Paleobiology, 42(4), 2016, pp. 643–658 DOI: 10.1017/pab.2016.13
Latitudinal body-mass trends in Oligo-Miocene mammals John D. Orcutt and Samantha S. B. Hopkins
Abstract.—Paleecological data allow not only the study of trends along deep-time chronological transects but can also be used to reconstruct ecological gradients through time, which can help identify causal factors that may be strongly correlated in modern ecosystems. We have applied such an analysis to Bergmann’s rule, which posits a causal relationship between temperature and body size in mammals. Bergmann’s rule predicts that latitudinal gradients should exist during any interval of time, with larger taxa toward the poles and smaller taxa toward the equator. It also predicts that the strength of these gradients should vary with time, becoming weaker during warmer periods and stronger during colder conditions. We tested these predictions by reconstructing body-mass trends within canid and equid genera at different intervals of the Oligo-Miocene along the West Coast of North America. To allow for comparisons with modern taxa, body mass was reconstructed along the same transect for modern Canis and Odocoileus.Of the 17 fossil genera analyzed, only two showed the expected positive relationship with latitude, nor was there consistent evidence for a relationship between paleotemperature and body mass. Likewise, the strength of body-size gradients does not change predictably with climate through time. The evidence for clear gradients is ambiguous even in the modern genera analyzed. These results suggest that, counter to Bergmann’s rule, temperature alone is not a primary driver of body size and underscore the importance of regional-scale paleoecological analyses in identifying such drivers.
John D. Orcutt. Department of Biology, Gonzaga University, Spokane, Washington 99258, U.S.A. E-mail:
orcutt@gonzaga.edu.
Samantha S. B. Hopkins. Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, U.S.A. E-mail:
shopkins@uoregon.edu.
Accepted: 11 February 2016 Published online: 3 May 2016 Supplemental material deposited at Dryad: doi:10.5061/dryad.q8bf1
Introduction Diversity and body-size clines have been
observed along a number of ecological gradi- ents: two of ecology’s foundational studies analyzed trends along elevational (Humboldt and Bonpland 1807) and latitudinal transects (Bergmann 1847), and modern researchers have traced patterns along climatic (e.g., Bradshaw and Holzapfel 2010), water depth (e.g., Smith and Brown 2002), chemical (e.g., Hollister et al. 2010), and other gradients. Such research lays the foundation for the formulation of ecological models: by observing how organisms respond to a wide range of environmental conditions in modern ecosystems, it is possible to predict how the same organisms will respond to environmental changes in the future. These models are critically important to anticipating and mitigating the effects of anthropogenic climate change, but in some cases lack pre- dictive power (as is the case, for example, with ecological niche models of range shifts; Guralnick and Pearman 2010; Davis et al.
© 2016 The Paleontological Society. All rights reserved.
2014). This ispartially because of the complexity of ecological interactions, in which several factors may influence biotic variables. It is also due in part to the complexity of the ecosystems themselves, in which many biotic and abiotic variables influence and are influenced by one another, making it difficult to tease out which variables are most important in shaping biotic patterns (Berteaux et al. 2006). Finally, models of future responses to environmental change based on neontological research are, of necessity, based on biotic variability across environmental regimes for which there is a historical precedent. Even the most conserva- tive estimates of future warming indicate a rapidly increasing divergence fromthe climatic conditions that have characterized the Holocene (Intergovernmental Panel on Climate Change 2014), meaning that any prediction of biotic responses to this change requires extra- polatingwell beyond the range ofmodern data. These last two concerns can be addressed by not only examining biotic clineswithinmodern
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