Abstract
The aim of this thesis is to investigate a number of potential causes and consequences of female extra-pair matings in a closed and wild population of house sparrows (Passer domesticus). First, I compared the morphology and genotypes of within-pair males with the extra-pair males to investigate female mating bias in extra-pair matings in the house sparrow, initially as a case study and then also as a contribution to a meta-analysis including data from all relevant studies on birds. I found that extra-pair males were in general older than within-pair males, but these two types of males did not differ from each other in body size, sexually selected traits or in their genetic compatibility with the focal females. These results suggest that extra-pair matings may not be adaptive to females in terms of gaining indirect benefits. Then, I compared the fitness performance of extra-pair offspring, within-pair offspring from genetically monogamous mothers, and within-pair offspring from polyandrous mothers. I found that extra-pair offspring had the lowest probability of recruiting into the breeding population and produced the fewest number of offspring in their lifetime, although these three types of offspring showed no difference in their probability of hatching and nestling survival. These results indicate that females suffer from indirect costs via extra-pair matings. Finally, I investigated the effects of age on within-pair and extra-pair paternity success. I found that there was an early increase and subsequent senescence-related decline in both within-pair and extra-pair paternity success, with the increase being more rapid and the decline more delayed in extra-pair paternity success. These results suggested that males might be under selection to partition their breeding effort differentially into one or the other type of mating pathway depending on their age. Overall, my thesis demonstrated that the non-adaptive explanations of female extra-pair matings (but adaptive to males) have the potential to improve our understanding of the evolution and maintenance of extra-pair matings. I discuss the ramifications of my thesis findings and suggest future avenues of research in female extra-pair mating behaviour, which continues to puzzle researchers.