When do Young Collared Lizards (Crotaphytus collaris) First Copulate with and Fertilize Females?

Abstract: In many species, males are larger and more colorful than females. This has often evolved by sexual selection, and the sexual dimorphism and dichromatism is observed once individuals reach adulthood. Such sexual differences are presented by Collared Lizards, Crotaphytus collaris. However, sexual dichromatism of a different sort than that expressed in adults is seen in juveniles before their first brumation. Late-season juvenile males develop bright orange dorsolateral bars and begin to defend territories and form pair bonds with females. Furthermore, the largest, late-season, putative juvenile males have mature sperm on their hemipenes. Paternity analyses using polymerase chain reaction of eight microsatellite loci revealed that a significant number of putative juvenile males, which were not seen during intensive fieldwork at our study site the following year, sired offspring with females who were juveniles, yearlings, or adults in the fall. Either females were fertilized by these males in the fall (and those females either stored sperm or arrested development of embryos overwinter), or they were fertilized by surviving fall juvenile males (i.e., yearlings) early the next spring, who subsequently died before we began our spring-summer field surveys. The majority of our data indicates the second possibility is true. First, adults and yearlings of both sexes begin brumation in the fall, weeks or months before October when larger putative juvenile males display sexual dichromatism and have mature sperm on their hemipenes. Females of these age classes are not available for copulation in October but are in early spring. Second, we collected juvenile females from the field in October and held them individually without exposure to males in the laboratory through brumation and beyond through spring and summer. None of these females became gravid. Our results indicate that although putative juvenile males possess mature sperm in the late fall, they wait until the following spring to fertilize adults of all age classes. Many of these males subsequently die early in the season but attain (posthumous) fitness by mating early in the spring before yearling and adult males become fully territorial and sexually active.