Female polyandry under male harassment: the case of the common toad (Bufo bufo)

Several recent studies have demonstrated the occurrence of multiple paternity in anuran amphibians, implying that it is more common than previously thought. However, an adaptive explanation for polyandry in anurans is still lacking. The common toad Bufo bufo is an explosively breeding species that releases its eggs in strings. The operational sex ratio (OSR) is male biased, causing strong scramble competition among males; females can even drown through harassment during multiple amplexi. We used microsatellite markers to determine patterns of paternity in natural B. bufo populations and experimentally mated individuals (females exposed to either two or six males). Thirty per cent of field-collected and 22% of experimentally produced egg strings were sired by at least two males; all others were sired by a single father. Multiple paternities arose only from multiple amplexi, and we found no indication of fertilization from non-amplexing males, for example through free-swimming sperm. Our results suggest that polyandry in B. bufo is likely to occur most often at high population densities, and under the most male-biased OSRs. Moreover, polyandry might be interpreted as being the consequence of females spawning when amplexed by a few males, to avoid the risk of drowning by amplexus with multiple males.

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