Abstract
Reproductive mode, ancestry, and climate are hypothesized to determine body size variation in reptiles but their effects have rarely been estimated simultaneously, especially at the intraspecific level. The common lizard (Zootoca vivipara) occupies almost the entire Northern Eurasia and includes viviparous and oviparous lineages, thus representing an excellent model for such studies. Using body length data for >10,000 individuals from 72 geographically distinct populations over the species' range, we analyzed how sex‐specific adult body size and sexual size dimorphism (SSD) is associated with reproductive mode, lineage identity, and several climatic variables. Variation in male size was low and poorly explained by our predictors. In contrast, female size and SSD varied considerably, demonstrating significant effects of reproductive mode and particularly seasonality. Populations of the western oviparous lineage (northern Spain, south‐western France) exhibited a smaller female size and less female‐biased SSD than those of the western viviparous (France to Eastern Europe) and the eastern viviparous (Eastern Europe to Far East) lineages; this pattern persisted even after controlling for climatic effects. The phenotypic response to seasonality was complex: across the lineages, as well as within the eastern viviparous lineage, female size and SSD increase with increasing seasonality, whereas the western viviparous lineage followed the opposing trends. Altogether, viviparous populations seem to follow a saw‐tooth geographic cline, which might reflect the nonmonotonic relationship of body size at maturity in females with the length of activity season. This relationship is predicted to arise in perennial ectotherms as a response to environmental constraints caused by seasonality of growth and reproduction. The SSD allometry followed the converse of Rensch's rule, a rare pattern for amniotes. Our results provide the first evidence of opposing body size—climate relationships in intraspecific units.
Highlights
The patterns and causes of geographic variation in body size are fundamental themes in studies on life-history evolution (Angilletta, Niewiarowski, Dunham, Leaché, & Porter, 2004; Arendt & Fairbairn, 2012; Roff, 2002)
As with female size, clade identity is a consistent predictor of sexual size dimorphism (SSD) in the three-clade analyses and becomes inconsistent in the two-clade analyses, where four of the six candidate models include climatic predictors only (Table 5); the top models explain a smaller proportion of the total variance in the two-clade analyses than in the three-clade analyses (26%–47% vs. 41%–58%, Table 5)
We show that the body size response to the seasonality gradient is strongly female-biased, more complex than previously thought, and is parsimoniously interpretable as a saw-tooth cline along a gradient of activity season lengths predicted by Adolph & Porter's models (Table 1; Figure 4)
Summary
The patterns and causes of geographic variation in body size are fundamental themes in studies on life-history evolution (Angilletta, Niewiarowski, Dunham, Leaché, & Porter, 2004; Arendt & Fairbairn, 2012; Roff, 2002). A number of recent studies have found that water availability (precipitation, humidity), and seasonality (within-year variation in temperature or precipitation), often explain a higher proportion of body size variation than does mean temperature (e.g., Ashton, 2001; Çağlar, Karacaoğlu, Kuyucu, & Sağlam, 2014; Stillwell, Morse, & Fox, 2007). For each of these factors, multiple mechanistic hypotheses have been proposed (see below). Such more complex patterns are unlikely to be revealed within limited spatial and environmental ranges (Ashton & Feldman, 2003; Gaston, Chown, & Evans, 2008)
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