Phylogenetic review of tonal sound production in whales in relation to sociality.

Laura J May-Collado,Douglas Wartzok,Ingi Agnarsson

doi:10.1186/1471-2148-7-136

Laura J May-Collado, Douglas Wartzok + Show 1 more

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https://doi.org/10.1186/1471-2148-7-136

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Abstract

BackgroundIt is widely held that in toothed whales, high frequency tonal sounds called 'whistles' evolved in association with 'sociality' because in delphinids they are used in a social context. Recently, whistles were hypothesized to be an evolutionary innovation of social dolphins (the 'dolphin hypothesis'). However, both 'whistles' and 'sociality' are broad concepts each representing a conglomerate of characters. Many non-delphinids, whether solitary or social, produce tonal sounds that share most of the acoustic characteristics of delphinid whistles. Furthermore, hypotheses of character correlation are best tested in a phylogenetic context, which has hitherto not been done. Here we summarize data from over 300 studies on cetacean tonal sounds and social structure and phylogenetically test existing hypotheses on their co-evolution.ResultsWhistles are 'complex' tonal sounds of toothed whales that demark a more inclusive clade than the social dolphins. Whistles are also used by some riverine species that live in simple societies, and have been lost twice within the social delphinoids, all observations that are inconsistent with the dolphin hypothesis as stated. However, cetacean tonal sounds and sociality are intertwined: (1) increased tonal sound modulation significantly correlates with group size and social structure; (2) changes in tonal sound complexity are significantly concentrated on social branches. Also, duration and minimum frequency correlate as do group size and mean minimum frequency.ConclusionStudying the evolutionary correlation of broad concepts, rather than that of their component characters, is fraught with difficulty, while limits of available data restrict the detail in which component character correlations can be analyzed in this case. Our results support the hypothesis that sociality influences the evolution of tonal sound complexity. The level of social and whistle complexity are correlated, suggesting that complex tonal sounds play an important role in social communication. Minimum frequency is higher in species with large groups, and correlates negatively with duration, which may reflect the increased distances over which non-social species communicate. Our findings are generally stable across a range of alternative phylogenies. Our study points to key species where future studies would be particularly valuable for enriching our understanding of the interplay of acoustic communication and sociality.

Highlights

It is widely held that in toothed whales, high frequency tonal sounds called 'whistles' evolved in association with 'sociality' because in delphinids they are used in a social context
Baleen whales produce sounds that have fundamental frequencies generally below 5 kHz [2,5], as do members of the sister lineage of Cetacea, the hippos [4]. In contrast, these sounds most commonly range from 5–20 kHz [2], and in some species, e.g. Delphinus delphis, Stenella attenuata, S. coeruleoalba, S. longirostris [6]Lagenorhynchus albirostris [7], Tursiops truncatus [8], fundamental frequencies can go as high as 48 kHz in Inia geoffrensis [9]
In delphinid toothed whales these high frequency tonal sounds, especially when complex, are often referred to as 'whistles', within the group whistle acoustic characteristics vary enormously. Several species produce both frequency modulated whistles and simple whistles that are relatively constant in frequency (e.g., Lagenorhynchus albirostris, [7]; Sotalia fluviatilis [10]; Stenella longirostris [11], others are limited to simple whistles (Lipotes vexillifer) [12] or to few frequency modulated whistles [9]

Summary

Introduction

It is widely held that in toothed whales, high frequency tonal sounds called 'whistles' evolved in association with 'sociality' because in delphinids they are used in a social context. Cetacean tonal signals are broadly defined as narrowband, frequency modulated sounds [1,2,3] Such sounds are produced by both baleen whales (Mysticeti) and toothed whales (Odontoceti) – sister clades containing all extant whales. In delphinid toothed whales these high frequency tonal sounds, especially when complex, are often referred to as 'whistles', within the group whistle acoustic characteristics vary enormously Several species produce both frequency modulated whistles (e.g., sine, convex, concave, upsweep, downsweep) and simple whistles that are relatively constant in frequency (e.g., Lagenorhynchus albirostris, [7]; Sotalia fluviatilis [10]; Stenella longirostris [11], others are limited to simple whistles (Lipotes vexillifer) [12] or to few frequency modulated whistles (e.g., mostly downsweep in Inia geoffrensis) [9]. Whistle frequency modulation and duration varies within species in relation to geography [e.g., [10,11,16,17]], and related species differ in many whistle frequency components (e.g., maximum, minimum, end, and start frequency) [e.g., [18,19,20,21,22]]

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