A study of Przewalski’s horses living wild in a reserve in Hungary have revealed fresh insights into the social structures adopted by the animals.
Researchers say they have observed remarkable changes in the demography of the horses kept in Pentezug Reserve over the years.
Globally, Przewalski’s horse numbers have recovered from only a handful of survivors, but it is still classed as endangered.
Like other wild horses, the Przewalski’s horses form groups of females, or harems, with a dominant stallion.
University of Debrecen researcher Viola Kerekes and her colleagues, reporting in the Global Ecology and Conservation journal, said they were curious whether certain characteristics of social structure also changed in parallel with increases in the size and density of the population in the 3000-hectare reserve.
“Remarkably, we observed that the growing number of individuals affected the total number of harems, but not their average size,” they reported.
“Horses are highly social individuals, their social bonds inside the harems are very strong and their behavior and activity are highly synchronized within a harem,” they noted.
“However, the number of these possible bonds and the number of females a harem stallion can protect are limited.”
They noted that the multi-male harem structure, observed as common among feral horses in a 1981 study, is extremely rare in Pentezug Reserve. Indeed, the authors observed it only twice between 1997 and 2018.
“Based on our observations and international data on Przewalski’s horse populations concerning harem numbers and sizes, we assume there is an optimal range of harem size, which seems to be independent of area and population size.”
The Pentezug reserve is a success story in the resurgence of the Przewalski’s horse. The population grew rapidly from 22 founder horses, who arrived between 1997 and 2007 to 329 individuals in 2017. By the end of 2018, the number had decreased to 267.
The 2017 population peak also coincided with a peak in the number of harems, at 30.
In the earlier years both harem and bachelor groups tended to have separate home ranges. As numbers grew, the whole population eventually formed a large herd moving together all year round, using almost the entire available area.
Of note, some harems, mainly the new ones, sometimes stayed isolated, keeping a large distance from the united herd.
“The reason for the large herd phenomenon and the manner in which animals interact is unknown, but is likely due to environmental (limited area, abundant food, and water supply) and social factors (long term social relationships and genetic relatedness).
“Interestingly, bachelor groups seemed to stay on the periphery of the large herd. Thus, the large herd formation may be a good defense strategy against bachelors.”
Similar social structures have been seen in feral horses in other countries, and plain zebras in Africa.
Pentezug Reserve, part of Hortobagy National Park, houses about 30% of the total European population of Przewalski’s horses.
The authors noted that the number of foals per year increased for 17 years, with the zenith in 2014, when 60 were born. It then started to decrease, due to both natural factors and the use of immunocontraception.
The peak of 328 horses in 2017 was followed by the decrease to 276 in 2018 because of a decreasing foaling rate, exports, and a population crash.
In 2018, 26.6% of the population died. The most obvious reason was extreme cold in March coupled with heavy snowfall. Fifty percent of juveniles died, as did all horses over 20.
The authors found that the inbreeding coefficient increased slightly after 2012, while gene diversity stabilized at a relatively high value.
“Today, many individuals from this well-monitored population can be found in Russia and Mongolia,” they said.
The study team comprised Kerekes, István Sándor, Dorina Nagy, Katalin Ozogány, Loránd Göczi, Benjamin Ibler, Lajos Széles and Zoltán Barta, from a range of institutions.
Trends in demography, genetics, and social structure of Przewalski’s horses in the Hortobagy National Park, Hungary over the last 22 years
Viola Kerekes, István Sándor, Dorina Nagy, Katalin Ozogány, Loránd Göczi, Benjamin Ibler, Lajos Széles and Zoltán Barta.
Global Ecology and Conservation Volume 25, January 2021, e01407 https://doi.org/10.1016/j.gecco.2020.e01407