| Summary: | Weddell seals ( Leptonychotes weddellii ) experience a synchronized annual reproductive cycle. Females give birth in the austral spring (October‐November) and are bred towards the end of lactation in November and December; following a brief period of embryonic diapause, active gestation begins. To examine endocrine dynamics during the breeding period and early gestation, we handled adult, post‐parturient Weddell seals with known pupping dates ~1 week prior to weaning (29–44 days post‐partum, n=32) in mid‐November to mid‐December (Nov/Dec) and approximately 60 days later, mid‐January to mid‐February (Jan/Feb). A second group of previously parous females that did not pup in the current season (non‐parturient) were also handled at equivalent dates (n=21). Seals were weighed, body composition (lipid as %body mass) was determined using isotopic dilution, and serum progesterone concentrations were measured using commercial radioimmunoassays validated for use in Weddell seals. Total estrogens were also measured by validated radioimmunoassay in a representative subset of animals. In Jan/Feb, transrectal ultrasonography was used to determine pregnancy status. Serum progesterone concentrations in Nov/Dec study animals fell into two distinct groups: a low‐concentration group, with values ranging from 0.80 to 2.50 ng•mL −1 (mean±SEM = 1.65±0.08 ng•mL −1 , n=26), and a high‐concentration group, with values between 20.60 and 146.40 ng•mL −1 (59.76±6.40 ng•mL −1 , n=27). Total estrogens were elevated in low progesterone seals (163.59±12.98 pg•mL −1 , n=8) relative to those with high progesterone (95.71 ±12.09 pg•mL −1 , n= 4) (t‐test, p value= 0.004). As elevated progesterone concentrations indicate that ovulation has occurred, we examined the likelihood that animals’ progesterone levels were in the high group using binomial logistic regressions to determine what factors may influence ovulation timing in Weddell seals. Females that did not give birth were more likely to exhibit high progesterone at later calendar dates, with ...
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