The Przewalski horse: walking on thin ice (Part 9)

“Twenty years ago a subspecies of wild horses, the Asian Przewalski horse, became extinct in the wild. There were still specimens in zoos but ten generations of inbreeding had weakened them and instead of infinite grasslands they knew only iron fences. The animals were merely a shadow of their former selves.”[1] It is with these words that Prince Bernhard of The Netherlands described the poor and seemingly hopeless condition of the species in the aftermath of its extirpation in the wild. Following this event, Przewalski horses experienced a dramatic population bottleneck. As the species survived through just a few captive individuals, the loss of founder genes became a terrible threat to their viability. Should they be saved, prompt action was required: breeding a healthy population through inter-zoo exchanges was a matter of urgency. From 1979, various conservation programs were launched in North America, Europe, Australia, and countries of the former USSR with the common objective to preserve the existing level of genetic diversity, specifically to “retain 95% of the current average individual heterozygosity for at least 200 years”[2].

Heinz Heck

Heinz Heck (1894-1982), Director of the Munich Tierpark Hellabrunn (Germany). Credit: Neumeister

The idea was inspiring yet awfully ambitious given there had been no effective population management in the past[3]. The 53 foals that had survived their brutal abduction from the East Asian steppe had been distributed between various zoos and private parks of different countries and no breeding exchange programs were in place. Each captive population was bred separately with only occasional addition of external blood. Breeding was often unsuccessful, especially in the beginnings: some mares did not procreate in captivity while others gave birth to a single offspring that sometimes failed to reproduce. During the first half of the 20th century, captive stock numbers were also affected by war; among other incidents, all Takhis from Askania Nova Reserve (Ukraine) perished in World War II bombings[4]. To make things worse, captive breeding took a prejudiced turn in the 1950’s when the Director of the Munich Tierpark Hellabrunn (Germany), Heinz Heck, began shaping the horses according to arbitrary morphological standards set by himself[5]. For some time, this aesthetic craze isolated the Munich population―which had the richest gene pool of all[6]―and prevented beneficial crossings with specimens from other establishments.

Around this time, concerns regarding the survival of the Przewalski horse justified by alarming reports of plummeting numbers in East Asia began to emerge. The equid’s first international studbook was created by Dr Erna Mohr at the Hamburg Zoological Museum (Germany) and in 1959 the Prague Zoo (Czech Republic), where the largest captive population lived at the time, held the First International Symposium on the Preservation of the Przewalski’s Horse[7]. More symposia followed and works and studies were published. Some horse exchanges took place during the 1960-1970’s, but these events remained unilateral and unsystematic. Efficient gene pool management did not materialize until the 1980’s.

This situation has been gravely detrimental to the species.

In small, isolated groups, genetic variation is quickly reduced and leads to inbreeding depression (i.e. consanguinity), which has deleterious effects on fitness and fecundity[8]. In captive Takhis, a 2-3% decrease in survival rate was observed for each 10% inbreeding increase, which resulted in higher juvenile mortality and lower life expectancy[9]. Genetic diseases also proliferated, like facial asymetry[10], an anomaly causing irregular teeth wear and misalignment of upper and lower jaws that prevents feeding, which has been particularly lethal in recent decades[11].

As a result, by the time the breeding programs started, nearly 60% of the unique genes of the studbook population had been irreversibly lost since the first Takhis were brought to Europe[12] and only four maternal lineages survived[13]. All living Przewakski horses descended from 14 individuals: 12 out of the 53 foals brought to Europe in 1899-1902 and 1947 and 2 captive-bred hybrids born from domestic mares in Halle (Germany) and Askania Nova (Ukraine)[14]. With only 14 founders, the genetic stock was meagre and some of the genes did not even pertain to the endangered species.

Przewalski foal conceived through artificial insemination

First foal born out of the insemination program launched by the Smithsonian Zoo, Washington DC, USA. Credit: Dolores Reed

Thanks to considerable conservation efforts since the 1980’s, some of these dramatic consanguinity problems have been contained and the remaining founder genes retained for future generations of Takhis. Regularly updated by the Prague Zoo since 1959, the International Studbook has been a precious database providing detailed information on each horse’s genealogy and inbreeding was lowered by mating together the most genetically viable individuals. Nowadays, modern techniques in the field of veterinary science and biology such as reverse vasectomy and artificial insemination, both successfully performed on Takhis at the Smithsonian Zoo (United States) in 2007 and 2013 respectively, help preserve genetic diversity for future generations; artificial insemination also eliminates the need to move the horses for breeding.

In extremis, Przewalski horses were saved from doom. However, since the gene pool used to breed the current population was limited, today’s specimens are an incomplete picture of their ancestors.

During the Holocene, Takhis populated the vast East Asian drylands and horses from different regions adapted differently to match their local habitat, a good example being coat color variations. About the foals he captured from three areas of western Mongolia[15], Hagenbeck noted: “Observation showed that there were no less than three varieties of the wild horse in the neighborhood [of Kobdo, Altai Mountains], closely ressembling one another in form but showing differences of colours”[16]. A century before, John Bell reported seing “wild horses of a chesnut color” in Southern Siberia[17]. Erna Mohr particularly insisted on such color differences: “When zoologists and lovers of horses judge the conformation coat and colour of the wild horse and criticise them from an ideal that they have conceived after seeing one or two examples of the species descended in a long line from captured animals, they forget or are in ignorance of the mixed up contingents of yearlings and foals that were imported from Mongolia. It would be completely wrong to judge Przevalsky horses in general hippological terms and ideas. One must not give preference to a particular compact or well-made type and assume that this is binding, but refer to the original diversity of form as they appeared when the imported yearlings were fully grown”[18]. Today, the horses’ coat does not accurately reflect the variety of phenotypes that existed in the wild. Only two color types have survived: bright yellowish-red-brown and pale grey-yellow, with manes ranging from dark brown to black[19].

Reintroducing the Przewalski horse in part of its historic East Asian range was seen as a way to mitigate the adverse effects of reduced genetic diversity through the positive action of natural selection.

Continue reading about the Przewalski horse: Part 1 I Part 2 I Part 3Part 4 I Part 5 I Part 6 I Part 7 I Part 8 I Part 10 I Part 11 I Part 12

REFERENCES – [1] Extract from the speech given by Prince Bernhard of The Netherlands at the Silver Carnation Award ceremony in honor of Jan and Inge Bouman in 1997. [2] Status and Action Plan for the Przewalski’s Horse, S. Wakefield et al., in Equids: Zebras, Asses, and Horses, Edited by Patricia Des Roses Moehlman, IUCN/SSC Equid Specialist Group IUCN, 2002, p. 86. [3] Foundation for the Preservation and Protection of the Przewalski Horse[4] Przewalski’s Horse: The History and Biology of an Endangered Species, Lee Boyd, Katherine A. Houpt, p 23. [5] Przewalski’s Horse: The History and Biology of an Endangered Species, Lee Boyd, Katherine A. Houpt, p 27-28. [6] Przewalski’s Horse: The History and Biology of an Endangered Species, Lee Boyd, Katherine A. Houpt, p 29. [7] IUCN Red List[8] Reduction of inbreeding in a natural herd of horses, P. Duncan et al., Animal Behavior 1984 (32) 520-527. [9] Studies by Ballou (1994), cited in Status and Action Plan for the Przewalski’s Horse, S. Wakefield et al., in Equids: Zebras, Asses, and Horses, Edited by Patricia Des Roses Moehlman, IUCN/SSC Equid Specialist Group IUCN, 2002, p. 85. [10] Anomaly identified by the paleontologist Laure Danilo, Institut des Sciences et de l’Evolution in Montpellier, France, according to Le cheval de Przewalski, a documentary by Guillaume Lévis (in French). [11] Le cheval de Przewalski, a video documentary by Guillaume Lévis (in French). [12] Przewalski’s Horse: The History and Biology of an Endangered Species, Lee Boyd, Katherine A. Houpt, p 88. [13] Horse Domestication and Conservation Genetics of Przewalski’s Horse Inferred from Sex Chromosomal and Autosomal Sequences, Lau et al., 2008. [14] IUCN Red List. [15] The Origin and Influence of the Thoroughbred Horse, William Ridgeway, Cambridge University Press, 1905, p 26-28. [16] Beasts and Men: Being Carl Hagenbeck’s Experiences for Half a Century Among Wild Animals, Carl Hagenbecks, p. 86. [17]A Journey from St Petersburg to Peking, John Bell, 1763, p 212. [18] The Asiatic Wild Horse, E. Mohr, by J. A. Allen, London, 1971,  pp 33-34. [19] Przewalski horse fact sheet, San Diego Zoo, 2008. Last update in 2013.

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