(CN) – In the extended families that make up the pods of endangered Southern Resident Killer Whales, males may be the primary hunters for Chinook salmon while females raise the young, according to a study published Monday.
Southern Resident Killer Whales, or orcas, are endangered. Their 75-member population continues to decline despite a major government effort to help them. Pollution and noise from shipping traffic both affect whales – but their biggest problem is a shortage of Chinook salmon to eat.
Researchers already knew that the whales emitted a specific “buzzing” noise when they were closing in on their prey and made certain tell-tale movements revealing when they were about to dive in pursuit of a fish. But the difficulty of observing whales’ underwater hunting practices limited available research.
A team led by Jennifer Tennessen with the National Oceanic Atmospheric Administration attached sensor tags with suction cups near the whales’ dorsal fins to record the underwater voices and movements of 21 whales.
The team collected data on 126 deep dives in which the whales hunted for Chinook salmon. The results, published in the Journal of Experimental Biology, showed that the whales’ herculean efforts to catch fish were not always successful, and that males and females hunted differently. Males made more prey dives, were more likely to successfully catch a fish when they dove and spent more time diving for prey overall than females did.
Southern Residents, which have lifespans similar to those of humans, live in matrilineal groups of extended families. Males can be twice the size of females and therefore require much more food. They often swim on the edges of the family groups, foraging on their own. Researchers speculated that males might hunt more often and at deeper depths than females to avoid competing with females and calves for food.
“Indeed, greater foraging effort by males may allow them to meet their own metabolic needs while not burdening the pod nutritionally, which may enable them to remain in their natal group,” the researchers wrote.
And males share food with females in a purposeful and frequent way, according to the researchers. So they might be hunting more often to help feed females who are caring for young.
“Given that Southern Resident Killer Whales engage in non-random prey sharing, and males and post-reproductive females disproportionately provision each other, particularly within their matriline, males and post-reproductive females may help offset the costs to the pod of additional mouths to feed, while increasing inclusive fitness through the benefits provided by post-reproductive females,” the team wrote.
The team suggested that their research methods could be used in the future to shed light on how shipping traffic affects the whales’ ability to hunt.