Rethinking biogeographic patterns: high local variation in relation to latitudinal clines for a widely distributed species

Abstract

Wide-ranging species typically differ morphologically across their ranges. Bergmann's rule suggests that taxa in colder environments are bigger than related taxa in warmer locations. We examined 767 painted turtles (Chrysemys picta) in ten populations near their northwestern range edge in south-central British Columbia, Canada, in conjunction with previous data, to test the hypotheses of (1) a Bergmann's latitudinal cline, and (2) that males and females show similar latitudinal variation in size. We also explicitly test the impact of high local variation on range-wide inference. Female and male turtles showed similar latitudinal clines in body size; the degree of sexual dimorphism did not change across the range. Importantly, local variation in sexual dimorphism across ponds was nearly as high as the previously observed continental variation. Indeed, we found both the lowest and the highest degrees of sexual size dimorphism that have ever been reported for this species. Further, differing criteria in the literature for identifying mature females compound the difficulty of interpreting latitudinal clines in size or dimorphism. Our results highlight the need for much more systematic local and regional sampling as inputs for latitudinal or other comparative analyses such as Rensch's rule because insufficient sampling of high local variation may mask important ecological and evolutionary patterns.

Geographic variation in thermal sensitivity of early life traits in a widespread reptile

“…We detected no clinal variation in our measures of offspring body size, besides the effect of latitude and the interaction between incubation temperature and latitude on hatchling mass that seems to be driven by lower incubation temperatures (Figure ; Supporting Information Figure S1 in Appendix ). Thus, among‐location variation in microenvironmental conditions might underpin this observation rather than a macroclimatic proxy (Cooley, Floyd, Dolinger, & Tucker, ; Tesche & Hodges, ).…”

“…Therefore, we predicted that eggs from northern populations would exhibit accelerated development rates, as an adaptation to potential constraints of a shorter growing season, compared to eggs from more southern locations, when incubated under constant conditions (sensu Ewert, ; Du et al, ). Emydid turtles (including C. picta ) exhibit an intraspecific pattern following Bergmann's Rule whereby adult females at higher latitudes are larger (Ashton & Feldman, ; Tesche & Hodges, ) and produce larger clutch sizes of smaller eggs (Iverson et al, ). Therefore, because egg size is a major determinant of offspring size in turtles (e.g., Mitchell, Warner, & Janzen, ), we predicted that hatchlings from higher latitudes would have smaller body sizes than conspecifics from lower latitudes (Figure ).…”

“…Yet, the close connections between temperature and physiology are especially apparent during the non-motile egg phase of embryonic development, particularly in species lacking post-parturition parental care (Refsnider & Janzen, 2010;Shine, Elphick, & Harlow, 1997;Telemeco et al, 2016). These early life thermal vulnerabilities-and eco-evolutionary solutions to resolve or minimize them-can perhaps be most readily revealed by examining multiple populations in taxa with broad geographic distributions (e.g., Tesche & Hodges, 2015).…”

Effects of urbanization on the population structure of freshwater turtles across the United States

“…Because common methods for capturing freshwater turtles (e.g., hoop traps, basking traps, dip netting) are all biased in some fashion (Frazer et al. ; Gamble ; Tesche & Hodges ), sampling by multiple methods has been recommended to minimize overall bias in a study (Ream & Ream ; Koper & Brooks ; Tesche & Hodges ). We used only one sampling method (hoop traps) given the large geographic scale and collaborative nature of this project.…”

“…Our single sampling method focused effort and consistency among all collaborators despite the inherent bias associated with hoop traps. This approach underestimates the abundance of all sex and age classes (Tesche & Hodges ) and may be biased against females (Frazer et al. ) and juveniles (Tesche & Hodges ).…”