Evaluating body size changes in relation to elevation and latitude in Torrent Ducks (Merganetta armata): a test on Bergmann?s and Allen?s rules

GUTIERREZ-PINTO, NATALIA; MCCRACKEN, KEVIN G.; ALZA, LUIS; TUBARO, PABLO LUIS; KOPUCHIAN, CECILIA; ASTIE, ANDREA; CADENA, CARLOS DANIEL

Encuentro; 2nd meeting of the Network for Neotropical Biogeography; 2013

Adaptive strategies to overcome challenges imposed by cold environments are diverse, and several mechanisms that help organisms to deal with such environments have been documented. Among the most studied patterns are Bergmann?s rule and Allen?s rule, which indicate that organisms living in colder habitats tend to be larger and to have smaller limbs, respectively, to minimize heat loss. Although evidence for geographic patterns in body dimensions supporting both rules has been found in different organisms, most of the work has been done studying either elevational or latitudinal temperature gradients, but has rarely considered the effect of both kinds of gradients. A good model to evaluate differential effects of both elevation and latitude would be a latitudinally widespread species that also occupies a wide range of elevations within a single mountain range. We tested for the effects of temperature on body size and appendage size in Torrent Ducks (Merganetta armata), a species restricted to whitewater rivers in the Andes.Using replicated elevational gradients at different latitudes from Colombia to Argentina,we found a strong, negative relationship between body size and temperature across latitude concordant with Bergmann?s rule, but observed a positive relationship between body size and temperature along elevational gradients. Likewise,we found evidence for variation in limb size supporting Allen?s rule in relation to variation in temperature with latitude, but not with elevation. In sum, Torrent Ducks inhabiting high-elevation habitats had smaller body size and longer wings than ducks inhabiting lower elevations within the same river system. We hypothesize that, while temperature is likely a dominating selective pressure acting on morphological variation associated with latitude, hypoxia and air density may be more important drivers of phenotypic evolution relative to temperature over elevational gradients.