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Lizards in the large neotropical genus Anolis exhibit an unusual mode of reproduction; all species lay a clutch of a single egg. In contrast, the more typical pattern for lizards is a multi—egg clutch in which the number of eggs increases with female size both inter— and intra—specifically. In Anolis low clutch number is associated with a potential for frequent ovipositions. Field rates for several tropical forest species are one egg every 1—2 wk. Thus, high rates of egg production coupled with a generation time of about 4 mo gives these species very high reproductive potentials. The evolution of low clutch number in relatively small tropical lizards has several non—exclusive explanations. One, which is specific to arboreal lizards (Anolis, geckonids, and some scincids), involves their adhesive toe pads, which facilitate climbing. Because pad area increases by the square of linear dimensions while body weight increases by the cube, pad area may put a limit both on lizard size and the weight of reproductive materials carried at any one time. A second and more general explanation involves the association between clutch size and climatic regime. Lizards in temperate and seasonal tropical habitats have a larger clutch number than do lizards of relatively equable tropical habitats. In these latter habitats short—term fluctuations in rainfall (associated with reproductive success) favor genotypes that are opportunistic reproducers, and relatively high predation intensity favors r—selected life history patterns. Anoline lizards are “extreme” examples of species living under such selective regimes.

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Summary1. Selected body temperatures of female lizards, Sceloporus jarrovi, were measured on a photothermal gradient during late pregnancy and again when postpartum, and pregnant females were subjected to one of three fluctuating temperature regimes that simulated body temperatures of (1) pregnant females, (2) postpartum females or (3) allowed normal thermoregulation. 2. Overall, females selected lower body temperatures when pregnant (mean = 32·0°C) than when postpartum (mean = 33·5°C). 3. Females regulated body temperature more precisely when pregnant than when postpartum as judged by their smaller variances in body temperature throughout the day. 4. When pregnant, females selected a lower mean maximum body temperature (mean: pregnant = 32·8°C; postpartum = 34·5°C) than when postpartum, but selected mean minimum body temperatures did not differ. 5. None of the experimental temperature treatments was detrimental to pregnant females. Female body length increased during pregnancy but the rate of increase did not differ among treatments. Moreover, length-adjusted body mass of postpartum females did not differ among treatments. 6. Pregnant females that experienced postpartum body temperatures produced neonates that were smaller in body mass and length than pregnant females that experienced pregnant body temperatures and females that were allowed to thermoregulate. 7. For neonates resulting from the postpartum body temperature treatment, the disparity in the body length, but not mass, was still observed at 9 days of age, although survival and growth of neonates was high and did not differ among treatments. 8. The results demonstrate that pregnant females could maintain higher postpartum body temperatures without compromising their physical condition, but select relatively low body temperatures, presumably to avoid decrements in offspring fitness.

To understand the process of natural selection, relationships between phenotype and fitness and sources of phenotypic variation must be known. We examined the importance of incubation moisture conditions, maternal yolk investment, and clutch (genotype) to phenotypic variation in hatchlings of the lizard Sceloporus undulatus. Eggs were distributed among two moisture treatments and a third treatment in which yolk was removed. After hatching, mass, snout–vent length, tail length, body shape, thermal preference, running speed, desiccation rate, and growth rate were measured for each hatchling in the laboratory. Hatchlings were then released at a field site in order to monitor growth and survival under natural conditions. Hatchlings from the dry and yolk‐removed treatments were significantly smaller than those from the wet treatment. However, neither performance nor survival were affected by moisture or yolk removal. All phenotypes were affected by clutch. Clutches that produced relatively large hatchlings had higher survival than clutches that produced relatively small hatchlings. Furthermore, clutches that produced relatively slow growing individuals and fast runners had higher survival rates than clutches that produced relatively rapid growing individuals and slow runners. Our results emphasize the overriding importance of clutch (genotype) to variation in phenotypes and survival in hatchling S. undulatus. © 2002 The Linnean Society of London, Biological Journal of the Linnean Society, 2002, 76, 105–124.

Viviparity in squamate reptiles is presumed to evolve in cold climates by selection for increasingly longer periods of egg retention. Longer periods of egg retention may require modifications to other reproductive features associated with the evolution of viviparity, including a reduction in eggshell thickness and clutch size. Field studies on the thermal and reproductive biology of high (HE) and low (LE) elevation populations of the oviparous lizard, Sceloporus scalaris, support these expectations. Both day and night-time temperatures at the HE site were considerably cooler than at the LE site, and the activity period was 2 h shorter at the HE than at the LE site. The median body temperature of active HE females was 2°C lower than that of LE females. HE females initiated reproduction earlier in the spring than LE females, apparently in order to compensate for relatively low temperatures during gestation. HE females retained eggs for about 20 days longer than LE females, which was reflected by differences in the degree of embryonic development at the time of oviposition (stages 35.5-37.0 versus stages 31.0-33.5, respectively). These results support the hypotheses that evolution of viviparity is a gradual process, and is favored in cold climates. Females in the HE population exhibited other traits consistent with presumed intermediate stages in the evolution of viviparity; mean eggshell thickness of HE eggs (19.3 μm) was significantly thinner than that of LE eggs (26.6 μm) and the size-adjusted clutch sizes of HE females (9.4) were smaller than those of LE females (11.2).