The natural regulation of giant tortoise populations on Aldabra Atoll: recruitment

Abstract

The reproductive ecology of the giant tortoise ( Geochelone gigantea Schweigger) in three isolated populations was studied for 2 years on Aldabra Atoll. Density-dependent recruitment was demonstrated. Nest destruction in the low density area was dependent on the density of mature females providing a mechanism for regulating population size. Increases in annual rainfall and the resultant increase in food availability induced an increase in mean egg mass in the low density area (and thereby total hatchling production), whereas in the high density population mean clutch size, mean egg mass, total number of nests and total hatchling production all increased significantly. Large eggs produced large hatchlings which survived better during the first year than hatchlings from small eggs. Hatchling mortality was 94 and 81 % in the first year in the high and low density populations respectively. Recruitment into the 5 year age class (after which predation is considered negligible) had almost ceased in the high density population compared with 0.44 per 100 breeding females per year in the low density population.

Hatching success, delay of emergence and hatchling biometry of the spur‐thighed tortoise,Testudo graeca, in south‐western Spain

“…The same was also observed for Geochelone elephantopus (MacFarland et al, 19746) and Geochelone gigantea (Swingland & Coe, 1978). Since temperature is the variable most affecting incubation of rigid-shelled turtle eggs (Packard et al, 1981;Packard, Packard & Boardman, 1982;Janzen, 1993a), the observed variation is very likely to reside in the different thermal environments experienced by clutches laid at different moments of the two-month nesting season.…”

“…The delay of emergence in Testudo graeca hatchlings is likely to be mediated by the absorption of external yolk sacs, which the new-borns still carry upon hatching, as has been observed in Gopherus polyphemus (Linley & Mushinsky, 1994) and Geochelone gigantea (Swingland & Coe, 1978). One T. graeca hatchling removed from the nest after hatching also carried a yolk sac weighing 11% of its body weight.…”

Experimental Analysis of an Early Life-History Stage: Selection on Size of Hatchling Turtles

“…Anecdotal reports have suggested that capture and/or ingestion of larger turtles may be difficult for certain predators (Barrows and Schwarz 1895, Bustard 1979, Swingland and Coe 1979. A more likely explanation, however, is differential exposure to predation.…”

The Evolution of Maternal Investment in Lizards: An Experimental and Comparative Analysis of Egg Size and Its Effects on Offspring Performance

“…Applications of Egg-Size Reduction to Other Groups.-Offspring size is correlated with determinants of offspring fitness in many groups including fish (Bagenal, 1969;Henrich, 1988), amphibians (Kaplan, 1980), reptiles (Ferguson and Fox, 1984;Swingland and Coe, 1979), and insects (Richards and Myers, 1980;Wiklund and Persson, 1983;Karlsson and Wiklund, 1984;Steinwascher, 1984). However, there have been few direct experimental tests demonstrating the causal relationships that link maternal investment (eggsize, yolk volume, etc.)…”