Birth synchronisation increases allonursing opportunities in free-ranging dogs

Over 50 years ago, the hypothesis emerged that co-resident women synchronise menstruation through pheromonal cues. This idea has persisted as a widespread narrative and has subsequently been extrapolated to other animals as well. Indeed, reproductive synchrony has many potential benefits, including creating allonursing opportunities and facilitating female mate choice. Nevertheless, little evidence exists of female vertebrates synchronising their oestrus beyond seasonal effects, through socially mediated cues. We analysed reproductive synchrony in free-ranging dogs, which combine weak, population-dependent reproductive seasonality with facultative cooperative breeding. Controlling for seasonality, we show that birth synchrony was higher among co-resident, maternally related females, suggesting synchronisation among individuals who are likely to cooperate. This was supported by our finding that birth synchrony and maternal kinship independently increased the probability and frequency of allonursing. Finally, preliminary evidence shows that oestrus synchrony decreased male monopolisation. These findings contribute to the debate on socially mediated reproductive synchronisation, suggesting that it may have mainly evolved because it facilitates cooperative breeding.