Bateman Gradients and Sexual Selection in the Rattlesnake Crotalus atrox and other North American Pi

Levine, Brenna A.* Department of Biological Sciences The University of Tulsa Tulsa, Oklahoma, USA

Schuett, Gordon W.* Department of Biology and Neuroscience Institute Georgia State University Atlanta, Georgia, USA

Clark, Rulon W.* Department of Biology San Diego State University San Diego, California, USA

Repp, Roger A.*

Herrmann, Hans-Werner* School of Natural Resources and the Environment University of Arizona Tucson, Arizona, USA

*Chiricahua Desert Museum, Rodeo, NM, USA

Sexual selection theory predicts that the sex in greater abundance and with “cheaper” gametes will experience increased reproductive success as a result of an increased number of matings, whereas the sex with more finite numbers of gametes and in lesser abundance will experience no such gain. Empirical data across taxa have supported the prevalence of conventional Darwinian sex roles, with significant relationships between mating success and reproductive success common in males but not females, as gauged by Bateman gradients (i.e., the slope of the least-squares regression line of relative reproductive success on to relative mating success). Yet, few of these studies have explored sexual selection in pitvipers mostly due to their cryptic behavior and complicated internal reproductive processes (e.g., multiple paternity, long-term sperm storage, facultative parthenogenesis) that make it difficult to accurately identify parentage from field observations alone. To help fill this gap, we estimated Bateman gradients and other sexual selection metrics [i.e., opportunities for selection (I) and sexual selection (Is)] in a population of Western Diamond-backed Rattlesnake (Crotalus atrox) in Southern Arizona using molecular parentage assignments derived for a previous study. In keeping with patterns of conventional sex roles prevalent across animal taxa, we found males to exhibit significantly higher I and IS than females (P<0.05). Yet, contrary to our expectations, both males and females exhibited significant Bateman gradients, and there was no significant difference between them (P=0.089). In the population we studied for > 10 years, sexual selection is thus not acting more strongly on male C. atrox than female C. atrox. This differs from a previous study of Bateman gradients in another North American pitviper, the Copperhead (Agkistrodon contortix), and we posit that mating system characteristics contribute to this difference.

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