Evol Ecol Res 13: 187-196 (2011)     Full PDF if your library subscribes.

Apparent selective advantage of leucism in a coastal population of Southern caracaras (Falconidae)

Pim Edelaar1, José Donazar1, Matias Soriano2, Miguel Ángel Santillán3, Diego González-Zevallos4, Pablo Garcia Borboroglu4,5, Nora Lisnizer4, Alejandro Javier Gatto4, María Laura Agüero4, Carlos A. Passera6, Luis Augosto Ebert7, Marcelo Bertellotti4, Guillermo Blanco8, Monica Abril9,10, Graciela Escudero4 and Flavio Quintana4,11

1Department of Conservation Biology, Estación Biológica de Doñana (CSIC), Sevilla, Spain, 2Estación Científica de Bahía Bustamante, Gaiman, Chubut, Argentina, 3Centro para el Estudio y Conservación de las Aves Rapaces en Argentina (CECARA), Universidad Nacional de la Pampa, Santa Rosa, La Pampa, Argentina, 4Departamento Biología y Manejo de Recursos Acuáticos, Centro Nacional Patagónico (CONICET), Puerto Madryn, Chubut, Argentina,  5Department of Biology, University of Washington, Seattle, Washington, USA,  6Causana Viajes, Puerto Madryn, Chubut, Argentina, 7Centro Universitário Leonardo da Vinci, Indaial, Santa Catarina, Brazil, 8Departamento de Ecología Evolutiva, Museo Nacional de Ciencias Naturales (CSIC), Madrid, Spain, 9Repositorio Científíco y Didáctico, Universidad Nacional de la Patagonia San Juan Bosco, Comodoro Rivadavia, Chubut, Argentina,  10Asociacion Patagonica de Ornitología, Comodoro Rivadavia, Chubut, Argentina and  11Wildlife Conservation Society, Buenos Aires, Argentina

Correspondence: Department of Conservation Biology, Estación Biológica de Doñana (CSIC), Avda. Americo Vespucio s/n, E-41092 Sevilla, Spain.
e-mail: edelaar@ebd.csic.es


Background: Southern caracaras are medium-sized raptors with a large range stretching to the southern tip of South America. An aberrant, leucistic plumage is found commonly along the coast of Chubut Province (Patagonia, Argentina). Leucistic birds do not produce dark melanin in their feathers. However, they are not albinos because their eyes are not red. No genetic studies of caracara plumages are known.

Hypothesis: The high frequency of leucistic birds in Chubut Province arises because of natural selection.

Methods: Map the distribution of leucistic individuals relative to normal individuals. Combine a variety of anecdotal natural history observations, collected over 20 years, into a logical inference.

Observations: Leucistic caracaras were found only along a 250-km stretch of rocky oceanic islands and continental outcrops with large seabird colonies in Chubut Province. In the rest of their range, Southern caracaras have dark plumage. Where they do occur, leucistic birds are frequent and co-occur with dark-plumaged birds. Intermediate individuals, presumably heterozygotes, exist. Leucism is not related to age or sex. Leucistic individuals are restricted to a particular habitat. Gene flow has not homogenized the coastal and inland populations.

Results: Leucism is not simply due to inbreeding producing more homozygous individuals. Leucism is not due to genetic drift. Leucism is not an environmental effect on individual physiology or development. Leucism is not a transient (plastic) phenomenon. Where they occur frequently, leucistic Southern caracaras are apparently favoured by natural selection, either directly or by pleiotropy.

Keywords: albinism, Caracara plancus, colour, inbreeding, leucism, polymorphism, selection.

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