Press release courtesy OSU
Toothed whales are known to form long-standing social groups – especially among females – and scientists have hypothesized that these bonds are created to fend off sub-dominant males interested in mating, and to watch after each other's offspring while individual females dive for food.
However, a new study of sperm whales suggests that these long-lasting female social groups – and short-term male aggregations – may actually be collaborations for coordinated feeding behavior, with individual whales taking turns making deep dives to "herd" bait balls of Humboldt squid higher in the water column so the other whales can feed.
Such coordinated feeding behavior has not been observed among sperm whales, scientists say.
Findings of the study, by researchers at Oregon State University and Universidad Autónoma de Baja California Sur in Mexico, were announced today at the American Geophysical Union's Ocean Sciences Meeting in Portland, Ore.
Using new sophisticated tagging instrumentation, the researchers were able to track the movements and diving patterns of sperm whales in the northern Gulf of California between Mexico and the Baja Peninsula. Three of the tagged animals traveled in a social group and took turns diving for squid, according to Bruce Mate, director of the Marine Mammal Institute at Oregon State, and principal investigator on the study.
"We expected their dives to be similar, but each time one of the three whales went deeper than the other two – suggestive of how sea lions coordinate their predation on bait balls of schooling fish," Mate said. "In both cases, it appears that some animals need to dive deeply to prevent the prey from escaping downward.
"It hasn't been possible to gather these types of data from multiple whales at the same to learn about possible coordinated feeding – and it is quite different from the collaboration of other whales, like humpbacks, that conduct 'bubble-net' feeding," Mate added. "There, the role of individual whales seems to be fixed from one dive to the next. We found the sperm whales appear to take turns making the deepest dive – perhaps as a way to share the physiologically demands of the task."
Sperm whales have been difficult to study in detail because they make deep dives and tags used to track their movements have been short-lived, or not accurate enough to adequately describe their dives. However new tags and a special tagging protocol, developed by Mate and Seattle-based Wildlife Computers, use ARGOS satellite transmitter components fitted with a GPS receiver and a time-depth recorder that allows the scientists to track the whales for up to 28 days.
The tags, called Wildlife Computers' Mark 10, are designed to detach and float to the surface, where they can be recovered and the data on dives can be downloaded and analyzed.
The researchers found that the whales frequently would dive 800 meters deep and went as deep as 1,500 meters, sometimes exhibiting fast, zigzag patterns that likely reflect foraging on Humboldt squid, a year-round fishery, Mate said. Frequently, a different whale would make the deepest dive.
"It's a stressful role that is not unlike the lead swan or goose in a V-formation changing positions to share the physiologically demanding role," Mate said. "It also explains why these social bonds form. Female bonds may still have a component that relates to thwarting the attention of sub-dominant males, or babysitting each other's offspring. But that didn't explain why males would bond for short periods of time."
Mate, a professor of fisheries and wildlife at OSU, is a pioneer in the use of satellites to track threatened and endangered whales and other marine mammal species. He directs the Marine Mammal Institute at Oregon State University's Hatfield Marine Science Center in Newport, Ore.