Hunting without an adult could seal an Orca's fate - Science - News - The Independent

Hunting without an adult could seal an Orca's fate

A killer whale's lunge on to a beach for food can mean death - for the whale. It takes practice, plus the odd nudge from Mum. Malcolm Smith reports

Success in hunting, even if you are a killer whale, doesn't come easy. Perfecting your technique can take years, especially if getting it wrong can seriously endanger your life. What is more, teaching such advanced skills to the point where young killer whales can hunt alone seems to alter substantially the reproductive rate of the adults involved.

The hunting technique in question, arguably one of the most dangerous used by any hunter, involves killer whales intentionally stranding themselves on beaches to grab elephant seal pups. On the beaches on the Crozet Archipelago in British Columbia, Canada, where such seals form a staple food for killer whales, two French biologists - Christopher Guinet and Jerome Bouvier - have been watching the long apprenticeships the young killers must endure.

Adult males - glossy black and white - can be up to 30 feet long and weigh tons. Females are smaller, usually reaching 20 feet. To capture an elephant seal pup lounging around on a beach and take itinto the sea is a remarkable feat for such huge creatures. Avoiding becoming stranded and succumbing to a slow death caused by dehydration and starvation requires considerable strength and agility. The whale must learn to push itself back into a sufficient depth of water to swim away.

What Drs Guinet and Bouvier have reported in the Canadian Journal of Zoology (Vol. 73) is that young killer whales - the calves - didn't even attempt to practise intentional stranding on their own until they were four or five years old. Before that, they were always accompanied by an adult, often the calf's mother. Once they were five or six, they made attempts to catch seal pups, but adults were usually on hand to help.

On some occasions, the adult's presence proved to be essential. Guinet and Bouvier recorded one such remarkable rescue:

"On one occasion, a calf was alone and almost stranded while practising intentional stranding [on a beach]. It experienced difficulties returning to sea. As it was struggling, its mother swam about 50 metres off shore, then turned and accelerated towards the beach. Five metres off the beach, she turned abruptly and produced a large wave that lifted the calf, who then managed to swim back to deep water."

Over a three-year period, the two calves in the pod (whale-watching term for group) were involved in 88 intentional strandings, all except seven taking placewhen no elephant seals were around. The assumption is that these were practice runs. The other seven strandings were done when seal pups were on the beaches, though none of the attempts were successful. Parents, or other adult killer whales, were nearby.

Successes for the whales (but not the seal pups) were recorded during some training runs. In one such event, a whale calf was pushed by its mother so that it grabbed the hapless pup. The female whale then positioned itself between the beach and her calf to prevent it from going too high up the beach. With some pushing by the adult, using her head and upper body, the young killer was able to return to deep water with its prey - truly a case of significant parental support.

It takes a young killer whale around six years, by which stage it is about three-quarters its eventual size, before it can safely catch a seal pup by beaching itself and refloating. Even then, help from a practised adult may be needed.

In other parts of the range of killer whales - throughout much of the Atlantic and Pacific oceans - calves usually don't associate with their mothers once they are about three years old. For instance, in the north Pacific, killer whale calves catch their first salmon within a year and are hunting independently within two years. But catching a large fish at sea is a lot easier for an enormously heavy mammal than grabbing a seal pup virtually on dry land.

The years of parental investment in teaching their apprentices how to catch seal pups - and survive - is the reason, the researchers believe, for the exceptionally low reproductive rate of those killer whales with seals high on their menus.

In waters around British Columbia generally, females, which have a life expectancy of about 50 years, first give birth when they are about 15 years old. Over the next quarter century, they might produce five live calves. At the research site used by Drs Guinet and Bouvier, where the whales seemed dependent upon seal pups, none of the 23 females monitored produced a live calf over a collective period of 47 female years.

No other apprenticeship was surely this costly.

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