Page 52 - Summer2017

Marine Mammal Acoustic Behavior
Killer Whales
Bottlenose dolphins have a social organization in which one animal may maintain strong associations with a few others but within an otherwise very fluid pattern of grouping in which groups are only stable for a few minutes at a time. By contrast, killer whales have the most stable groups known for any mammal. Neither sex disperses from its natal group as they mature. Closely related groups of killer whales that are matrilineally related (called a matriline) and that associ- ate most of the time together are called pods. Each pod has a unique group-distinctive repertoire of stereotyped calls, which are thought to be used to maintain group cohesion (http://www.dosits.org/files/dosits/orca5.mp3; http:// acousticstoday.org/kwhale). If a pod grows too big, it may separate into several different matrilineally related pods. Pods that have some overlap in their repertoire of stereotyped calls are described as an acoustic clan, and acoustic clans tend to share mitochondrial DNA, suggesting that they descended from the same matriline (Yurk et al., 2002).
The calls of killer whales do not change as rapidly as the songs of humpback whales, but some calls do change over the course of a decade or so. There appears to be variability in the rate of change in different call types, with some show- ing divergence between matrilines over a period of a decade and others showing little change and divergence (Deecke et al., 2000). This suggests that the pattern of acoustic clans may result from cultural drift of the acoustic features of calls coupled with vocal matching of calls from matrilines that regularly associate with one another. This kind of matching across matrilines is called horizontal transmission of infor- mation, in contrast to vertical transmission that goes from parent to offspring.
Within the same geographical area, different ecotypes of killer whale may have completely different foraging patterns. For example, in the Pacific Northwest, there are populations of killer whales that specialize in foraging on salmon in the same area as populations of killer whales that specialize in foraging on marine mammals. When killer whales use echo- location to hunt marine mammal prey that have good hear- ing, they are much more cryptic in their use of echolocation and calling than killer whales that are hunting less acousti- cally sensitive salmon. Even when these different ecotypes of killer whale live in the same place, they have no overlap in their repertoires of stereotyped calls, indicating that hori- zontal transmission of calls requires social association and interaction.
Figure 6. Waveform (top) and spectrogram (bottom) of a sperm whale coda containing 5 clicks (from Rendell and Whitehead, 2005). Note how each coda click contains a series of pulses. ICI, interclick interval.
Sperm Whale Vocalizations
Sperm whales do not make vocalizations as complex as the songs of humpback whales or the stereotyped calls of killer whales. Their communication signals, called codas (Figure 6), are formed of rhythmic repetitions of an altered version of their echolocation click (Madsen et al., 2002; audio file, http://acousticstoday.org/swhale). Analysis of the coda rep- ertoires of groups of sperm whales from the Caribbean and the South Pacific revealed vocal clans. Each vocal clan is a set of sperm whale groups that share the same coda repertoire (Rendell and Whitehead, 2003). Only one vocal clan was recorded in the Caribbean, but five different clans were re- corded in the South Pacific. Each clan is composed of thou- sands of whales spread over thousands of kilometers. Dif- ferent clans could share the same geographical region, but groups producing one set of codas have only been observed joining with other groups that produced the same codas.
Codas are not the only tradition passed down within a sperm whale clan. Different clans had different movement and feeding patterns even when they co-occurred in the same area; different clans also had better foraging success at different phases of the El Niño oceanographic cycle (White- head and Rendell, 2004). These results suggest that cultural differences allow multiple social groupings of sperm and killer whales, each with specific ecological and foraging ad- aptations, to coexist using the same habitat in different ways. A fascinating feature of the culture of whales is that it is cu- mulative; the cultural traits such as vocalizations build on the features of earlier versions. In humans, this accumulation of
50 | Acoustics Today | Summer 2017