After witnessing a humpback protecting a seal from orcas in Antarctica in 2009, marine ecologist Robert Pitman started searching for comparable accounts and came across 115 instances of humpbacks interacting with orcas, reported by 54 observers since 1951. His scientific team’s survey has just been published in Marine Mammal Science.
Pitman, who works with the National Oceanic & Atmospheric Administration (NOAA) in California, was inspired to undertake the research when he saw the humpback rising from the sea with the threatened Weddell seal on its upturned belly, and keeping it safe from the orcas that had just tipped it off an ice-floe. It then used its flippers to hold the seal in place until it was safe for it to swim away.
Other than humans, adult humpback whales have no natural predators. However, killer whales do sometimes prey on their more vulnerable offspring.
In the survey the scientists found that humpbacks initiated 57% of the interactions with orcas, and in 87% of cases the orcas were hunting or starting to feed on prey at the time. In only 11% of cases were they attacking humpback calves or juveniles – usually they were preying on other species, including grey and minke whales, porpoises, sea-lions, seals and in one case an ocean sunfish.
The humpbacks, frequently working in pairs, would respond by harassing or mobbing the attacking killer whales, action that sometimes allowed the prey to escape.
Pitman believes that the humpbacks initially react to the calls of the hunting orcas without knowing which prey species is being targeted but then harass the orcas anyway, suggesting that humpbacks’ anti-predator behaviour has evolved beyond basic defence.
But other than when young humpbacks are under attack – and not necessarily the adults’ own offspring – there is no apparent benefit for humpbacks continuing to fight off the orcas other than altruism or revenge (neither traits associated with the animal kingdom) – or perhaps they are issuing a general warning to the species.
Read the report here
DIVERNET – The Biggest Online Resource for Scuba Divers