ASHTON, Kyle/G.; Kutztown University: Are There General Body Size-Latitude (and Temperature) Trends in Vertebrates?

Biologists have long been interested in identifying general patterns of geographic variation in body size, and its relationship with environmental parameters. Meta-analytical techniques provide an excellent tool for testing for such patterns. Using these techniques, I have shown strong evidence for general size-latitude (and temperature) trends in tetrapod vertebrates. In particular, most tetrapod groups (mammals, birds, chelonians, amphibians) show general intraspecific tendencies for increasing 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 investigating the relationship between strength of size trends (correlation coefficient between body size and latitude; r) and sample size (number of populations studied), range of latitudes studied, average latitude studied, and body size. For these analyses I used both nonphylogenetic (standard regression) and phylogenetic (independent contrasts) approaches. Based on independent contrasts analysis, the absolute value of r was negatively related to sample size. The absolute value of r was not significantly related to range of latitudes sampled, mean latitude sampled, or body size. Further, r was not significantly related to mean latitude or body size. In summary, species occurring over wider ranges, or at higher latitudes, do not have stronger size trends than those sampled over smaller ranges or at lower latitudes. Larger bodied species do not have stronger size trends than smaller bodied species. Thus, these size trends appear general for tetrapods. The significant negative relationship between absolute value of r and sample size is probably due to increased precision of estimating the overall effect size (correlation coefficient between size and latitude) with increased sampling.