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JOHN D. ORCUTT AND SAMANTHA S. B. HOPKINS
size between Southern California and Oregon. Likewise, sample size cannot explain most of the patterns observed in equids, which are both common and well sampled. Even the most poorly sampled equid in the database (late early Hemphillian Pliohippus) has a sample size of 10, and Merychippus is represented by more than 100 specimens. Significant results such as the positive correlations between body mass and latitude in hipparionin Merychippus (n=102) and Dinohippus (n=56) are likely not tapho- nomic artifacts but can be interpreted as ecological trends. The same is true, though, for the patterns observed in other well-sampled taxa such as Acritohippus (n=36), Hipparion (n=43), and Astrohippus (n=49), in which no latitudinal gradient is visible. Taphonomic bias is also unlikely to affect these patterns too strongly; in many cases, taxa sampled in similar numbers and from the same sites (e.g., Dinohippus and Astrohippus)showdiffer- ent patterns. Perhaps the most striking result of this study
is the absence of latitudinal gradients among the modern genera examined. A negative correlation with temperature and a positive correlation with latitude, both in keeping with Bergmann’s rule, are visible in Odocoileus, but the sample is insufficiently large to establish whether or not the trend is significant (while deer are common, they also often lack body-mass data in collections, as individuals tend to be large and difficult to measure accurately). Only within Canis are significant trends apparent. However, these patterns are likely a sampling artifact: specimens from the contiguous United States and southern Canada are almost exclusively coyotes (C. latrans), while specimens from northern Canada and southeast Alaska are almost all wolves (C. lupus). While neither of these species shows a significant relationship with temperature or latitude, wolves are larger than coyotes, and their presence at the north end of the transect accounts for the negative correlation with temperature and positive correlation with latitude. At first glance, this seems to support Bergmann’s rule sensu stricto, as it is a case of larger species within a genus occupying colder climates.However,itisunlikely thatthisisa truly natural signal, as wolves have been
extirpated over large areas of the contiguous United States, and many of these extirpations took place before systematic specimen collecting had taken hold or at the hands of individuals with no scientificinterestinpreservingdata about the animals they had killed.Were reliable data to exist for wolf populations along the southern end of the coastal transect, they would very likely obscure the trend currently visible in the data. In fact, previous analyses of geographic trends within extant canid species, all of which includeatleast onespecies of Canis, show that these species do not show monotonic relation- ships with latitude (McNab 1970; Geist 1987; Thurber and Peterson 1991) or temperature (Meachen and Samuels 2012). If, as these studies suggest, modern latitudinal transects in Canis are shaped by extirpation rather than by climate, both Recent genera included in this studywould fail to conform to the predictions made by Bergmann’srule, andone of thefew linesof evidence obtained in this analysis that seemingly supports Bergmann’srulewould disappear.
Drivers of Body-Mass Gradients Many neontological studies of Bergmann’s
rule have shown evidence of latitudinal body- mass gradients (Ashton et al. 2000), which our results do not support. This may be due in part to the level at which the studies were con- ducted: almost all recent research on body-size evolution has focused on patterns within species. It may be that temperature and body mass interact at a very fine scale and that geographic trends become obscured at higher taxonomic levels. This would run counter to Bergmann’s (1847) observation of body-mass gradients within genera and would contradict his suggestion that the forces driving trends within genera should drive similar trends at all taxonomic levels. Another possibility is that the source of the data for these studies is influencing the patterns observed in them. Bergmann’s research, and several landmark studies in the field since (e.g., Erlinge 1987; Korpimäki and Norrdahl 1989), focused on mammals in Europe. As is the case with wolves in North America, many large animals have long since been extirpated from the southern, temperate parts of Europe and, if they survive
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