Evolution of sexual dimorphism in the forelimb muscles of Andrew's toad (Bufo andrewsi) in response to putative sexual selection

Abstract

Sexual differences in morphological traits are widespread among animals. Theory predicts that dimorphism in secondary sexual characters evolves as a consequence of sexual selection. We investigated the intersexual difference in mass of forelimb muscles of the Andrew's toad, Bufo andrewsi, an elongated species inhabiting montane regions in western China. Our results showed that average body size of females was significantly larger than males. However, when the influence of body size was removed, the forelimb muscle mass of males significantly exceeded that of females, and total mass of forelimb muscles of amplectant males was significantly larger than that of non-amplectant males. These results suggest that the robustness of the forelimbs can allow amplectant males to retain a firm grip on the female in amplexus, which may aid resistance to inference by conspecific males. Our findings are consistent with the prediction that sexual selection favors large forelimb muscle mass in males.

Diet composition, body condition and sexual size dimorphism of the common African toad(Amietophrynus regularis)in urban and agricultural landscape

“…It is widespread among animals (Seglie et al ., 2010; Di Cerbo and Biancardi, 2012; Ivanovic and Kalevic, 2012; Labus et al ., 2013), varying widely even within closely-related groups (Radojicic et al ., 2002; Paoletti et al ., 2009; De Lisle and Rowe, 2015). Sexual dimorphism arises from different selective pressures, such as natural selection, sexual selection and fecundity selection (Vargas-Salinas, 2006; Kupfer, 2009; Liao et al ., 2012; Martin et al ., 2012; Colleoni et al, 2014; Altunisik, 2017). Natural selection theory posits sexual dimorphism as due to differential interactions of each sex with the environment (Lovich and Gibbons, 1992).…”

No sexual dimorphism in limb muscles of a frog not engaging in amplexus

“…In all other anurans with conventional inguinal or axillary amplexus (see below), males have heavier forelimb muscles than females (Kirby, 1983;Oka et al, 1984;Yekta & Blackburn, 1992;Sidor & Blackburn, 1998;Peters & Aulner, 2000;Clark & Peters, 2006;Navas & James, 2007;Liao et al, 2012a;Mi, 2012). Sexual selection can promote the evolution of morphology and behavior that may increase male mating success (Andersson, 1994).…”

“…They were sexed after dissection and their body size (snout to vent length, SVL) was measured to the nearest 0.1 mm with digital calipers. Four fore-limb muscles (pectoralis, deltoideus, triceps branchii, and flexor carpi radialis) were dissected, dried to constant mass, and weighed to the nearest milligram following Liao et al (2012a). We then used a similar procedure to assess size of the three major hind-limb muscles: triceps femoris, sartorius, plantaris longus (Lee & Corrales, 2002).…”

“…Most likely it is a combination of those two factors. Male-biased limb muscle dimorphism has also been found in hindlimbs which are not involved in the clasping movements (Lee & Corrales, 2002;Vargas, 2005;Mi, 2013; but see Liao et al, 2012b). This latter pattern may be driven by scramble competition in which amplectant males kick the competing males with their hindlimbs (Wells, 1979;Halliday, 1980).…”

“…Males have much more robust forelimbs than females, even though the females are larger in body size (Kirby, 1983;Oka et al, 1984;Yekta & Blackburn, 1992;Peters & Aulner, 2000;Lee, 2001;Clark & Peters, 2006;Navas & James, 2007;Liao et al, 2012a;Mi, 2012). The cause of this sexual divergence is generally thought to be associated with amplexus: the common mating strategy in anurans where the male typically mounts the female and adopts a "clasping" posture while awaiting and during oviposition (Duellman, 1992;Lee, 2001).…”