The effect of coachwhip presence on body size of North American racers suggests competition between these sympatric snakes D. A. Steen 1,2 *, C. J. W. McClure 2 , L. L. Smith 1 , B. J. Halstead 3 , C. K. Dodd Jr 4 , W. B. Sutton 5 , J. R. Lee 6 , D. L. Baxley 7 , W. J. Humphries 8 & C. Guyer 2 1 Joseph W. Jones Ecological Research Center, Newton, GA, USA 2 Department of Biological Sciences, Auburn University, Auburn, AL, USA 3 US Geological Survey, Western Ecological Research Center, Dixon, CA, USA 4 Department of Wildlife Ecology and Conservation, University of Florida, Gainesville, FL, USA 5 Department of Forestry, Wildlife, and Fisheries, University of Tennessee, Knoxville, TN, USA 6 The Nature Conservancy, Camp Shelby, MI, USA 7 Kentucky Department of Fish and Wildlife Resources, Frankfort, KY, USA 8 North Carolina Wildlife Resources Commission, Chapel Hill, NC, USA Keywords character displacement; competition; morphology; reptile; squamate; snake. Correspondence *Current address: Virginia Polytechnic Institute and State University, 100 Cheatham Hall, Blacksburg, VA 24061, USA. Email: davidasteen@gmail.com Editor: Mark-Oliver Rödel Received 9 April 2012; revised 19 August 2012; accepted 21 August 2012 doi:10.1111/j.1469-7998.2012.00965.x Abstract When sympatric species compete, character divergence may help maintain coex- istence. Snakes are often found in species-rich assemblages while exploiting similar resources; because snake body size is a relatively plastic trait that determines the range of prey sizes an individual may consume, divergence in body size between sympatric species may arise as a result of interspecific interactions. The North American racer, Coluber constrictor, and the larger coachwhip, Coluber flagellum, have a close taxonomic relationship and similar foraging strategies. Therefore, we hypothesized that C. constrictor would be smaller where they co-occur with C. flagellum, as compared to where C. flagellum is absent, throughout the south- eastern extent of their range. To evaluate this hypothesis, we obtained data on body size for 2321 adult C. constrictor and 526 adult C. flagellum, along with habitat data and other potentially important factors influencing body size. Coluber constrictor was smaller than elsewhere when in peninsular Florida, in pine forests, on hydric soils and in the presence of the larger and potentially competing C. flagellum. Body size of C. flagellum did not vary by any measured habitat variables. The trends we documented are consistent with the hypothesis that C. constrictor body size is influenced by several variables, including co-occurrence with C. flagellum. Introduction Given a limited resource, interspecific competition may occur between closely related species. Morphological divergence is one potential outcome of this competition (e.g. Grant & Grant, 2006), a process referred to as character displacement (Brown & Wilson, 1956). By influencing resource use, charac- ter displacement may facilitate coexistence over evolutionary time (Bowers & Brown, 1982). However, it is difficult to dif- ferentiate the past effects of competition from ongoing processes. For example, studies of resource partitioning or morphological variation may demonstrate that sympatric species use different resources or exhibit differences in mor- phology. However, it is often unknown whether these differ- ences result from past evolutionary divergence to reduce competition, random processes or from plastic adaptations resulting from current interactions (e.g. Connell, 1980). Studies that quantify both the diet and morphology of sym- patric species may provide powerful evidence for character displacement (e.g. Huey, 1974). However, it is logistically challenging to measure the availability and consumption of prey on a large scale. As a result, morphological variation may be used as a proxy for resource use. If the morphological traits related to catching and consuming prey differ in relation to the presence or abundance of potential competitors, this is con- sistent with the hypothesis that interspecific interactions influ- ence resource use (e.g. Simberloff et al., 2000; Davies et al., 2007). Body sizes may differ among members of species-rich snake assemblages (Guyer & Donnelly, 1990). Although body size may not always be an appropriate means to infer differences in resource use among wildlife species (Wilson, 1975), snakes swallow prey whole; thus, body size has an important influ- ence on the maximum size of potential prey (King, 2002). In Journal of Zoology Journal of Zoology. Print ISSN 0952-8369 86 Journal of Zoology 289 (2013) 86–93 © 2012 The Authors. Journal of Zoology © 2012 The Zoological Society of London