HOST RECOGNITION OF BROOD PARASITES: IMPLICATIONS FOR METHODOLOGY IN STUDIES OF ENEMY RECOGNITION

Auk, The, Apr 2005 by Grim, Tomás

ABSTRACT

Various studies have shown that experiments on nest defense and enemy recognition (e.g. recognition of adult brood parasites) can be confounded by many factors. However, no study has described a confounding effect of control dummy type. Here, I show experimentally that the choice of control dummy may influence the results of an experiment and lead to erroneous conclusions. I tested recognition abilities of the Blackcap (Sylvia atricapilla), currently a host rarely used by the Common Cuckoo (Cuculus canorus). Blackcaps responded very differently to two kinds of control dummies: they ignored the Eurasian Blackbird (Turdus merula) dummy, but attacked the Rock Pigeon (Columba livia) dummy as frequently as they attacked the Common Cuckoo. The differing results may be explained by the fact that the Rock Pigeon is more similar to the Common Cuckoo than the Eurasian Blackbird is, and consequently elicited more aggressive behavior than the latter. Thus, absence of discrimination in enemy-recognition studies may reflect a methodological artifact resulting from varying abilities of particular hosts to discriminate along a continuum of recognition cues. This result has serious methodological implications for further research on enemy recognition and aggression in general: a control dummy should not be too similar to the dummy brood parasite; otherwise, the chance of detecting existing recognition abilities is low. Further, I argue that coevolution only increases pre-existing aggression in the particular host species. Therefore, increment analysis (assessing changes in host antiparasitic responses during the nesting cycle while controlling for background aggression to control dummies) provides a more accurate picture of hosts' recognition abilities than the traditional approach (when the total level of antiparasitic response is analyzed). Received 30 January 2004, accepted 3 November 2004.

Key words: brood parasitism, coevolution, discrimination, methodology, nest defense, recognition.

Reconocimiento del Hospedero de los Parásitas de Nidada: Consecuencias para las Metodologías de Estudios sobre Reconocimiento del Enemigo

RESUMEN.-Varios estudios han mostrado que los experimentos sobre defensa de nidos y reconocimiento del enemigo (e.g. reconocimiento de parásitos de nidadas adultos) pueden ser equivocos por muchos factures. Sin embargo, ningún estudio ha descrito un efecto equívoco dado por el tipo de modelo usado como control. En este estudio, demuestro experimentalmente que la elección del modelo puede influenciar los resultados de un experimente, conduciendo a una conclusión errónea. Probé las habilidades de reconocimiento de Sylvia atricapilla, que es actualmente un hospedero raramente usado por Cuculus canorus. S. atricapilla respondió de manera muy diferente ante dos tipos de modelos control: ignoraron a modelos de Turdus merula, pero atacaron a modelos de Columba livia tan frecuentemente como atacaron a C. canorus, Las respuestas diferenciales pueden ser explicadas por el hecho que C. livia es más similar a C. canorus que a T. merula, por lo que provocó un comportamiento mas agresivo que contra T. merula. Por lo tanto, la falta de discriminacion en estudios sobre el reconocimiento de enemigos puede estar reflejando un artefacto metodologico que resulta de la variation en la habilidad de un hospedero en particular de discriminar a lo largo de un continuo de senales de reconocimiento. Este resultado presenta serias consecuencias metodológicas para estudios futures sobre reconocimiento de enemigos y sobre agresividad en general: el modelo utilizado como control no debe ser muy parecido al modelo del parásite de nidada; de lo contrario, la probabilidad de detectar la habilidad de reconocimiento existente es muy baja. Además, argumenta que la coevolución sólo aumenta la agresión preexistente en una especie hospedera en particular. Por Io tanto, los análisis de incremento (la determinación de los cambios en las respuestas antiparasitarias de los hospederos durante el ciclo de nidificación mientras se controla por la agresión de fondo hacia modelos control) brindan una mejor idea sobre las habilidades de reconocimiento de los hospederos que el enfoque tradicional (cuando el nivel total de la respuesta antiparasitaria es analizado).

ALL ANIMALS ARE subject to strong selection pressure from various environmental factors, including predators (Montgomerie and Weatherhead 1988) and brood parasites (Rothstein and Robinson 1998). Response or absence of response to those stimuli may have an important effect on an individual's fitness. Given that brood parasites may reduce host reproductive success (Rothstein 1990), the best defense against parasitism should be to deter a parasitic female from laying her egg in a host nest in the first place (Sealy et al. 1998). However, nest defense, like any other activity around the nest, can be costly-it can attract predators (Martin et al. 2000) or brood parasites (Banks and Martin 2001), there may be a trade-off with parental care (Ueta 1999), and the defending parent risks injury (McLean et al. 1986, McLean 1987, Montgomerie and Weatherhead 1988). Selection, then, should favor recognition of specific intruders. Importantly, the difference between the generalized nest defense and a specific response to the parasite is not relevant, per se, to host avoidance by the parasite-both responses could reduce the probability of parasitism. However, specific enemy recognition can be important evidence of host-parasite coevolution (Sealy et al. 1998).