (Equus caballus),” Behavioural Processes, March 2016

This article first appeared in EQUUS issue #464, May 2016. 

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