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

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

Many animals breed in large mating aggregations, where males must rapidly discriminate between prospective mates and rivals. Selection may favour features that facilitate rapid discrimination in these aggregations. The explosively breeding Neotropical Yellow Toad, Incilius luetkenii, exhibits a rapid and dramatic colour change; males change from a cryptic brown to a conspicuous lemon yellow for their brief breeding period. Females, in contrast, remain cryptic brown throughout the year. The function of this temporary, sex-specific colour change is unknown. We tested the hypothesis that yellow colouration displayed by male I. luetkenii facilitates sex recognition during both daytime and nighttime mating aggregations. We created yellow and brown model toads and presented them to males during a breeding event. Male I. luetkenii responded significantly more intensely to brown models compared to yellow models, approaching them and making more amplexus attempts on the brown versus yellow models. This strong pattern held true regardless of ambient light intensity, making this the first study to expose a dynamic colour signal that operates during both day and night. Our results indicate that male I. luetkenii use colouration to quickly discriminate between males and females during their brief, explosive mating aggregations. Our findings suggest that the rapid, dramatic colour change of male I. luetkenii facilitates sex recognition, which could provide a significant fitness advantage to males in the form of reduced energy expenditure and reduced risk of injury by other males. Dynamic dichromatism may provide similar fitness benefits in any organisms that mate in large, competitive aggregations.

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

Males in polygamous species often engage in intrasexual competition for mates. If females actively evade mating attempts, it may benefit males to cooperate to restrict female movement, as has been found in some mammals. We tested if male dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) cooperate or compete during group mating chases. If they cooperate, then the per-male probability of copulating should increase with group size; if they compete, then the probability should decrease. We followed mating groups by boat during the breeding season (October 2013 – January 2014) off Kaikoura, New Zealand. The copulation rate per male decreased with increasing group size and with the number of noncopulating males in proximity to a copulating female. Male dusky dolphins have multiple mates and appear to use sperm and exploitative scramble competition. Males may remain in mating groups despite competition because there are alliances within the groups, they are unable to exclude rivals from joining a group, the time and energy costs of searching for unescorted females exceed the costs of reduced mating opportunities in a group, they receive other direct or indirect benefits that offset the costs of reduced mating opportunities, or they are in the group largely for social learning rather than procreation.

Sex-specific variation in conspecific-acquired marking prevalence among dusky dolphins (Lagenorhynchus obscurus)

Despite logistical challenges that limit direct observations of behavior for some species, physical scars can provide indirect evidence of aggression. Dusky dolphins (Lagenorhynchus obscurus (Gray, 1828)) off Kaikoura, New Zealand, derive most of the notches and scars on their dorsal fins from conspecifics. However, aggressive encounters have rarely been observed in this sexually monomorphic species. If injurious interactions are associated with intrasexual competition among males but not females, we predicted males would have more conspecific-acquired markings than females. High-quality dorsal fin photographs of dolphins of known sex (n = 79) were collected during boat-based surveys off Kaikoura. We measured three quantitative variables (notch height ratio, notch depth ratio, and dorsal fin rake direction) and one qualitative variable that scored the overall prevalence of markings (notches and scars). All four measures yielded consistent results. Males had significantly more conspecific-acquired markings than females. We reject the hypothesis that there is no difference between the sexes in marking prevalence. We propose that brief events of direct competition are missed by observers but recorded in accumulated patterns of conspecific-acquired markings that differ between the sexes.

Evasive behaviours of female dusky dolphins (Lagenorhynchus obscurus) during exploitative scramble competition

When males engage in scramble competition, are females non-evasive recipients of male coercion or evasive? Small groups of male dusky dolphins (Lagenorhynchus obscurus) engage in exploitative (non-interference) scramble competition for access to oestrous females near Kaikoura, New Zealand. We conducted behavioural sequence analyses of videos of 48 mating groups with continuous records and focal individual follows of females. We determined (1) the frequency and type of behaviours (non-evasive vs. evasive), (2) the variation in simple transition probabilities of behaviours leading to a copulatory position associated with female resistance and non-resistance and (3) the variation in female responses to male behaviours throughout focal follows. Females exhibited frequent active evasive manoeuvers following male behaviours. Copulation sequences were highly variable and most complex when terminated by females. Females altered responses to male signals, one aspect of mate choice potentially favouring male endurance.