ABSTRACT
Differences between males and females are generally divided into two categories: primary sexual characteristics, which include the gonads, efferent ducts and copulatory organs; and secondary sexual characteristics, which include all other physical and behavioral differences. Secondary sexual characteristics are generally thought to be brought about by sexual selection. A common type of sexual selection in snakes is precopulatory intrasexual selection, in which the trait or behavior provides an advantage to the male in competition with other males (i.e., body size, Shine 1978). Within snakes, secondary sexual characteristics include: differences in adult size (Shine 1994), color (Northern Viper, Vipera berus, Shine and Madsen 1994; Rock Rattlesnake, Crotalus lepidus klauberi, Jacob and Altenbach 1977), body proportions (V. berus, Madsen 1988), special ornamentations (nasal extension, Langaha, Greene 1997; spurs of boids, Carpenter et al. 1978) and behaviors (male-male combat, Shine 1994; mate searching behaviors, Duvall and Schuett 1997; Shine et al. 2005). The majority of these differences are developed to assist males in prenuptial mating endeavors. However, not all sexual dimorphism is a result of sexual selection. Shine and Madsen (1994) suggest that the brightly colored male V. berus may be a result of natural selection acting selectively on males during the mating season. They suggest that the bright color of males causes a “flicker-fusion” effect in which moving males, while searching for females, become difficult for predators to focus on and to assess the direction and velocity of the viper’s movement. Lindell and Forsman (1996) found support for this hypothesis in a field study of V. berus on islands east of Sweden. They reported that
the survival rate of brightly colored males was significantly higher than melanistic males.
