SREL Reprint #3561

Geographic variation in body size and sexual size dimorphism of North American Ratsnakes (Pantherophis spp. s.l.)

Brett A. DeGregorio1, Gabriel Blouin-Demers2, Gerardo L.F. Carfagno3, J. Whitfield Gibbons4,
Stephen J. Mullin5, Jinelle H. Sperry1, John D. Willson6, Kenny Wray7, and Patrick J. Weatherhead8

1Engineer Research and Development Center, 2902 Newmark Drive, Champaign, IL 61822, USA
2Department of Biology, University of Ottawa, 30 Marie-Curie Private, Ottawa, ON K1N 6N5, Canada
3Department of Biology, Manhattan College, 4513 Manhattan College Parkway, Riverdale, NY 10471, USA
4Savannah River Ecology Lab, University of Georgia, Drawer E, Aiken, SC 29802, USA
5Department of Biology, Stephen F. Austin State University, SFA Box 13003, Nacogdoches, TX 75962, USA
6Department of Biological Sciences, University of Arkansas, Fayetteville, AR 72701, USA
7Department of Biological Science, Florida State University, 319 Stadium Drive, Tallahassee, FL 32306, USA
8Department of Natural Resources and Environmental Science, University of Illinois at Urbana – Champaign,
1102 South Goodwin Avenue, Urbana, IL 29801, USA

Abstract: Because body size affects nearly all facets of an organism’s life history, ecologists have long been interested in large-scale patterns of body-size variation, as well as why those large-scale patterns often differ between sexes. We explored body-size variation across the range of the sexually dimorphic Ratsnake complex (species of the genus Pantherophis Fitzinger, 1843 s.l.; formerly Elaphe obsoleta (Say in James, 1823)) in North America. We specifically explored whether variation in body size followed latitudinal patterns or varied with climatic variables. We found that body size did not conform to a climatic or latitudinal gradient, but instead, some of the populations with the largest snakes occurred near the core of the geographic range and some with the smallest occurred near the northern, western, and southern peripheries of the range. Males averaged 14% larger than females, although the degree of sexual size dimorphism varied between populations (range: 2%–25%). There was a weak trend for male body size to change in relation to temperature, whereas female body size did not. Our results indicate that relationships between climate and an ectotherm’s body size are more complicated than linear latitudinal clines and likely differ for males and females.

Keywords: Bergmann’s rule, body size, climatic variability, latitudinal clines, North American Ratsnake, Pantherophis spp., sexual dimorphism, temperature

SREL Reprint #3561

DeGregorio, B. A., G. Blouin-Demers, G. L.F. Carfagno, J. W. Gibbons, S. J. Mullin, J. H. Sperry, J. D. Willson, K. Wray, and P. J. Weatherhead. 2018. Geographic variation in body size and sexual size dimorphism of North American Ratsnakes (Pantherophis spp. s.l.). Canadian Journal of Zoology 96(11): 1196-1202.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).