Summary: EVOLUTION OF EGG RETENTION IN THE SCELOPORUS SPINOSUS
GROUP: EXPLORING THE ROLE OF PHYSIOLOGICAL,
ENVIRONMENTAL, AND PHYLOGENETIC FACTORS
M. L. CALDEROŽ N-ESPINOSA
, R. M. ANDREWS
, AND F. R. MEŽ NDEZ DE LA CRUZ
Laboratorio de HerpetologiŽa, Instituto de BiologiŽa, UNAM, CP 04510, Ciudad Universitaria, MeŽxico D.F.
Virginia Polytechnic Institute, Blacksburg, Virginia, USA
ABSTRACT: The evolution of viviparity in squamates has involved intermediate stages of egg retention.
Reduction in the thickness of the eggshell, in relative clutch mass (RCM), and in clutch frequency would
have facilitated the transition from oviparity to viviparity, while low temperatures are likely the ultimate
selective force that promoted this evolutionary shift. We tested these ideas using the Sceloporus spinosus
group. Because it is the sister clade of the viviparous Sceloporus formosus group, we predicted that mem-
bers of the S. spinosus group would exhibit extended egg retention and other features associated with
the evolution of viviparity. To test this idea, we examined the ability to retain eggs past the time of