We used radiotelemetric data and behavioural observations to characterize seasonal (mating versus post-mating seasons) and sexual variation in movement patterns, as well as to examine some of the ecological factors contributing to the evolution of the mating system in a venomous predator from the Mojave Desert of North America, the speckled rattlesnake, Crotalus mitchellii. Mating occurs in spring from late April to early June, shortly after emergence from hibernation, when snakes are predictably aggregated around the dens. Males and females travelled further per unit time in the mating season compared to the post-mating season. Males also travelled longer distances per unit time than females in the mating and post-mating seasons, and males with larger home ranges during the mating season had more potential mating partners. The results obtained suggest that males actively locate females during the mating season, and that the drastic increase in distance travelled by males during the mating season may be caused by strong male–male competition for access to females, probably because of the limited availability of sexually receptive females. Furthermore, males fight for access to females, and males of larger size are more likely to acquire females. Therefore, sexual selection apparently acts on two different male phenotypic traits: investment in mate-searching activities and male body size. The present study demonstrates that combining quantitative spatial analyses and behavioural observations in an explicit temporal context can significantly advance our understanding of the ecology and evolution of organismal mating systems. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 103, 681–695.