Mating Group Size in Dusky Dolphins (Lagenorhynchus 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 (Lagenorhynchus obscurus) off Kaikoura, New Zealand, 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 November 2011 through January 2012. We used video and a GPS to record group size, behaviour and movement. For each group, we measured potential costs (Swim Speed, Loss of Monopolization Potential) and benefits (Copulation Rate and Duration, Energy Savings). Only Loss of Monopolization Potential was positively correlated with group size, while Energy Savings 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.

Copulation rate declines with mating group size in dusky dolphins (Lagenorhynchusobscurus)

“…Mating groups were identified by observed attempted copulation events or by males swimming inverted with their penises everted (Markowitz 2004;Orbach et al 2014). Because it was not possible to confirm if ejaculation occurred, all ventral contacts with body alignment between males and females were classified as copulation events (Orbach et al 2014).…”

“…Dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) off Kaikoura, New Zealand, also form mating groups with several males chasing one sexually mature female Orbach et al 2014). The modal mating group size consists of four adult males and one adult female (Orbach et al 2014).…”

“…The modal mating group size consists of four adult males and one adult female (Orbach et al 2014). Only one-third of males in mating groups copulate with a female ) and only one offspring results per fertilization.…”

“…Only one-third of males in mating groups copulate with a female ) and only one offspring results per fertilization. Orbach et al (2014) observed that males were positioned directly beneath and in close proximity to a copulating pair, and engaged in immediate copulations with the female when the initial copulating male stopped mating. Markowitz et al (2010) observed the longest copulation duration and highest copulation rate in a small group of dusky dolphins with synchronous male behaviors.…”

Dynamic sexual dichromatism produces a sex signal in an explosively breeding Neotropical toad: A model presentation experiment

“…Many animals breed in dense mating aggregations comprising tens to thousands of individuals, including diverse insects (Antolin and Strand, 1992;Beani and Turillazzi, 1990), fish (Foote et al, 1997), anurans (Wells, 2007), snakes (Shine et al, 2005), birds (Ligon, 1999), and mammals (Orbach et al, 2014). Within these aggregations, males often engage in scramble competition polygyny, roaming to find and secure a breeding partner before their rivals do (Thornhill and Alcock, 1983).…”