Studying the movements of Przewalski’s horses makes it possible to anticipate which harem a female will go to or from 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 a collective defense exercise seen in very few species other than hominids.
There are barely 3,000 copies of the Przewalski. Most live in Mongolia, their homeland. But there are also scattered herds in eastern Europe, like in Chernobyl. One of the largest herds is found in the Hortobágy National Park in Hungary. Some twenty adult horses arrived there at the end of the last century with the idea of increasing the chances of survival of these horses and it worked. Barely 25 years later, there are already 278 specimens in the park grouped in 28 harems. From what was known of these animals, in the Mongolian steppes they form a harem of a dozen or fewer females and their young led by a stallion.
Male foals are driven out as they approach breeding age and join other youngsters in the hope that they can form their own harem or wrest one from one of the stallions. Females often leave the family group. But in Hortobágy something happens not seen in Mongolia: the harems move together, forming a large herd. Now, the detailed study of that herd reveals that they do not move in a chaotic or random way, but rather following a complex structure of a social network.
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 to observe the movement of the herd from the air, with drones. The result of this work, published in the scientific journal Nature Communicationsconfirms several of the things that were already known, but it has also thrown several novelties.
“Their social structure, that is, that the horses live in stable harems and bachelor groups, was known from long-term monitoring by park staff,” says Katalin Ozogány, a researcher in animal behavior at the University of Debrecen, Hungary. ) and first author of the paper. “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 staying at a greater distance from others. And this is more or less maintained during the movements as well. We also expected that past affiliations in the same harem might have an effect, as friendships can develop over time.” But what they didn’t expect, she adds, is that “future social relationships could be related to the movement.” Indeed, the drone recordings made it possible to determine where a female could end up or what harem she came from.
The study confirms the central position of stallions, especially older ones with a larger harem. It further shows that his brothers’ harems tend to be around his. In a second position are those of their half brothers, then those of the unrelated males with which they occupy the center and finally the groups of non-breeding males. “We hypothesize that the 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 web represent these alliances,” Ozogány says. “Based on this, the advantage for stallions is that they have to expend less energy in battles with bachelors, so they can probably keep their harems longer. In addition, it is also beneficial for females to be surrounded by allied harems, as this can reduce the chance of bachelor harassment and foal murder,” she adds.
This hierarchical social organization based on the dominant stallions runs parallel to another, more liquid structure, of the females. It is summarized by the researcher at the Hungarian Academy of Sciences and senior author of the paper, 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 switch 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 harems 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 have no answer for is the difference between Przewalski’s horses from Mongolia, which do not form large herds, and Hortobágy’s. Ozogány points out some possibilities: “Presumably, various environmental and social factors can influence the formation of large herds from harems. For example, the abundance of food, the availability of water, the presence of predators, and bachelor groups can influence herd formation. To better understand their formation under more natural conditions, our study should be repeated in Mongolia.”
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