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1 April 2009 Biometrics of Iberian Dippers Cinclus cinclus: Environmental Sources of Among-Population Variation
Juan Arizaga, M. Ángeles Hernández, José Rivas, Rafael Miranda
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Abstract

Biometry of Iberian Dippers Cinclus cinclus has typically been addressed from a geographic perspective, often considering small local populations. In this work, we explored environmental correlates of among-population phenotypic variation in Dipper morphology throughout Iberia. We tested for effect of river slope, altitude, temperature and precipitation (summarized using Principal Component Analyses, PCA) on absolute and relative (controlling for tarsus length) wing length and tarsus length. We used data of 222 Dippers captured from 35 rivers in eight breeding areas in N and S Iberia. All biometric variables were larger for older birds and males, though tarsus length was not affected by age. Wing length was longer in S Iberia, even when controlling for tarsus length, All biometric variables co-varied negatively with one of the PCA components of abiotic factors, owing to large-scale environment differences between N and S Iberia (zone). The abiotic factors predicted mean tarsus length of birds within zones, with tarsus increasing with increasing river slope and decreasing temperatures and precipitation. Furthermore, Dippers from S Iberia were not larger but had longer absolute and relative wings than those from N Iberia. Altogether, these findings imply that the morphological diversification of Iberian Dippers is affected by the environment and that body size-wing length allometric relationship differs between N and S Iberia.

INTRODUCTION

Studying how size and shape of animals are determined by selective pressures, such as habitat (Schluter 1993, Langerhans et al. 2003), mates (Székely et al. 2000), foraging (Marchetti et al. 1995, Alonso et al. 2006) or migration (Mönkkönen 1995, Calmaestra & Moreno 2001) can be of key importance to understand the ultimate causes promoting the biometric and morphological diversification of populations, and, finally, the processes involving speciation (e.g. Benkman 1993).

The Dipper Cinclus cinclus is a very suitable species for studying morphological diversification. This polytypic Palaearctic passerine breeds from W Europe to E Asia (Cramp 1988), and in the W Palaearctic it is present between Fennoscandia and S Europe, reaching also the main mountain ranges of N Africa (Cramp 1988, Wilson & Breitenmoser-Würsten 1997). It is associated to well-oxygenated waters of rivers with beds of stones where it finds macroinvertebrates that constitute its main diet (Cramp 1988, Tyler & Ormerod 1994). Dipper populations vary from sedentary to migratory, with migration distances increasing with latitude (Cramp 1988).

Within Iberia, the species has been suggested to be sedentary, performing only some post-breeding or post-juvenile movements, often from high- to lowlands in autumn or winter (Cramp 1988, Tellenía et al. 1999). This, as well as its confined occurrence in mountain rivers, suggests that populations from Iberia might be quite isolated among each other. Therefore, Dipper is a suitable model species for evaluating how environmental conditions affect morphology.

Two subspecies are currently suggested to breed in Iberia (though their status is still unresolved from a taxonomic viewpoint; Campos et al. 2005a): C. c. cinclus in NW Iberia, and C. c. aquaticus in S and E Iberia (Ormerod & Tyler 2005; for further details see Campos et al. 2005a). Noteworthy, several studies have investigated biometric variation between subspecies or populations (Marsá 1988, Esteban et al. 2000, Villarán et al. 2001, Campos et al. 2005a). However, most of these studies focus on small populations on a relatively small geographic scale; with the exception being a study by Campos et al. (2005a), who analysed the biometry of four populations from Central, N and S Iberia, finding that Dippers from S Iberia (inhabiting the higher mountain ranges of Iberia) were larger than the rest. However, one of the issues unsolved in that work was the question of how the environment promoted this variation in morphology. Recently, changes in river features have been reported to affect Dippers' morphology (Moreno-Rueda & Rivas 2007), suggesting that the morphological diversification may be affected by river-specific environmental features.

Iberia is a highly mountainous region extending over an area of nearly 583 000 km2, resulting in a high environmental diversification. Dippers, hence, occur in rivers with very different environmental conditions. In this work, we investigated whether the biometry can be predicted from population-specific environmental factors. We used data from eight populations from N and S Iberia, and made the following predictions.

(1) Body size increases with river slope steepness. Dippers feed on benthic insects and macroinvertebrates, captured with the bill whilst walking on the river bed (Cramp 1988). For a given river, the water speed increases as the river slope becomes steeper (Armantrout 1998). Therefore, its slope could be used as a proxy of water speed. This variable is one of the most important hydraulic factors shaping morphology of aquatic animals (Jowett & Duncan 1990). For instance, fish living in fast-flowing streams tend to be more robust (i.e. have a larger body size; Pakkasmaa & Piironen 2001) or more streamlined (i.e. have larger fins and a more hydrodynamic body; Riddell & Leggett 1981, Beacham 1985, Pakkasmaa & Piironen 2001). Similarly, Dippers inhabiting rivers with a steep slope (i.e. fast-flowing streams) should thus be expected to have a larger body size, since birds with larger body size could swim and dive more efficiently than smaller birds in such type of waters.

(2) Body size increases with decreasing temperature. Bergmann's rule states that body size increases with decreasing temperatures in homeothermic animals, because heat loss increases with body area (Futuyma 1998). Larger bodies have a relatively small surface which is advantageous in cold climates, whereas smaller bodies are favoured in warm climates. Mechanisms underlying such a relationship seem likely to be associated with fluctuations in over-winter survival (Marchant et al. 1990, Peach et al. 1995) in relation to body size, with larger birds being more likely to survive in harsh winters. In addition, Allen's rule posits that, in warm climates, homeothermic animals have longer, higher limbs, since this allows them