Pairing and spawning patterns in the common toad, Bufo bufo: the effects of sex ratios and the time available for male-male competition

Abstract

The time available for male-male competition and the sex ratios of common toads were manipulated within enclosures in a natural pond. At an even sex ratio, pairs formed at random with respect to body length of pair members. Pairs were still randomly composed at spawning. By contrast, when sex ratios were experimentally male-biased (2:1), after 3 days of male-male competition large males were over-represented in spawning pairs. When the time available for competition was longer (10 days), pairs were size-assorted. The probability of taking over a female in amplexus and resisting displacement attempts when in amplexus was a function not only of male size but also of the relative sizes of the pair members. Small males in amplexus with large females were easily displaced, as were large males coupled with small females. Pairs where the male and the female were matched in size had a higher probability of staying in amplexus than those less closely matched. Males did not discriminate between females of different sizes. These findings are in line with a hypothesis that non-random spawning patterns in the common toad are generated primarily by male-male competition. Mate choice (male or female) does not have to be invoked to explain non-random spawning patterns in this species.