Sensitivity of intraspecific latitudinal clines of body size for tetrapods to sampling, latitude and body size

Recent studies have shown that most tetrapod groups (mammals, birds, chelonians, amphibians) show general intraspecific tendencies for increasing body size with latitude, whereas squamates (lizards and snakes) show an intraspecific tendency towards decreasing body size with latitude. Here I evaluate whether these size trends are general by using independent contrasts analysis to investigate the dependence of intraspecific size-latitude relationships (r), and the magnitude alone of size-latitude relationships ([r]), for tetrapod vertebrates, on sample size, range of latitudes sampled, average latitude sampled, and body size. Range of latitudes sampled, average latitude sampled, and body size did not influence body size-latitude relationships (r) or the magnitude alone of body size-latitude relationship ([r]). Sample size did not influence size-latitude relationships (r), but did influence the magnitude alone of size-latitude relationships ([r]), possibly indicating increased precision of estimating size-latitude relationships with increased sampling. In short, intraspecific size-latitude relationships are similar for species of different sizes, occurring at different latitudes, sampled over different latitudinal ranges, and differing in number of populations sampled (though magnitude alone is influenced by sample size). These results suggest that intraspecific size-latitude trends are general, and biologically significant (i.e., are not artifacts of sampling), thus deserving explanation.