Sound Science

In the spring of 1964, two American scientists carved a hole in the ice of McMurdo Sound, Antarctica, to submerge an underwater speaker that played back recorded sounds of Weddell seals to the animals under the ice. This simple experiment became one of the first-ever “playbacks” to a marine mammal in its natural habitat.

Since this landmark study four decades ago, there has been a slow but steady increase in the use of sound playback to study acoustic cognition and communication among marine animals. Dr. Volker Deecke, of the Marine Mammal Research Unit at the University of British Columbia, conducted a review of the published literature on field playback experiments with marine mammals. His findings were recently published in the journal Aquatic Mammals.

Suite of Sounds
“Playback is probably the most commonly used method to study sound cognition and communication in wild animals,” Deecke writes. “While its use is common in research on terrestrial animals, such as birds and anuran amphibians, playback is still only used infrequently to study marine mammal cognition, especially in the field.”

In his review of 46 published studies of field playback with marine mammals, Deecke cites a number of factors that make these types of studies difficult – including the logistical challenges and the expense of conducting research at sea, and the difficulty in quantifying behavioral responses to playback. But he also lists a number of reasons why this important branch of marine mammal research should continue, including its role in conservation.

Click on spectograms below to sample the sounds of killer whale calls: (the files are wav files) Spectogram (a visual representation of sound) of Alaska resident killer whale calls. Spectogram (a visual representation of sound) of transient killer whale calls. Spectogram (a visual representation of sound) of B.C. resident killer whale calls.

“The role of playback in the management of marine mammals is to elicit behavioral responses in certain individuals to prevent them from interfering with human industrial activities (e.g., fishing), to save them from negative impacts caused by human activity (e.g., seismic testing, underwater explosions), or to prevent stranding or other accidental death,” Deecke writes.

Playback experiments have helped researchers to explore many aspects of marine mammal science, including wildlife management, the impact of man-made noise, the recognition of individuals and kin by sound, the study of predator-prey interactions, and the study of communicative vocalizations.

Deecke’s own research on the subject identified an ability among Pacific harbor seals to distinguish between the calls of local fish-eating killer whales and their mammal-eating cousins – thus deducing which whale populations are benign and which are predatory. Deecke notes: “Given the fact that the fish-eating populations show a complex system of group-specific variation in vocal repertoires, the seals’ ability to discriminate between populations is an impressive cognitive feat.”

Future Potential
Scientists like Deecke rely on research using sound playback to provide much of what we know about how marine mammals read and respond to their aquatic environment. “Playback is one of only a handful of experimental tools to study cognition in free-ranging animals,” he writes, “and sound playbacks appear to be especially suited for learning about marine mammals, a group of animals that rely extensively on sound to obtain information about their environment and to transfer information through it.”

In addition to offering a thorough history on the use of sound playback in marine mammal science, Deecke offers his own expertise in a section on playback protocol, in which he points out ways to overcome some of the challenges of designing an experiment based on sound playback. He also advocates the continuation of long-term studies that incorporate acoustic components, suggesting that the resulting databases of recordings will only increase the wealth of acoustic stimuli that could be used in future research.

The branch of marine mammal science that began with a simple hole in the Antarctic ice has snowballed into a worldwide effort involving acoustic telemetry, datalogging devices, and impressive libraries of recorded sounds. What is learned about cognition and communication through sound inevitably feeds a global effort to understand and conserve the spectrum of marine mammal life in the world’s oceans.

As Volker Deecke enthusiastically concludes, “The potential for future sound playback experiments in the field of marine mammal cognition research, as well as the larger field of marine mammalogy, is bound to be substantial.”

July 5, 2007

Publication:
Studying marine mammal cognition in the wild - a review of four decades of playback experiments. Deecke, V.B. 2006. Aquatic Mammals 32(4):461-482. abstract The playback of sounds to animals to assess their behavioural responses presents a powerful tool to study animal cognition in the wild. While playbacks are commonly used to study acoustic responses in birds and other terrestrial animals, their application to the study of marine mammal cognition so far has been limited. A survey of the published literature on field playback experiments with marine mammals identified 46 studies, with a trend towards increased use of playback approaches in recent years. Field playbacks to marine mammals have been used to address questions of wildlife management, the impact of anthropogenic noise, acoustic interactions between predators and prey, individual and kin recognition, as well as the function of communicative sounds. This paper summarizes the major findings of marine mammal playbacks to date and reviews recent advances in the design and execution of playback experiments, with special reference to marine mammals. Issues concer! ning appropriate presentation of acoustic stimuli, appropriate quantification of behavioural responses, as well as appropriate control and replication of treatments are discussed. An analysis of replication in marine mammal playbacks showed that the use of a small number of playback stimuli to conduct multiple playback trials (pseudoreplication) was common. This overview of playback experiments in the study of marine mammal cognition in the wild showed that such approaches contribute significantly to the field; however, in many cases there appears to be substantial room for improvement of playback procedure and experimental design