Killer whales create and visit social clubs just like people do, scientists have discovered.
Up to 100 fish-eating killer whales come together in the Avacha Gulf, off the coast of Russia.
But no-one knew why these orcas form these huge superpods, when they normally live in smaller groups.
Now scientists report in the Journal of Ethology that these groups act as clubs in which the killer whales form and maintain social ties.
Fish-eating killer whales (Orcinus orca) in the Avacha Gulf live in stable groups called pods that contain an average of ten individuals and up to 20 in the largest pods.
But researchers have seen up to eight of these pods coming together to form large groups of up to 100 animals.
As far as the eye can see, in every direction you see killer whales surfacing
Erich Hoyt, WDCS
These large aggregations of pods are seen in numerous places around the world where large numbers of killer whales occur such as British Columbia, Alaska, Iceland and Antarctica.
It is unlikely that the whales gather for protection as they have no natural predators.
In the past researchers have suggested that the killer whales meet to increase their foraging success or to breed.
But the behaviour has not been quantified before.
To investigate, Olga Filatova of the Moscow State University and colleagues from the Far East Russia Orca project observed and photographed whales in the Avacha Gulf from a 4m long boat.
"At first we might see just a few spouts on the horizon. Then quickly we move among them, keeping a distance of a hundred metres so as not to bother them," explained project co-director Erich Hoyt of the Whale and Dolphin Conservation Society (WDCS), which provided the majority of funding for the project.
"As far as the eye can see, in every direction you see groupings of two to six killer whales surfacing, spouting then dipping below the surface."
"Each grouping has a focal mother figure surrounded by her offspring, some of whom may be full grown males with up to 2m dorsal fins that tower over the females," he says.
A small pod goes in search of others
They also used a special underwater directional microphone called a hydrophone to record the sound of the killer whale vocalisations.
Each pod of fish-eating killer whales in the Avacha Gulf has a specific vocal dialect which could be pinpointed by the hydrophone, while individual killer whales can also be identified by the shape of their dorsal fins and markings.
That allowed the scientists to analyse the killer whales' behaviour.
The whales rarely forage and feed when they gather into a much larger superpod, the scientists found.
That suggests they do not gather to herd fish or increase their foraging success. In fact, say the scientists, depending on the type of prey, a superpod might have the effect of decreasing the feeding success of each whale making it unproductive to feed in large groups.
However, the killer whales did interact much more during these large gatherings that lasted from a few hours to almost half a day.
When meeting killer whales from other family pods, they made contact with each other, swam in synchrony and rubbed flippers much more often, the researchers found.
Sexual activity also increased, suggesting that these big aggregations provide a chance to assess potential breeding partners.
However, these behaviours likely have a greater function beyond reproduction, the scientists believe.
They enable the killer whales to establish and maintain social bonds and it is for that reason that the killer whales gather in core meeting areas and form large aggregations.
"The superpods are like big social clubs," says Hoyt. "These clubs could help them stay acquainted, could be part of the courting process but could have other functions that we need to learn about."
Maintaining social bonds is crucial for many social mammals which live and hunt together.
Looking for a mate?
But maintaining connections with the wider community may be especially important for killer whales, which tend to live long lives in relatively small communities with low birth rates.
Killer whales also face challenges with high calf mortality rates which may be as high as 50% in the first six months.
"Understanding more about their social lives, including their reproduction, will be crucial to our future understanding of them and our ability to keep their population healthy," Hoyt says.
The team has also recently extended the study outside of the Avacha Gulf further north and south along the Kamchatka coast and to the offshore Commander Islands to learn more about other killer whale groups, including the mammal-eating type of killer whale that feeds on porpoises, seals and sea otters.