Environmental influences on geographic variation in body size of western bobcats

Abstract

Hypotheses that explain geographic variation in body size were examined using cranial measurements of 950 bobcats (Lynx rufus) from western North America. Bobcats were divided into 25 geographic localities of similar habitats and landform (based on ecoregions). Principal component analyses were used to derive a single estimate of size from scores on the first principal component. Males and females were examined separately, because they were significantly dimorphic in body size and because sex and locality exhibited a significant interaction. We expected that female body size would best reflect environmental influences, because male size may be influenced by sexual selection. We found significant geographic variation in bobcat body size, with about 44% of the variation in males and 47% of the variation in females accounted for by comparison among the localities. We also found that variation in body size was associated with Bergmann's rule, as indicated by significant multiple regression of body size of males (R2 = 0.426) and females (R2 = 0.480) on latitude and elevation. Using correlation and regression analyses, we examined the association of body size with selected environmental variables that represent the classical physiological explanation of Bergmann's rule, James' moisture-humidity modification of Bergmann's rule, Rosenzweig's productivity hypothesis, and Boyce's seasonality hypothesis. Only the productivity hypothesis was not supported. The relative strengths of associations suggested, however, that James' modification was better supported than the classical explanation for Bergmann's rule. Path analyses permitted further discrimination of hypotheses, and only the seasonality hypothesis received significant support. As expected, this support was only evident for females. Path analysis may provide a tool for evaluating relative strengths of competing but correlated explanations of geographic variation.

A test of Allen's rule in ectotherms: the case of two south American Melanopline Grasshoppers (Orthoptera: Acrididae) with partially overlapping geographic ranges

“…Longitude (LON) was considered because several climatic variables show strong longitudinal gradients in the surveyed area (see Results). Altitude (ALT) in metres above sea level was determined for all localities and was included because frequently, an increase in elevation is ecologically equivalent to an increase in latitude (Wigginton & Dobson 1999). However, this kind of one-dimensional analyses have no explanatory power (Hawkins & Diniz Filho 2004), since size clines (e.g.…”

BERGMANN's RULE IN NONAVIAN REPTILES: TURTLES FOLLOW IT, LIZARDS AND SNAKES REVERSE IT

“…In particular, combined effects of temperature and humidity (James 1970), annual evapotranspiration as a measure of productivity (Rosenzweig 1968a,b), and seasonality (Boyce 1978(Boyce , 1979Lindstet and Boyce 1985) are hypothesized to better explain size variation in vertebrates. Few studies have compared all of these variables as predictors of size variation in single species, yet in each case seasonality better explained size variation than other variables (Boyce 1978;Murphy 1985;Wigginton and Dobson 1999;Ashton 2001b).…”

Seasonality, Climate Cycles and Body Size Evolution

“…Positive relations between body size and seasonality were found for muskrats [2], western bobcats [44], western rattlesnake [1], weevils [4] and sifakas [23]. In contrast, data on moose [10] and thirteen species of western Palaearctic carnivores [29] did not show clear correlations and thus did not support the hypothesis.…”