In most mammals, individual body sizes tend to be smaller in warmer regions and larger in cooler regions. Secord et al. (p. 959; see the Perspective by Smith) examined a high resolution 175,000-year record of equid fossils deposited over a past climate shift-the Paleo cene-Eocene Thermal Maximum-for changes in body size. Using oxygen isotopes collected from the teeth of co-occurring mammal species to track prevailing environmental temperature, a clear decrease in equid body size was seen during 130,000 years of warming, followed by a distinct increase as the climate cooled at the end of the period.
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