One glacial cycle may seem like a short interval for
the evolution of a large, long-lived vertebrate with
a low reproductive rate, but the even larger modern
tortoise Geochelone gigantea, endemic to Aldabra Island
in the Indian Ocean, provides a close analogy.
Aldabra was completely submerged in the last interglacial
(MIS 5), and tortoises had to colonize and
evolve since then (Braithwaite et al., 1973), giving the
species G. gigantea an approximate 100,000-yr life span
similar to that postulated for the Bermuda tortoise.
With the reduction in land area of Aldabra in the
present interglacial, tortoise populations have begun
to exert a negative effect on food plants and may
exceed ‘‘the stable carrying capacity’’ (Bourn and Coe,
1978:140). Overpopulation has also resulted in loss of
shade plants resulting in significant mortality from
hyperthermia, both conditions being exacerbated by
drought (Bourn et al., 1999). Similar circumstances on
Bermuda may have led to the extinction of H.
bermudae during the MIS 9 highstand