Extraordinary sperm to egg ratios in seabirds

ABSTRACT Following copulation, females of many seabird species spend a prolonged period of time away from the colony, building up reserves for egg formation and incubation. Here, we report that the number of sperm associated with eggs of single-egg clutch seabirds was almost an order of magnitude greater than predicted from the relationship between ovum size and sperm numbers in multi-egg clutch non-seabirds. Sperm numbers were also several times greater than the estimated number necessary for maximal fertilization success. Our results are consistent with 3 unusual features of seabird reproduction: (1) single-egg clutches, (2) prolonged sperm storage, and (3) a lag period between the end of yolk formation and ovulation. We hypothesize that sperm release from storage is under precise temporal control in these species, with high sperm numbers acting as an insurance against infertility in single-egg clutches. If true, the lag period may have evolved to provide sufficient time for sperm to be released simultaneously from storage and accumulate at the site of fertilization prior to ovulation. LAY SUMMARY After copulation, female birds store sperm in their reproductive systems for some time before egg production and fertilization. It is thought that sperm are gradually lost during storage, meaning that later in the storage period, if there are no further inseminations, fewer sperm should be available for insemination. We studied the number of sperm reaching eggs in 6 seabird species, in which females spend a long time feeding at sea between mating and producing eggs. Surprisingly, we found that the number of sperm that reached eggs in these species was much higher than expected, based on a known relationship between egg size and sperm number. We suggest that, in these species, sperm are released simultaneously from storage just before ovulation. This may be particularly important in the species we studied, because they all lay only a single egg. Our hypothesis implies that female birds have greater control over sperm storage and use than previously thought.