Climatic temperature and precipitation jointly influence body size in species of western rattlesnakes

Both the metabolic theory of ecology and dynamic energy budget theory predict that climate influences body size through its effects on first order determinants of energetics: reactive temperatures, carbon resources, and oxygen availability. Although oxygen is seldom limiting in terrestrial systems, temperature and resources vary spatially. While controlling for effects of spatial autocorrelation, we used redundancy analyses and variation partitioning to evaluate the influence of climatic temperature, precipitation, and their seasonalities, on multivariate body size across the distributions of four species of the western rattlesnake group in North America (Crotalus pyrrhus, C. scutulatus, C. oreganus, C. viridis). Most species showed a pattern of increased body size in cooler, mesic climates, and decreased body size in warmer, xeric climates. Exceptions to the pattern provided additional context through climatic idiosyncrasies in the distributions of each species. For example, the general pattern of a negative influence of temperature on body size was not apparent for C. oreganus, which among the four species ranges across the mildest climates overall. Moreover, excepting C. oreganus, complimentary multiple regression analyses indicated that the negative influence of high temperatures on body size was likely stronger than the positive influence of greater precipitation. In contrast to previous studies, our results found that seasonality had negligible effects on body size. We suggest that precipitation gradients correlate positively with resource availability in driving intraspecific body size, and that temperature compounds this gradient by increasing baseline metabolic demands and restricting activity in particularly warm or otherwise extreme climates.

Methods
The dataset includes species identifiers, geographic coordinates of origin for specimens (in decimal degrees), sex, and linear measurements for 1531 specimens of four species of rattlesnakes (Crotalus pyrrhus, C. scutulatus, C. oreganus, and C. viridis). All data are raw (i.e., untransformed) and in mm scale. Abbreviated column headings for morphometric variables are defined in electronic supplementary material.