Clinal variation in body and cell size in a widely distributed vertebrate ectotherm

Abstract

Bergmann's rule states that, among conspecific populations, individuals are larger in cooler than in warmer environments as a consequence of selection related to heat conservation. Many of the most comprehensive assessments of Bergmann's rule to date have examined clinal patterns in body size among species assemblages. Our study is a more direct test of Bergmann's rule because we examine the pattern within a single, widely distributed species. We examined geographic variation in body and cell size in the spotted turtle ( Clemmys guttata). Our analysis of 818 turtles collected from the entire range (45-28 degrees N), indicated that body size increased with latitude; however, the relationship was driven by a population of large turtles at the northern extreme of the species' range. When the northern population was removed from the analyses, Bergmann's rule was not supported, and the smallest turtles occurred near the central part of the species' distribution. Recent literature has suggested that latitudinal clines in body size may simply be a physiological byproduct of the effects of temperature on cell division, resulting in larger cells, and hence larger organisms, from cooler temperatures. Measurements of the diameter of skin cells did not support the hypothesis that cell size increases with latitude and decreases with temperature in the spotted turtle, nor was there a significant relationship between body size and cell size. Our study suggests that neither Bergmann's rule nor cell size variation sufficiently explain the body size cline observed in the spotted turtle. We hypothesize that patterns in body size are related to variation in female size at maturity and reproductive cycles.

Variation in life‐history characteristics among populations of North American wood turtles: a view from the north

“…Foscarini, 1994; Tuttle, 1996). This nonlinear body size pattern matches that of spotted turtles Clemmys guttata for which it was proposed, in part, to reflect changes in energy investment into reproduction, manifesting as multiple clutches per year in the southern part of the range (Litzgus et al ., 2004). Wood turtles are not known to produce more than one clutch of eggs per year (Brooks et al ., 1992; Ernst et al ., 1994; Tuttle, 1996; Walde, 1998; Smith, 2002); therefore the explanation proposed for spotted turtles does not appear to explain body size variation in wood turtles.…”

“…However, selection pressure in southern populations, where climate is more predictable, may be acting on individuals to reproduce earlier. Turtles are a good model species for testing patterns of geographic variation in life‐history traits and body size (Gibbons, 1976) because their rigid shell is not altered during brief fluctuations in health, reproductive condition or during times of low food availability (Litzgus et al ., 2004).…”

Mineralized soft-tissue structure and chemistry in a mummified hadrosaur from the Hell Creek Formation, North Dakota (USA)

“…The size of the cell-like structures (approx. 5-30 mm) is within the size range expected for skin cells (Fusenig 1986;Litzgus et al 2004). The imaged partial epidermal .…”

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

“…Western pond turtles (Actinemys marmorata) also varied in growth and size at maturity in relation to habitat quality (Germano and Bury 2009). In the lab, spotted turtles (Clemmys guttata) exposed to different foods and developmental temperatures showed plasticity in body and cell size (Litzgus et al 2004).…”