Paternity assurance through frequent copulations in a wild passerine with intense sperm competition

Abstract

Frequent copulation is assumed to be an important male reproductive strategy for paternity assurance in species with female sexual promiscuity. However, the empirical evidence supporting this hypothesis is limited. We examined copulation behaviour in relation to within-pair paternity in the socially monogamous tree swallow, Tachycineta bicolor, a passerine bird species with frequent copulation, a lack of mate guarding and high levels of extrapair paternity. We found that the frequency of within-pair copulation attempts increased and peaked just before the appearance of the first egg. A marked drop in the frequency of copulation attempts was observed once the first egg was laid. Furthermore, the frequency of successful within-pair copulations was highly repeatable for particular pairs within the 3 days prior to appearance of the first egg. Finally, males that obtained a higher number of successful within-pair copulations during this time also sired a larger proportion of the offspring in their own brood. Our findings suggest that frequent copulations just before the start of egg laying is an adaptation to decrease the risk of being cuckolded in the presence of sperm competition, and thus provide empirical support for the paternity assurance hypothesis. Crown Copyright 2008 Published by Elsevier Ltd on behalf of The Association for the Study of Animal Behaviour. All rights reserved. Paternity assurance is critical to male fitness, particularly in species in which females frequently participate in extrapair copulations (EPCs) and males provide costly parental care. In response to fitness loss due to cuckoldry, several types of paternity assurance behaviour have evolved across taxa. One strategy is constant mate guarding by following, as males can protect their paternity by ensuring that no other male has an opportunity to mate with their paired female (Moller & Birkhead 1991). However, for some species, constant mate guarding may be constrained by territory protection or other activities. The paternity assurance hypothesis (Moller 1987; Simmons 1990), also known as the paternity confidence hypothesis (Eens & Pinxten 1995) and the sperm competition hypothesis (Birkhead et al. 1987), suggests that for species in which EPCs are common, males copulate frequently with their pair females to avoid being cuckolded. Males may minimize cuckoldry through frequent copulation by devaluing EPCs that their mates