Acoustic crypsis in southern right whale mother-calf pairs: infrequent, low-output calls to avoid predation?

Behavioural context of call production in humpback whale calves: identification of potential begging calls in a mysticetes species

Baleen whale calves vocalize, but the behavioural context and role of their social calls in mother-calf interactions are yet to be documented further. We investigated the context of call production in humpback whale (Megaptera novaeangliae) calves using camera-equipped animal-borne multi-sensor tags. Behavioural states, including suckling sessions, were identified using accelerometer, depth and video data. Call types were categorized through clustering techniques. We found that call types and rates predict the occurrence of a given state. Milling, resting and travelling were associated with a median call rate of 0 calls min-1, while surface play, tagging responses and suckling were associated with higher call rates, averaging up to a median of 0.5 calls min-1 for suckling. Suckling sessions were mainly associated with two sets of low-frequency calls corresponding to previously described burping, barking and snorting sounds. Surface play sessions featured mid-frequency calls with whoop-like sounds and other call types. These results address the significance of vocal signalling in mother-calf communication and the calf's development, including the first identification of potential begging calls. Overall, this study offers new insights into baleen whale behaviour, underscores the importance of social calls in mother-calf interactions and enhances our understanding of communication systems in aquatic mammalian mother-young pairs.

Social calls in humpback whale mother-calf groups off Sainte Marie breeding ground (Madagascar, Indian Ocean)

Humpback whales (Megaptera novaeangliae) use vocalizations during diverse social interactions or activities such as foraging or mating. Unlike songs produced only by males, social calls are produced by all types of individuals (adult males and females, juveniles and calves). Several studies have described social calls in the humpback whale's breeding and the feeding grounds and from different geographic areas. We aimed to investigate for the first time the vocal repertoire of humpback whale mother-calf groups during the breeding season off Sainte Marie island, Madagascar, South Western Indian Ocean using data collected in 2013, 2014, 2016, and 2017. We recorded social calls using Acousonde tags deployed on the mother or the calf in mother-calf groups. A total of 21 deployments were analyzed. We visually and aurally identified 30 social call types and classified them into five categories: low, medium, high-frequency sounds, amplitude-modulated sounds, and pulsed sounds. The aural-visual classifications have been validated using random forest (RF) analyses. Low-frequency sounds constituted 46% of all social calls, mid-frequency 35%, and high frequency 10%. Amplitude-modulated sounds constituted 8% of all vocalizations, and pulsed sounds constituted 1%. While some social call types seemed specific to our study area, others presented similarities with social calls described in other geographic areas, on breeding and foraging grounds, and during migrating routes. Among the call types described in this study, nine call types were also found in humpback whale songs recorded in the same region. The 30 call types highlight the diversity of the social calls recorded in mother-calf groups and thus the importance of acoustic interactions in the relationships between the mother and her calf and between the mother-calf pair and escorts.

Preferred shallow-water nursery sites provide acoustic crypsis to southern right whale mother-calf pairs

Adaptations to sound production behaviour can reduce the detectability of animal signals by eavesdroppers in a phenomenon known as acoustic crypsis. We propose that acoustic crypsis can include selection of locations that affect how sound transmits through the environment: habitats with poor acoustic propagation can minimize the range of detectability of animal signals. We investigated the potential for the preferred habitats of southern right whales to confer acoustic crypsis. We modelled acoustic propagation and range of detection of calls from southern right whales in the shallow, sandy, near shore waters where mothers and calves aggregate during the calving season. At three nursery sites across three continents in the southern hemisphere, results showed that the depth at which right whales are most commonly sighted has the most limited acoustic detection range for their calls. Thus, these habitats allow mother-calf pairs to remain acoustically cryptic from potential eavesdroppers, both predators and conspecifics, when their calves are the most vulnerable. Our results provide preliminary evidence that, in addition to other behavioural strategies, the use of habitats with poor acoustic propagation can contribute to acoustic crypsis. This adaptation may be a widespread and underappreciated mechanism for avoidance of eavesdroppers.

An overview of North Atlantic right whale acoustic behavior, hearing capabilities, and responses to sound

Creating a baseline understanding of communicative signals and perceptual abilities is imperative for gaining insight into a species' life history. This is especially relevant for at-risk species, as it can aid in monitoring and conservation efforts. Marine mammals communicate predominately through acoustic modalities for a variety of functions, including foraging and reproduction. The acoustic signals produced by marine mammals, and their ability to perceive signals produced by conspecifics, are directly impacted by the level of ambient noise in the underwater environment in which they inhabit. Modern ocean noise levels are considerably louder than historical levels, and noise is therefore considered to be a threat to acoustically communicating marine mammal species. This review summarizes the documented acoustic signals, hearing abilities, and responses to sound of a critically endangered baleen whale, the North Atlantic right whale (Eubalaena glacialis), highlights gaps in the current body of literature, and identifies priorities for future research.

Cryptic vocal behavior of foraging humpback whales on feeding grounds in West Greenland

Male humpback whales (Megaptera novaeangliae) sing in mating aggregations in the form of song displays, but much less is known about how both sexes use sound on their feeding grounds. Here, we test different hypotheses about the function of vocalizations in 14 foraging humpback whales tagged with sound and movement recording Dtags in Greenland. We show that this population of foraging humpback whales have an overall low call rate of 11.9 calls h^-1 (inter-quartile range = 12.1) with no support for the hypotheses that they employ sound in the localization or manipulation of prey nor in the coordination of lunge feeding. The calls had a mean received level of 135 ± 5dB re 1 μPa, which is some 30 dB lower than maximum levels of song recorded on similar deployed tags, suggesting a much smaller active space of these vocalizations. This reduced active space might, in concert with low call rates, serve to mitigate eavesdropping by predatory killer whales or conspecifics competing for the same prey resources. We conclude that feeding humpback whales in Greenland produce low level, infrequent calls suggesting that calling is not a prerequisite for successful feeding, but likely serves to mediate within group social interactions.

Non-visual camouflage

Rohan Brooker and Bob Wong introduce the ways animals conceal themselves using non-visual sensory stimuli.

Animal-borne tags provide insights into the acoustic communication of southern right whales (Eubalaena australis) on the calving grounds

This study investigated the repertoire, call-type variability and call rates of southern right whales on a calving ground off Brazil in the western South Atlantic. Acoustic tag data were collected from four lactating females and one juvenile. Pulsive, hybrid, and upcalls showed the greatest variability among call-types with up to 23% of non-standard forms detected. Quiet sounds (grunt, single, and double pulse) were detected for the first time in this species on the calving grounds. Although the sample size was limited, results suggest that social interaction increased call-type diversity and call rates, in line with other acoustic studies on right whales.

Variations in received levels on a sound and movement tag on a singing humpback whale: Implications for caller identification

Bio-logging devices are advancing the understanding of marine animal behavior, but linking sound production and behavior of individual baleen whales is still unreliable. Tag placement potentially within the near field of the sound source creates uncertainty about how tagged animal sounds will register on recorders. This study used data from a tagged singing humpback whale to evaluate this question of how sound levels present on a tag when calls are produced by a tagged animal. Root-mean-square (rms) received levels (RLs) of song units ranged from 112 to 164 dB re 1 μPa rms, with some, but not all, of the lower frequency units registering on the tag's 800 Hz accelerometer sensor. Fifty-nine percent of recorded units measured lower acoustic RLs than previously reported source levels for humpback song, but signal-to-noise ratios (SNRs) were 30-45 dB during periods of the dive with low noise. This research highlights that tag RL does not alone predict caller identity, argues for higher SNR thresholds if using SNR to inform decisions about the source of a call, and provides a baseline for future research identifying diagnostic properties of tagged animal calls in cetacean bioacoustic tag datasets.

Acoustic crypsis in communication by North Atlantic right whale mother-calf pairs on the calving grounds

Mammals with dependent young often rely on cryptic behaviour to avoid detection by potential predators. In the mysticetes, large baleen whales, young calves are known to be vulnerable to direct predation from both shark and orca predators; therefore, it is possible that mother–calf pairs may show cryptic behaviours to avoid the attention of predators. Baleen whales primarily communicate through low-frequency acoustic signals, which can travel over long ranges. In this study, we explore the potential for acoustic crypsis, a form of cryptic behaviour to avoid predator detection, in North Atlantic right whale mother–calf pairs. We predicted that mother–calf pairs would either show reduced calling rates, reduced call amplitude or a combination of these behavioural modifications when compared with other demographic groups in the same habitat. Our results show that right whale mother–calf pairs have a strong shift in repertoire usage, significantly reducing the number of higher amplitude, long-distance communication signals they produced when compared with juvenile and pregnant whales in the same habitat. These observations show that right whale mother–calf pairs rely upon acoustic crypsis, potentially to minimize the risk of acoustic eavesdropping by predators.