Mate attraction in frogs and toads typically involves multiple males calling and females responding to these acoustic signals, often choosing which prospective male to approach based at least in part on differences among the calls of different males. Mating typically involves the male grasping the female in amplexus (a tight "embrace" in which the male mounts the female, wrapping his front legs around her). In most frogs and toads, including Cane Toads, fertilization is external, with males depositing sperm on eggs as they are laid. In many species, "satellite males" may also be present at breeding sites. A satellite male may remain silent, but rather than compete with other males to attract a female, he may instead intercept a female attracted by another male and attempt to force a mating by grasping her in amplexus and fertilizing her eggs.
Bruning et al. (2010) investigated the possibility that female Cane Toads may be able to affect the outcome of competition among males for the primary amplexus position by making it more or less difficult for particular aspiring mates to maintain amplexus. This would allow females to retain a greater degree of female mate choice. The authors suggest that female Cane Toads (and presumably some other species as well) have co-opted a common anti-predator strategy for this purpose. Frogs and toads often defend themselves against predators by inflating their body: the increased girth may deter predators by both increasing the apparent size of the animal and by rendering it too large to ingest (e.g., Williams et al. 2000). Bruning et al. carried out experiments in which male frogs were induced to clasp model females with adjustable balloons inserted inside them. The researchers then measured the force required to pull males off females inflated to varying degrees. They also carried out mating trials using live females that had had their ability to inflate eliminated by placing a catheter into the trachea, preventing the tracheal glottis from closing (which is necessary to keep the body inflated). The results reported by Bruning et al. indicate that inflated female toads are indeed more difficult for males to hold on to, and that the ability of a female in amplexus to inflate her body can facilitate takeovers by larger rival males. In females who were unable to inflate, a small male could maintain his amplectant position despite takeover attempts by larger rivals. Thus, a female toad’s ability to inflate her body can influence the body size of her eventual mate. Given that females may often benefit from mating with larger-than-average males (Davies and Halliday 1977), females might use their ability to inflate to make it easier for a rival male to dislodge a smaller male. This could explain why field studies typically find that larger males are able to dislodge smaller ones.
