ABSTRACT: The sand lizard, Lacerta agilis occupies a large part of temperate Eurasia from the Pyrenees to the Baikal Lake. This chapter presents an analysis of geographic variation in SSD within this species based on original and published data on SVL of adult males and females in 52 local or regional samples. The major pattern, distinctive differences between the consistently female-larger L. a. agilis (West Europe) and the predominantly male-larger L. a. boemica (the south-eastern North Caucasus), is primarily determined by divergence in male size (Rensch’s rule). The other subspecies (L. a. chersonensis, L. a. exigua, and the three Transcaucasian forms) tend to occupy intermediate positions along the SSD axis. Within subspecies, the variation in SSD is characterized by latitudinal (L. a. exigua) and altitudinal (L. a. boemica) clines towards a male-biased SSD in warmer climates, with female size varying as much or more than male size. Data on age-specific SVLs and age compositions for L. a. agilis and L. a. boemica show that sex differences in body growth are the major proximate determinant of adult SSD, the sex-biased adult survival being of minor importance. Selective and proximate-level factors are discussed as possible determinants of the geographic patterns in SSD. These include sexual, fecundity and viability selection; growth limitations by environmental constraints for energy intake; and a trade-off between growth and egg production in females. The available correlational data are not sufficient to permit adequate evaluation of these hypotheses, but future directions for research are proposed.
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