Hormones reflect the impacts of reproductive rest on subsequent pregnancy in a marine predator

Abstract After reaching sexual maturity, iteroparous female vertebrates may intermittently skip producing offspring, effectively taking a year of reproductive rest. This may confer benefits to lifetime fitness by enhancing future reproductive success. To identify links between reproductive rest and subsequent reproductive outcomes, the physiological status of female Weddell seals ( Leptonychotes weddellii ) that skipped pupping in the current year (skip females, n = 45) and seals that gave birth (postpartum females, n = 87) was compared. Serum concentrations of hormones (progesterone, oestrogen, luteinizing hormone, follicle‐stimulating hormone, prolactin, thyroxine [T 4 ] and cortisol) were measured during the pupping period, breeding period and early gestation. Transrectal ultrasonography was used to image ovaries and detect early pregnancies. Range‐wide censuses conducted the next year determined if females successfully gave birth. Skip females ovulated up to 4 weeks earlier than postpartum seals and gave birth earlier the following year. Postpartum seals that gave birth late in the pupping season ovulated at fewer days postpartum than seals that gave birth early and had the lowest rates of pregnancy and successful birth. Late‐birthing females were therefore most likely to skip reproduction the following year, likely shifting subsequent reproductive timing forward. Skip females had higher serum concentrations of T 4 , oestrogen and progesterone than postpartum females in both breeding and early gestation periods, plus lower serum cortisol concentrations during early gestation. Serum T 4 concentrations were positively correlated with higher likelihoods that seals were pregnant and would successfully give birth. Postpartum females had higher serum prolactin concentrations than skip females in the pupping and breeding periods. However, prolactin declined across the summer in all animals, likely reflecting a seasonal transition from a breeding to a non‐breeding physiological state. This study revealed that physiological differences between skip and postpartum female Weddell seals persisted long after pupping and into the next reproductive cycle. Findings on trade‐offs between current and future reproduction in a polar pinniped suggest that reproductive rest allows birth timing to advance the next season, helping improve reproductive fitness and maintaining a tight temporal window for birth in the Antarctic marine environment. Read the free Plain Language Summary for this article on the Journal blog.