Do males offset the cost of female aggression? An experimental test in a biparental songbird

Aggressive behavior in females is thought to be costly due to a trade-off between aggression and parental care. In biparental systems, resolution of this trade-off may depend on the extent to which males mitigate the cost of female aggression. Using a population of tree swallows (Tachycineta bicolor) in which intrasexual aggression has been shown to be beneficial to females in acquiring a nesting cavity, but costly in terms of offspring quality, I asked if the cost of female aggression is offset by her partner. First, I determined if pairs mate disassortatively by aggressiveness and whether the degree of dissimilarity of aggressiveness correlates with parameters of reproductive success. I then experimentally handicapped males to test whether female aggressiveness becomes more costly when males provision young less. I found no evidence of disassortative mating, although pairs differing more in aggressiveness laid more and larger eggs. When male provisioning was reduced, offspring were no worse in quality, but nestling mortality increased. Aggressive behavior was only associated with a fitness cost in control nests. Therefore, males may mitigate the cost of aggression for their female partners indirectly, not by compensating for poor parenting by aggressive females, but instead by females investing more heavily in reproduction when mated to a male that is more different from her own phenotype. To the extent this differential allocation outweighs the cost of aggressiveness, male phenotype may play a key role in understanding the selective pressures shaping the evolution of aggressive behavior in females.