The measurement of circulating reproductive hormone levels provides a relatively noninvasive method for assessing reproductive status. We assessed seasonal changes in concentrations of progesterone and total estrogens from serum samples collected from live stranded (n = 199) and captive (n = 5) California sea lions (Zalophus californianus). Progesterone concentrations increased in the fall (September–November) in both pregnant and nonpregnant animals, with no significant difference associated with pregnancy. Progesterone concentrations were greater in pregnant animals in the spring (February–April) than in nonpregnant animals. Circulating estrogen concentrations in captive, nonpregnant sea lions increased in July and November, correlating with periods of estrus and implantation. These data provide a baseline for studying reproduction in California sea lions.
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