Ontogeny of vocal rhythms in harbor seal pups: an exploratory study

Small apes adjust rhythms to facilitate song coordination

Song coordination is a universal characteristic of human music. Many animals also produce well-coordinated duets or choruses that resemble human music. However, the mechanism and evolution of song coordination have only recently been studied in animals. Here, we studied the mechanism of song coordination in three closely related species of wild Nomascus gibbons that live in polygynous groups. In each species, song bouts were dominated by male solo sequences (referred to hereafter as male sequence), and females contributed stereotyped great calls to coordinate with males. Considering the function of rhythm in facilitating song coordination in human music and animal vocalizations, we predicted that adult males adjust their song rhythm to facilitate song coordination with females. In support of this prediction, we found that adult males produced significantly more isochronous rhythms with a faster tempo in male sequences that were followed by successful female great calls (a complete sequence with "introductory" and "wa" notes). The difference in isochrony and tempos between successful great call sequences and male sequences was smaller in N. concolor compared with the other two species, which may make it difficult for females to predict a male's precise temporal pattern. Consequently, adult females of N. concolor produced more failed great call (an incomplete sequence with only introductory notes) sequences. We propose that the high degree of rhythm change functions as an unambiguous signal that can be easily perceived by receivers. In this regard, gibbon vocalizations offer an instructive model to understand the origins and evolution of human music.

Isochrony in barks of Cape fur seal (Arctocephalus pusillus pusillus) pups and adults

Animal vocal communication often relies on call sequences. The temporal patterns of such sequences can be adjusted to other callers, follow complex rhythmic structures or exhibit a metronome-like pattern (i.e., isochronous). How regular are the temporal patterns in animal signals, and what influences their precision? If present, are rhythms already there early in ontogeny? Here, we describe an exploratory study of Cape fur seal (Arctocephalus pusillus pusillus) barks-a vocalisation type produced across many pinniped species in rhythmic, percussive bouts. This study is the first quantitative description of barking in Cape fur seal pups. We analysed the rhythmic structures of spontaneous barking bouts of pups and adult females from the breeding colony in Cape Cross, Namibia. Barks of adult females exhibited isochrony, that is they were produced at fairly regular points in time. Instead, intervals between pup barks were more variable, that is skipping a bark in the isochronous series occasionally. In both age classes, beat precision, that is how well the barks followed a perfect template, was worse when barking at higher rates. Differences could be explained by physiological factors, such as respiration or arousal. Whether, and how, isochrony develops in this species remains an open question. This study provides evidence towards a rhythmic production of barks in Cape fur seal pups and lays the groundwork for future studies to investigate the development of rhythm using multidimensional metrics.

Measuring rhythms of vocal interactions: a proof of principle in harbour seal pups

Rhythmic patterns in interactive contexts characterize human behaviours such as conversational turn-taking. These timed patterns are also present in other animals, and often described as rhythm. Understanding fine-grained temporal adjustments in interaction requires complementary quantitative methodologies. Here, we showcase how vocal interactive rhythmicity in a non-human animal can be quantified using a multi-method approach. We record vocal interactions in harbour seal pups (Phoca vitulina) under controlled conditions. We analyse these data by combining analytical approaches, namely categorical rhythm analysis, circular statistics and time series analyses. We test whether pups' vocal rhythmicity varies across behavioural contexts depending on the absence or presence of a calling partner. Four research questions illustrate which analytical approaches are complementary versus orthogonal. For our data, circular statistics and categorical rhythms suggest that a calling partner affects a pup's call timing. Granger causality suggests that pups predictively adjust their call timing when interacting with a real partner. Lastly, the ADaptation and Anticipation Model estimates statistical parameters for a potential mechanism of temporal adaptation and anticipation. Our analytical complementary approach constitutes a proof of concept; it shows feasibility in applying typically unrelated techniques to seals to quantify vocal rhythmic interactivity across behavioural contexts. This article is part of a discussion meeting issue 'Face2face: advancing the science of social interaction'.

Greater Horseshoe Bats Recognize the Sex and Individual Identity of Conspecifics from Their Echolocation Calls

The echolocation calls of bats are mainly used for navigation and foraging; however, they may also contain social information about the emitter and facilitate social interactions. In this study, we recorded the echolocation calls of greater horseshoe bats (Rhinolophus ferrumequinum) and analyzed the acoustic parameter differences between the sexes and among individuals. Then, we performed habituation-discrimination playback experiments to test whether greater horseshoe bats could recognize the sex and individual identity of conspecifics from their echolocation calls. The results showed that there were significant differences in the echolocation call parameters between sexes and among individuals. When we switched playback files from a habituated stimuli to a dishabituated stimuli, the tested bats exhibited obvious behavioral responses, including nodding, ear or body movement, and echolocation emission. The results showed that R. ferrumequinum can recognize the sex and individual identity of conspecifics from their echolocation calls alone, which indicates that the echolocation calls of R. ferrumequinum may have potential communication functions. The results of this study improve our understanding of the communication function of the echolocation calls of bats.

Manatee calf call contour and acoustic structure varies by species and body size

Vocal activity and signal characteristics of mammals are driven by several factors that result in both stability and plasticity over multiple time scales. All three extant species of manatee communicate with several calls that are especially important for maintaining contact between cows and calves. Determining if calf calls differ across manatee species will provide insights into the evolution of species-specific acoustic communication traits. We investigated the interspecific differences in the vocalizations of calves of Amazonian manatees (Trichechus inunguis) and the two subspecies of the West Indian manatee (T. manatus). Vocalizations of individual calves were recorded in rehabilitation centers in Brazil, Puerto Rico, the United States, and Mexico. The acoustic structure of calls produced by manatee calves varied between species and with body size. Amazonian manatee calves produced shorter calls with multiple notes at higher frequency while West Indian calves produced modulated calls that were lower in frequency and longer in duration. Smaller West Indian calves produced frequency modulated, hill-shaped calls that flattened with an increase in body length. Our results provide evidence for divergence in the ontogeny of vocalizations across T. manatus and T. inunguis and suggest variation in body size contributed to the evolution of differences in the characteristics of their calls.

Vocal learning as a preadaptation for the evolution of human beat perception and synchronization

The human capacity to synchronize movements to an auditory beat is central to musical behaviour and to debates over the evolution of human musicality. Have humans evolved any neural specializations for music processing, or does music rely entirely on brain circuits that evolved for other reasons? The vocal learning and rhythmic synchronization hypothesis proposes that our ability to move in time with an auditory beat in a precise, predictive and tempo-flexible manner originated in the neural circuitry for complex vocal learning. In the 15 years, since the hypothesis was proposed a variety of studies have supported it. However, one study has provided a significant challenge to the hypothesis. Furthermore, it is increasingly clear that vocal learning is not a binary trait animals have or lack, but varies more continuously across species. In the light of these developments and of recent progress in the neurobiology of beat processing and of vocal learning, the current paper revises the vocal learning hypothesis. It argues that an advanced form of vocal learning acts as a preadaptation for sporadic beat perception and synchronization (BPS), providing intrinsic rewards for predicting the temporal structure of complex acoustic sequences. It further proposes that in humans, mechanisms of gene-culture coevolution transformed this preadaptation into a genuine neural adaptation for sustained BPS. The larger significance of this proposal is that it outlines a hypothesis of cognitive gene-culture coevolution which makes testable predictions for neuroscience, cross-species studies and genetics. This article is part of the theme issue 'Synchrony and rhythm interaction: from the brain to behavioural ecology'.

Isochrony, vocal learning, and the acquisition of rhythm and melody

A cross-species perspective can extend and provide testable predictions for Savage et al.'s framework. Rhythm and melody, I argue, could bootstrap each other in the evolution of musicality. Isochrony may function as a temporal grid to support rehearsing and learning modulated, pitched vocalizations. Once this melodic plasticity is acquired, focus can shift back to refining rhythm processing and beat induction.

Animal Signals, Music and Emotional Well-Being

Playing music or natural sounds to animals in human care is thought to have beneficial effects. An analysis of published papers on the use of human-based music with animals demonstrates a variety of different results even within the same species. These mixed results suggest the value of tailoring music to the sensory systems of the species involved and in selecting musical structures that are likely to produce the desired effects. I provide a conceptual framework based on the combined knowledge of the natural communication system of a species coupled with musical structures known to differentially influence emotional states, e.g., calming an agitated animal versus stimulating a lethargic animal. This new concept of animal-based music, which is based on understanding animal communication, will lead to more consistent and specific effects of music. Knowledge and appropriate use of animal-based music are important in future research and applications if we are to improve the well-being of animals that are dependent upon human care for their survival.

The Relevance of Ecological Transitions to Intelligence in Marine Mammals

Macphail's comparative approach to intelligence focused on associative processes, an orientation inconsistent with more multifaceted lay and scientific understandings of the term. His ultimate emphasis on associative processes indicated few differences in intelligence among vertebrates. We explore options more attuned to common definitions by considering intelligence in terms of richness of representations of the world, the interconnectivity of those representations, the ability to flexibly change those connections, and knowledge. We focus on marine mammals, represented by the amphibious pinnipeds and the aquatic cetaceans and sirenians, as animals that transitioned from a terrestrial existence to an aquatic one, experiencing major changes in ecological pressures. They adapted with morphological transformations related to streamlining the body, physiological changes in respiration and thermoregulation, and sensory/perceptual changes, including echolocation capabilities and diminished olfaction in many cetaceans, both in-air and underwater visual focus, and enhanced senses of touch in pinnipeds and sirenians. Having a terrestrial foundation on which aquatic capacities were overlaid likely affected their cognitive abilities, especially as a new reliance on sound and touch, and the need to surface to breath changed their interactions with the world. Vocal and behavioral observational learning capabilities in the wild and in laboratory experiments suggest versatility in group coordination. Empirical reports on aspects of intelligent behavior like problem-solving, spatial learning, and concept learning by various species of cetaceans and pinnipeds suggest rich cognitive abilities. The high energy demands of the brain suggest that brain-intelligence relationships might be fruitful areas for study when specific hypotheses are considered, e.g., brain mapping indicates hypertrophy of specific sensory areas in marine mammals. Modern neuroimaging techniques provide ways to study neural connectivity, and the patterns of connections between sensory, motor, and other cortical regions provide a biological framework for exploring how animals represent and flexibly use information in navigating and learning about their environment. At this stage of marine mammal research, it would still be prudent to follow Macphail's caution that it is premature to make strong comparative statements without more empirical evidence, but an approach that includes learning more about how animals flexibly link information across multiple representations could be a productive way of comparing species by allowing them to use their specific strengths within comparative tasks.

Rhythm in speech and animal vocalizations: a cross-species perspective

Why does human speech have rhythm? As we cannot travel back in time to witness how speech developed its rhythmic properties and why humans have the cognitive skills to process them, we rely on alternative methods to find out. One powerful tool is the comparative approach: studying the presence or absence of cognitive/behavioral traits in other species to determine which traits are shared between species and which are recent human inventions. Vocalizations of many species exhibit temporal structure, but little is known about how these rhythmic structures evolved, are perceived and produced, their biological and developmental bases, and communicative functions. We review the literature on rhythm in speech and animal vocalizations as a first step toward understanding similarities and differences across species. We extend this review to quantitative techniques that are useful for computing rhythmic structure in acoustic sequences and hence facilitate cross-species research. We report links between vocal perception and motor coordination and the differentiation of rhythm based on hierarchical temporal structure. While still far from a complete cross-species perspective of speech rhythm, our review puts some pieces of the puzzle together.