Morphological variation in the specialist Dupont’s Lark Chersophilus duponti: geographical clines vs. local ecological determinants

Abstract

Intraspecific geographic variation in morphology is common in animals along geographic or climatic clines. Local ecological factors are likely to act simultaneously at smaller spatial scales, but have hardly been contrasted with wide-ranging predictors. We tested here whether the morphological variation of Dupont’s Larks (Chersophilus duponti) responded to ecological parameters at two different spatial scales. First, we investigated the effects of geographic and climatic gradients over its breeding range. Second, we focussed at a smaller spatial scale on a fragmented population and tested additionally several fine-grained ecological factors related to key components in the species’ habitat. Contrary to Bergmann’s rule, wing length and cranium size decreased with rainfall and increased with aridity and maximum temperature at the large scale, so birds tend to be larger at lower latitudes. At the same time, wing and tarsus length increased at high elevations where minimum temperatures are lower, providing some support to Bergmann’s rule. At the small spatial scale we failed to detect any relationship between body size and positional or climatic variables, nor did food availability, intra- and inter-specific competition and predation pressure produce any significant effect on morphology. Nevertheless, cranium size and wing length differed between habitats as measured by soil and vegetation types, and wing length decreased with patch size. This later result could be explained in the context of strong habitat fragmentation if larger individuals have a higher propensity of dispersing or a higher probability of surviving dispersal events. Our study indicates that several geographic and environmental sources may occur simultaneously at different spatial scales. Further, even at the same scale, intraspecific morphological variation may show contrasting patterns for climatic, latitudinal, and elevational gradients that make their interpretation difficult in the context of ecogeographical rules. The effects elicited by aridity, habitat loss, and fragmentation on body size should be considered in future studies of global change, as they may have serious consequences for the distribution, abundance, and ultimately the persistence of birds in arid environments.