Dietary Changes and The Evolution of The Horse

Published in Horses and People Magazine

You are what you eat!

 By Mariette van den Berg, BAppSc. (Hons), MSc. (Equine Nutrition)

In a number of nutrition and health articles it has been emphasised that the horse evolved primarily as a grazing herbivore,eating a diet based on fibre, mainly grasses and when available or in seasonal shortage horses will even browse trees, shrubs and other foliage to maintain their dietary needs. In this article we will highlight some new findings about how dietary change influenced the evolution of the horse and why horses are more adapted to a grassland life. Although natural selection and adaptation takes millions of years we should review the way we feed our horses today and how we might influence the horse’s continuous evolution.

Paleo (prehistoric)-dental researchers at the New York College of Osteopathic Medicine and the American Museum of Natural History, have completed an important new study ⁽¹⁾ that shows that the evolutionary path of horses as we know them today was affected by the food available to their prehistoric ancestors.”

So the old proverb “you are what you eat” holds true — if you give it about a million years.

The evolution of the horse, Equus ferus caballus, has occurred for approximately 55-60 million years. The Equus became extinct in the New World (America) 10,000 years ago, but persisted in the Old World and was domesticated in central Asia about 6000 years ago from stock probably similar to the present day Przewalski’s wild horses, Equus (ferus) przewalski ⁽²⁾. From studies of fossils, knowledge about the phylogeny of the horse as an herbivore have been derived and show that the first 35 million years (Eocene to early Miocene) of equine phylogeny are characterized by browsing species with a relative small body size (~10-50 kg). The remaining 20 million years (middle Miocene until the present day) are characterized by either primarily browsing/grazing or mixed feeders with a large diversification in body size (~50-500kg) ^(3,4). The evolutionary transformations in equids (hoofed mammals of the family Equidae, like horses, donkeys, zebras and extinct animals) may have been triggered by dramatic climatic changes ⁽³⁾. The areas where equids lived may have become drier, developing grasslands and savannas. These environmental changes may have contributed to the diversification of equids. The equids developed not only elongated limbs and rigid vertebral columns, which allow the animals to move quickly and economically. They also developed teeth with an increased capacity to grind grass, through hypsodonty (high crowned), increased complexity of the grinding surface and molarization of the premolars.

The influence of changes in paleoclimate on equine diet and evolution as described above has been suspected for many years but was not previously confirmed because of limitations in traditional approaches to the study of diet and tooth wear. Previous studies used chemical analyses of teeth and microscopic wear to understand what the animals were eating, but because these approaches require such laborious work, studies were limited to a few specimens at a time. In the present study Matthew Mihlbachler, Ph.D., and Nikos Solounias, Ph.D.  used a new approach called mesowear that relies on the shape of the tooth, specifically sharpness of the cusps of molars (bumps on mammalian teeth) to see how they’ve been worn down by chewing. By using this mesowear analysis, they were able to examine a much large sample size. They studied the teeth of 6,500 fossil horses representing 222 different populations of more than 70 extinct horse species, from 55 million years ago in North America to the extinction at the end of the last ice age.

To better understand how horses’ teeth evolved over time due to diet and climate change, researchers ranked the sharpness of the cusps of the molars of fossil horses. CREDIT: Matthew Mihlbachler

The researchers analysed the amount of tooth wear and later analysed their data alongside records of North American climate changes that would have shifted the diets from rainforest fruits and woody, leafy vegetation to the more abrasive diets found in grasslands. Horses originally evolved in North America, but became extinct about 10,000 years ago. The first horses in North America emerged about 55-60 million years ago. They were small, fox-size animals with four toes and low-crowned teeth (brachydont). They lived in a warm, moist, forest environment, and the wear on their rounded cusps matches those of fruit-eaters. Approximately 33 million years ago, the shape of horse species teeth changed, from the rounded cusps of fruit eaters, to the sharper points characteristically of leaf eaters. Around this time the climate had cooled and the forests would have displayed more open habitats.

Markedly changes in the teeth of ancestral horses, which are closely related to modern horses, took place around 18 million years ago. The change in climate favouring the spread of open grasslands and required the horse species to adapt to tougher plant diets, particularly grasses. Because grasses contain silica, which is abrasive and causes increased tooth wear, horse teeth compensated by growing longer (developing high-crowned teeth).

Around 10 million years ago, horse species that maintained a fruit and leaf diet went extinct, and about 4-5 million years later also species that had a intermediate diets (part leafy and part grassy) disappeared. Only the grass-eating equids that eventually became the modern day horse (Equus ferus caballus) survived. Although the researchers underline that there were leaves and trees throughout all that time period, from 55 million years ago to the extinction. They don’t know why horses left those niches.

The researchers emphasize that the modern horse is in fact a classic example of evolution through the processes of natural selection and adaptation. However, the dietary changes resulting from the changing climate took time to lead to adaptation in the horse’s ancestors. They found that evolutionary changes in tooth anatomy lag behind the dietary changes by a million years or more.

Conversely, some researchers have some reservation about the data and how it was interpreted.  The researchers of the present study say that mesowear scores must equal grass eating, however it may also be that it indicates the openness of the environment.  The idea is that vast open areas are windier and become contaminated with dust and sand, which account for higher levels of abrasive materials in the available vegetation.

The new findings are of great value to biologists and zoologists to understand more about the natural selection and adaptation processes in animal species. Nevertheless it will take more data and research to get a clearer picture of this very slow adaptation process and even then we know the history of life is not simple and never be fully determined. The study does emphasise that diet change will influence the horse evolution and so the way we feed our horse nowadays will also affect this adaptation process. We must be aware that we, humans, change diets of horses much more rapid then climate changes ever did over those millions of years. If adaptation takes more then million years then we can also understand that we are creating problems in our horses when we feeding diets that are low in fibre and high in concentrate which causes less wear of the teeth. It’s therefore important we keep feeding our horses according their design, providing a basal diet high in fibre.

References:

¹ Mihlbachler, M.C., Rivals, F., Solounias, N.,  Semprebon, G.M. 2011.  Dietary change and evolution of horses in North America.   Science 331: 1178-1181

² MacFadden, B.J. 2005. Fossil Horses: Evidence for Evolution. Evolution, 307:1728-1730.

³ MacFadden, B.J. 1992. Fossil Horses: Systematics, Paleobiology, and Evolution of the family Equidae. Cambridge University Press, New York, USA.

⁴ Simpson, G.G. 1953. Major Features of Evolution. Columbia University Press, New York, USA.

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