Horses still have the five toes per foot passed on by their distant ancestors, a new study has suggested.
While just the one “toe” is clearly visible in the modern horse, scientists have long accepted that remnants of two other toes remain as small bones at the side of the foot.
But research on equine embryos undertaken by Kathryn D Kavanagh, C Scott Bailey and Karen E Sears at the University of Massachusets has demonstrated that at a brief phase of horses’ development, all five toes are present in the foetus.
Previous studies have looked at the mechanisms by which animals have evolved to have fewer digits, and concluded that the horse retains one central digit flanked by two “splint metapodials” — the remains of digits two and four.
“[The] evolution of the modern horse limb, and in particular reduction of the number of toes during the evolution of the horse lineage, is one of the most iconic evolutionary transitional stories documented in the fossil record,” the study’s authors said.
They added that the idea that modern horses only ever formed three toes had become “an important cornerstone of the general view of the evolutionary developmental biology of digits”.
But their work, published last week by the Royal Society, indicates that the two flanking toes are actually fusions of digits one and two and four and five — and that all five toes can be seen in early development.
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While previously limited embryology data suggested that horses only ever form three digits, a 2018 paper (by the New York Institute of Technology College of Osteopathic Medicine) was the first to challenge this belief, suggesting there were signs of five in the modern adult horse.
By looking at younger horse embryos than had previously been studied, the new research has provided the missing evidence to support this idea.
Horse embryos used in the study were obtained from mares at North Carolina State University who had been bred using artificial insemination within 48 hours of documented ovulation. Several embryonic limbs from horses of estimated ages of 29 to 35 days-post-insemination were fixed, dehydrated, paraffin-embedded, serially transverse-sectioned across the distal limb, stained and digitally imaged.
Researchers now believe the development of digits in the horse embryo mirrors the evolutionary progression from the five-toed Phenacodus through to the three-toed Protorohippus, Mesohippus and Hypohippus and ultimately the monodactyl modern horse.
“We have shown that five digit condensations form in the embryonic equus limb, and their subsequent reduction follows a striking parallel with evolution in the famous fossil transition series in the horse lineage,” they said.
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