Sexual Size Dimorphism and Growth Plasticity in Snakes: an Experiment on the Western Diamondâ•’Backed Rattlesnake (Crotalus atrox)

We conducted an experiment to examine the effects of sex and food intake on growth, mass gain, and attainment of sexual maturity in Western Diamond‐backed Rattlesnakes (Crotalus atrox). We also measured testosterone levels to determine whether testosterone might be involved in the male‐biased sexual size dimorphism observed in this species. We collected neonate rattlesnakes and raised them in the laboratory for 2 years on either a high‐intake diet (fed one mouse per week) or a low‐intake diet (fed one mouse every 3 weeks). High‐intake snakes grew and gained mass more rapidly than low‐intake snakes, but males did not grow or gain mass more rapidly than females in either treatment group. High‐intake snakes attained reproductive maturity earlier than low‐intake snakes, indicating that size, not age, is the critical determinant of reproductive maturity. Males had higher levels of testosterone than females but did not grow more quickly, suggesting that testosterone may not affect growth in this species and may therefore not be the proximate determinant of sexual size dimorphism. Darwin (1871) recognized that sexual size dimorphism (SSD) was prevalent among animal species and varied in both direction and magnitude. Mammals and birds tend most often to show male‐biased SSD where males are larger than females (Andersson, ’94). Reptiles are variable but typically show female‐biased SSD where females are larger than males (Fitch, ’81). Numerous ultimate explanations of SSD have been proposed,but Darwin’s original hypotheses are still the most widely discussed: in species where females are larger, size may provide a fecundity advantage, whereas in species where males are larger, size may lend an intrasexual competition advantage (Darwin, 1871). A recent idea emerging from movements to integrate the fields of physiology, ecology, and evolutionary biology is that SSD and other life history characteristics cannot be fully understood without elucidation of the physiological mechanisms responsible for them (e.g., the “physiology/life‐history nexus”, Ricklefs andWikelski, 2002). Specifically, to understand howselection operates upon a trait such as body size, we must understand how body size is determined on a proximate level in an organism’s life (Duvalland Beaupre, ’98). In ectotherms, body size is strongly dependent on resource availability. Numerous studies have experimentally confirmed that snakes exhibit resource‐dependent growth; that is, snakes that consume more food grow more quickly and/or gain mass more quickly than snakes that consume less food (Forsman and Lindell, ’96; Scudder‐Davis and Burghardt, ’96; Bonnet et al., 2001). Repro‐ duction may be similarly regulated by resource availability: snakes that consume more food may reproduce earlier or more often than snakes that face lower resource availability (Ford and Seigel, ’94; Lourdais et al., 2002). The plasticity of growth in snakes may be an adaptation to variable environments, in which the ability to speed or slow growth in response to fluctuating resource availability may be favored by natural selection (Partridge and Harvey, ’88; Forsman and Lindell, ’96). However, the sexes may face different selection pressures that favor different growth patterns. For example, in most species of snakes, females are larger than males, presumably because large body