Sound travels much farther than visual cues in the marine environment. As a result, many marine mammals rely on sound to gather information about their surroundings, search for prey, avoid predators, and communicate with others of their species. For walruses, vocalizations play an important role in communication and mating displays during the breeding season. As their Arctic environments continue to experience an influx of human activities (and associated noise), it is important to consider how these changes may impact wild populations. But how do we know what walruses can hear, or how anthropogenic noise affects them?
A recent study investigated the hearing ability of walruses by behaviorally evaluating their in-air hearing range, or the span of frequencies that walruses can detect in air. This noninvasive study was conducted by scientists at the University of California, Santa Cruz and Oceans Initiative in Seattle, WA, in collaboration with animal care specialists at Point Defiance Zoo and Aquarium in Tacoma, WA and Six Flags Discovery Kingdom in Vallejo, CA. The team worked with two adult female Pacific walruses (Odobenus rosmarus divergens) named Uquq and Joan, who were trained using seafood rewards to cooperate in hearing tests similar to those given to people. The researchers played tones at different frequencies (i.e., sounds of varying pitch) but constant volume, and the walruses were trained to report detection of these sounds by touching a response target. The researchers mixed in control or blank trials with no sounds; the well-trained walruses learned their task quickly and responded only when they heard a tone.
This study revealed that walruses can detect sounds between 60 Hz and 23 kHz in air, with their underwater hearing range expected to be the same or broader. While this hearing range is narrower than that of related species (e.g., seals, sea lions, sea otters), it is wider than suggested by prior research. It turns out that walruses can hear lower and much higher frequency sounds than scientists previously thought! Importantly, this means that anthropogenic noise in this frequency range may disturb wild individuals. Now that we have this information, it can be considered in the development of conservation plans for this species.
This study of hearing in walruses is ongoing. A second phase is underway that focuses on auditory masking, or how background noise interferes with the detection of relevant sounds like conspecific vocalizations. The research team is currently partnering with SeaWorld San Diego to improve understanding of how well walruses can hear in noisy environments. Ultimately, this work with captive individuals will inform best management practices related to anthropogenic noise in the Arctic habitats on which wild walruses depend.
This simple study also highlights the ability of scientists to work closely with zoological partners to obtain information relevant to walrus conservation that cannot readily be obtained from free-ranging animals. Trained animals living in zoos can learn to support a variety of different research projects, and their lives can be enriched through this interactive process. Behavioral tasks that involve active decision making and learning from experiences—in particular—can benefit the welfare of individuals and inspire meaningful research.
For more information:
Reichmuth, C., Sills, J.M., Brewer, A. Triggs, L., Ferguson, R., Ashe, E., & Williams, R. (2020). Behavioral assessment of in-air hearing range for the Pacific walrus (Odobenus rosmarus divergens). Polar Biology, 43, 767–772. https://doi.org/10.1007/s00300-020-02667-6
Cover photo source: PDZA
[All images provided by Jillian Sills; Edited by Lindsey Broadus]