How Did Horses Get to Just One Toe?

Mechanics of evolutionary digit reduction in fossil horses (Equidae) *
Brianna K. McHorse, Andrew A. Biewener, Stephanie E. Pierce
Summarized by Time Scavengers contributor, Maggie Limbeck

What data were used? This study used metapodials (toe bones) from 12 fossil horse genera as well as from a tapir (herbivorous mammal that looks similar to a pig, but that also has an odd number of toes) to collect data. The metapodials were imaged in cross sectional views to determine load strength (how was weight distributed among the main three toes of fossil horses and the one toe of recent horses) and geometry of the metapodials.

Methods: The metapodials from the fossil horses and tapir were micro-CT scanned (3D x-ray scanning, like the human procedure but on a smaller scale) and the images were manipulated to see the cross sectional area and other views using the open source program ImageJ with the plugin BoneJ. The images were then measured and corrected for evolutionary changes using the open source statistical software, R. Estimates for bone stress were calculated using a toe reduction index (TRI), reconstructed body weights, and angle of metapodial during ground reaction at two speeds of forward locomotion. Additionally, the amount of stress that the metapodials could support was estimated using beam mechanics (an engineering process that looks at how much stress a hypothetical beam could withstand before bending and/or breaking).

Results: Looking at the geometry of the metapodials, it was determined that as the fossil horses grew in both size and weight, their need for four front and three back toes was decreased, and as such the digits gradually decreased to one on all four limbs. For the stress experiments, as the fossils moved forward in time to recent horses, it is seen that the amount of stress that can be placed on metapodial III (what we see expressed as the hoof) increases through time and the dependence on the two metapodials on either side of digit III decreases. This statement is true for both front and back metapodials at both a moderate speed (trotting) and performance (acceleration, jumping).

Figure 1. Image of the toe reduction index (TRI) shown across a phylogenetic tree (evolutionary tree) with the cross sectional view of the metapodial being analyzed. Based on the TRI it is apparent that there is a gradient for toe loss and that there is only one genus of horse, Equus, that truly has one toe. You can also see that for those early horses that still had side toes that the shape of the toe in cross section has a much different shape and therefore still needs side toes to some extent.

Why is this study important? This study is important because it supports two hypotheses that were held about digit reduction in horses. That a) the increased body mass of horses selected for a single, strong metapodial and b) that as horses grew taller, the cost of speed from the side toes outweighed their use in stabilization. This also contradicts the commonly held belief that horses experienced digit reduction as an adaptation to the replacement of forests by grasslands.

The big picture: The big picture here is sort of two-fold. Digit reduction in tetrapods (four-legged creatures) has been of interest to many scientists because as tetrapods emerged onto land 5, 8, even more digits was the ancestral state for these organisms. As we see today, that is not the case. The vast majority of the organisms that we think of have 5 or less digits on their hands and feet, so we want to understand what drove the process of digit reduction in every animal. Second, this study highlights that it is important to keep testing hypotheses even if they have been held for a while. The additional lines of evidence provided by this study give more credibility to two commonly held hypotheses while continuing to falsify the common explanation for digit reduction in horses.

Citation: McHorse BK, Biewener AA, Pierce SE. 2017. Mechanics of evolutionary digit
reduction in fossil horses (Equidae)
. Proceedings of the Royal Society B 284: 20171174.

*all samples in this study were fossils, no live animals were used

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