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Cuckoos Combat Socially Transmitted Defenses of Reed Warbler Hosts with a Plumage Polymorphism

Science  03 Aug 2012:
Vol. 337, Issue 6094, pp. 578-580
DOI: 10.1126/science.1220759

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Learning to Recognize a Cuckoo

Species that are parasitized by cuckoos have evolved several strategies for trying to avoid having their nests hijacked—one of the most obvious being outright attacking, or mobbing, of cuckoos that enter the area. However, cuckoos are not without evolved defenses—most common cuckoo females look remarkably similar to a small hawk, and this mimicry deters mobbing. Thorogood and Davies (p. 578; see the Perspective by Mappes and Lindström) show that social learning in parasitized birds can thwart this protective mimicry. When hosts observe mimics being mobbed, they are more likely to mob them, themselves, later. However, the hosts will only mob the color morph that they observed being mobbed. This specificity may have allowed for the evolution and maintenance of two female morphs within common cuckoos.

Abstract

In predator-prey and host-parasite interactions, an individual’s ability to combat an opponent often improves with experience—for example, by learning to identify enemy signals. Although learning occurs through individual experience, individuals can also assess threats from social information. Such recognition could promote the evolution of polymorphisms if socially transmitted defenses depend on enemy morph frequency. This would allow rare variants to evade detection. Female brood parasitic common cuckoos, Cuculus canorus, are either gray or rufous. The gray morph is a Batesian mimic whose hawk-like appearance deters host attack. Hosts reject this disguise through social learning, increasing their own defenses when they witness neighbors mobbing a cuckoo. Our experiments reveal that social learning is specific to the cuckoo morph that neighbors mob. Therefore, while neighbors alert hosts to local cuckoo activity, frequency-dependent social information selects for a cuckoo plumage polymorphism to thwart host detection. Our results suggest that selection for mimicry and polymorphisms comes not only from personal experience but also from social learning.

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