We examined body size, litter size and evidence for Bergmann's and Rensch's rules among eight closely related viviparous Phrynocephalus (Agamidae) lineages from the Qinghai-Xizang (Tibetan) Plateau. Mean snout-vent length and (relative) abdomen length was greater in females than males, whereas absolute and relative mean head size, fore – and hindlimb length was larger in males. These patterns suggest that body size may reflect sexual evolutionary conservatism. The lizards are smaller at higher elevations or in colder climates, representing the converse of Bergmann's rule. Absolute differences in female body size among taxa are not equal to the male differences among taxa. This results in a slope less than one when male size is regressed on female size, which allows rejection of Rensch's rule for this group. The female body size elevation-related cline was steeper than the corresponding male cline. Litter sizes were both smaller and less variable at higher elevations. These elevational clines remained after application of phylogenetic comparative methods, indicating that ecological processes play a more important role than phylogeny in shaping patterns of size and reproductive variation in these lizards. It is suggested that seasonal activity and temperature are important environmental factors that contribute to the converse Bergmann's cline, while fecundity selection in females and sex-specific differential-plasticity likely explain why patterns do not conform to Rensch's rule.
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