Morphological differences in a population of Rufous-collared Sparrow (Zonotrichia capensis, Statius Muller, 1776) (Passerine, Emberizidae) at different elevations in the Tropical Andes

Abstract

Populations that breed along steep elevation gradients show diverse physiological and morphological changes in response to the different environmental conditions. The latter has been discussed by Bergmann's and Allen's ecogeographic rules about body and appendage sizes and environmental temperature. We compared morphometric measures (mass, bill width, tarsus, wing, and tail length) of a Zonotrichia capensis population in two localities at different elevations with similar latitudes and photoperiods on the western slope of the Colombian Central Andes. We compared a Low Elevation locality (LE) at 1800 m a.s.l. and a High Elevation locality (HE) at 3853 m a.s.l. that have approximate wind speeds of 1.3 m/s and 8.4 m/s, respectively. During 12 months of sampling, we captured 46 adults using mist-nets; 26 in the LE and 20 in the HE. Each individual was sexed using molecular techniques at the Laboratory of Genetics of the Department of Biological Sciences of Universidad de Caldas. Individuals (males + females) from the HE had longer wings and tails than those from the LE (F-1,F-44 = 5.93; P = 0.019). Also, wings of males in the HE were longer than those of females in both localities and tails of males in the HE were longer than those of LE males. Our results did not agree with what was expected according to Allen's and Bergmann's ecogeographic rules. Longer wings and tails increase sustainment, maneuverability, and balance in low atmospheric pressures and strong air currents and these conditions are found at high elevation habitats. Most likely, the longer wings found for HE males allow greater movement during territorial behavior. Further, these differences in morphological traits along elevational gradients could result from micro-evolutionary changes between localities or phenotypic plasticity of individuals exposed to different environmental conditions.

Las poblaciones que se reproducen en gradientes altitudinales, adoptan diversos cambios morfológicos para afrontar las condiciones ambientales. En el presente estudio se compararon las medidas morfológicas (peso corporal, longitud del tarso, ala, cola y culmen) de una población de Zonotrichia capensis, a diferente altitud en los Andes colombianos. Las localidades de tierra baja (TB) y alta (TA) se encuentran a 1800 m.s.n.m. y 3853 m.s.n.m., con velocidad aproximada del viento de 1.3 m/s y 8.4 m/s, respectivamente. Durante 12 meses se realizó la captura de 46 individuos (TB n=26, TA n=20) para la medición de los rasgos morfológicos. La longitud del ala de los individuos de TA fue mayor que en TB. Así mismo, en machos de TA la longitud del ala fue mayor que en hembras en general. Es posible que a las más grandes incrementen la eficiencia del vuelo en zonas ventosas y con baja presión atmosférica, como ocurre en TA. Probablemente la diferencia del tamaño del ala entre los machos de TA y las hembras, se deba a un mayor desplazamiento dentro de la conducta territorial. Estas diferencias podrían ser el resultado de cambios microevolutivos entre localidades o la plasticidad fenotípica de individuos expuestos a diferentes condiciones ambientales.