Changing Seasons, Changing Harems: Studying Wild Horse Social Structure
by Karola Szemán
What do lions and horses have in common? After dark, they can both become nightmares. But another similarity shared by these species, along with hippos, gorillas, and fruit bats, is that they are polygynous and form harems. These harems consist of a single dominant male that has exclusive reproductive access to a group of females.
All these species form harems because they offer reproductive and survival benefits for the population. For males, harems grant access to multiple reproductively available females, while for females, they provide protection against potential invaders and foster bonding and socialization among members. This bonding enables cooperative behaviors such as grooming and territorial defense, which bring resilience in harsh times.
A herd of Przewalski horses in Eastern Hungary. Photo by Karola Szemán.
When polygynous species confront threats such as habitat fragmentation, poaching, or anthropogenic droughts, park rangers' immediate conservation practice is to determine the annual harem size to identify any upward or downward trend in the number of individuals. Nonetheless, harem size fluctuates throughout the year, akin to the blooming of flowers in spring or the shedding of leaves in autumn, as it also follows a phenological cycle.
So, if harem formation has a seasonal pattern, what information can we extract from the process and timing of harem formation to improve conservation plans for these polygynous species? Should harem size be measured by more than the sum of its parts? To answer this question, our research group teamed up with equine experts of Hortobágy National Park Directorate (Hungary) to estimate the phenological cycle of harems of an endangered species with a year-round female-defense polygyny.
Biologist Karola Szemán looking for wild horses in the Pentezug Reserve. Photo by Gergely Katona.
Przewalski horses became extinct in the wild in 1968, but fortunately, individuals were kept in captivity in zoos worldwide. Thirty years later, a population of 21 adults was reintroduced to the Pentezug Biosphere Reserve in Eastern Hungary by the Hortobágy National Park Directorate, where it has since thrived within semi-wild circumstances. The reserve boasts grassland steppe terrain with diverse vegetation communities and a continental climate. It is fenced to restrict large mammal movement while allowing smaller wildlife to roam freely.
Small harem of Przewalski horses in the Pentezug Reserve, Hungary. Photo by Karola Szemán.
We were interested in determining the factors that influence the phenology of harem size in these true wild horses. Specifically, we focused on intrinsic population factors such as adult sex ratio, adult mortality, and the mean age of harem-holding stallions, as well as extrinsic population factors such as the severity of annual droughts.
Karola Szemán looking at Przewalski horses in the Pentezug Reserve. Photo by Gergely Katona.
Using a 15-year long demographic dataset (1998-2012), provided by the Hortobágy National Park Directorate where the number and composition of harems were registered each month, we described four phenology-based harem size metrics. First, the "annual mean harem size," which tells us the average number of adult females in a harem throughout the year. We thought that in larger animal populations, more females might join harems, so the average harem size might also increase. Second, the "maximum harem size" which is the largest number of females observed in a harem during any month of the year. We suggested that when there are many male stallions competing for females, it might be harder for one male to manage a large harem, so the maximum harem size could be smaller. Third, the "dispersion of harem sizes", that measures how much harem sizes vary within a year. We thought that during droughts, when there's less water available, competition among males for females might increase, causing more variability in harem sizes. Fourth, the "harem size monthly departure index" that compares the harem size in a particular year to the average harem size over many years. If the harem size in a year is different from the long-term average, it's noted in this index.
Karola Szemán walking among a harem of Przewalski horses in the Pentezug Reserve. Photo by Gergely Katona.
First of all, we observed that despite that the Przewalski horse population significantly grew along with the number of harems, the harem sizes did not increase over time. Harem sizes remained relatively stable each year, ranging from 1 to 11 mares, with different peaks and stable periods observed within the 12-month phenological harem cycle. This revealed to us that population size alone does not explain the size of the harems and that possibly what explains their variation is not how many individuals there are, but rather what sex they are.
Changes in the number of horses (a), number of harems (b) and mean harem size (c) of Przewalski horses between 1998 and 2012. 95% confidence intervals are represented by black lines.
So, if we looked at the other factors we observed that, the annual mean harem size was regulated by the adult sex ratio and by the annual adult mortality. To clarify, we observed that when there were more males in the population, the harem size decreased, and when the adult mortality was high the harem size increased. Further, annual maximum harem size was also strongly predicted by adult sex ratio.
Annual mean harem size of Przewalski horses predicted by the additive effects of adult sex ratio and annual adult mortality between 1998 and 2012.
This confirms that the male-to-female ratio is the key piece to understand the temporal variation of harem size in populations of polygynous species. An irregular harem size can have important negative effects on population growth. Therefore, our results highlight that the adult sex ratio should be a crucial metric for any threatened species recovery program, especially for those aiming to protect and recover polygynous species.
Large harem of Przewalski horses in the Pentezug Reserve, Hungary. Photo by Karola Szemán.
Another self-regulatory demographic process that emerged from our analysis was that the number of harems in a given year mostly determines how much the harem size changes over the course of that year. So, if there are more harems, the harem sizes tend to change more, and if there are fewer harems, the harem sizes tend to stay more consistent. This reiterates that when there are more harems, and therefore more stallions, competition for females becomes tougher, so it is expected that harem sizes vary. Therefore, it is not surprising that the adult sex ratio has also been an explanatory factor for the dispersion of harem size.
Harem of Przewalski horses enjoying the shade and having a bath in the small water bodies of the Pentezug Reserve. Photo by Karola Szemán.
The second major important observation for the conservation of polygynous species emerged when we analyzed the extrinsic climatic factor that could affect harem size phenology. Drought severity had a positive effect on maximum harem size, demonstrating the impact of climate and resources on polygyny capabilities of feral horses. The effect of droughts on social organization was mild, and this is because the Pentezug provides them with ideal environmental conditions for their population growth, with a large foraging area and ample water resources throughout the year. However, it is notable that despite living in ideal conditions, the effect of droughts on harem size has been significant. This suggests that in areas where environmental resources are scarce, a severe drought in one year can profoundly harm the social organization of species with female-defense harems.
Graphical representations of an ordinary least squares regression model predicting the effects of adult sex ratio + drought severity index on annual maximum harem size of Przewalski horses between 1998 and 2012.
“These are wonderful results and further strengthen the strong links between conservationists at the National Park and ecologists at Debrecen University”
In conclusion, studying the phenology of harem size in a Przewalski horse population, we discovered two crucial factors to consider for the proper management of polygynous species: the adult sex ratio and the severity of droughts. We hope that our results will prompt park rangers, conservationists, and researchers, when a polygynous species faces habitat fragmentation, poaching, or illegal hunting, not only inquire about the harem size but also about the adult sex ratio of the population.
Przewalski horses grazing in the Pentezug Reserve, Hungary. Photo by Karola Szemán.
The social structure of iconic harem-forming species, such as hippos, gorillas, and wild horses, varies throughout the year in a seasonal manner. Just like the blooming of flowers, the organization of these mammals fundamentally depends on water. The population of Przewalski horses in Pentezug Hortobágy Biosphere Reserve is fortunate to live in an area that withstands years of severe drought, but not all species are as fortunate. In the coming years, it is predicted that the severity of droughts will increase globally, putting the resilience of this demographic mechanism in some large and small mammals to the test.
Przewalski’s foal with mother at Pentezug, Hortobágy Biosphere Reserve. Photo by Karola Szemán.
The true nightmare is losing species of such great ecological and cultural importance in such a short period. What wild horses, lions, hippos, and gorillas have in common is that they are endangered. The tremendous efforts of those dedicating their lives to studying, conserving, and maintaining the Przewalski horses in Hungary have led to a significant success story in biodiversity conservation. We hope that their recovery narrative does not become their distinguishing feature.
ABOUT THE AUTHOR
Karola Szemán
I’m finishing my PhD at the Department of Evolutionary Zoology and Human Biology, University of Debrecen, Hungary. My broad research interest is behaviour, social organization, social evolution and conservation, with a special interest in large-bodied herbivores. I studied the agonistic behaviour of male Przewalski’s horses in a population in Hortobágy National Park, Hungary. This project was part of my master's thesis. After my master's, I studied the same population as part of my PhD. I aimed to investigate the social organization and reproductive patterns of Przewalski’s horses. I processed archive data and also collected some new data on the field. The information we gain from this long-term, individual-based data could help plan conservation programs for the species or other related species.
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Funding for this project was provided by Hortobágy National Park Directorate, University of Debrecen, ÉLVONAL KKP-126949 and HUN-REN-UD Evolution of Reproductive Strategies Research Group.
