Mating Group Size in Dusky Dolphins ( L agenorhynchus obscurus ): Costs and Benefits of Scramble Competition

Abstract In fission–fusion social systems with scramble competition between males, multiple males join mating groups while surrounding an oestrous female. If male decisions to join a mating group have been shaped by natural selection, then there should be an optimal group size resulting from the trade‐offs between the benefits of monopolizing a female in small groups and the energy lost in defending her from rivals in large groups. Male dusky dolphins ( L agenorhynchus obscurus ) off K aikoura, N ew Z ealand, provide a unique opportunity to assess the optimum mating group size because they join transient mating groups not confounded by foraging or predator evasion. Within aggregations of up to 1000 individuals, males search for oestrous females, encountering choices of staying with a large mating group or leaving to find a smaller group. Mating groups typically involve multiple males mating with a single female. We conducted focal follows of mating groups (N = 44) by vessel from N ovember 2011 through J anuary 2012. We used video and a GPS to record group size, behaviour and movement. For each group, we measured potential costs ( S wim S peed, L oss of M onopolization P otential) and benefits ( C opulation R ate and D uration, E nergy S avings). Only L oss of M onopolization P otential was positively correlated with group size, while E nergy S avings was negatively correlated. Using these two factors as utility functions, we constructed an optimality model and predicted the optimal mating group size to be seven individuals with a range of 4–11 individuals due to variance. The observed modal mating group size was five dolphins, with a range of 2–15. We compare variation in mating group currencies and sizes to past studies. We discuss potential limitations of applying optimality models to predict mating group size for socially complex and behaviourally plastic species such as dolphins.