MALE REPRODUCTIVE SUCCESS IS CORRELATED WITH BLOOD PARASITE LEVELS AND BODY CONDITION IN THE PROMISCUOUS AQUATIC WARBLER (ACROCEPHALUS PALUDICOLA)

Auk, The, Apr 2005 by Dyrcz, Andrzej, Wink, Michael, Kruszewicz, Andrzej, Leisler, Bernd

ABSTRACT

The Aquatic Warbler (Acrocephalus paludicola) has a unique mating system. Males are free of any parental duties and can fertilize many females during a single breeding season. Females also usually copulate with more than one male, and nestlings in a single nest may be sired by as many as five males. Paternal success of 33 Aquatic Warbler males in a Polish population was determined by microsatellite polymerase-chain-reaction analysis. Males infected by blood parasites (trypanosomes) sired fewer offspring, weighed less, and probably arrived later to breeding grounds than uninfected males. Number of nestlings sired by a male correlated positively with its fat deposits and wing length. These findings indicate that male body condition directly influences paternal success. Number of sired nestlings per individual male ranged from one to eight. Reproductive success among males was uneven, with six males (18%) fathering 44% of nestlings in the study area. Males with high reproductive success arrived at the breeding grounds earlier in spring than males with low reproductive success. Received 23 April 2003, accepted 10 November 2004.

Key words: Aquatic Warbler, Acrocephalus paludicola, blood parasites, male reproductive success.

El �xito Reproductivo de los Machos Est� Correlacionado con los Niveles de Par�sitos en la Sangre y la Condici�n Corporal en la Especie Promiscua Acrocephalus paludicota

RESUMEN.-Acrocephalus paludicola tiene un sistema de apareamiento �nico. Los machos se ven liberados de toda responsabilidad parental y pueden fecundar a muchas hembras durante una �poca reproductiva. Las hembras tambi�n copulan generalmente con m�s de un macho, y los polluelos en un nido pueden haber sido engendrados por un m�ximo de 5 machos. Se determin� el �xito parental de 33 machos de A. paludicola en una poblaci�n polaca, utilizando un an�lisis de microsat�lites por medio de la reacci�n en cadena de la polimerasa. Los machos infectados por par�sitos de sangre (tripanosomas) engendraron menos progenie, pesaron menos y probablemente llegaron m�s tarde a los territories de cr�a que los machos no infectados. El n�mero de polluelos engendrados por un macho se correlacion� positivamente con sus dep�sitos de grasa y longitud del ala. Estos hallazgos indican que la condici�n corporal del macho influencia directamente el �xito parental. El n�mero de polluelos engendrados por cada macho vari� entre 1 y 8. El �xito reproductivo entre los machos no fue parejo; seis machos (18%) fueron los padres del 44% de los polluelos en el �rea de estudio. Los machos con alto �xito reproductivo llegaron antes a los territories de cr�a en la primavera que los machos con un �xito reproductivo menor.

BLOOD PARASITES HAVE been shown to negatively affect reproductive success of birds (especially females) in several studies (discussed below). In the present study, we analyzed variation in reproductive success of individual males in a Polish population of Aquatic Warblers (Acrocephalus paludicola). Specifically, we asked whether male reproductive success is affected by blood parasites and body parameters. Hamilton and Zuk's (1982) hypothesis predicts that resistance to endoparasites is inherited. Data supporting such inheritance are scarce. M011er (1990), in his summary article, listed only three papers that proved intraspecific heritable resistance to parasites. In any case, resistance to parasites can be an important factor in female choice, and thus influence male reproductive success. Some male body parameters may have a similar influence.

The Aquatic Warbler inhabits Carex marshlands, which are particularly rich in arthropods, in fertile river valleys, mainly in Belarus, Poland, and Ukraine. Nests of the Aquatic Warbler are often clustered in places where food resources are abundant (Dyrcz and Zdunek 1993). Within clusters, females are aggressive toward one another, and their foraging grounds are exclusive during the nestling-feeding period. Males have defined singing posts; however, no distinct agonistic behavior among males was observed in the field. Telemetric data indicate that males regularly visit large areas throughout the population; as a consequence, their home ranges often overlap (Schmidt et al. 1999, Schaefer et al. 2000). The Aquatic Warbler has a unique mating system, which is classified as a kind of promiscuity (Heise 1970; Birkhead 1993; Schulze-Hagen et al. 1993, 1995; Dyrcz et al. 2002). Males can sire nestlings in the nests of many females, and eggs in one nest may be fertilized by more than one male. There are no leks or arenas, but males sing during the whole breeding season from more-or-less fixed singing posts. Males do not participate in parental duties. The high density of potential prey may explain and facilitate the uniparental rearing of offspring by the female. The percentage of broods with nestlings sired by more than one male is high, ranging from 54% to 92% (n = 62) across years; a maximum of five fathers per brood was detected in four broods (Dyrcz et al. 2002).