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Sexual selection in common toads: correlates with age and body size

Anurans have been the subject of many studies of sexual selection (Wells, 1977; Arak, 1983; Halliday, 1983). The most commonly studied morphological correlate of male mating success has been overall body length (snout-vent length) (Shine, 1979; Woolbright, 1983; Halliday and Verrell, 1986) probably because body length is easy to measure and reflects overall size well. However, because of correlations between characters, an observed correlation between body length and mating success may reflect selection on a covarying character, for example, arm length (Lee, 1986; Howard, 1988). Thus there is a need for studies of more characters than size and how these correspond with male fitness. The age of individual males may also have important implications in studies of sexual selection. For example, Trivers (1972) proposed that age might be correlated with genetic quality, since old individuals have proved their ability to survive. If it is to be used in mate choice, however, it must be possible to assess the age of individual males from other traits. Body size has been proposed as an appropriate trait for assessing age, since toads continue to grow throughout their lives (Barker Jorgensen, 1983). Here, we examine the correlation between age and size in the common toad (Bufo hufo) in order to test the feasibility of this hypothesis. The breeding period of this species is two to three weeks, the sex ratio is male biased (approximately 2 : 1) and male-male competition is the major determinant of male mating success (e. g. Davies and Halliday, 1979; Gittins et al., 1980; Loman and Madsen, 1986). In years when the breeding period is short, mating is random with respect to size (Hoglund and Robertson, 1987; Hoglund, in press). In this study we consider body size and other characters in relation to male mating success. The traits studied are forearm length and thumb-pad width, two characters that may affect a male’s ability to hold on to a female during amplexus, independently of body length (Howard and Kluge, 1985; Lee, 1986).