Zoo Biology 00: 1–8 (2012) RESEARCH ARTICLE Endocrine Changes in Harbor Seal (Phoca vitulina) Pups Undergoing Rehabilitation Stephen J. Trumble, 1 Danielle O’Neil, 2 Leslie A. Cornick, 3 Frances M. D. Gulland, 4 Michael A. Castellini, 2 and Shannon Atkinson 2* 1 Department of Biology, Baylor University, Waco, Texas 2 School of Fisheries and Ocean Sciences, University of Alaska Fairbanks, Fairbanks, Alaska 3 Department of Environmental Science, Alaska Pacific University, Anchorage, Alaska 4 The Marine Mammal Center, Marin Headlands, Sausalito, California Rehabilitating pinniped pups are often admitted to care centers as neonates and generally lack maternal investment and are in poor body condition. Upon admittance to a rehabilitation facility, pups are typically fed a milk replacement formula via gavage, which is switched to frozen fish upon weaning. While rehabilitation has been successful in terms of recovery and release, preweaning growth rates in captivity are consistently lower than in the wild. Indicators of stress (cortisol and total thyroxine; TT4), and standard morphometrics, of harbor seal pups in rehabilitation (n = 20) were determined for both preweaned and weaned pups. Hormone concentrations and standard morphometrics from pups in care were compared with free-ranging harbor seal pups (n = 59). Pups in rehabilitation gained mass on both milk and fish diets. Preweaned pups had greater mean serum cortisol and similar TT4 concentrations than weaned pups. Free- ranging harbor seal pups were heavier and longer than preweaned and weaned pups in rehabilitation. The free-ranging pups had the lowest cortisol and highest TT4 concentrations of any of the pups. These results suggest that weaned pups that have undergone rehabilitation are not physiologically equivalent to free-ranging weaned pups. Additional research is needed regarding physiological changes in endocrine values during early development under captive care conditions. This information should be useful to marine mammal rehabilitation centers in their development of care protocols and release criteria for rehabilitating harbor seal pups. Zoo Biol. 00:1–8, 2012. C  2012 Wiley Periodicals, Inc. Keywords: harbor seal; stress hormones; cortisol; thyroxine INTRODUCTION Harbor seals (Phoca vitulina) are annual breeders that give birth to pups that are typically weaned from 4 to 6 weeks postpartum, at which time they switch from a high-fat (40–50%) milk diet to a solid diet consisting of mostly fish (Atkinson, 1997; Muelbert and Bowen, 1993). Pups average 7–12 kg at birth and accumulate mass at a rate of 0.6–0.7 kg/d throughout nursing, which cor- responds to 5–9% daily increases in body mass (Bowen et al., 1992; Lang et al., 2005; Schulz and Bowen, 2004). This rapid increase in body mass results in deposition of subcutaneous blubber, the thickness of which has been positively correlated with survival rate (Muelbert et al., 2003). Following an abrupt weaning, phocid pups enter a fasting period, lose body mass, and remain in an ener- getic deficit for some weeks after they begin to feed on fish (Bowen, 1991). Characteristic of mammalian physiology, stress pathways in pinnipeds activate the hypothalamo– pituitary–adrenal axis (HPA) resulting in a significant release of the steroid hormone cortisol from the cortex of the adrenal glands (Mashburn and Atkinson, 2004). Therefore, cortisol concentrations recovered from serum, Grant sponsor: Exxon Valdez Oil Spill Trustee Council; Grant sponsor: NOAA/NMFS Office of Protected Resources John H. Prescott Marine Mammal Rescue Assistance Grant Program. ∗ Correspondence to: Shannon Atkinson, School of Fisheries and Ocean Sciences, Fisheries Division, Juneau Center, 17101 Pt: Lena Loop Road, Juneau, AK 99801. E-mail: shannon.atkinson@alaska.edu Received 15 December 2011; Revised 1 May 2012; Accepted 18 June 2012 DOI 10.1002/zoo.21036 Published online in Wiley Online Library (wileyonlinelibrary.com). C  2012 Wiley Periodicals, Inc.