The Natural Regulation of Giant Tortoise Populations on Aldabra Atoll. Movement Polymorphism, Reproductive Success and Mortality

(1) Transect censuses and resightings of marked giant tortoises from two island populations on Aldabra show that in one population, on Grande Terre, some of the tortoises migrate to the coast during the rainy season although many stay inland. (2) Mortality is higher on the coast than it is inland; due to the lack of shade on the coast many tortoises die from heat exposure. (3) The coastal vegetation is quicker to respond to the beginning of the rains than the `tortoise turf' found inland. Migrant tortoises gain from the higher productivity of the coastal vegetation at the beginning of the rains. (4) The reproductive output (assessed by ovarian examination) of females on the coast is higher than of those that remain inland. (5) It is argued that migrant and non-migrant individuals persist together in the population because they receive approximately equal pay-offs; although coastal individuals risk a greater probability of death, the migrants achieve a higher reproductive success mediated by the greater primary productivity of coastal vegetation. The costs and benefits of migration will be frequency dependent, so the equilibrium ratio of migrants to non-migrants should be stable. (6) Resighting frequencies of tortoises on the coast in different wet seasons suggest that migrant tortoises go to the coast in a relatively high proportion of years and that there are some `inland' tortoises that never go to the coast. (7) The system controlling the determination of the movement morph is unknown.