Simultaneous mating with multiple males reduces fertilization success in the myobatrachid frog Crinia georgiana

Approximately 50% of matings in the frog Crinia georgiana involve two or mo re males. We report reduced fertilization success as a major cost of mating with multiple males. For single-male matings, fertilization success was co nsistently high averaging 96%. Only 68% of eggs were fertilized when female s were amplexed by two males and this dropped to 64% when females were ampl exed by three to five males, Multiple regression analysis revealed thr redu ction in fertilization success was significantly related to the number of a mplectant males but not to clutch size or three measures of water quality ( depth, temperature and oxygen concentration) at the site of oviposition. Th e most likely cause of reduced fertilization success is struggles amongst m ales which interfere with effective sperm transfer.