The Evolution of a Single Toe in Horses: Causes, Consequences, and the Way Forward.
Abstract: Horses are a classic example of macroevolution in three major traits-large body size, tall-crowned teeth (hypsodonty), and a single toe (monodactyly)-but how and why monodactyly evolved is still poorly understood. Existing hypotheses usually connect digit reduction in horses to the spread and eventual dominance of open-habitat grasslands, which took over from forests during the Cenozoic; digit reduction has been argued to be an adaptation for speed, locomotor economy, stability, and/or increased body size. In this review, we assess the evidence for these (not necessarily mutually exclusive) hypotheses from a variety of related fields, including paleoecology, phylogenetic comparative methods, and biomechanics. Convergent evolution of digit reduction, including in litopterns and artiodactyls, is also considered. We find it unlikely that a single evolutionary driver was responsible for the evolution of monodactyly, because changes in body size, foot posture, habitat, and substrate are frequently found to influence one another (and to connect to broader potential drivers, such as changing climate). We conclude with suggestions for future research to help untangle the complex dynamics of this remarkable morphological change in extinct horses. A path forward should combine regional paleoecology studies, quantitative biomechanical work, and make use of convergence and modern analogs to estimate the relative contributions of potential evolutionary drivers for digit reduction.
© The Author(s) 2019. Published by Oxford University Press on behalf of the Society for Integrative and Comparative Biology. All rights reserved. For permissions please email: journals.permissions@oup.com.
Publication Date: 2019-05-28 PubMed ID: 31127281DOI: 10.1093/icb/icz050Google Scholar: Lookup
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- Journal Article
- Research Support
- U.S. Gov't
- Non-P.H.S.
- Review
Summary
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This research review article explores various theories behind the evolution of a single toe (monodactyly) in horses. The article takes into account various factors like changing habitats, body size, foot posture, and climate to analyze their contribution towards the digit reduction.
Overview of the Research
The research discusses why and how the evolution of monodactyly in horses occurred, a subject that is still not fully understood. Horses are considered a classic case of macroevolution, particularly concerning their large body size, tall-crowned teeth or hypsodonty, and monodactyly. Several hypotheses linked the reduction of digits in horses to the spread of open grassland habitats dominating over forests during the Cenozoic era. The digit reduction has been proposed to be for improved speed, better locomotor economy, stability, and/or increased body size.
Theories behind the Evolution of Monodactyly in Horses
- The review assesses a variety of hypotheses, not necessarily exclusive to each other, using evidence from multiple fields. These fields include paleoecology, biomechanics, as well as phylogenetic comparative methods.
- The authors also consider the evidence of convergent evolution of digit reduction observed in litopterns and artiodactyls.
- Such convergence suggests that similar environmental pressures may have led to similar evolutionary outcomes in different groups, reinforcing the likely influence of factors like habitat change.
Key Findings of the Research
- The research finds that it is unlikely that a single factor drove the evolution of monodactyly in horses.
- Instead, it is more probable that changes in body size, foot posture, habitat, and substrate frequently influenced each other, and might be linked to broader drivers like changing climate.
Future Directions
- The review assesses a variety of hypotheses, not necessarily exclusive to each other, using evidence from multiple fields. These fields include paleoecology, biomechanics, as well as phylogenetic comparative methods.
- The authors also consider the evidence of convergent evolution of digit reduction observed in litopterns and artiodactyls.
- Such convergence suggests that similar environmental pressures may have led to similar evolutionary outcomes in different groups, reinforcing the likely influence of factors like habitat change.
Key Findings of the Research
- The research finds that it is unlikely that a single factor drove the evolution of monodactyly in horses.
- Instead, it is more probable that changes in body size, foot posture, habitat, and substrate frequently influenced each other, and might be linked to broader drivers like changing climate.
Future Directions
- The research finds that it is unlikely that a single factor drove the evolution of monodactyly in horses.
- Instead, it is more probable that changes in body size, foot posture, habitat, and substrate frequently influenced each other, and might be linked to broader drivers like changing climate.
Future Directions
The authors concluded by outlining future research directions to further understand the complex dynamics of this significant morphological change in extinct horses. They recommend:
- Combining regional paleoecology studies with quantitative biomechanical work.
- Making use of modern analogs and convergent evolution to estimate the relative contributions of potential evolutionary drivers for digit reduction.
Cite This Article
APA
McHorse BK, Biewener AA, Pierce SE.
(2019).
The Evolution of a Single Toe in Horses: Causes, Consequences, and the Way Forward.
Integr Comp Biol, 59(3), 638-655.
https://doi.org/10.1093/icb/icz050 Publication
Researcher Affiliations
- Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
- Concord Field Station, Harvard University, Bedford, MA 01730, USA.
- Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA.
- Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
- Concord Field Station, Harvard University, Bedford, MA 01730, USA.
- Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
- Museum of Comparative Zoology, Harvard University, Cambridge, MA 02138, USA.
MeSH Terms
- Animals
- Biological Evolution
- Equidae / anatomy & histology
- Equidae / physiology
- Fossils / anatomy & histology
- Life History Traits
- Phenotype
- Phylogeny
- Toes / anatomy & histology
Citations
This article has been cited 10 times.- Vincelette A. The Characteristics, Distribution, Function, and Origin of Alternative Lateral Horse Gaits. Animals (Basel) 2023 Aug 8;13(16).
- Batcher K, Varney S, Raudsepp T, Jevit M, Dickinson P, Jagannathan V, Leeb T, Bannasch D. Ancient segmentally duplicated LCORL retrocopies in equids. PLoS One 2023;18(6):e0286861.
- Rothier PS, Fabre AC, Clavel J, Benson RBJ, Herrel A. Mammalian forelimb evolution is driven by uneven proximal-to-distal morphological diversity. Elife 2023 Jan 26;12.
- MacLaren JA. Biogeography a key influence on distal forelimb variation in horses through the Cenozoic. Proc Biol Sci 2021 Jan 13;288(1942):20202465.
- Mallet C, Billet G, Houssaye A, Cornette R. A first glimpse at the influence of body mass in the morphological integration of the limb long bones: an investigation in modern rhinoceroses. J Anat 2020 Oct;237(4):704-726.
- Jafari H, Abebe BK, Cong L, Ahmed Z, Zhaofei W, Sun M, Muhatai G, Chuzhao L, Dang R. Review: Genomic insights into the adaptive traits and stress resistance in modern horses. Stress Biol 2026 Jan 12;6(1):5.
- McGrath AJ, Croft DA, Carrillo JD, Gabriela Suárez M, Vanegas A, Cooke SB, Link A. New Miocene litoptern remains from Colombia and ecological structure of American Neogene herbivore guilds. BMC Zool 2025 Aug 20;10(1):17.
- Rothier PS, Herrel A, Benson RBJ, Hedrick BP. Body mass evolution as a driver of morphological and ecological diversity in terrestrial mammals. BMC Ecol Evol 2025 Jul 11;25(1):69.
- van Bijlert PA, Geijtenbeek T, Smit IH, Schulp AS, Bates KT. Muscle-Driven Predictive Physics Simulations of Quadrupedal Locomotion in the Horse. Integr Comp Biol 2024 Sep 27;64(3):694-714.
- Vincelette AR, Renders E, Scott KM, Falkingham PL, Janis CM. Hipparion tracks and horses' toes: the evolution of the equid single hoof. R Soc Open Sci 2023 Jun;10(6):230358.
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