Male and female reproductive tactics in mallards (anas platyrhynchos l.): sperm competition and cryptic female choice

Mallards, Anas platyrhynchos, are among the most common waterfowl species in theNorthern Hemisphere. Despite their abundance and despite growing interest of behavioralecologists and evolutionary biologists in key aspects of their behavior, few studies haveused genetic tools to investigate their mating system. We studied the breeding biology ofmallards by examining 41 clutches from two areas that differ in breeding density. Wefocus on three aspects of mallard reproductive behavior. First, adult sex ratios in mallardsare often reported to be male-biased. In our population, the proportion of males observedduring autumn and winter counts varied between 59% and 67%. Here we show that thisbias is already present in the primary sex ratio: on average 60% of eggs in a clutch aremales. Second, intra-specific brood parasitism is observed in many duck species. Wefound egg dumping in 53% of mallard clutches in a high breeding density area, whereasbrood parasitism was entirely absent in an area with low breeding density. Third,although mallards are socially monogamous, forced extra-pair copulations are frequentlyobserved. Using microsatellites, we estimate that a minimum of 56% of broods containedat least one extra-pair young. Overall, at least 14% of fertilized eggs were sired by anextra-pair male. Breeding density did not influence the proportion of broods with extrapairpaternity. However, broods from the high density area contained significantly moreextra-pair young than broods from the low density area.

Angelika G. Denk & Bart Kempenaers, Dissertation der Fakultat fur Biologie der Ludwig-Maximilian-Universitat Munchen