The Galápagos Islands are home to 11 subspecies of large terrestrial tortoises (Geochelone nigra). All Galápagos tortoises are considered endangered and approximately 12,000 animals still exist. Until now, the reproductive cycle of the Galápagos 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, Galápagos. Plasma steroid hormones were measured by radioimmunoassays (RIAs). In males, plasma testosterone and corticosterone increased a few months before the onset of the mating season. Peak levels were observed while most copulations occurred and environmental temperatures were highest. Both testosterone and corticosterone showed low levels during the cold and dry nesting season and high levels during the hot and rainy mating season. In females, testosterone and corticosterone also rose during the hot and rainy mating season. Both hormones peaked during the second half of the mating season and decreased during the 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 slowly decreased within the next months and rapidly dropped at the onset of the nesting season when temperatures decreased. Progesterone levels were high 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 clutch. Progesterone decreased during the nesting season, when ambient temperatures began to decrease, and reached minimal levels in the postbreeding period shortly before the onset of the next mating season. There were significant annual variations in plasma testosterone in both males and females. Plasma corticosterone was generally higher in males than in females and varied throughout the year in both sexes.
Copyright 1999 Academic Press.