1 The evolution of a single toe in horses: causes, consequences, and the way forward 2 Brianna K. McHorse,1,*,†,‡ Andrew A. Biewener,*,† and Stephanie E. Pierce*,‡ 3 4 *Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA; 5 †Concord Field Station, Harvard University, Bedford, MA 01730, USA; ‡Museum of Comparative Zoology, 6 Harvard University, Cambridge, MA 02138, USA 7 8 1
[email protected] 9 10 Abstract 11 Horses are a classic example of macroevolution in three major traits—large body size, tall- 12 crowned teeth (hypsodonty), and a single toe (monodactyly)—but how and why monodactyly evolved is 13 still poorly understood. Existing hypotheses usually connect digit reduction in horses to the spread and 14 eventual dominance of open-habitat grasslands, which took over from forests during the Cenozoic; digit 15 reduction has been argued to be an adaptation for speed, locomotor economy, stability, and/or 16 increased body size. In this review, we assess the evidence for these (not necessarily mutually exclusive) 17 hypotheses from a variety of related fields, including paleoecology, phylogenetic comparative methods, 18 and biomechanics. Convergent evolution of digit reduction, including in litopterns and artiodactyls, is 19 also considered. We find it unlikely that a single evolutionary driver was responsible for the evolution of 20 monodactyly, because changes in body size, foot posture, habitat, and substrate are frequently found to 21 influence one another (and to connect to broader potential drivers, such as changing climate). We 22 conclude with suggestions for future research to help untangle the complex dynamics of this remarkable 23 morphological change in extinct horses.