Abstract
Effective conservation and management of wild populations is predicated on our knowledge of the primary cause(s) of variation in reproductive parameters. Yet, the drivers of many global reproductive patterns remain unknown. For terrestrial vertebrates, latitude and clutch size are often positively correlated, with animals producing larger clutches at higher absolute latitudes. This latitudinal gradient in clutch size has generated decades of scientific investigation, and 3 hypotheses have been proposed as the underlying mechanism (Lack's, Skutch's, and Ashmole's hypotheses). Prior efforts to test these hypotheses (primarily in birds) have been hindered by the highly mobile behavior of most birds and, consequently, the underlying cause of the latitudinal clutch size gradient is unclear. Our study employs comparative methods within a phylogenetic framework to identify the causes of variation in >1,000 species of extant snakes, a novel study system with previously undescribed patterns of latitudinal clutch-size variation. We sought to test predictions generated from the 3 hypotheses above and to identify the mechanism underlying the latitudinal clutch-size gradient. Our results show nuanced, phylogenetically influenced relationships between latitude and clutch size in snakes, possibly moderated by life-history traits such as activity period and habitat. We also examined intraspecific variation in clutch size in a widespread, viviparous snake (Northern Pacific Rattlesnake, Crotalus viridis oreganus) to quantify clutch-size plasticity and identify the mechanism(s) that drive geographic variation in clutch sizes of North American snakes. This ongoing study will improve our understanding of life-history evolution in vertebrates and contribute novel findings to numerous disciplines.