Evolution of Viviparity: Constraints on Egg Retention

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Abstract
Extended egg retention is an important component of the cold climate model for the evolution of viviparity in squamate reptiles. The presumptive selective benefits of extended egg retention are that (1) eggs retained in utero are subject to a warmer thermal environment than those laid in nests, and (2) embryonic development in utero is faster than embryonic development in nests as a result of these temperature differences. We tested these assumptions with observations on the montane lizard, Sceloporus virgatus. We measured the diel temperature cycles of gravid females and of nest sites in the field and contrasted the embryonic development of eggs laid at the normal time and eggs retained for an additional 13 d, 20 d, and 30 d in laboratory experiments. The mean temperatures of gravid females and of nest sites were very similar at 25.4° and 25.2°C, respectively. Thus, egg retention did not alter the thermal environment of developing embryos of S. virgatus. Moreover, embryonic development was retarded by egg retention; embryos from retained eggs were smaller and less differentiated than control eggs. Egg retention also lengthened the incubation period from 57 d for eggs laid at the normal time to 65 d, 66 d, and 71 d for eggs retained 13 d, 20 d, and 30 d, respectively. Thus, egg retention reduces the time available for hatchlings to grow prior to winter and delays sexual maturation.

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