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

Several recent studies have demonstrated the occurrence of multiple paternity inanuran amphibians, implying that it is more common than previously thought.However, an adaptive explanation for polyandry in anurans is still lacking. Thecommon toad Bufo bufo is an explosively breeding species that releases its eggs instrings. The operational sex ratio (OSR) is male biased, causing strong scramblecompetition among males; females can even drown through harassment duringmultiple amplexi. We used microsatellite markers to determine patterns ofpaternity in natural B. bufo populations and experimentally mated individuals(females exposed to either two or six males). Thirty per cent of field-collected and22% of experimentally produced egg strings were sired by at least two males; allothers were sired by a single father. Multiple paternities arose only from multipleamplexi, and we found no indication of fertilization from non-amplexing males,for example through free-swimming sperm. Our results suggest that polyandry inB. bufo is likely to occur most often at high population densities, and under themost male-biased OSRs. Moreover, polyandry might be interpreted as being theconsequence of females spawning when amplexed by a few males, to avoid the riskof drowning by amplexus with multiple males