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

EDELAAR, PIMM; DONAZAR, JOSE; SORIANO, MATIAS; SANTILLAN, MIGUEL ANGES; GONZALES ZEBALLOS, DIEGO; GARCÍA BORBOROGLU, PABLO; LISNIZER, NORA; GATTO, ALEJANDRO; AGUERO, LAURA; PASSERA, CARLOS; EBERT, LUIS; BERTELLOTTI, M; BLANCO, GUILLERMO; ABRIL, MONICA; ESCUDERO, GRACIELA; QUINTANA, FLAVIO

EVOLUTIONARY ECOLOGY RESEARCH

EVOLUTIONARY ECOLOGY LTD

Lugar: Tucson; Año: 2011 p. 187 - 196

1522-0613

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 50 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. 55 Methods: Map the distribution of leucistic individuals relative to normal individuals. Combine a variety of anecdotal natural history observations, collected over twenty years, into a logical inference. Observations: Leucistic caracaras were found only along a 250 km stretch of rocky 60 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 65 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 70 caracaras are apparently favored by natural selection, either directly or by pleiotropy.