Patterns in the vocalizations of male harbor seals

Are Icelandic harbor seals acoustically cryptic to avoid predation?

Male harbor seals (Phoca vitulina) produce stereotypic underwater roars during the mating season. It remains unclear to what extent roar structures vary due to predation levels. Here, seal roars from waters with many (Iceland) and few (Denmark and Sweden) predators were compared. Most Icelandic roars included a long pulse train and a pause. Icelandic roars occurred less frequently, lasted longer (20.3 ± 6.5 s), and were recorded with lower received sound levels (98.3 ± 8.9 dB re 1 μPa root mean square) than roars from Denmark and Sweden. Local extrinsic factors may shape sound production in harbor seals more than previously reported.

The underwater vocal complexity of seals (Phocidae) is not related to their phylogeny

Closely related mammalian species often make similar vocalizations, but this is not so with the underwater calls of the true seals. Some seal species have diverse underwater vocal repertoires, whereas others only make pulsed calls. Vocal complexity scores of underwater calls of 13 seal species were compared with their phylogeny and life-history traits. Waveform types, repertoire sizes, repetition and rhythm patterns, and frequency and duration measures (15 attributes, scaled 0 to 1) were summed to give a vocal complexity score. The lowest complexity group use low frequency, burst pulse or irregular waveforms and have small repertoires. The intermediate group have both sinusoidal and noisy waveforms, songs, and a single rhythm pattern in repeated element calls. The most complex group have large repertoires, sinusoidal and noisy waveforms, songs, and two or more rhythm patterns in repeated element calls. There is no evidence of a relationship between phylogeny and vocal complexity. The low vocal complexity species are serially monogamous, do not form breeding groups, breed on beaches or pack ice, and are subject to higher predation risk. Species with higher vocal complexity are promiscuous or polygamous, form breeding groups on pack or landfast ice, and have a lower predation risk.

Clear diel patterns in breeding calls of harbor seals (Phoca vitulina) at Hornby Island, British Columbia, Canada

During the breeding season, male harbor seals (Phoca vitulina Linnaeus, 1758) produce underwater calls for sexual advertisement. The daily and seasonal timing of these calls is influenced by female availability (i.e., tidal haul-out patterns, foraging behavior, and oestrus cycle). Therefore, temporal patterns of male calling can provide clues about patterns of female behavior. We collected underwater recordings during the 2014 breeding season at Hornby Island, British Columbia, Canada, and examined the relationships of light level, progression of breeding season, and tide relative to call presence or absence and calling rate. Calls were 15 times more likely to occur at night than during the day. Nocturnal peaks in calling rate have been observed in other harbor seal populations and have been attributed to tidal haul-out patterns and nocturnal foraging of females. In this study, tide level did not have a significant effect on calling rate, and female foraging behavior was not monitored. One acoustic observation of mammal-eating killer whale (Orcinus orca (Linnaeus, 1758)) calls was followed by 48 h of decreased calling rate. We infer that predation risk influences the temporal pattern of male calling at this location and suggest further study to support this hypothesis.

Hearing thresholds, for underwater sounds, of harbor seals (Phoca vitulina) at the water surface

High-amplitude impulsive sounds produced by pile driving and airguns may result in hearing damage in nearby seals. By swimming at the water surface, seals may reduce their exposure to underwater sound, as sound pressure levels (SPLs) are often lower just below the surface than deeper in the water column. Seals can make physiological adjustments such that they can switch between having maximum sensitivity for either aerial or underwater sounds. This could mean that hearing sensitivity for underwater sounds is lower when swimming at the water surface (when hearing may be focused on aerial sounds) than when swimming at depth. To investigate this possibility, hearing thresholds of two female harbor seals were quantified psychophysically, while their heads were in the position normally adopted while swimming at the surface. The seals' hearing thresholds at the water surface were similar to each other and to previous measurements made at 1 m depth. When calculating the cumulative sound exposure level for hearing damage assessment, the SPL just below the water surface needs to be measured or modeled, and the proportion of time seals normally swim at the water surface needs to be estimated, to estimate the sound energy that reaches the seals' ears.

Female harbor seal (Phoca vitulina) behavioral response to playbacks of underwater male acoustic advertisement displays

During the breeding season, male harbor seals (Phoca vitulina) make underwater acoustic displays using vocalizations known as roars. These roars have been shown to function in territory establishment in some breeding areas and have been hypothesized to be important for female choice, but the function of these sounds remains unresolved. This study consisted of a series of playback experiments in which captive female harbor seals were exposed to recordings of male roars to determine if females respond to recordings of male vocalizations and whether or not they respond differently to roars from categories with different acoustic characteristics. The categories included roars with characteristics of dominant males (longest duration, lowest frequency), subordinate males (shortest duration, highest frequency), combinations of call parameters from dominant and subordinate males (long duration, high frequency and short duration, low frequency), and control playbacks of water noise and water noise with tonal signals in the same frequency range as male signals. Results indicate that overall females have a significantly higher level of response to playbacks that imitate male vocalizations when compared to control playbacks of water noise. Specifically, there was a higher level of response to playbacks representing dominant male vocalization when compared to the control playbacks. For most individuals, there was a greater response to playbacks representing dominant male vocalizations compared to playbacks representing subordinate male vocalizations; however, there was no statistical difference between those two playback types. Additionally, there was no difference between the playbacks of call parameter combinations and the controls. Investigating female preference for male harbor seal vocalizations is a critical step in understanding the harbor seal mating system and further studies expanding on this captive study will help shed light on this important issue.

Geographic vocal variation and perceptual discrimination abilities in male Australian sea lions

Vocal characteristics can vary among and within populations. In species with geographic variation in the structure of vocalizations, individuals may have the ability to discriminate between calls from local and non-local individuals. The ability to distinguish differences in acoustic signals is likely to have a significant influence on the outcome of social interactions between individuals, including potentially mate selection and breeding success. Pinnipeds (seals, fur seals, sea lions and walruses) are highly vocal yet the Australian sea lion (Neophoca cinerea) is the only eared seal known to show geographic vocal variation in male barks. Barks are produced in many social interactions and encode sufficient information for both individual and colony identity to be discriminable. Yet until now, whether males could themselves discriminate these bark differences was unclear. We performed playback experiments in four breeding colonies to investigate whether males can discriminate local from non-local barks. Overall, males responded more strongly to barks from their own colony compared to barks from other colonies regardless of whether those other colonies were close or distant. Competition for females is high in Australian sea lions, but mating periods are asynchronous across colonies. The ability to correctly assess whether a male is from the same colony, thus representing a potential competitor for mates, or merely a visitor from elsewhere, may influence how males interact with others. Given the high cost of fighting, the ability to discern competitors may influence the nature of male-male interactions and ultimately influence how they allocate reproductive effort.

Comment on "Temporal and spatial variation in harbor seal (Phoca vitulina L.) roar calls from southern Scandinavia" [J. Acoust. Soc. Am. 141, 1824-1834 (2017)]

In their recent article, Sabinsky and colleagues investigated heterogeneity in harbor seals' vocalizations. The authors found seasonal and geographical variation in acoustic parameters, warning readers that recording conditions might account for some of their results. This paper expands on the temporal aspect of the encountered heterogeneity in harbor seals' vocalizations. Temporal information is the least susceptible to variable recording conditions. Hence geographical and seasonal variability in roar timing constitutes the most robust finding in the target article. In pinnipeds, evidence of timing and rhythm in the millisecond range-as opposed to circadian and seasonal rhythms-has theoretical and interdisciplinary relevance. In fact, the study of rhythm and timing in harbor seals is particularly decisive to support or confute a cross-species hypothesis, causally linking the evolution of vocal production learning and rhythm. The results by Sabinsky and colleagues can shed light on current scientific questions beyond pinniped bioacoustics, and help formulate empirically testable predictions.

How small could a pup sound? The physical bases of signaling body size in harbor seals

Vocal communication is a crucial aspect of animal behavior. The mechanism which most mammals use to vocalize relies on three anatomical components. First, air overpressure is generated inside the lower vocal tract. Second, as the airstream goes through the glottis, sound is produced via vocal fold vibration. Third, this sound is further filtered by the geometry and length of the upper vocal tract. Evidence from mammalian anatomy and bioacoustics suggests that some of these three components may covary with an animal's body size. The framework provided by acoustic allometry suggests that, because vocal tract length (VTL) is more strongly constrained by the growth of the body than vocal fold length (VFL), VTL generates more reliable acoustic cues to an animal's size. This hypothesis is often tested acoustically but rarely anatomically, especially in pinnipeds. Here, we test the anatomical bases of the acoustic allometry hypothesis in harbor seal pups Phoca vitulina. We dissected and measured vocal tract, vocal folds, and other anatomical features of 15 harbor seals post-mortem. We found that, while VTL correlates with body size, VFL does not. This suggests that, while body growth puts anatomical constraints on how vocalizations are filtered by harbor seals' vocal tract, no such constraints appear to exist on vocal folds, at least during puppyhood. It is particularly interesting to find anatomical constraints on harbor seals' vocal tracts, the same anatomical region partially enabling pups to produce individually distinctive vocalizations.

Temporal and spatial variation in harbor seal (Phoca vitulina L.) roar calls from southern Scandinavia

Male harbor seals gather around breeding sites for competitive mating displays. Here, they produce underwater vocalizations possibly to attract females and/or scare off other males. These calls offer prospects for passive acoustic monitoring. Acoustic monitoring requires a good understanding of natural variation in calling behavior both temporally and among geographically separate sites. Such variation in call structure and calling patterns were studied in harbor seal vocalizations recorded at three locations in Danish and Swedish waters. There was a strong seasonality in the calls from end of June to early August. Vocalizations at two locations followed a diel pattern, with an activity peak at night. Recordings from one location also showed a peak in call rate at high tide. Large geographic variations were obvious in the total duration of the so-called roar call, the duration of the most prominent part of the call (the roar burst), and of percentage of energy in roar burst. A similarly large variation was also found when comparing the recordings from two consecutive years at the same site. Thus, great care must be taken to separate variation attributable to recording conditions from genuine biological differences when comparing harbor seal roars among recording sites and between years.

Classification of underwater vocalizations of wild spotted seals (Phoca largha) in Liaodong Bay, China

Underwater vocalizations were recorded and classified from wild spotted seals (Phoca largha) in Liaodong Bay, China. The spotted seals exhibited an extensive underwater vocal repertoire but with limited complexity. Four major call types, representing 77.8% of all calls recorded, were identified using multivariate analyses of ten acoustic parameters; knock, growl, drum, and sweep. The calls were relatively brief (12-270 ms, mean of -10 dB duration) pulsating sounds of low-frequency (peak frequency <600 Hz) and narrow bandwidth (169-232 Hz, mean of -3 dB bandwidth; 237-435 Hz, mean of -6 dB bandwidth). Frequency variables (-3/-6 dB frequency bandwidth, center frequency, and top three peak frequencies) were the primary descriptors used to differentiate the call types. Comparing the spotted seal underwater vocalizations with those of the closely related Pacific harbor seal (Phoca vitulina richardii) indicated that the two species use similar bandwidths and peak frequencies but spotted seal calls were generally shorter. Knowledge of underwater vocalizations of wild spotted seals is important for understanding the species behavior and for planning future acoustic surveys of its distribution and occurrence.

Aerial low-frequency hearing in captive and free-ranging harbour seals (Phoca vitulina) measured using auditory brainstem responses

The hearing sensitivity of 18 free-ranging and 10 captive harbour seals (Phoca vitulina) to aerial sounds was measured in the presence of typical environmental noise through auditory brainstem response measurements. A focus was put on the comparative hearing sensitivity at low frequencies. Low- and mid-frequency thresholds appeared to be elevated in both captive and free-ranging seals, but this is likely due to masking effects and limitations of the methodology used. The data also showed individual variability in hearing sensitivity with probable age-related hearing loss found in two old harbour seals. These results suggest that the acoustic sensitivity of free-ranging animals was not negatively affected by the soundscape they experienced in the wild.

Quantitative classification of harbor seal breeding calls in Georgia Strait, Canada

During breeding season, male harbor seals (Phoca vitulina) produce underwater calls used in sexual competition and advertisement. Call characteristics vary among populations, and within-population differences are thought to represent individual variation. However, vocalizations have not been described for several populations of this widely-distributed and genetically diverse species. This study describes the vocal repertoire of harbor seals from British Columbia, Canada. Underwater recordings were made near Hornby Island during the summer of 2014 using a single hydrophone. A wide variability was detected in breeding vocalizations within this single breeding site. Four candidate call types were identified, containing six subtypes. Linear discriminant analysis showed 88% agreement with subjective classification of call types, and 74% agreement for call subtypes. Classification tree analysis gave a 92% agreement with candidate call types, with all splits made on the basis of call duration. Differences in duration may have reflected individual differences among seals. This study suggests that the vocal repertoire of harbor seals in this area comprises a vocal continuum rather than discrete call types. Further work with the ability to localize calls may help to determine whether this complexity represents variability due to propagation conditions, animal orientation, or differences among individual seals.

What Pinnipeds Have to Say about Human Speech, Music, and the Evolution of Rhythm

Research on the evolution of human speech and music benefits from hypotheses and data generated in a number of disciplines. The purpose of this article is to illustrate the high relevance of pinniped research for the study of speech, musical rhythm, and their origins, bridging and complementing current research on primates and birds. We briefly discuss speech, vocal learning, and rhythm from an evolutionary and comparative perspective. We review the current state of the art on pinniped communication and behavior relevant to the evolution of human speech and music, showing interesting parallels to hypotheses on rhythmic behavior in early hominids. We suggest future research directions in terms of species to test and empirical data needed.

Vocal learning in seals, sea lions, and walruses

The pinnipeds provide a variety of clues to those interested in the vocal learning capabilities of non-human animals. Observational and experimental studies of seals, sea lions, and walruses reveal elements of vocal development, contextual control, plasticity in expression and learning, and even imitation of complex sounds. Consideration of the factors that influence the expression of these capabilities informs understanding of the behavioral and structural mechanisms that support vocal learning in mammals and the evolutionary forces shaping these capabilities.

Mating system and reproductive success in eastern Pacific harbour seals

Harbour seals sometimes breed along inland travel corridors where females become clustered in space and time and males establish underwater acoustic display territories similar to terrestrial arenas known as resource-based leks. Under these conditions, we predicted that higher levels of polygyny would be observed than has been previously reported for this species mating in open coast environments without travel corridors. Reproductive success (RS) of 70 males was measured using 20 microsatellite DNA loci and likelihood-based paternity analysis of 136 offspring collected over 3 years. Most males were assigned either zero or one paternity with 80% confidence. The greatest number of pups assigned to one male in a season was two. Variance in RS was higher for males than females (which are biologically limited to one offspring per year) indicating low to mild polygyny. In addition, distributions of relatedness values among pups within year classes did not differ significantly from a simulated distribution with R = 0, indicating that half-siblings were uncommon. Overall, polygyny levels were low relative to terrestrial pinniped mating systems and similar to observations from a harbour seal population along an open coast. Due to large confidence intervals associated with our results, we cannot rule out the hypothesis that a travel corridor might increase the degree of polygyny skew relative to that observed in open coast environments. Habitat appeared to influence male strategies as the most successful males in open coast environments patrolled offshore, while the most successful male in this study defended a territory along the travel corridor.

Geographic variation and acoustic structure of the underwater vocalization of harbor seal (Phoca vitulina) in Norway, Sweden and Scotland

The male harbor seal (Phoca vitulina) produces broadband nonharmonic vocalizations underwater during the breeding season. In total, 120 vocalizations from six colonies were analyzed to provide a description of the acoustic structure and for the presence of geographic variation. The complex harbor seal vocalizations may be described by how the frequency bandwidth varies over time. An algorithm that identifies the boundaries between noise and signal from digital spectrograms was developed in order to extract a frequency bandwidth contour. The contours were used as inputs for multivariate analysis. The vocalizations' sound types (e.g., pulsed sound, whistle, and broadband nonharmonic sound) were determined by comparing the vocalizations' spectrographic representations with sound waves produced by known sound sources. Comparison between colonies revealed differences in the frequency contours, as well as some geographical variation in use of sound types. The vocal differences may reflect a limited exchange of individuals between the six colonies due to long distances and strong site fidelity. Geographically different vocal repertoires have potential for identifying discrete breeding colonies of harbor seals, but more information is needed on the nature and extent of early movements of young, the degree of learning, and the stability of the vocal repertoire. A characteristic feature of many vocalizations in this study was the presence of tonal-like introductory phrases that fit into the categories pulsed sound and whistles. The functions of these phrases are unknown but may be important in distance perception and localization of the sound source. The potential behavioral consequences of the observed variability may be indicative of adaptations to different environmental properties influencing determination of distance and direction and plausible different male mating tactics.