Proximate Determinants of Sexual Size Dimorphism in the Western Diamond-Backed Rattlesnake (Crotalus atrox)

—Sexual size dimorphism (SSD), where males and females differ in body size, is prevalent among animal species. 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. In most rattlesnake species, adult males are heavier and longer than females. The predominant hypothesis for this SSD is sexual selection for large males, since male rattlesnakes fight for access to females. However, ultimate explanations for the occurrence of SSD cannot be fully understood without knowledge of the proximate, physiological mechanisms responsible for them. We conducted a series of experiments designed to understand the proximate determinants of SSD in the Western Diamond-backed Rattlesnake (Crotalus atrox). Testosterone does not appear to stimulate growth in males, as it does in many other vertebrates. Males and females raised on controlled diets in the laboratory grow at the same rates and never diverge in size, with both sexes growing to sizes much in excess of free-ranging snakes. Similarly, supplementation of the diets of free-ranging females leads to dramatic increases in growth, showing that they are capable of growing like males if they increase their energy intake. It therefore appears that SSD in C. atrox is plastic, since females are capable of growing to male-like sizes. Sexual size dimorphism is facultative, not fixed, and results largely from the high cost of reproduction of female snakes living in food-limited environments.