When it comes to understanding herd dynamics that help build happy horse groups, it’s hard to beat 15 years of cumulative, ongoing research.

And that’s exactly what Icelandic scientists have done. They’ve spent a decade and a half studying hundreds of horses living in different kinds of groups in spacious pastures around farms. Observing the details of all their social interactions—both friendly and aggressive—they’ve been able to come up with some never-before-seen conclusions that give useful insight into herd management.

“It’s important to know what kind of groups to have in our pastures if we want to increase their welfare,” said Hrefna Sigurjonsdottir, professor and behavior ecologist at the School of Education in the University of Iceland, in Reykjavik. “When data from studies on many different groups were combined the opportunity arose to answer the question: ‘How can we put together groups that have low agonistic behaviors?’

Specifically, Sigurjonsdottir and her fellow researchers looked at incidences of aggressive behavior—namely, fighting, biting, threats to bite, chasing, attacking, and especially laying the ears back (which was by far the most common aggressive behavior) and friendly behavior in the form of mutual grooming. They also watched out for submissive behavior, in the form of horses giving way to other horses who act dominant towards them. Sigurjonsdottir presented her team’s work at the 14th Conference of the International Society for Equitation Science (ISES) in Rome, Italy.

Counting 426 Icelandic horses, 20 social groups in large pasture settings in Iceland, and more than 2600 hours of scientific observation, the scientists have reached key findings that could be applied to domestic horse groups worldwide.

Here’s a look at the top five trends from their study that were related to less aggression and more friendliness in the groups:

  • Having a stallion in the herd

Yes, you read that right. A single stallion with a group of mares (and in some cases sexually immature young horses) made for the most peaceful herd settings of all. Better even than keeping a stable group, having a stallion in the herd seemed to make inter-horse interactions the friendliest, according to Sigurjonsdottir. While the stallions showed herding behaviors with their mares, they didn’t interfere in the mare-to-mare relationships, and they didn’t act dominant towards the mares. And even though the stallion was generally the most “aggressive” member of these groups, he was still usually less aggressive than other horses in other groups without a stallion. “The finding agrees with other studies where it has been shown that stallions are neither especially dominant over females nor more aggressive than other horses,” she said. (Even so, keeping a stallion requires specific training and responsibility, and isn’t necessarily a solution for the average horse owner, she added.)

  • Keeping a stable herd

While this isn’t a new finding, it was certainly confirmed by this study. “When there’s stability, the horses know their relationships with each other and don’t need aggressive interactions and shows of submission to establish a peaceful social network, but when a newcomer arrives, those conflicts have to occur until the newcomer knows his place in the group,” she said. Far from being a case of simple linear hierarchy, equine social groups have complex structures, with various levels of “rank” that can apply in certain situations and not others, and are relative from individual to individual. (In other words, for example, a horse might have dominance over another horse which has dominance over a third, which might have dominance over the first, depending on the context.) In order to sort out the multitude of different levels of relationships with individual members of a group, a new horse can cause disruption to a stable group. The loss of a member can also cause instability, leading to more frequent aggressive interactions among the remaining members. Even so, interestingly enough, when a stallion joins a group of mares who are unfamiliar with each other, that doesn’t seem to incite more aggression, Sigurjonsdottir added (see point 1).

  • Having foals in the herd

Although mares tended to show slight aggression towards other horses to protect their foals, the presence of foals seems to somehow make groups generally less aggressive, Sigurjonsdottir said. (Again, keeping foals requires specific training and responsibility on the part of the herd manager.)

  • Avoiding groups entirely made up of young horses

There’s a relatively high amount of aggression that occurs when groups of young horses, like yearlings, are formed. “Presumably, the horses (in these groups) experienced very high level of stress, a situation eliciting frequent acts of aggression and submission,” she said.

  • Taking special precautions in winter

Winter seems to bring out the worst in horses—probably because of the limited access to food, according to Sigurjonsdottir. Higher ranking individuals show more aggression towards lower-ranking individuals, who show more submissive behavior by backing away from food sources. To combat this issue, owners can make efforts to provide hay in various places in the pasture and ensure that less dominant horses have peaceful access to food. “Placing one large round bale in the middle of the pasture is a good way to incite aggression,” she said.

Additionally, the researchers noticed that younger horses performed more mutual grooming than adult horses—possibly because they were still learning to build social relationships, according to Sigurjonsdottir. Also, the presence of mutual grooming depended on the season and parasite loads. It depended on group size as well, with mutual grooming more common in smaller groups than larger ones. “We see through this work the need horses have to establish close bonds with a few horses in particular,” she said.

“Our results can be of use when planning further studies on equine social networks and, practically speaking, when putting together domestic horse groups with their social welfare in mind,” Sigurjonsdottir said.

This research was presented at the 14th International Equitation Science Conference in Rome, Italy. To download the full proceedings click here.

Recommended reading: Horse Welfare: A paradigm shift with link to download The Horse Welfare Assessment Guide poster.