Steroid levels and reproductive cycle of the Galapagos tortoise, Geochelone nigra, living under seminatural conditions on Santa Cruz Island (Galapagos)

The Galapagos Islands are home to 11 subspecies of large terrestrial tortoi ses (Geochelone nigra). All Galapagos tortoises are considered endangered a nd approximately 12,000 animals still exist. Until now, the reproductive cy cle of the Galapagos tortoise has been studied only in captive animals, and no data from free-ranging tortoises have been available. During a one-year period, blood samples were collected from male and female G. nigra living under seminatural conditions on Santa Cruz Island, Galapagos. Plasma steroi d hormones were measured by radioimmunoassays (RIAs). Tn males, plasma test osterone and corticosterone increased a few months before the onset of the mating season. Peak levels were observed while most copulations occurred an d environmental temperatures were highest. Both testosterone and corticoste rone showed low levels during the cold and dry nesting season and high leve ls during the hot and rainy mating season. In females, testosterone and cor ticosterone also rose during the hot and rainy mating season. Both hormones peaked during the second half of the mating season and decreased during th e cooler dry season, Female estradiol levels increased at the onset of the mating season, reaching the highest level at the peak of the mating season, which coincided with the highest annual temperatures measured. Estradiol s lowly decreased within the next months and rapidly dropped at the onset of the nesting season when temperatures decreased. Progesterone levels were hi gh close to the time of ovulation and showed clearly elevated levels at the beginning of the nesting season after some females had laid their first cl utch. Progesterone decreased during the nesting season, when ambient temper atures began to decrease, and reached minimal levels in the postbreeding pe riod shortly before the onset of the next mating season. There were signifi cant annual variations in plasma testosterone in both males and females. Pl asma corticosterone was generally higher in males than in females and varie d throughout the year in both sexes. (C) 1999 Academic Press.