For a few short days in the womb, horses have five tiny toes, research reveals.
Horses are born with feet that are effectively a “single digit”, with two flanking remnants identified as digits 2 and 4.
Now, Kathryn Kavanagh, a biologist at the University of Massachusetts, Dartmouth, and her colleagues have shown that early horse embryos go through a stage with five-digit condensations.
These effectively condense during fetal development into the foot structure familiar to all horse owners.
Surprised by their discovery, the researchers re-examined the initial stages of digit condensation of all digit-reduced four-limbed animals where suitable information was available.
They found that, in all cases, early embryos have five or four digits, with the first digit missing in the case of the latter.
The persistent five-digit initiation in the horse and other digit-reduced modern animals underscores a durable developmental stability at the initiation of digits.
Kavanagh launched into the research after sorting through preserved horse embryos. She noticed that, in early gestation, there were unmistakeably five clusters of developing cells in the area where the foot goes on to develop.
As Kavanagh and her colleagues noted in their paper in the Proceedings of the Royal Society B, the evolution of the modern horse limb is one of the most iconic evolutionary transitional stories documented in the fossil record, in particular the reduction of the number of toes during the evolution of the horse lineage.
Recent work has concurred with older hypotheses that modern horses arose from a five-toed ancestor with intermediate descendants that have reduced numbers of digits.
Modern horses are called single-toed because they possess an enlarged central digit 3 with a thick metapodial called the cannon bone and three smaller distal phalanges that complete the main digit in both their front and bag legs.
Alongside this large third digit lie two very small “splint” metapodials, identified as remnants of digits.
Research has shown that just a month or so after the formation of the embryo, the central digit of the modern horse is already relatively large and the two digits that will become the remnants are being reduced.
The limited data on horse embryology suggested that horses only ever formed three digit remnants during their development and it became an important cornerstone of the general view of the evolutionary developmental biology of digits.
By contrast, a recent palaeontology paper proposed a novel hypothesis, based on bone articulations and ridges in fossil horses and vasculature in late foetal horses, that the identities of all five ancestral digits might be preserved in the metacarpal anatomy of the modern horse adult, but direct evidence was lacking.
This same study also proposed that the frog in the hoof is a remnant of all five digit condensations, although embryological evidence was again lacking.
For their study, Kavanagh and fellow researchers Scott Bailey and Karen Sears assessed early digit development by studying embryonic limbs from horses aged 29 to 35 days after the stallion was mated to the mare.
They found evidence of five digits in the embryonic horse, but this formation lasted only a couple of days. “Their subsequent reduction follows a striking parallel with evolution in the famous fossil transition series in the horse lineage,” they reported.
Looking at the data on a wider array of four-legged animals, it appears there is both a developmentally favoured maximum of five and a developmentally favoured minimum of four for early digit condensations, with digit 1 missing.
“Careful scrutiny is warranted to determine whether digit 1 initiation is truly missing in species in which only four digit condensations are thus far reported.”
Kavanagh Kathryn D., Bailey C. Scott and Sears Karen E. Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number.
287 Proc. R. Soc. B http://doi.org/10.1098/rspb.2019.2756
The study can be read here.