Annual androgen rhythm in the spiny-tailed lizard, Uromastix hardwicki

Abstract

Monthly changes in plasma and testicular androgen concentrations of the spiny-tailed lizard, Uromastix hardwicki, were measured using a specific testosterone radioimmunoassay. A peak in plasma testosterone levels (22.4 ± 2.6 ng/ml) was associated with maximal testicular weights in late March, preceding the mating period in April. Simultaneous with testicular regression in May, there was a marked drop in plasma testosterone values. From June to January, plasma testosterone levels remained more or less uniform, and the mean monthly estimations made during this period ranged between 4.6 and 7.2 ng/ml. Plasma testosterone concentrations tended to increase again in February. Testicular testosterone concentrations, expressed per 100 mg of tissue, were low during the breeding period (March and April) and fell to minimal levels (3.1 ± 0.3 ng/100 mg) in the second week of May. In June, tissue testosterone levels began to rise, reaching a peak by late August or early September, just prior to the fall recrudescence of the testis. There was a marked fall in tissue testosterone levels during October and November and a more gradual decrease was recorded from January to May. Tissue testosterone content calculated per testis had two peaks, one coinciding with the recrudescent phase of the testis and the second larger peak coinciding with the breeding period when the testicular weights were maximal. Rise in relative as well as absolute testosterone concentrations, in late summer and fall, were not reflected in plasma testosterone levels. The androgen rhythm in this lizard is discussed in relation to changes in testicular weight and the spermatogenic cycle.