Acoustic coordination by allied male dolphins in a cooperative context

Abstract

Synchronous displays are hallmarks of many animal societies, ranging from the pulsing flashes of fireflies, to military marching in humans. Such displays are known to facilitate mate attraction or signal relationship quality. Across many taxa, synchronous male displays appear to be driven by competition, while synchronous displays in humans are thought to be unique in that they serve a cooperative function. Indeed, it is well established that human synchrony promotes cooperative endeavours and increases success in joint action tasks. We examine another system in which synchrony is tightly linked to cooperative behaviour. Male bottlenose dolphins form long-lasting, multi-level, cooperative alliances in which they engage in coordinated efforts to coerce single oestrus females. Previous work has revealed the importance of motor synchrony in dolphin alliance behaviour. Here, we demonstrate that allied dolphins also engage in acoustic coordination whereby males will actively match the tempo and, in some cases, synchronize the production of their threat vocalization when coercing females. This finding demonstrates that male dolphins are capable of acoustic coordination in a cooperative context and, moreover, suggests that both motor and acoustic coordination are features of coalitionary behaviour that are not limited to humans.

Keywords: alliance formation; bottlenose dolphin; communication; cooperation; coordinated behaviour; synchrony.

Figure 1.

Spectrograms of individual and multi-male pop trains from one second-order alliance (down-sampled to 48 kHz, FFT length: 1024, Hamming window function). (a) Individual male pop trains. (b) Multi-male pop trains. (Online version in colour.)

Figure 2.

Localized multi-male pop trains from two different second-order alliances. Spectrogram of multi-male pop trains; (a) sampled at 96 kHz, FFT length: 1024, Hamming window function and (b) down-sampled to 48 kHz, FFT length: 1024, Hamming window function. Both panels show pops produced by two different males (A in white and B in yellow) from two different second-order alliances, and the localized bearing of each pop in relation to the research vessel (0° is the research vessel's bow). Each localized pop is also identified with a coloured dot (corresponding to male A or B) at the base of the pop. (Online version in colour.)

Figure 3.

Variation in individual male pop trains. (a) Density plot of the slopes from the linear regression of 279 individual male pop trains (total of 2692 pops) recorded from seven different second-order alliances (one value not visible in density plot, x = 0.028); (b) smoothed linear change in IPI over time in a subset of localized pop trains produced by six individual males from four different second-order alliances. (Online version in colour.)

Figure 4.

Analysis of individual pop tempo when produced in multi-male pop trains. (a) IPI of male A (n = 96) against the IPI of male B immediately following male A (n = 96), across multiple pop trains in the same pop sequence, with corresponding Pearson correlation coefficient; colour-coded by second-order alliance membership (green, RR alliance; grey, KS alliance; red, AC alliance; blue, SB alliance; yellow, PB alliance). Note, this figure does not represent the change in IPI over time but the correlation of consecutive IPIs between male dyads. (b) Null model of the expected correlation coefficients based on 1000 permutations of pop trains produced in multi-male pop trains with pop trains produced by an individual male (matched for number of pops). The dotted line indicates the observed correlation coefficient, with plots colour-coded by second-order alliance membership. Note, given the lack of correlation for the AC alliance, no null model was created. (Online version in colour.)