The Biology of Rattlesnakes II Edited by: Michael J. Dreslik • William K. Hayes • Steven J. Beaupre • Stephen P. Mackessy Copyright © 2017 by ECO Herpetological Publishing and Distribution All rights reserved. No portion of this book may be reproduced, stored in a database or retrieval system, or distributed in any form or by any means—electronic, mechanical, photocopying, recording, or any other—except for brief quotations in printed reviews, without the prior written permission of the publisher. Published by ECO Herpetological Publishing and Distribution 4 Rattlesnake Canyon Rd. Rodeo, NM 88056 Printed in the United States of America Copies may be ordered from http://www.williamkhayes.com/rattlesnakes/volume.htm ISBN 978-1-938850-54-7 Dust jacket illustration An adult Tiger Rattlesnake (Crotalus tigris) set beautifully in its Sonoran desert habitat. Tiger Rattlesnakes occur from south-Central Arizona into southern Sonora, Mexico. The image titled, “Tiger Rattlesnake (in situ),” was painted by Tell Hicks and commissioned for the cover of Biology of the Rattlesnakes II. Limited edition prints of this painting are available at http://tellhicksprints.weebly.com/index.html. Manufactured in the United States of America Dreslik, M. J., D. B. Shepard, S. J. Baker, B. C. Jellen, and C. A. Phillips. 2017. Body Size, Growth, and Sexual Size Dimorphism in the Eastern Massasauga (Sistrurus catenatus). Pp. 66–78 in Dreslik, M. J., W. K. Hayes, S. J. Beaupre, and S. P. Mackessy (eds.), The Biology of Rattlesnakes II. ECO Herpetological Publishing and Distribution, Rodeo, New Mexico. Body Size, Growth, and Sexual Size Dimorphism in the Eastern Massasauga (Sistrurus catenatus) Michael J. Dreslik1,2, Donald B. Shepard1,3, Sarah J. Baker1, Benjamin C. Jellen1,4, and Christopher A. Phillips1 1 Illinois Natural History Survey, Prairie Research Institute, University of Illinois Urbana-Champaign, Champaign, Illinois 61820, USA ABSTRACT.—Body size varies with many life history and ecological traits. Assessing intraspecific variation in body size, particularly in species with broad distributions, can reveal how selective pressures vary geographically and how populations have adapted locally. We examined body size, growth, and sexual size dimorphism in a population of Eastern Massasauga (Sistrurus catenatus) at the species’ southern range limit. Females averaged 46.7 cm snout-vent length (SVL) whereas males averaged 43.8 cm. Males reached a larger maximum SVL (77.8 cm) compared to females (71.5 cm). Size structure (SVL) of both sexes was multimodal with females having a bi- or trimodal distribu- tion and males having a tri-, penta-, or hexamodal distribution. Females had a faster instantaneous growth rate than males and that pattern held for nonlinear growth curves. Females grew faster to a smaller adult body size compared to males. Growth analyses establish a pattern of age-specific sexual size dimorphism (SSD). Females were slightly longer at birth, grew faster, and reached a maximum size disparity as the larger sex by age 2. As female growth decreased at sexual maturity, SSD became absent by age 5 and males were the larger sex after age 6. Post-maturational differences in growth rates are likely due to higher reproductive costs in females; however, larger male size also provides an advantage in agonistic encounters. Finally, we found that males had slightly longer tail lengths (TLs) at birth and dimorphism in TL increased with SVL. INTRODUCTION (Blueweiss et al., 1978; Calder, 1984). Growth, a temporal component of body size, is critical in predicting life history Body size is a fundamental trait in life history studies traits such as the age of sexual maturity and can have (Stearns and Koella, 1986; Stearns, 1989, 1992) and often important population-level implications (Stearns, 1992). displays geographic (Ashton and Feldman, 2003), sexual Several studies on viperids have examined growth (Heyrend (Shine, 1978a,b, 1993, 1994), and ontogenetic variation and Call, 1951; Barbour, 1956; Fitch, 1960; Gibbons, 1972; (Andrews, 1982). In organisms with indeterminate growth, Klauber, 1972; Fitch, 1985; Martin, 1988; Macartney et al., like most ectotherms, maximum body size varies along 1990); however, few have taken advantage of nonlinear environmental and resource gradients, leading to size modeling approaches (e.g., Madsen and Shine, 2000; Blou- differences among populations (Andrews, 1982). Assessing in-Demers et al., 2002). Unlike other approaches, nonlinear variation in body size within species is important because models can yield biologically important parameters that size varies with many life history and ecological traits, allow quantitative comparisons (Andrews, 1982). and thus reflects adaptive variation across a species’ range Determining which sex is larger in a species provides insight 2 Correspondence e-mail: [email protected] into selective forces driving reproductive success (Shine, 3 Present address: School of Biological Sciences, Louisiana Tech 1978, 1993, 1994). Sexual size dimorphism (SSD) can occur University, Ruston, Louisiana 71272, USA at any life stage and may vary ontogenetically, being present 4 Present address: Urban Chestnut Brewing Company, St. Louis, at one life stage but absent at others (Shine, 1978b, 1993, Missouri 63110, USA 1994). SSD in neonate snakes is presumably rare because 66 Dreslik et al. 2017 dimorphic body plans have not yet developed as many MATERIALS AND METHODS dimorphic traits are linked to expression of sex hormones (Shine, 1978b; Fitch, 1981; King, 1989; Shine, 1993, 1994). Study area.—Carlyle Lake, an impoundment of the In adult snakes, SSD is common, but the direction and Kaskaskia River in south-central Illinois, is bordered by extent varies among taxa. Sexual bimaturism and fecundity 4,455 ha of state and federally managed lands, consisting selection explain cases where females are larger than males of upland and bottomland forest, old-field, and restored (Shine, 1978a,b; Bull, 1980; Fitch, 1981; Parker and Plummer, prairie within a larger agricultural matrix. For a more 1987). Alternatively, male-male competition for mates and detailed habitat description, see Dreslik (2005). aggressive interactions incited by dense mating aggrega- tions or a male-skewed operational sex ratio may explain General methods.—We captured live snakes through larger male body sizes (Shine, 1978b; Fitch, 1981; King, visual encounter surveys (Heyer et al., 1994) during the 1989; Shine, 1993, 1994). In species with fecundity select - spring egress from 1999 to 2010, and also included snakes ion in females and mate competition in males, the pattern encountered opportunistically throughout the active of SSD will be the net result of selection on both of these season and captive born snakes. To measure snout-vent important determinants of individual reproductive success. length (SVL), we restrained the snake’s head in a clear PVC tube, took repeated measurements from the tip of the snout Over the last few decades, research on snake ecology has to the cloaca with a flexible tape until three measurements increased to levels rivaling that of many endotherms (Shine were within 1 cm, and then averaged these measurements. and Bonnet, 2000) with several species (e.g., Crotalus viridis, We measured tail length (TL) from the cloaca to the base Vipera berus, Python molurus, and Thamnophis sirtalis) of the rattle with a plastic ruler to the nearest 1 mm and emerging as models. The Eastern Massasauga Sistrurus( to determine total length (TOL) we summed SVL and TL. catenatus) is a relatively small-bodied pitviper that occu- We determined the sex of individuals by cloacal probing pies a diversity of habitats across a broad geographic range (Schaefer, 1934). We classified snakes as adults if their SVL (Ernst, 1992; Ernst and Ernst, 2003). This ecological niche exceeded the size of the smallest individual observed exhib- variation provides an opportunity to compare ecological iting mating behavior or gravidity, which was 49.7 cm SVL and life history patterns to increase our understanding of for males (Jellen et al., 2007) and 48.4 cm SVL for females plasticity and adaptability in wide-ranging ectotherms. (Dreslik, unpubl. data). We uniquely marked snakes by Most research on S. catenatus has focused on spatial ecology clipping ventral scales (Brown and Parker, 1976), painting and a range-wide view of this aspect of their biology is rattle segments with nail polish (Brown et al., 1984), and available (Reinert and Kodrich, 1982; Weatherhead and injecting a PIT tag subcutaneously. For individuals too Prior, 1992; Johnson, 2000; Parent and Weatherhead, 2000; small to be PIT-tagged (<35 cm SVL), we photographed King et al., 2004; Harvey and Weatherhead, 2006a; Marshall dorsal pattern to ensure proper identification. We released et al., 2006; Dreslik et al., in press). Reproductive biology all snakes at their initial point of capture and released indi- has also received attention (Keenlyne, 1978; Reinert and viduals born in captivity at their mother’s gestation site. Kodrich, 1982; Jellen et al., 2007; Aldridge et al., 2008), but few studies have focused on other aspects of life history Size structure.—We constructed size frequency histo- such as body size (Seigel, 1986; Ernst and Ernst, 2011). This grams for snakes captured at our most intensively moni- lack of data limits our ability to determine how ecology and tored site, South Shore State Park (SSSP), by grouping life history influence large-scale evolutionary trends. To fill snakes into 5-cm size classes by sex. We then decomposed this gap, more population-level studies on body size and the size frequency distributions for each sex into one to other life history traits are needed. six unimodal normal distributions following the methods outlined in Ebert (1999). We selected breakpoints in the size Here, we examine body size of a population of S. catenatus distributions for each component, then calculated means, at its southern range limit. First, we determine the size standard deviations, and the proportion of individuals for structure and number of size classes. Second, we determine each component (Ebert, 1999). To represent multimodal individual growth patterns for S.