Sex differences in plasma corticosterone in desert tortoises, Gopherus agassizii, during the reproductive cycle

Blood samples from 30 female and 20 male adult desert tortoises, Gopherus a gassizii, were collected at monthly intervals during the annual reproductiv e cycle (April to October). Plasma corticosterone and the sex steroids in e ach of the samples were analyzed by radioimmunoassay. Mean corticosterone l evels in males were significantly higher than in females (P < 0.001) in eve ry month. Male tortoises showed a marked seasonal pattern in plasma cortico sterone with a highly significant peak in July, August, September, and Octo ber that corresponded with a similar peak in plasma testosterone. Testoster one and corticosterone in the male showed a highly significant correlation (P < 0.0001). The pattern of corticosterone in the female was less marked, with a significant peak in May during the mating and nesting season, but no association with the peak in estradiol in late summer was apparent. The hi ghest levels of corticosterone in the males were associated with the peak i n spermatogenesis and intense male-male combat. These results support simil ar data from other reptiles that suggest increased glucocorticoid secretion during periods of increased activity and metabolism. J. Exp. Zool. 289:285 -289, 2001. (C) 2001 Wiley-Liss, Inc.