Data from: Male medaka continue to mate with females despite sperm depletion

In animals where males engage in multiple matings, sperm depletion can
substantially reduce the reproductive success of both sexes. However,
little is known about how successive matings affect sperm depletion,
fertilisation rates, and mating behaviour. Here, we investigated this
phenomenon under laboratory conditions. Medaka (Oryzias latipes), an
externally fertilising fish, is an ideal model to test predictions of
sperm depletion because there are established methods to observe its
mating and count sperm. Medaka males mated with multiple females (19 per
day, on average; range, 4–27), experiencing significant sperm depletion,
with sperm release declining markedly after the first few matings,
reaching only 0.5–6.3% by the last mating of the day. Fertilisation rates
decreased, particularly after approximately 10 consecutive matings,
although there was some recovery in the next-day’s matings. The decline in
courtship effort and mating duration probably resulted from the males
becoming increasingly fatigued. Despite the reduced sperm availability,
females did not adjust their clutch size as a counterstrategy. These
results suggest substantial reproductive costs for males and the potential
for sexual conflict owing to limited sperm availability. For species with
frequent successive mating, these findings highlight the need to
reconsider reproductive strategies and their impact on sexual selection.