Herd movements show the complex society of the last wild horses

Studying the movements of Przewalski’s horses makes it possible to anticipate which harem a female will go to or which one she comes from. Far from moving chaotically, a herd of the last subspecies of wild equids does so following a rigid and hierarchical social structure. The oldest stallions with the largest harems group together in an exercise of collective defense seen in very few species, apart from hominids.

There are only about 3,000 Przewalski specimens. Most live in Mongolia, their native land. But there are also scattered herds in eastern Europe, such as in Chernobyl. One of the largest herds is in the Hortobágy National Park in Hungary. Twenty adult horses arrived there at the end of the last century with the idea of ​​expanding the survival possibilities of these horses and it worked. Barely 25 years later, there are already 278 specimens grouped in 28 harems in the park. From what was known about these animals, in the Mongolian steppes they form a harem of a dozen females or less and their offspring led by a stallion.

Male foals are expelled when they approach reproductive age and join other youngsters waiting to form their own harem or snatch one from one of the stallions. Females usually go outside the family group. But in Hortobágy something unseen in Mongolia happens: the harems move together, forming a large herd. Now, the detailed study of this herd reveals that they do not move in a chaotic or random manner, but rather following a complex social network structure.

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A group of Hungarian researchers has taken advantage of the fact that almost everything is known about the Hortobágy herd, the age of each animal, its gender, its relationship with others, its genetics… to study the society of these horses. To all this data, they added the information that they believed could offer observing the movement of the herd from the air, with drones. The result of this work, published in the scientific journal Nature Communications, confirms several things that were already known, but has also revealed several new things.

“Their social structure, that is, the horses live in stable harems and in bachelor groups, was known through long-term monitoring by park staff,” says Katalin Ozogány, an animal behavior researcher at the University of Debrecen in Hungary. ) and first author of the work. “We were confident that the movement patterns could be related to that social structure, because members of harems generally stay close to each other, while they stay at a greater distance from others. And this is more or less maintained during movements as well. “We also hoped that past affiliations in the same harem might have an effect, as friendships can develop over time.” But what they did not expect, she adds, is that “future social relations could be related to the movement.” Indeed, the drone recordings made it possible to determine where a female could end up or what harem it came from.

Two Przewalski horses, in the Hortobagy National Park, 184 kilometers east of Budapest, Hungary.ZSOLT CZEGLEDI (EFE)

The study confirms the central position of stallions, especially the older ones with a larger harem. It also shows that his brothers’ harems tend to be around his own. In a second position are those of their half-siblings, then those of unrelated males with which they occupy the center and finally the groups of non-reproductive males. “We assume that harem stallions form a kind of alliance with each other to protect their harems more effectively against single males, and the edges in the detected network represent these alliances,” says Ozogány. “Based on this, the advantage for stallions is that they have to expend less energy in battles with bachelors, so they can probably hold onto their harems longer. Additionally, it is also beneficial for females to be surrounded by allied harems, as this can reduce the possibility of harassment of bachelors and murder of foals,” she adds.

This hierarchical social organization based on dominant stallions runs parallel to another, more liquid, structure of females. It is summarized by researcher at the Hungarian Academy of Sciences and senior author of the work, Máté Nagy: “At first glance, one might think that harem stallions simply keep several females with occasional fights with other stallions that can end up with a new male. , previously single, gaining control of the harem. But in reality, the dynamics are much more complex. Harems can split or merge, and females can decide to change harems. What they have discovered is that kinship does not have a network effect, but friendship does: related females do not usually group together in the same harem. Something that helps genetic diversity. But mares that shared a harem in the past tend to move together. They also tend to go to another harem together as well. “The fact that future female exchanges can already be predicted by the joint movements of individuals exceeded our expectations,” says Nagy.

What these researchers do not have an answer to is the difference between the Przewalski horses of Mongolia, which do not form large herds, and those of Hortobágy. Ozogány points out some possibilities: “Presumably, various environmental and social factors can influence the formation of large herds from harems. For example, food abundance, water availability, the presence of predators, and bachelor groups can influence flock formation. To better understand their formation under more natural conditions, our study should be repeated in Mongolia.”

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