that support the scenario of drought of subtle var- iability with high impact. In addition, the data and modeling support an interpretation of decreased rainfall during the summer, associated with a re- duction in the severity and frequency of tropical storms. This study suggests that there is substan- tial potential for establishing a relationship be- tween the actual climatic variability over the region and the spatially complex historical events (30) that shaped the demise of the Maya civilization. References and Notes 1. L. Schele, M. E. Miller, in The Blood of Kings: Dynasty and Ritual in Maya Art, G. I. Braziller, Ed. (George Braziller, New York, and Kimbell Art Museum, Fort Worth, TX, 1986), p. 335. 2. A. A. Demarest, P. M. Rice, D. S. Rice, in The Terminal Classic in the Maya Lowlands: Collapse, Termination, and Transformation, A. A. Demarest, P. M. Rice, D. S. Rice, Eds. (Univ. Press of Colorado, Boulder, CO, 2004), pp. 1–11. 3. T. P. Culbert, in Precolumbian Population History in the Maya Lowlands, T. P. Culbert, D. S. Rice, Eds. (Univ. of New Mexico Press, Albuquerque, NM, 1990), p. 395. 4. W. J. Folan, Información Universidad Autónoma Ciudad de México (UACM) 13, 122 (1988). 5. B. L. Turner, in Population Reconstruction for the Central Maya Lowlands: 1000 B.C. to A.D. 1500, T. P. Culbert, D. S. Rice, Eds. (Univ. of New Mexico Press, Albuquerque, NM, 1990), pp. 301–324. 6. T. P. Culbert, L. J. Kosakowsky, R. E. Fry, W. A. Haviland, in Precolumbian Population History in the Maya Lowlands, T. P. Culbert, D. S. Rice, Eds. (Univ. of New Mexico Press, Albuquerque, NM, 1990), pp. 103–121. 7. P. A. Andrews, E. W. Andrews, F. R. Castellanos, Ancient Mesoam. 14, 151 (2003). 8. J. Aimers, D. A. Hodell, Nature 479, 44 (2011). 9. D. A. Hodell, J. H. Curtis, M. Brenner, Nature 375, 391 (1995). 10. J. H. Curtis, D. A. Hodell, M. Brenner, Quat. Res. 46, 37 (1996). 11. J. H. Curtis et al., J. Paleolimnol. 19, 139 (1998). 12. M. 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Socki, Hydrogeology of the Yucatán Peninsula, 21st Symposium on Plant Biology, Arturo Gomez Pompa, Scott Fedick, Eds. (Haworth Press, Binghamton, NY, 2003), pp. 115–138. 21. CONAGUA, Servicio meteorológico nacional, México; available at http://smn.cna.gob.mx/ (2011). 22. J. H. Christensen et al., Regional Climate Projections. Climate Change 2007: The Physical Science Basis (Cambridge Univ. Press, Cambridge, 2007). 23. A. V. Karmalkar, R. S. Bradley, H. F. Diaz, Clim. Dyn. 37, 605 (2011). 24. R. J. Lawrence, D. S. Gedzelman, Geophys. Res. Lett. 23, 527 (1996). 25. A. B. Frappier, D. Sahagian, S. J. Carpenter, L. A. Gonzalez, B. R. Frappier, Geology 35, 111 (2007). 26. W. K. Michener, E. R. Blood, K. L. Bildstein, M. M. Brinson, L. R. Gardener, Agric. Appl. 7, 770 (1997). 27. H. Jiang, E. J. Zipser, J. Clim. 23, 1526 (2010). 28. F. N. Scatena, M. C. Larsen, Biotropica 23, 317 (1991). 29. NOAA, National Weather Service/National Hurricane Center; available at: www.nhc.noaa.gov/ (2011). 30. R. B. Gill, The Great Maya Droughts: Water, Life, and Death (Univ. of New Mexico Press, Albuquerque, NM, 2000. Acknowledgments: This paper contributes to UK Natural Environment Research Council projects NE/C003152/1, NE/I009906/1, and NE/H004424/1. We thank two anonymous reviewers for their valuable comments and suggestions. Supporting Online Material www.sciencemag.org/cgi/content/full/335/6071/956/DC1 Materials and Methods SOM Text Table S1 References (31–34) 14 November 2011; accepted 23 January 2012 10.1126/science.1216629 Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum Ross Secord, 1,2 * Jonathan I. Bloch, 2 Stephen G. B. Chester, 3 Doug M. Boyer, 4 Aaron R. Wood, 5,2 Scott L. Wing, 6 Mary J. Kraus, 7 Francesca A. McInerney, 8 John Krigbaum 9 Body size plays a critical role in mammalian ecology and physiology. Previous research has shown that many mammals became smaller during the Paleocene-Eocene Thermal Maximum (PETM), but the timing and magnitude of that change relative to climate change have been unclear. A high-resolution record of continental climate and equid body size change shows a directional size decrease of ~30% over the first ~130,000 years of the PETM, followed by a ~76% increase in the recovery phase of the PETM. These size changes are negatively correlated with temperature inferred from oxygen isotopes in mammal teeth and were probably driven by shifts in temperature and possibly high atmospheric CO 2 concentrations. These findings could be important for understanding mammalian evolutionary responses to future global warming. I nterest in how organisms respond to climate change has intensified in recent years with projected warming of ~2° to 4°C over the next century (1). Although models can be developed to predict evolutionary responses to warming of this magnitude, empirical examples must be drawn from fossil or historical records. Here we report a dramatic example of shifts in body size in the earliest known horses (family Equidae) during the Paleocene-Eocene Thermal Maximum (PETM) (~56 million years ago). The PETM is recognized in marine and continental records by an abrupt negative carbon isotope excursion (CIE) that lasted ~175 thousand years (ky), caused by the release of thousands of gigatons of carbon to the ocean- atmosphere system (2, 3). Some marine records suggest that although d 13 C values shifted rapid- ly at the onset of the CIE in 21 ky or less (2), temperature increase was slower, peaking 60 ky or more into the CIE (4) at ~5° to 10°C above pre-CIE levels (5, 6). We use oxygen isotope values in mammal teeth as a proxy for local temperature change in the continental interior of North America, and we show that equid body size during the PETM was negatively correlated with temperature. In extant mammals and birds (endotherms), closely related species or populations within a species are generally smaller-bodied at lower lat- itudes, where ambient temperature is greater (7). This relationship, known as Bergmann’ s rule, is followed by ~65 to 75% of studied extant mam- mals (8, 9). The cause of Bergmann’ s rule is usually attributed to thermoregulation and the optimization of body size (10) and/or the availa- bility of food resources related to primary pro- ductivity (11). Bergmann’ s rule predicts that average mammalian body size should decrease with warming climate, and smaller size in endo- therms has even been suggested as a third “uni- versal” response to warming, along with changes in phenology and species distribution (10). De- clining body size has been attributed to warming over decadal and millennial scales in some living endotherms (12, 13), but many counterexamples also exist (10). Furthermore, it is difficult to dis- tinguish natural selection (genetic change) from ecophenotypic plasticity (morphological response 1 Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, NE 68588, USA. 2 Florida Museum of Nat- ural History, University of Florida, Gainesville, FL 32611–7800, USA. 3 Department of Anthropology, Yale University, New Haven, CT, 06520, USA. 4 Department of Anthropology and Archaeol- ogy, Brooklyn College, City University of New York, New York, NY 11210, USA. 5 Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701, USA. 6 Department of Paleobiology, Smithsonian Museum of Natural History, Washington, DC 20560, USA. 7 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA. 8 Department of Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA. 9 Department of Anthropology, University of Florida, Gainesville, FL 32611–7305, USA. *To whom correspondence should be addressed. 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