Reproductive study of Giant tortoises on Aldabra

Evidence for seasonal shift in the reproduction of Aldabra giant tortoises (Aldabrachelys gigantea) in managed care in the Northern hemisphere compared to the natural habitat in the Southern hemisphere

Ex situ breeding constitutes an important tool for species conservation; however, many reptile species are not managed sustainably under human care due to poor fecundity in ex situ settings. In this study, we tested whether the translocation of a seasonally reproducing species to a different environment results in decoupling of extrinsic signals and intrinsic conditions. The endocrinological patterns of plasma steroid sex hormones, follicular development, and mating behaviour of two female and two male sexually mature Aldabra tortoises (Aldabrachelys gigantea) in a zoological institution in the Northern hemisphere was aligned with enclosure climate data (mean monthly daylight duration, temperature, and precipitation) and compared with respective hormone patterns of wild individuals and climate conditions in the native habitat on the Aldabra Atoll in the Southern hemisphere. Whereas occurrence of mating behaviour was not considered a limiting factor, lack of ovulation and subsequent follicular atresia was the main reason for the lack of reproductive output. While it was impossible to elucidate the triggering factors of ovulation and the multifactorial complexity of reproduction was not fully addressed, this study indicates suboptimal temperature conditions and relative temporal shifts of interacting external triggers (temperature and photoperiod) in the zoo setting.

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

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.

The size, structure and distribution of the giant tortoise population of Aldabra

The results of a two year field study of the endemic giant tortoise population ( Geochelone gigantea ) on the western Indian Ocean atoll of Aldabra are described. The work entailed an extensive sample census from which the size of the population was estimated to be 150000 individuals. A large scale marking programme was also carried out. 6882 tortoises (4.6% of the total population) have now been individually marked and measured. Tortoises are unevenly distributed over the atoll with local densities ranging from 0 to 217 per hectare. Various environmental factors are considered in relation to their distribution. A method of age estimation, based on growth ring counts, is described and the characteristics of populations in high and low density areas are compared. Analysis of preliminary recapture information shows that the majority of tortoises are relatively sedentary in their habits, although some are capable of long distance movements; and that individual growth rate is density dependent. The remains of 643 natural tortoise mortalities were examined. Life tables of survivorship and mortality were calculated for the tortoise population in the southeast of the atoll. Crude estimates of mortality and recruitment indicate that the tortoise population in that area is on the decline. Tortoise biomass estimates are far in excess of those for other natural tropical ecosystems with similar rainfall. The vegetation in the southeast is being greatly modified by tortoise activity and there is every indication that the tortoise population there exceeds ‘the stable carrying capacity’.