Serum levels of reproductive steroid hormones in captive sand tiger sharks, Carcharias taurus (Rafinesque), and comments on their relation to sexual conflicts
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Levels of reproductively-related steroids were determined in captive male sand tiger sharks, Carcharias taurus, maintained at two institutions: SeaWorld Adventure Park Orlando and the National Aquarium in Baltimore. Sexual conflicts were absent at the former, but were documented at the latter. Serum titers of 17β-estradiol, progesterone, testosterone, and 5α-dihydrotestosterone were determined via radioimmunoassay in adult male sharks from 1988 to 2000. Sampling overlap between the two institutions occurred for 3 months of the year, but steroid concentrations were compared only for April due to the occurrence of sexual conflicts in the sharks at the National Aquarium in Baltimore in that month. For April, testosterone and dihydrotestosterone were significantly higher in the SeaWorld males, and progesterone was significantly higher in the National Aquarium in Baltimore males, while estradiol was not significantly different. Steroid levels were also determined from serial samples taken monthly over 17 months from three male sharks and one female shark at the National Aquarium in Baltimore in 2001–2002 and were compared with corresponding observed sexual conflicts. The steroid levels obtained showed distinct annual hormonal cycles in the male sharks and corroborated a biennial cycle for the single serially-sampled female shark. Furthermore, the steroid levels for individual males correlated with sexual conflicts as well as their position within the male dominance hierarchy. As this species is depleted in some regions globally, insight into the steroid profile of mature sand tiger sharks is important for a greater understanding of the relationship between their reproductive physiology and behavior, and may aid in captive management and reproduction.
KeywordsDihydrotestosterone Elasmobranch Hormone Progesterone Reproduction Sexual conflict Testosterone
This work was supported by a NAIB Biological Programs Research Grant to ADH. We thank the Fishes and Animal Health departments at the NAIB for invaluable assistance in obtaining samples from the sharks. In addition, we thank SeaWorld Adventure Parks for the use of data from their sharks for comparison. Valuable suggestions provided by two anonymous reviewers and V. Lounsbury are sincerely appreciated. Finally, the paper is dedicated to the memory of Bets Rasmussen, a wonderful scientist whose dedication and insight led to significant advances in our understanding of the reproductive biology of both elasmobranchs and elephants.
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