No Evidence for Pace of Life Evolution Along Elevational Gradients in Squamate Reptiles

Ecological conditions can significantly influence the trade-off between survival, growth and reproduction, driving life-history divergence among populations in different environments. The pace of life syndrome hypothesis (POLS) proposes that additional suites of traits, from physiology to behaviour, adaptively co-evolve with life-history traits along a slow-fast continuum. Using data from 192 studies representing 104 squamate species, we performed phylogenetic meta-analyses testing whether traits vary predictably across elevation within species in accordance with POLS. Results show there is no clear evidence for an overall intraspecific elevational pace-of-life syndrome in squamate reptiles. While high elevation populations had significantly lower body temperatures and larger egg sizes, most traits-including body size, longevity, fecundity and thermal tolerance-exhibited non-significant elevational patterns. Critically, lizards and snakes responded fundamentally differently: snakes showed significant differences in adult female and neonate body size and fecundity across elevation, while lizards showed no significant divergence. Elevation-latitude interactions provide further evidence against a single pace-of-life solution to elevation. Our findings challenge the applicability of POLS theory within species, revealing that life-history evolution is more context-dependent and taxonomically constrained than syndrome-based approaches suggest. These results contribute to growing evidence that universal trait syndromes rarely emerge across environmental gradients in diverse taxonomic groups.