Experts confirm that modern horses lost their additional toes

The distant ancestors of modern horses – such as the Eocene Hyracotherium – had feet similar to those of modern tapirs, consisting of four toes in front and three behind, each hooved with an underlying foot pad. 

By contrast, modern equids like horses, asses, and zebras, have just a single toe – corresponding to the original third toe on each foot – encased in a thick-walled keratinous hoof, with a triangular frog on the sole acting as a shock absorber. 

Recently, by analyzing hoof prints and foot bones from modern horses, as well as a collection of fossil records, an international team of scientists led by the University of Bristol has shed new light on what happened to the lost digits.

“The upper portions – the remains of the additional hand and foot bones – remain as ‘splint bones’ fused with the remaining central one, but where are the fingers and toes? In later fossil horses there were only three toes front and back. The extra toes, known as side toes, in these horses were smaller and shorter than in a tapir, and likely did not touch the ground under normal circumstances, but they may have provided support in exceptional situations, such as sliding or forceful impact,” said study senior author Christine Janis, a professor of Earth Sciences at Bristol.

Previous theory overturned 

In a previous study from 2018, scientists proposed that in modern horses these side toes are retained within the hoof of the central toe, still partly contributing to the functions of the frog (although the frog does not actually contain any bones). 

This hypothesis was based on an interpretation of the 3.7 million-years-old hoof prints of an extinct three-toed horse, Hipparion, from Laetoli in Tanzania (the same site where the foot prints of the hominid Australopithecus were found). The foot prints apparently lacked a frog, adding weight to the notion that the side toes of such horses now contribute to the frog of modern horses.

However, since the presence of the frog can be seen in many hoof prints known to have been made by three-toed horses, it is doubtful that the frog of modern horse hooves formed out of the side toes of tridactyl equids.

What the researchers learned 

“While the notion that modern horses have retained all of their original toes as within-hoof remnants is a novel one, and so rather appealing, it can be shown to be incorrect,” Janis said.

“Although it does seem that remainders of the proximal (upper portions) of the side digits have been retained in modern horses, as the earlier 2018 paper claimed, the distal (lower portions, or toes) have simply been lost. The frog of the horse’s hoof evolved independently of the side toes as a unique structure providing shock absorption and traction during locomotion,” added lead author Alan Vincelette, an associate professor of Philosophy at St. John’s Seminary in Camarillo, California.

Finally, the experts also discovered that the feet of one-toed horses have a different shape from the main toe of the foot of three-toed horses (round rather than oval) – a difference most likely related to differences in weight distribution and/or ecological habitats. The study is published in the journal Royal Society Open Science.

More about modern horses

Modern horses, scientifically known as Equus ferus caballus, are descendants of a line of equids (the family that also includes zebras and donkeys) that started about 4 million years ago. Here are some key points about modern horses:

Size and breeds

Modern horses come in a variety of breeds, sizes, and colors. They can range in size from small ponies like the Shetland Pony (around 10 hands or 40 inches at the shoulder) to large draft breeds like the Shire horse, which can exceed 18 hands (72 inches).

Domestication

The domestication of horses started around 5,500 years ago, probably in the Eurasian steppes. Horses were initially used for meat and milk, and as their speed and strength were recognized, they began to be used for transportation and warfare.

Roles

Today, horses are used for a variety of purposes, including recreational activities (e.g., riding and racing), therapeutic uses (e.g., equine-assisted therapy), and work (e.g., farming, policing, and pulling carriages).

Diet

Horses are herbivores with a diet mainly consisting of grasses. However, when grasses aren’t available, they can also consume other plant materials like leaves and stems. Many domestic horses are also fed hay, grain, and supplements.

Behavior and social structure

Horses are social animals, typically living in small groups consisting of a single stallion, several mares, and their offspring. They have a clear hierarchy within their groups, and they use a range of vocalizations, body language, and even touch to communicate with each other.

Lifespan

The lifespan of a horse can vary based on factors such as breed, diet, and care, but typically ranges from 25 to 30 years. Some horses have been known to live into their 40s.

Physiology and speed

Horses have been bred for centuries for various traits including strength, endurance, and speed. A horse’s speed can vary depending on the breed and individual, but many horses can run at speeds up to around 40-48 km/h (25-30 mph). The fastest recorded sprinting speed of a horse was 70.76 km/h (43.97 mph).

Reproduction

Horses have a gestation period of approximately 11-12 months, and typically produce a single foal, although twins are not unheard of.

Genetics

Modern genomics has allowed scientists to delve into the genetic makeup of horses, leading to insights about their domestication, evolution, and even specific traits and diseases.

Despite their evolution and domestication, horses still retain many of their natural instincts and behaviors, and caring for them requires an understanding of these natural traits.

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By Andrei Ionescu, Earth.com Staff Writer

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