Plasma Concentrations of Vitellogenin and Sex Steroids in Female Tuatara ( Sphenodon punctatus punctatus) from Northern New Zealand

Abstract

Vitellogenesis in tuatara ( Sphenodon punctatus) on Stephens Island, New Zealand, at the southern (coolest) end of the geographical range of this species involves a prolonged period of about 3 years of the average 4-year reproductive cycle. These studies used a semiquantitative electrophoretic assay for vitellogenin (Vg). In the present study an ELISA was used to measure plasma levels of Vg in 138 female northern tuatara ( S.p. punctatus) on 11 islands at the warmest end of the tuatara's range. Blood samples were collected in late summer-early autumn, just prior to the expected time of ovulation in those females that would nest the following spring. Plasma Vg levels ranged from nondetectable to 2.9 mg/ml but were not significantly correlated with plasma estradiol, testosterone, or progesterone levels. There was also no significant correlation among the three sex steroids. Plasma Vg and hormone levels were further examined as to whether females were ovulating. Incipient ovulation was inferred using endocrinological criteria derived from studies of tuatara on Stephens Island (i.e., elevated plasma testosterone). Plasma levels of Vg differed significantly between inferred ovulators and nonovulators when data for all islands were combined, although similar ranges were observed in the two groups of females (inferred ovulators: nondetectable to 2382 μg/ml, mean = 505 ± 75; inferred nonovulators: nondetectable to 2864 μg/ml, mean = 460 ± 63). Plasma levels of estradiol, but not progesterone, differed significantly between inferred ovulators and nonovulators when data for all islands were combined. Mean levels in inferred ovulators were: estradiol, 157 ± 16 pg/ml (vs. 34 ± 5 pg/ml in inferred nonovulators); progesterone, 1.1 ± 0.2 ng/ml (vs. 0.7 ± 0.1 ng/m in inferred nonovulators); and testosterone, 5.6 ± 0.5 ng/ml (vs. 0.2 ± 0 ng/ml in inferred nonovulators). Only 37% of the females sampled met the minimum hormonal criteria indicative of ovulation. This suggests that, as in S. punctatus on Stephens Island, female S.p. punctatus on northern islands do not ovulate each year.