LOW SYNCHRONY IN THE BREEDING CYCLE OF GALAPAGOS SEA LIONS REVEALED BY SEASONAL PROGESTERONE CONCENTRATIONS S. VILLEGAS-AMTMANN,* S. ATKINSON, AND D. P. COSTA Department of Ecology and Evolutionary Biology, Long Marine Laboratory, 100 Shaffer Road, University of California Santa Cruz, Santa Cruz, CA 95060, USA (SV, DPC) University of Alaska Fairbanks, School of Fisheries and Ocean Sciences and Alaska SeaLife Center, 301 Railway Avenue, Seward, AK 99664, USA (SA) Mammalian reproductive biology is modified consistently across pinnipeds to achieve a synchronous and seasonal reproductive pattern. This synchrony relies on an embryonic diapause and ensures optimal conditions for offspring survival. Behavioral observations on Galapagos sea lions (Zalophus wollebaeki) indicate little synchrony and variable breeding seasons, making this species unique among otariids. We studied the reproductive physiology of wild female Galapagos sea lions in 2 months in contrasting seasons, March 2005 (n 5 11) and August 2006 (n 5 12) by examining progesterone and estrogen concentrations in serum and plasma. We provide physiological evidence for remarkably low synchrony and minor seasonality in the breeding cycle of Galapagos sea lions. Specifically, we found females in intermediate or advanced pregnancy during both seasons, as determined by high progesterone concentrations coupled with physical examination. Possible causes for this low synchrony are absence of strong photoperiodic change throughout the year, thought to regulate embryonic diapause, or adaptation to an environment with variable productivity and prey availability, or both. Key words: breeding cycle, estrogen concentration, Galapagos sea lions, pregnancy hormones, progesterone concentration, reproductive physiology, Zalophus wollebaeki The reproductive biology of pinnipeds has several special features. Embryonic diapause in most species of seals and sea lions allows for a tight synchrony of births to ensure that pups are born at the optimal time of year for weather, prey abundance, and general availability of resources necessary for maximal survival of offspring (Atkinson 1997; Boyd 1991b). Lactation and mating strategies have evolved to ensure that adult females can make an optimal maternal investment in their offspring while maintaining the ability to reproduce annually. This may be 1 of the reasons why some populations such as the California sea lions (Zalophus californianus) of the west coast of North America have increased so rapidly after decimation (Costa et al. 2006; National Marine Fisheries Service 1997). Hormone concentrations in wild mammals are increasingly used to estimate demographic parameters such as pregnancy rate and age at 1st reproduction (Greig et al. 2007). Knowledge of the endocrinology of these parameters needs to be improved and applied to species whose populations are fluctuating or in decline (Atkinson 1997), such as the Australian sea lion (Neophoca cinerea—Shaughnessy et al. 2006), Galapagos sea lion (Zalophus wollebaeki—Trillmich and Limberger 1985), and Hawaiian monk seal (Monachus schauinslandi—Pietraszek and Atkinson 1994). Breeding biology of the first 2 species differs from typical otariid reproductive biology. Most otariid species reproduce annually and have narrowly defined breeding seasons lasting approximately 2 months (Atkinson 1997). In contrast, Australian sea lions exhibit an 18-month aseasonal breeding cycle (Gales et al. 1992; Higgins 1993; Ling and Walker 1978), and Galapagos sea lions exhibit a protracted breeding season lasting about 4 months and varying between 2 and 11 months among the different islands (Wolf and Trillmich 2007). Reproductive hormones of Australian sea lions have been previously studied (Gales et al. 1997) but nothing yet is published about reproductive hormones in Galapagos sea lions. The breeding biology of Galapagos sea lions is unique in that their breeding season is much longer than for sympatric Galapagos fur seals (Arctocephalus galapagoensis) or closely related California sea lions. Their reproductive period shifts slightly from year to year and from island to island. On average, female Galapagos sea lions could successfully give * Correspondent: stella@biology.ucsc.edu E 2009 American Society of Mammalogists www.mammalogy.org Journal of Mammalogy, 90(5):1232–1237, 2009 1232