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A Comprehensive Analysis of Sex Hormones and Their Role in Reproductive Suppression in Female Northern Elephant Seals (Mirounga angustirostris)
Purpose of the Study: Long-lived iteroparous females may suppress reproduction during unfavorable conditions to maximize lifetime reproductive success. Capital breeders must accrue sufficient body reserves to support reproductive effort or they may exhibit reproductive failure. The basis of these reproductive decisions is not well understood. Northern elephant seals are seasonal, colonial breeders with a period of embryonic diapause and biannual foraging trips divided by a breeding and molting haul out. Being capital breeders, they use the post-breeding foraging trip to recover body mass lost during lactation and the gestational foraging trip to accumulate resources for lactation. Females exhibit high natality, roughly 15% of females skip breeding each year. A previous study suggested alteration in baseline cortisol concentrations may be the proximate mechanism linking foraging success and reproductive suppression, but evidence of an interaction between cortisol and reproductive hormones is currently lacking. Procedure: We investigated a suite of reproductive hormones (progesterone, 17β- estradiol, inhibin A, prolactin) and cortisol, to determine patterns across life-history stages. We analyzed 1,029 serum samples from 441 adult female NES during their biannual haul outs. Included in this dataset were 30 females known to have skipped breeding. Findings: The data confirmed a highly seasonal pattern to changes in sex hormones. Estradiol and inhibin were high during the late breeding haul out, suggesting the timing of ovulation. Progesterone was highest at the end of the molt, indicating implantation had occurred. Path analysis revealed strong influences of estradiol and prolactin on progesterone, indicating these three hormones are key for implantation. Individual variation suggests important regulatory roles for prolactin, inhibin, and cortisol in modulating implantation. The data suggest that some females suppress implantation under the modulation of cortisol and inhibin. Most females that skip breeding appear to implant but fail to maintain gestation. Conclusions: Failure to recover body reserves from the preceding reproductive event may inhibit implantation, but most non-breeding females fail to maintain pregnancy during the gestational foraging trip. This bet-hedging strategy is likely one component of a capital breeding life-history that buffers females against short-term changes in foraging success, but limits fitness costs to reproduction through multiple check-points for body reserves.