Incest at the Zoo: Saving and (in)Breeding the Przewalski’s Horse

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Shocking news on CNN and Deutsche Welle: Polar bear cub Hertha, born at Berlin Zoo in 2018, was recently revealed to be the baby of full siblings Tonja and Wolodja. This was apparently a misfortune due to poor record keeping at Moscow Zoo; at birth, Tonja was mixed up with another baby polar bear because they both looked like pinkish white puffballs.

Incest is more common than one would expect amongst zoo-bred animals, or at least it was. The studbook—the register of captive-bred polar bears—reveals that not that long ago most polar bears in zoos, if not wild-born, were likely to be children of incest. Especially with endangered species, inbreeding was a common practice up until the 1970s, despite awareness of its risks and consequences. Why did the casual mating of siblings occur and how did the practice become a newsworthy shocking mistake? In the following lines, I discuss the case of the Przewalski’s horse—considered the last species of wild horse, one of the best documented and longest captive-breeding stories in the world—and trace how practices and ideas related to inbreeding have evolved.

The scruffy-looking foals strained after tedious journeys arrive at London Zoo, 1902.
The scruffy-looking foals strained after tedious journeys arrive at London Zoo, 1902. (Photo by P. Dando, reproduced from Erna Mohr, the Asiatic Wild Horse, 1971).

Sixty Przewalski’s Horses Brought into Captivity

At the turn of the twentieth century, roughly sixty Przewalski’s horses were caught in the mountainous deserts in what was called at the time Inner Asia. The scruffy-looking foals (some supposed to be hybrids with domestic horses1), strained after violent capture and tedious journeys, were sold around the world—usually in pairs–to zoos, animal collectors, and scientists, by the animal trader Carl Hagenbeck. Since then, only one more wild-caught horse joined the captive population in 1947. The captive horses circulated the globe. From Ukraine to Germany, from the US to Prague, from the UK to Australia, from Australia to Germany. Some of the yearlings perished enroute. Others died of trivial causes, e.g. a cut from a pitchfork. Many new-born foals died soon after birth.

“Until the mid-twentieth century, the most convenient way to breed the animals was to breed the resident pairs, who were often siblings or parents and children.”

Successful captive breeding was rare. Stallions put in small enclosures would often be aggressive, kicking and biting the mares introduced into their paddocks for mating. Even when mating did occur, fertility was an issue. Until the mid-twentieth century, the most convenient way to breed the animals was to breed the resident pairs, who were often siblings or parents and children. Given that many of the horses, as well as most other animals held in zoos, did in fact not breed at all, keepers paid somewhat less heed to consanguinity. Only thirteen wild Przewalskis bred and passed on their genes, and given all the hardships of capture and captivity, including the bombs of the Second World War, the continuation of the species was very fortunate. But with many species tethering on the brink of extinction, questions linger about whether survival is good enough, or if inbreeding and low genetic diversity will strike a lethal blow sooner or later.

The Munich Hellabrunn herd of Przewalski's Horses, 1957
The Munich Hellabrunn herd, 1957. From left to right, Sitta, Sira, Romana, Sidor, Roger, Rosa. (Photo by H. Heck, reproduced from Erna Mohr, 1971, The Asiatic Wild Horse)

Fresh Blood

Ideas about the wicked effects of inbreeding circulated in the worlds of naturalists and breeders in the nineteenth century. In 1868, Darwin said of livestock breeding that all the practical breeders were aware of the evils that followed from the ‘closest inbreeding.’ The converse phenomena, that of ‘hybrid vigour’, or ‘fresh blood,’ i.e. the benefits that accrue with the crossing of unrelated animals, was also known. But in the case of the wild horse there was not much ‘fresh blood’ available.

In the 1930s, Munich Zoo was breeding Przewalski’s horses. Its director, Heinz Heck, son-in-law of the animal trader Hagenbeck, showed a lot of interest in contributing to saving the Przewalski’s horses from impending extinction. He wrote in his zoo magazine ‘Das Tier und Wir’: “It would be a shame for our century if … these last primeval horses disappear … It is the last minute for something to happen to the dying steppe wild horses.”2 Heck had some success with breeding and thanks to his good connections he imported Przewalski’s horses from Ukraine, Australia, and from the US, thus securing a stock of genetically diverse horses. Nevertheless, he feared a bleak future: “We bred until now six wild horses, but we were advised about inbreeding, if we do not find a stallion with a completely different blood in the following years.”3 Heck complained that it was nearly impossible to avoid inbreeding because there were so few available wild horses. Yet, two years after he wrote the article, Heck found the desired stallion—Neville, a yearling from the Duke of Bedford’s estate in Woburn, UK, who would have many offspring (but who also carried genes that would cause inbreeding depression, as later studies have shown).4

Orlica 3, with son Pegas and daughter Volga in Askania Nova, Ukraine.
Orlica 3, with son Pegas and daughter Volga in Askania Nova, Ukraine. (Photo by N. V. Lobanov, reproduced from Bannikov and Treus, 1967, Die Zucht des Przewalskipferdes in der Sowjetunion)

Demographic Ups and Downs

Severe losses during World War Two would diminish the population again. Thirty-one horses were alive in 1945. The Munich line survived, despite Hellabrunn Zoo being nearly destroyed. The only other horses that survived were at Prague Zoo, where bombing raids did not hit. Here the breeding stallion, Horymir, who came from Washington Zoo, was the son of full siblings. One of the mares, Helus, was also a daughter of full siblings, and the other mare was her mother, Minka, an offspring of half siblings. To make things even more complicated, Minka was the daughter of a hybrid stallion, a situation that would create a rift between the two zoos, Munich and Prague, as ten years after World War Two, Heck decided to breed only ‘purebloods,’ to exclude horses with Prague ancestry from ‘his’ breeding, even to ‘eliminate’ them.

Heck was a complex character. An early preservationist and a member of the Nazi party (although his genuine allegiance to its ideology was doubtful), he was interested in heredity and in back-breeding extinct primeval animals, such as aurochs and tarpans, through hybridisation and trait selection.5 After 1950, he boasted about his ‘pure’ line of Przewalski’s horses, and showed a marked interest in preserving the ideal species’ looks (phenotype) through selection: a sandy bay horse with a whitish potbelly, mealy muzzle, dun upright mane without a forelock and without any white marks, dorsal black stripe going to the root of a bushy tail, as well as dark zebra-like stripes on the legs.6 But Heck became marginal in the zoo breeders community, because of post-war politics, but more so because of his back-breeding experiments, as well as his aesthetic craze. The ‘Munich line’ practices would later become a negative example.7

A stallion who sired 56 Przewalski's Horses in captivity.
Bars, December 1968. Orlica 3’s son, the ‘prolific stallion,’ sired 56 offspring. He was born in Askania Nova in 1963, was imported to Prague Zoo in 1965, and went to Hellabrunn Munich nineteen years later, in a historical exchange when the two lines would cross again. (Photo by J. Volf, reproduced from Volf, 2019, article in journal Lynx, n. s., 50, with permission by journal).

A much-acclaimed ‘injection of fresh blood’ occurred in 1947, when a wild-caught mare, Orlica 3, was brought to Askania Nova in Ukraine. A stallion, Robert, was imported from Munich to be her partner, and breeding of Przewalski horses again held some promise.

In the 1950s and 60s, breeding took off. Some attributed this success to Orlica’s ‘new blood.’8 Others, to the publication of the studbook in 19589 by Erna Mohr, a zoologist in Hamburg, who staunchly collected data on the entire population since the original founders.10 A unique dataset at the time, the studbook allowed the breeders to imagine the physically scattered horses as one population.

A genetic tree of the Munich line of Przewalski's horses.
Part of the genealogical tree of the Munich line, illustrating inbreeding. Red lines represent first-degree incest relations (e.g. Minoi, father – Lori, daughter; their daughter Ella again mates with Minoi, her grandfather; or Sitta – Sidor, full siblings). Blue lines represent second-degree incest relations (e.g. Severin – Sitta, half-siblings with common father Neville; their daughter Sira mates with uncle Sidor and have seven offspring). Red dots in triangles represent ‘prolific stallions.’ Triangles with star are stallions diagnosed with ataxia and cryptorchidism. (Drawn by author according to studbook information).

But this demographic success had a quandary: the rise of prolific stallions. A handful of stallions sired most of the offspring, which did not help with genetic diversity. The other males were deemed ‘surplus’. A son of Orlica 3 and Robert, Bars, went to Prague. He sired fifty-six offspring. Another son, Pegas, stayed in Askania, where he fathered sixty-one children. In Prague, stallion Uran sired thirty-five offspring, with his full or half-sisters. In Munich, stallion Severin had thirty-eight children, nearly all with his half-sisters, and his son Basil would father no less than seventy-two offspring. Demographics rose, but there were several catches. Few stallions meant less genetic diversity. And larger herds allowed for confusion. In many cases, keepers could not tell individuals apart. The wild horses were not freeze-branded, because they were not tame enough to submit, and immobilization was risky and difficult to perform. For this reason, and largely because of poor record keeping practices, inbreeding would unintentionally occur.

The Scientific Community Reacts

How did zoo people think about what was going on? At the first conference that brought together zoo directors, conservationists, and animal traders, held in London in 1964, the delegates were concerned with inbreeding, which they thought reduced the stamina of the progeny.11 But these concerns seemed not to touch upon Przewalski’s horse, the poster child of captive-breeding. Zdenek Veselovsky, the director of Prague Zoo, declared at the conference, “I do not believe that there is any real danger of a degeneration in the physical characteristics of the species because even where in-breeding has occurred, lethal genes have not yet appeared. A good reproductive rate and healthy progeny are in themselves a good guarantee for further successful breeding.”12

The 1st International Symposium on Przewalski Horse
The 1st International Symposium on Przewalski Horse, Prague, 5–8 September 1959. From left to right: Jiri Volf (speaking, studbook keeper) Erna Mohr, Zdenek Veselovsky, A. G. Bannikov, F. J. Appelman. Photo by J. V. Stanek, reproduced from Volf, 2019, article in journal Lynx, n. s., 50, with permission by journal).

The post-World War Two demographic boost annihilated inbreeding concerns. Zoo breeders were somewhat reassured by the belief that in the ‘original’ wild horse population incest avoidance was not practiced, and within herd structures, daughters would often stay in the same herd as their mothers and mate with their fathers. This would mean that inbreeding was ‘naturally’ tolerated. Indeed, studies have shown that in species where group boundaries are strict, incest occurs frequently, such as in the banded mongoose.13 Others, like Veselovsky, believed that the captive-bred Przewalskis did not show any signs of inbreeding depression, so there was nothing to worry about.

The Realities of Inbreeding

Inbreeding indeed operates by probability. The offspring of related parents are more likely to inherit and manifest a deleterious feature, as well as to pass it on and spread it until fixation. It is theoretically possible that the original founders of a captive-breeding population have no significant deleterious genetic features, thereby there would be no negative effects of inbreeding. For a long time, the apparently successful captive survival of Przewalski’s horses, as well as of other inbred endangered species—Pere David’s deer and the golden hamster, the latter being all descended from one wild female—drove zoo breeders to be skeptical about the harmful effects of inbreeding. 

But after the mid-seventies undeniable evidence for inbreeding trouble began to gather. What did not meet the eye at first became visible at a closer analysis of the records, enabled by a datafication revolution brought about by the use of computers. Driven by a desire to reintroduce Przewalski’s horses back to Mongolia, a Dutch couple, Inge and Jan Bouman, outsiders to both scientific and zoo worlds, became critically preoccupied with the soundness of captive-breeding. They partnered with Utrecht University and, based on painstaking study of zoo records, they proved there was reason for alarm.

Single-gene pathologies were present. They showed that inbreeding correlated with diseases such as ataxia, lack of muscle control that results in inconsistent gait, or cryptorchidism, a condition in which one or both testicles does not drop into the scrotal sac. They also showed that after a few generations of inbred offspring, fertility and longevity were diminishing.14 In parallel, Nate Flesness, a US-based scientist, also analyzed the data with computer software and arrived at the same worrisome results. He showed that highly inbred individuals were reproducing less.15 But both Flesness and the Boumans were seen as outsiders meddling in zoo business, and their reports were seen as exaggerated. Yet, studies showing problems would multiply. Katherine Ralls and Jon Ballou analysed thirty-eight well-recorded species of mammals kept at the Smithsonian’s National Zoo, and their results were dramatic: higher mortality of inbred offspring was present in most species.16

“Avoidance of inbreeding was the only way to ensure the long-term survival of captive bred endangered species. This meant accurate record-keeping, close cooperation and calculated exchanges.”

The bottom line of all these analyses was that avoidance of inbreeding was the only way to ensure the long-term survival of captive bred endangered species. This meant accurate record-keeping, close cooperation and calculated exchanges. Nonetheless, analysis would show that, in line with the zoo people’s earlier beliefs, the Przewalski’s horse was indeed a lucky species: “Although statistically significant, this severity of inbreeding depression is substantially less than that found for many other mammalian species”, concluded a study by Ballou. He explained that the reduced susceptibility to inbreeding depression could be attributed to chance alone – the founders of the captive population might have been individuals who were free of deleterious alleles.17 He showed that the slight inbreeding effect that did exist could be traced primarily to one of the founders, ancestor of many horses in the Munich line, Neville’s grandfather.18 In retrospect, Heck’s desired stallion proved unlucky; even though he was brought to Munich as ‘fresh blood’, his genes caused unforeseen problems down the line.

Reintroduced Przewalski's Horses in Mongolia
Przewalski’s horses reintroduced to Mongolia, Takhin Tal, February 1994, given supplementary feeding. (Photo from J. Volf’s archive, reproduced from Volf, 2019, article in journal Lynx, n. s., 50, with permission by journal).

In parallel with scientific studies, a few other things would completely change the face of zoo breeding in the late 1970s. First, the rise of the International Species Information System (later Species360), a computerized global database first based at Minnesota Zoo, initiated by Ulysses Seal, Dale Mackey, and Nate Flesness, helped zoos coordinate transparent exchanges of unrelated individuals. International breeding programmes started to ensure coordinated genetic management: in the early 1980s a Species Survival Plan for the Przewalski’s horse was established in the US.19 Later, in 1986, the European Endangered Species Programme for Przewalski’s horse was established.20 At the same time, soaring criticism on the part of animal welfare groups would call attention to zoo practices, and drive increasingly restrictive regulations. In this conjuncture, incest at the zoo, a practice with a long history, would become banned, or shocking news. Since the 1980s, Przewalski’s horse numbers have increased significantly; there are now around three thousand, and nearly half live free in Mongolia.

Feature Image: “Mongolei: Przewalski-Pferd – Mongolia: Przewalski’s horse” by Daniela Hartmann (alles-schlumpf) is licensed under CC BY-NC-SA 2.0.


  1. More on this topic in Nigel Rothfels, “(Re)Introducing the Przewalski’s Horse,” in The Ark and Beyond: The Evolution of Zoo and Aquarium Conservation, by Ben Minteer, Jane Maienschein, and James P. Collins (Chicago: University of Chicago Press, 2019), 77–89.
  2. Heinz Heck, “Die Letzten Und Die Ersten Urpferde,” Das Tier Und Wir. Tierparkzeitung. Tierpark Hellabrunn June (1936): 4.
  3. Heck, 5.
  4.  Jonathan D Ballou, “Population Biology,” in Przewalski’s Horse: The History and Biology of an Endangered Species, by Lee Boyd and Katherine Houpt (Albany: SUNY Press, 1994), 93–113.
  5. See Clemens Driessen and Jamie Lorimer, “Back-Breeding the Aurochs: The Heck Brothers, National Socialism and Imagined Geographies for Nonhuman Lebensraum,” in Hitler’s Geographies, by P. Giaccaria and C. Minca (Chicago: University of Chicago Press, 2016), 138–57; also for a critical take on Heinz Heck and his practices see Susanna Forrest, The Age of the Horse: An Equine Journey through Human History (Grove Press, 2018), ch. 5.
  6. This phenotype was in accordance with the taxonomic descriptions and samples of animals from the last wild populations of the species. Thus, the Munich line  selection has attempted to remove ‘atypical’ animals from the population. Heinz Heck, “Die Merkmale Des Przewalskipferdes” in Equus. Proceedings of the 2nd International Symposium on the Preservation of the Przewalski’s Horse. (Berlin, 1967), 295–301.
  7. For a long time though the owners of Munich line Przewalski’s horses would be reluctant to introduce ‘impure’ horses with Prague domestic ancestry, as asserted by  Oliver A. Ryder and Elizabeth A. Wedemeyer, “A Cooperative Breeding Programme for the Mongolian Wild Horse Equus Przewalskii in the United States,” Biological Conservation 22, no. 4 (April 1982): 259–71.
  8. J. Bouman and I. Bouman, Breeding Przewalski Horses: In Captivity for Release into the Wild (Foundation for the Preservation and Protection of the Przewalski Horse, 1982),
  9. The English edition is Erna Mohr, The Asiatic Wild Horse (London: J. A. Allen, 1971).
  10.  Jiri Volf, “The Studbook,” in Przewalski’s Horse: The History and Biology of an Endangered Species, by Lee Boyd and KA Houpt (Albany: SUNY Press, 1994), 61–75.
  11. ‘Symposium “Zoos and Conservation.” IUCN Publications, New Series: Supplementary Paper No. 3. Report (IUCN in conjunction with ICBP and IUDZG, 1964), 14.
  12. Zdenek Veselovsky “Conservation of Asian Equids, with particular reference to the Przewalski Horse.” in ‘Symposium “Zoos and Conservation.” IUCN Publications, New Series: Supplementary Paper No. 3. Report’, 38.
  13. H. J. Nichols et al., “Evidence for Frequent Incest in a Cooperatively Breeding Mammal,” Biology Letters 10, no. 12 (December 2014): 20140898,
  14.  Jan Bouman, ‘The Future of Przewalski Horses Equus Przewalskii [Plate 10] in Captivity,” International Zoo Yearbook 17, no. 1 (1977): 62–68; J. Bouman and I. Bouman, Breeding Przewalski Horses: In Captivity for Release into the Wild (Foundation for the Preservation and Protection of the Przewalski Horse, 1982).
  15. Nathan R Flesness, “Gene Pool Conservation and Computer Analysis,” International Zoo Yearbook 17, no. 1 (1977): 77–81.
  16. Katherine Ralls, Jonathan D Ballou, and Alan Templeton, “Estimates of Lethal Equivalents and the Cost of Inbreeding in Mammals,” Conservation Biology 2, no. 2 (1988): 185–93; Jonathan Ballou and Katherine Ralls, “Inbreeding and Juvenile Mortality in Small Populations of Ungulates: A Detailed Analysis,” Biological Conservation 24, no. 4 (1982): 239–72.
  17. Ballou, “Population Biology.”
  18. Ballou.
  19. Ryder and Wedemeyer, “A Cooperative Breeding Programme for the Mongolian Wild Horse Equus Przewalskii in the United States.”
  20.  Lydia Kolter and Waltraut Zimmermann, “Die Haltung von Junggesellengruppen Für Das EEP-Przewalskipferd–Hengste in Gehegen Und Reservaten,” Zeitschrift Des Kölner Zoo 3 (2001): 44.
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Monica Vasile

I am an environmental historian at Maastricht University. I currently research the history of reintroductions of endangered species, and I am a PhD candidate in the project 'Moving Animals: A history of Science, Media and Policy in the 20th century'. My approach integrates animals' biographies with a history of science and conservation.

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