Seasonal changes in plasma sex steroid levels in the male tuatara, Sphenodon punctatus, from Stephens Island, New Zealand

Abstract

A unique opportunity, occasioned by the sacrifice of five male tuataras ( Sphenodon punctatus), was taken to measure the circulating levels of a number of sex steroids, and other parameters, at different seasons throughout the year on Stephens Island, New Zealand. Male tuataras exhibit continuous spermatocytogenesis throughout the year (H. Saint Girons and D. G. Newman, 1987, N.Z. J. Zool. 14, 231–237) but levels of sex steroids showed significant seasonal variation nonetheless, with testosterone concentrations being positively correlated with the height of the epithelium lining the epididymal ducts. Plasma testosterone and 5α-dihydrotestosterone levels were highest in February, coincident with the mating season when spermiogenesis is maximal and epididymal ducts are hypertrophied, but DHT concentrations were generally lower and more constant throughout the year. Circulating levels of progesterone, 17β-oestradiol, and corticosterone showed no significant changes over the period of sampling which extended from February until August. The tuatara is the only surviving member of the ancient Sub-Order Rhynchocephalia, and it seems likely that testosterone functions as a male sex hormone in this species, as it does in other members of the Class Reptilia.