Socially Complex Breeding Interactions in Humpback Whales Are Mediated Using a Complex Acoustic Repertoire

Intraspecific conflict can be costly; therefore, many species engage in ritualized contests composed of several stages. Each stage is typically characterized by different levels of aggression, arousal, and physical conflict. During these different levels of “intensity,” animals benefit from communicating potential information related to features such as resource holding potential, relative fighting ability, level of aggression, intent (i.e., fight or flight), and whether or not the competitor currently holds the resource (e.g., a receptive female). This information may be conveyed using both visual displays and a complex acoustic repertoire containing fixed (e.g., age, sex, and body size) and flexible information (e.g., motivation or arousal). Calls that contain fixed information are generally considered “discrete” or stereotyped, while calls that convey flexible information are more “graded,” existing along an acoustic continuum. The use of displays and calls, and the potential information they convey, is likely dependent on factors like intensity level. The breeding system of humpback whales (Megaptera novaeangliae) involves intense male competition for access to a relatively limited number of breeding females (the resource). Here, we investigated the behavior and acoustic repertoire of competitive groups of humpback whales to determine if an increase in intensity level of the group was correlated with an increase in the complexity of the vocal repertoire. We categorized the behavior of humpback whales in competitive groups into three mutually exclusive stages from low to high intensity. While discrete calls were infrequent compared to graded calls overall, their use was highest in “low” and “moderate” intensity groups, which may indicate that this stage of contest is important for assessing the relative resource holding potential of competitors. In contrast, visual displays, call rates, and the use of graded call types, were highest during “high intensity” competitive groups. This suggests that flexible information may be more important in “high intensity” levels as males continue to assess the motivation and intent of competitors while actively engaged in costly conflict. We have shown that the relatively complex social call repertoire and visual displays of humpback whales in competitive groups likely functions to mediate frequently changing within-group relationships.

Stereotyped whistles in southern resident killer whales

The endangered Southern Resident killer whales (Orcinus orca) of the northeast Pacific region use two main types of vocal signals to communicate: discrete calls and whistles. Despite being one of the most-studied cetacean populations in the world, whistles have not been as heavily analyzed due to their relatively low occurrence compared to discrete calls. The aim of the current study is to further investigate the whistle repertoire and characteristics of the Southern Resident killer whale population. Acoustic data were collected between 2006–2007 and 2015–2017 in the waters around San Juan Island, Washington State, USA from boats and from shore. A total of 228 whistles were extracted and analyzed with 53.5% of them found to be stereotyped. Three of the four stereotyped whistles identified by a previous study using recordings from 1979–1982 were still occurring, demonstrating that whistles are stable vocalizations for a period of more than 35 years. The presence of three new stereotyped whistles was also documented. These results demonstrate that whistles share the longevity and vocal tradition of discrete calls, and warrant further study as a key element of Southern Resident killer whale communication and cultural transmission.

Vocal traditions among resident killer whales (Orcinus orca) in coastal waters of British Columbia

Underwater vocalizations were recorded during repeated encounters with 16 pods, or stable kin groups, of resident killer whales (Orcinus orca) off the coast of British Columbia. Pods were identified from unique natural markings on individuals. Vocal exchanges within pods were dominated by repetitious, discrete calls. Pods each produced 7–17 (mean 10.7) types of discrete calls. Individuals appear to acquire their pod's call repertoire by learning, and repertoires can persist with little change for over 25 years. Call repertoires differed significantly among pods in the resident population. The 16 pods formed four distinct acoustic associations, or clans, each having a unique repertoire of discrete calls, or vocal tradition. Pods within a clan shared several call types, but no sharing took place among clans. Shared calls often contained structural variations specific to each pod or group of pods in the clan. These variants and other differences in acoustic behaviour formed a system of related pod-specific dialects within the vocal tradition of each clan. Pods from different clans often travelled together, but observed patterns of social associations were often independent of acoustic relationships. It is proposed that each clan comprises related pods that have descended from a common ancestral group. New pods formed from this ancestral group through growth and matrilineal division of the lineage. The formation of new pods was accompanied by divergence of the call repertoire of the founding group. Such divergence resulted from the accumulation of errors in call learning across generations, call innovation, and call extinction. Pod-specific repertoires probably serve to enhance the efficiency of vocal communication within the group and act as behavioural indicators of pod affiliation. The striking differences among the vocal traditions of different clans suggest that each is an independent matriline.

Acoustic behaviour of resident killer whales (Orcinus orca) off Vancouver Island, British Columbia

Recordings of underwater vocalizations and behavioural observations were collected from 16 photographically identified resident pods of killer whales (Orcinus orca) off Vancouver Island, British Columbia, during 1978–1983. Vocalizations recorded during 43 days spent with three representative pods were analyzed for each of five activity states: foraging, travelling, group-resting, socializing, and beach-rubbing. The whales produced three types of sounds that were assumed to be social signals: discrete calls, variable calls, and aberrant calls. Discrete calls dominated vocalization in most contexts. An increase in production of variable and aberrant calls was observed during socializing and beach-rubbing activities. Each resident pod had a group-specific repertoire of 7–17 discrete call types (mean = 10.7) that was consistent over a number of years. The relative use of different calls varied with activity, but no call type was correlated exclusively with any behaviour or circumstance that could be identified. Discrete calls probably function as intragroup contact signals to maintain pod cohesion and coordinate activities. Structural modulations of standard call formats and variations in frequencies of call use appear to carry information on the emotional state of vocalizing individuals. Group-specific repertoires of calls may increase the effectiveness and reliability of this contact system and function as indicators of pod affiliation.