Body Size and Sex Ratio in a Population of Spotted Turtles (Clemmys guttata) in Southwestern Michigan

摘要

Body size within populations of reptiles can vary as a function of age, genetics, sex, and random variation in resource acquisition among individuals (Andrews 1982; Gibbons and Lovich 1990). Within freshwater turtle populations, body size distributions show a preponderance of large adults due to either low recruitment (Browne and Hecnar 2007) or sampling methods that favor the collection of adults relative to juveniles (Ream and Ream 1966; Reeves and Litzgus 2008). Among populations, differences in mean, minimum, and maximum body sizes may be influenced by variation in environmental factors such as diet, primary productivity, and climatic factors that affect environmental temperature and growing season length (Iverson 1992; Iverson and Smith 1993). Among populations of some species of freshwater turtles for example, mean population body size (carapace length or mass) and some measures of reproductive output (e.g., clutch size) are positively correlated with longitude, latitude, or both (Chelydra serpentina, Iverson 1992; Iverson et. al. 1997; Chrysemys picta, Iverson and Smith 1993). Relatively large body size at more northern latitudes (Bergmann's Rule) may be related to heat retention (Ashton and Feldman 2003; Mayr 1963) or to enhanced energy acquisition and storage for growth or reproduction in a seasonal environment (Iverson and Smith 1993; Iverson et al. 1993; Olalla-Tárraga et al. 2006; Rollinson and Brooks 2007).