Straight from the (wild) horse’s mouth: Researchers compare captive and wild horses’ dental condition

Our horses’ mouths have enjoyed the progress in equine dental medicine and diligent maintenance. Are there parallels between the mouths of domestic horses and wild equids living in captivity? The conservation of numerous wild equids depends on protection and captive breeding programs. Yet while their diets in the wild may be somewhat meager, consisting of hardy herbs and grasses, their diets in captivity may be too soft to wear down their teeth.

“Don’t look a gift horse in the mouth.” “I got this straight from the horse’s mouth.” “He’s a bit long in the tooth.” “She took the bit between her teeth and ran.”

Colloquial English is enlivened with colorful expressions about horse mouths. The art of horse trading in days gone by created euphemisms about the actual age of a horse and the condition of its teeth; only the most naïve buyer would neglect looking inside the mouth for evidence of the horse’s age and presence of any dental problems. 

In recent years, equine dentistry has advanced so that more horses are not handicapped by dental issues.

But what about horses in the wild? They survive without power rasps and obsessions over hooks and ramps and wolf teeth. Is the wild horse mouth a gift of nature? Or do they have problems, too? And when wild equids live in captivity, do their mouths begin to have the same problems that domestic horses have?

For the past 20 years, the wild horse has come under scrutiny to see what we might learn about care of domestic horses from their wild or feral cousins. The wild horse is, after all, as natural as a horse can be. Equine behavior, in particular, has benefited from studying feral and wild horses, as well as their wild cousins like zebras, in their natural environments. Study of the feral horse hoof allowed us the revelation of what a “real” hoof looks like, without the effects of the stabled environment or human intervention.

Scientists could observe feral equids from afar, thanks to concealed cameras and long lenses. They could obtain the hooves of dead horses to study their morphology, which was a lot easier than the early studies that required very fast photography work while a live horse was being held in a chute. 

But what about the wild horse’s mouth? Wild horse studies have their limits; restraining a wild or feral horse so you could look inside its mouth to do a dental exam was not a possibility. Is it self-maintaining? Does the natural environment have a positive or negative effect on dental health? How do different environments and diets impact dental health?

How “natural” is a horse with perfect dental alignment and smooth surfaces? Researchers wonders how much variation is acceptable, and how much deviation it takes to affect a horse’s health.

We already know that statistics show domestic equids suffer from more dental problems than wild equids. Research from more than 30 years ago (Penzhorn, 1984) found that only 19% of wild zebras had evidence of dental abnormality. Compare that with Du Toit et al’s 2008 tally of dental problems in 93% of domestic donkeys in Great Britain, and in 62% of Mexican donkeys.

Having many choices of wild and feral horses living in different natural environments around the world, and being able to compare them to domestic horses, is an advantage of equine science today. While most of the truly “wild” horses are gone from our planet, we still have the subspecies Przewalski’s Horse (Equus ferus ssp. przewalskii), and we can compare its health and characteristics in both captive-bred programs in zoos and sanctuaries around the world, and after re-introduction in the wild in Mongolia and China.

The same is true of other equids, such as zebras, and scientists are able to study species that are gone from the wild, like the African quaggas, because their tissue still exists or is well-documented. In addition, molds have been made of equids to preserve their morphology (body shape and size characteristics) for the scientific record. 

The problem with the molds is that they are sometimes oceans apart. They belong to different zoos, natural history museums and universities. So if you are a researcher who wants to study and compare a wide selection of preserved wild equid tissues with their captive counterparts, you will surely bank some frequent flyer miles.

German researcher Ellen Schulz-Korna, PhD (Dr rer. nat.) knows her way around the archives at the world’s wild equid study centers. Her work at the Max Planck Weizmann Center for Integrative Archaeology and Anthropology at the University of Leipzig in Germany is, like much of the research there, devoted to how humans and animals chew, and what this simple function means to the animal’s wellbeing. The researchers especially study the influence of abrasion on animal’s teeth and jaws. How teeth wear over time is not well understood in science, so researchers study both the teeth of animals and what they eat, and try to connect the dots. Schulz-Korna researches carnivores, herbivores, frugivores, compares the past with present, and–fortunately for us–compares the effects of specific diets on teeth.

Go ahead, ask her about guinea pig teeth. Or, what a gazelle eats–and how it chews what it eats. 

A short video from the American Museum of Natural History in New York documents the re-introduction of some Przewalski’s horses to the wild in Mongolia.

This spring, Dr Schulz-Korna saw her latest research on wild vs captive equid teeth published in the Equine Veterinary Journal. “Comparative analyses of tooth wear in free-ranging and captive wild equids” compiles her data from studying 228 different equids’ jaw and teeth models. Her previous research had already determined that a difference existed between captive and wild rhinoceroses. Would the same be true of equids?

Schulz-Korna was part of an international team that tackled the question; her co-authors include researchers from the Bristol Zoological Society and the School of Biological Sciences, University of Bristol, in the UK; Clinic for Zoo Animals at the University of Zurich, Switzerland; National Park ‘Bavarian Forest’, the Centre of Natural History (CeNak) at the University of Hamburg, and Veterinary Clinic Gessertshaus in Germany. 

She and her colleagues studied seven different wild species. While they weren’t looking for specific dental abnormalities, they did record them; they found them. Out of 312 “hooks” found in the teeth of equids, 206 were in the mouths of captive-held equids.

More important, Schulz-Korna found a significant difference in the effects of abrasion on the pre-molars of captive equids. There was less difference int he molars, and no difference in the distal cusps of the molars of the wild horses and zebras. Overall, she observed, “Captive equids exhibited more homogeneous wear along the tooth row whereas free-ranging equids exhibited a tooth wear gradient, with more abrasion on premolars than molars.” The difference between captive and wild equids was not as pronounced as what had been documented in the rhinoceros.

While that may be good news, the research warns, “Factors leading to hook formation, in particular feeding height, should receive special attention.”

In 2010, researchers O’Neil, Keen and Dumbell at Hartpury College in Great Britain documented differences between Thoroughbred-type horses that lived in stables with no turnout with horses that enjoyed free grazing for 16 hours a day. The results? “Stable-kept horses had a significantly higher total occurrence of abnormalities ( P < 0.001) than free-living horses," the researchers wrote. "The stable-kept group had a significantly higher prevalence of exaggerated transverse ridging across the occlusal surface of the cheek teeth, focal or ramped overgrowths of the cheek teeth and periodontal disease ( P < 0.01 in all cases). All horses in both groups had some occurrence of sharp edges of the buccal and lingual edges of the cheek teeth." 

A Przewalski’s horse in a zoo eats his forage through the mesh of a hanging hay bag. Researchers remark that head position during eating, as well as the abrasive characteristics of what is being eaten, can affect the wear on teeth.

Do our domestic horses need more and better dental care, or do they need a change in diet? There are things that dental care can do, but there are good reasons to analyze degrees of irregularity and to understand a horse’s lifestyle and diet.

Schulz-Korna may have some answers in the future. She is working with equine nutritionists and dentists to find answers to some of the big questions, such as the relationship between malnutrition and tooth illness. The nature of this specific research precluded making any speculation about domestic horses. 

Even so, the research recommends a forage-based diet, including access to pasture, for improved dental health in equids. The condition of their mouths suggested that captive equids ingest lower abrasion diets and/or employ a different feeding position, than their free-ranging conspecifics.

That said, her research has a satisfying observation buried in the data. Since some of the molds she was examining in museums dated back to the truly wild Przewalski’s horses, before the captive breeding programs, she was able to document that today’s re-introduced wild horses in Mongolia or China were in the same dietary niche as the “real” wild horses who had never set foot in a crate or lived even in a sanctuary.

There’s no doubt that equine research needs data from truly wild, as well as feral, horses for comparison with different regimen employed to manage domestic horses. The work being done to document the similarities and differences is both fascinating and inspiring. Looking a horse in the mouth–gift horse or not, wild or domestic–was never more compelling.

To access the original paper:

Taylor, L. A., Müller, D. W. H., Schwitzer, C., Kaiser, T. M., Castell, J. C., Clauss, M. and Schulz-Kornas, E. (2016), Comparative analyses of tooth wear in free-ranging and captive wild equids. Equine Veterinary Journal, 48: 240–245. doi: 10.1111/evj.12408 (access by subscription only)

Further reading (all papers accessible online):

Penzhorn BL. Dental abnormalities in free-ranging Cape mountain zebras (Equus zebra zebra). Journal of Wildlife Diseases. 1984 Apr;20(2):161-6.

Masey O’Neill HV, Keen J, Dumbell L. A comparison of the occurrence of common  dental abnormalities in stabled and free-grazing horses. Animal. 2010 Oct;4(10):1697-701. doi: 10.1017/S1751731110000893. (Open Access)

du Toit N, Burden FA, Dixon PM. Clinical dental examinations of 357 donkeys in the UK. Part 1: prevalence of dental disorders. Equine Veterinary Journal. 2009 Apr;41(4):390-4. 

du Toit N, Burden FA, Dixon PM. Clinical dental examinations of 357 donkeys in the UK. Part 2: epidemiological studies on the potential relationships between different dental disorders, and between dental disease and systemic disorders. Equine Veterinary Journal. 2009 Apr;41(4):395-400.




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