Whether diving for food in the deep waters of the open ocean or foraging along the seafloor in the shallower depths of the continental shelf, pilot whales adjust their hunting strategies to suit the local environment, a new Duke University-led study shows.
This flexible approach likely contributes to the species' success in adapting to shifting prey distributions and other ecological disturbances as climate change and human activity increasingly alter life beneath the ocean surface.
To conduct the new study, the researchers reviewed thousands of high-pitched echolocation clicks produced by short-finned pilot whales on 287 foraging dives off Cape Hatteras, N.C.
Whales use these clicks, and the echoes produced as they bounce off surrounding objects, to navigate their environment and locate prey. The researchers recorded both the clicks and echoes using acoustic tags that were attached with suction cups to the whales' fins prior to dives. The harmless tags where then recovered hours later after falling off in area waters.
"The tags allowed us to eavesdrop on the whales and 'see' for the first time how close to the bottom they were diving and how their foraging behaviors changed to take advantage of the seafloor environment," said Jeanne Shearer, a doctoral student in marine biology at the Duke University Marine Lab, who was lead author of the study.
Using a whale's echolocation to document its behavior and location is possible because whales emit different types and patterns of clicks for different reasons. The rapid-fire clicks or buzzes a whale emits when it spots prey, for instance, are easily distinguishable from the slower patterns of clicks it uses to gauge distances, map features in its surrounding environment, or locate other whales in its pod.
The characteristics of different echoes likewise can help scientists 'see' where a whale is, since echoes off a nearby object bounce back more quickly, and echoes bouncing off large objects, like the seafloor, sound different than those bouncing off small ones.
By analyzing the distinctive characteristics of the clicks and echoes from the 287 dives, Shearer found that pilot whales off Cape Hatteras, which sits at the edge of the continental shelf, do more of their foraging in shallow waters near the shelf's floor than previously thought and do more of it during the daytime than pilot whales elsewhere who primarily feed in deep oceanic waters.
She also found that when feeding near the bottom, the whales emit more rapid-fire buzzes, which may mean they are finding prey there that are easier to catch or in denser groups, resulting in higher foraging rates than in the deep ocean.
These behaviors may be adaptations the whales have made in response to "the potentially more consistent prey distributions found near the continental shelf seafloor, and illustrate how much behavioral flexibility pilot whales have – they use different strategies in different habitats," Shearer said.
She and her colleagues published their peer-reviewed findings Aug. 25 in Marine Ecology Progress Series.
The Duke Marine Lab is part of Duke's Nicholas School of the Environment.
Researchers at Duke's Nicholas School and its Pratt School of Engineering; Syracuse University; Woods Hole Oceanographic Institution; the University of California Santa Cruz; Southall Environmental Associates and Neptune & Company coauthored the paper with Shearer.
Funding came from North Carolina Sea Grant, NOAA's Strategic Environmental Research and Development Program, and the U.S. Navy Atlantic Fleet Forces Command.
CITATION: "Short-finned Pilot Whales Exhibit Behavioral Plasticity in Foraging Strategies Mediated by Their Environment," Jeanne M. Shearer, Frants H. Jensen, Nicola J. Quick, Ari Friedlaender, Brandon Southall, Douglas J. Nowacek, Matthew Bowers, Heather J. Foley, Zachary T. Swaim, Danielle M. Waples, and Andrew J. Read. Marine Ecology Progress Series, Aug. 25, 2022. DOI: https://doi.org/10.3354/meps14132
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