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Evolution of viviparity in squamate reptiles (Sceloporus spp.): a variant of the cold-climate model

Summary: Evolution of viviparity in squamate reptiles (Sceloporus spp.):
a variant of the cold-climate model
Robin M. Andrews
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, U.S.A. E-mail: randrews@vt.edu
(Accepted 28 April 1999)
An alternative to the cold-climate model for the evolution of viviparity is that the impetus for the initial
transition from oviparity to viviparity is not an increase in the duration of egg retention but a shift in the
location of nests to more superŽcial and thus warmer locations in the soil proŽle as temperature declines
with increasing altitude or latitude. Shallower nest placement, however, would lead to increased egg
mortality as a result of physiological stress or predation, and enhanced egg mortality would thus provide
the initial beneŽt of extended egg retention. To test this hypothesis, I examined the thermal biology of
three species of Sceloporus lizards living at high altitudes: S. virgatus (1800 m), S. aeneus (2800 m), and
S. bicanthalis (3200 m). The oviparous S. virgatus and S. aeneus females laid eggs at depths of 6 and 2 cm,
and mean body/nest temperatures were 24.6/25.2 8C and 19.9/20.8 8C, respectively. Because the initial
increment in the duration of egg retention is presumably short, females of these oviparous species cannot
initially keep their eggs appreciably warmer than they would be in nests. In contrast, mean temperatures of
simulated nests (17.6 8C) of the viviparous S. bicanthalis were similar to the 17 8C low temperature
threshold for development, and temperatures in some nests fell below freezing at night and rose to lethal
levels during the day. Mean body temperatures of S. bicanthalis females were 20.1 8C; eggs retained during


Source: Andrews, Robin - Department of Biological Sciences, Virginia Tech


Collections: Environmental Sciences and Ecology