BODY-MASS TRENDS


at all, are present only in the more inaccessible regions of the continent. These regions tend to be cold and are, for the most part, located far to the north, and this alone could explain the appearance of latitudinal gradients and of a negative correlation between temperature and body size. Studies of Bergmann’s rule have, of course, been conducted in other areas as well, but the example of the wolves suggests that even on relatively “wild” continents such as North America, extirpation and extinction can strongly influence body-mass patterns. If mean annual temperature has not been the


primary driver of mammalian body size through time, what factor or factors have been? Precipitation and seasonality are the two climatic variables besides temperature that have been proposed to influence body-mass evolution (James 1970; Millar and Hickling 1990). Unlike temperature, precipitation and seasonality should not be expected to vary predictably with latitude. Rather, the preva- lence of arid, seasonal environments in continental interiors suggests that tracking body-size trends along longitudinal gradients would be a more appropriate test of the effect of these variables. The comparison of coastal and inland sites conducted as part of this analysis is a small-scale longitudinal analysis and suggests that significant differences in body size between marine-mediated and rain shadow sites are unlikely to be found. However, a larger-scale analysis of longitudi- nal trends from the Pacific Coast, across the Western Cordillera, and onto the Great Plains might reveal trends that could prove useful in identifying the role precipitation and seasonality have played in driving body-size evolution. As with temperature, paleoclimatic reconstructions for a wider range of localities would allow for direct comparisons between climate and body size through time. Amajority of neontological analyses indicate


a biotic driver of body-size trends. The biotic interactions most frequently hypothesized to have a causal relationship with body mass are competition (Damuth 1993, McNab 1970), predation (Korpimäki and Norrdahl 1989), and food supply (Rosenzweig 1968; McNab 1970; Geist 1987; Erlinge 1987; Thurber and


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Peterson 1991). While biotic interactions can be difficult to quantify in paleoecosystems, the fossil record does provide opportunities for testing the influence of biotic interactions. Analyses of morphological change through time in Pleistocene and Holocene coyotes (Meachen and Samuels 2012; Meachen et al. 2014a) represent an example of such a test. These analyses suggest that while Canis latrans did not go extinct along with several other Pleistocene megafauna, the ecological reorga- nization brought about by these extinctions had a major influence on coyote size and morphology. Similar analyses linking size with morphological traits associated with biotic variables (e.g., using relative blade length as a proxy for carnivory; Van Valkenburgh 1988) along chronoclines could be useful in identifying the influence of these variables on body-size evolution. Lovegrove and Mowoe (2013) present


another chronocline analysis of body size and suggest that, rather than being driven by one ecological variable, body-size evolution is the product of complex interactions between vari- ables that affect some taxa differently than others. Orcutt and Hopkins (2013) reached a similar conclusion, showing that three families of mammals from the same age and region showed very different body-size trends through the Oligo-Miocene. The data pre- sented here suggest that biotic patterns can vary considerably even between closely related genera; Merychippus and Acritohippus, for instance, are both Merychippus-grade equids, but while the former shows a strong latitudinal gradient, the latter does not. The same is true of Dinohippus and its close (but smaller) relative Astrohippus. Not only do body-size patterns vary between coeval taxa, but they also often vary between closely related taxa through time. The late Miocene hipparionins Hipparion and Neohipparion, for example, are both likely descended from the hipparionin merychip- pines of the mid-Miocene (MacFadden 1992), but whereas Merychippus exhibits a strong latitudinal gradient, its probable descendants do not. Bergmann’s “rule,” then, not only does not apply to most taxa examined here, but it does not apply to related taxa at different points in time.


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