This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Happy Year of the Horse! The new year began in China on Friday, but celebrations continue for a full week, meaning that I can still wish you a happy new year.
In honor of the Year of the Horse, here are 10 things you didn't know about my favorite kind of horse, Przewalski's horse.
The what horse? The first thing you should know about Przewalski's horse is how to say it. Przewalski is a Polish word, and it belongs to Nikolai Przhevalsky. But we're getting ahead of ourselves. Przewalski is pronounced shuh-VAL-skee. But you can call it a "P-horse" and most conservationists, zoologists, and zoo keepers and curators will know what you're talking about.
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Okay, so Przewalski or Przhevalsky? At one time it was thought that the Przewalski horse was first "discovered" by the Russian explorer Colonel Nikolai Przhevalsky, who lived from 1839 to 1888. It turns out that it was actually discovered and described earlier, but the name stuck. For some reason - and I'm not quite certain why - the Polish spelling became associated with the animal, rather than the Russian spelling. It was actually in the 15th century that the P-horse was first sighted by a European. A German writer named Johann Schiltberger recorded a description of the animal in one of his diaries, "A Journal Into Heathen Parts," while traveling through Mongolia as a prisoner of a Mongol Khan named Egedi. Mongolians, presumably, were quite familiar with the Przewalski's horse prior to Schiltberger's visit, but they may have called it the tahki. Other acceptable names are: Asian wild horse, Przewalski's wild horse, and Mongolian wild horse. There was a time when it was called a "tarpan," but pretty much everybody agrees that it's not a tarpan.
What is a P-horse? Everybody might agree that they're not tarpans, but that's about where the agreement ends. It is clear that the Przewalski's horse is a wild, undomesticated horse. In fact, it's the only surviving species of wild horse. Other "wild" horses, like the American mustang, are actually descended from feral domesticated horses who escaped from their herds and adapted to life outside of direct human influence. Much like their equid cousins, the zebras and African wild asses, Przewalski's horses have never been successfully domesticated.
While there are those who would argue that all domestic horses (Equus caballus) are descended from Przewalski's horses (Equus przewalskii), recent genetic evidence suggests otherwise. In 2011, a group of researchers used a powerful sequencing technique to determine that P-horses form their own clade, separate from the lineage that includes domestic horses. "Our results suggest that Przewalski's horses have ancient origins and are not the direct progenitors of domestic horses," they write. "The analysis of the vast amount of sequence data presented here suggests that Przewalski's and domestic horse lineages diverged at least [117,000 years ago]." (Other research puts the divergence more recently, 38–72 thousand years ago). The consensus is that both domestic and Przewalski's horses are derived from a common ancestor, similar to the way in which humans and chimpanzees share a common ancestor, rather than either species being derived from the other.
Viable breeding. Usually, species that have a different number of chromosomes can't breed and create viable offspring. For example, domestic horses have 64 pairs of chromosomes and donkeys have 62. When they breed and give birth to a mule, with 63 chromosome pairs, it is usually sterile. The Przewalski's horse has 66 chromosomes, the most of any equid species. When a P-horse and a domestic horse breed, their offspring are born with 65 chromosomes. Surprisingly, those offspring are usually viable. Still, the P-horse and the domestic horse are usually considered separate species.
The fall of the P-horse. The Przewalski's horse became well known to Western science only in 1881 when Przhevalsky described it. By 1900, a German merchant named Carl Hagenbeck had captured most of them. Hagenbeck was a seller of exotic animals, providing critters for zoos throughout Europe and for P.T. Barnum. His legacy for the zoo world is mixed - he was among the first to advocate for more naturalistic enclosures, for example - but the Przewalski's horse undoubtedly suffered. By the time Hagenbeck died in 1913, most of the world's P-horses lived in captivity. But it isn't all his fault. The P-horse was already suffering from over-hunting before Hagenbeck got his hands on them, and the few remaining wild herds continued to suffer from habitat loss and from a handful of particularly harsh winters in the mid-1900s. One herd, who lived in the Askania Nova region of Ukraine, was slaughtered by German soldiers during their World War II occupation. In 1945, there were just 31 remaining P-horses in the world, in just two zoos, in Munich and in Prague. By the end of the 1950s, only 12 individuals remained.
A conservation success story. All P-horses still alive today are descended from nine of those 31 captive horses in 1945. Since then, the Zoological Society of London has worked together with teams of Mongolian researchers to conserve the species. The captive breeding programs were so successful that in just fifty years the species rebounded to over 1500 individuals by the early 1990s. Some 300 Przewalski's horses have been reintroduced to their native Mongolian habitat. Those herds now graze the fields of the Khustain Nuruu National Park, Takhin Tal Nature Reserve, Khar Us Nuur National Park, and Khomiin Tal Reserve. Chinese researchers, who had their own captive breeding program, reintroduced a group into a reserve near the Gobi desert. The largest herd of reintroduced P-horses is once again found in the Askania Nova reserve in southern Russia. Another group has been introduced to the Hungarian Hortobágy National Park. Oh, and there's a herd that is successfully reproducing on its own within the Chernobyl exclusion zone, an area that has effectively become a wildlife refuge. Less than a decade ago, the IUCN reclassified the species from "extinct in the wild" to "endangered."
International Cooperation
Thanks to the work of the Netherlands-based Foundation for the Preservation and Protection of the Przewalski horse, horses were traded between the different breeding programs to maximize genetic diversity. As a result, despite being founded by just nine individuals, the current population of P-horses is genetically sustainable. The Prague Zoo continues to maintain the studbook for the species, a record of the parentage of every individual Przewalski's horse on the planet.
Continuous Monitoring
The horses that were introduced into Hungary's Hortobágy National Park have been constantly monitored by scientists, who are working to understand their natural behaviors. Studies of Przewalski's horse social structure and behavior conducted there continue to help in husbandry and management efforts worldwide. Researchers have learned that P-horses live in small, permanent family groups, which are comprised of a mature stallion, one to three mature females, and their offspring. The juveniles stay within the family group for two to three years before they go off in search of potential mates. Multiple family groups combine to form herds that move together in search of food.
A surgical first. In 2007, veterinary researchers from the National Zoo successfully conducted the first-ever reverse vasectomy on a Przewalski's horse. It wasn't just a first for the species, but the first time such a procedure had been successfully completed on any endangered species. Minnesota - that's his name - originally had the vasectomy in 1999 while he was at the Minnesota Zoo. It was only later that researchers realized how genetically valuable he was, given his ancestry.
An artificial first. It was only a few months ago that the first Przewalski's horse was born as the result of artificial insemination. The insemination process and subsequent birth (a whopping 340 days later) took place at the Smithsonian Conservation Biology Institute (SCBI) in Front Royal, Virginia. The foal was born to a mare named Anne; the first time mother also grew up at SCBI.
It isn't as simple as collecting some semen and depositing it into a mare. Reproductive physiologist Budhan Pukazhenthi who worked on the project told National Geographic News that "the team learned how to successfully collect semen from stallions, monitored hormone levels in mares, and studied how estrus cycles of Przewalski's horses compared with those of domestic horses." Even then it took seven years to result in a viable pregnancy.
Goto H., Ryder O.A., Fisher A.R., Schultz B., Kosakovsky Pond S.L., Nekrutenko A. & Makova K.D. (2011). A Massively Parallel Sequencing Approach Uncovers Ancient Origins and High Genetic Variability of Endangered Przewalski's Horses, Genome Biology and Evolution, 3 1096-1106. DOI: 10.1093/gbe/evr067
Lau A.N., Peng L., Goto H., Chemnick L., Ryder O.A. & Makova K.D. (2008). Horse Domestication and Conservation Genetics of Przewalski's Horse Inferred from Sex Chromosomal and Autosomal Sequences, Molecular Biology and Evolution, 26 (1) 199-208. DOI: 10.1093/molbev/msn239
Ryder O.A. & Wedemeyer E.A. (1982). A cooperative breeding programme for the Mongolian wild horse Equus przewalskii in the United States, Biological Conservation, 22 (4) 259-271. DOI: 10.1016/0006-3207(82)90021-0
Header image via Wikimedia Commons/Chinneeb. Diagram adapted from Goto et al., (2011). Juvenile P-horse image via Smithsonian National Zoo.
