In species that exhibit lifelong monogamous pair bonds, even outside of the breeding season, there is no bias in the sex that is philopatric. However, among polygynous species that disperse (including those that find only a single mate per breeding season), there is a much higher rate of breeding-site philopatry in males than females among birds, and the opposite bias among mammals. Many possible explanations for this sex bias have been posited, with the earliest accepted hypothesis attributing the bias to intrasexual competition, and territory choice. The most widely accepted hypothesis is that proposed by Greenwood (1980). Among birds, males invest highly in protecting resources – a territory – against other males. Over consecutive seasons, a male that returns to the same territory has higher fitness than one that is not philopatric. Females are free to disperse, and assess males. Conversely, in mammals, the predominant mating system is one of matrilineal social organisation.