Itchy and scratchy: Mutual horse grooming put under the spotlight

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If you’ve got an itch, you’re unlikely to get anyone other than a friend to scratch it. The same goes for the horse world, according to fresh research in Japan.

Teikyo University of Science researchers Masaki Shimada and Nae Suzuki set out to learn more about the contribution of mutual grooming to relationships within a feral horse herd.

Their work, reported in the journal Animals, focused on a group of nine horses who live in a protected area of about 550 hectares at Cape Toi, in Kushima City.

The pair noted that, among social primates, social grooming strengthens friendly relationships.

Their work with horses examined three hypotheses: That mutual grooming helped strengthen already friendly relationships; that it helped to restore troubled relationships; and that it had a role in parasite removal.

The horses were monitored for a total of 33.5 hours over 15 days, and mathematical models were applied to learn about the roles of kinship, aggression, proximity, social rank, and social networking in mutual grooming.

Self-grooming was observed in all members of the herd. The average total time each individual spent grooming themselves during the study period was 2174.8 seconds (36 minutes).

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Six of the nine horses were seen to engage in mutual grooming during the observation periods, comprising two stallions, three adult mares and one colt.

There were 84 bouts, which only ever involved two individuals. No instances were observed of a third individual joining a grooming pair.

The average duration of a single mutual grooming bout was about 85 seconds.

Mutual grooming between horses was almost completely and perfectly symmetrical, the researchers found. One horse received grooming from the other as soon as the former started grooming the latter.

As soon as one horse stopped grooming, the other stopped as well. These behaviors applied both to related and unrelated pairs of horses.

In all cases, the horses faced each other and mutually groomed nearly the same body parts simultaneously.

Shimada and Suzuki said their observations and analysis supported the hypothesis that mutual grooming helped strengthen friendly relationships, and parasite removal. However, the findings did not corroborate the relationship-restoring hypothesis. For example, the top-ranked horse frequently directed aggression toward the second-ranked herd member. The two were never seen grooming each other.

The pair also found that individuals who spent less time on self-grooming invested longer times receiving grooming from other individuals.

“In a feral horse population, mutual grooming maintains hygiene by controlling ectoparasites and forges affiliative interactions between herd members,” they concluded.

Shimada, M.; Suzuki, N. The Contribution of Mutual Grooming to Affiliative Relationships in a Feral Misaki Horse Herd. Animals 2020, 10, 1564.

The study, published under a Creative Commons License, can be read here.

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