Richard Collins: A whale of a time off the Beara Peninsula

Spring was late this year, migrant birds being slow to arrive. Things were different at sea; whales and basking sharks turned up earlier than usual, writes Richard Collins.

Richard Collins: A whale of a time off the Beara Peninsula

Spring was late this year, migrant birds being slow to arrive. Things were different at sea; whales and basking sharks turned up earlier than usual, writes Richard Collins.

Two orcas visited the Blaskets at the beginning of March and there were humpbacks off the Beara Peninsula on the 26th of the month, the earliest arrival date of recent years, say the Irish Whale and Dolphin Group.

Cold winds kept the birds away but sea temperatures change slowly; even prolonged cold snaps have little effect. In any case, sea mammals cope well with cold. That’s mostly down to Bergmann Rule.

Creatures living in cold environments, it states, tend to be bigger than those in warmer ones. A body’s ability to retain heat depends on its volume, while heat is lost through the surface. The volume of a sphere depends on the cube of its radius, but the surface area is only a square function.

Big objects, therefore, hold on to heat longer than small ones. Tea cools more slowly in the pot than in the cup and the water of the Gulf Stream stays warm for months. In cold environments it pays to be big.

It’s not just whales that are large. All sea mammals are. Elephant seals and manatees make obese humans look slim.

The various marine mammals, however, have different evolutionary origins. Whales shared a hippo-like ancestor with deer and cattle fifty million years ago, seals and sealions are related to dogs, while manatees are distant cousins of elephants. Growth in the sizes of these creatures, therefore, developed independently.

William Geary and colleagues at the Stanford School of Earth Sciences, intrigued by this curious evolutionary history, calculated the average body masses of 3,859 mammal species alive today and 2,999 fossil ones. Those living full-time in water, they found, are bigger than their equivalents on land.

Being able to float was a major incentive to becoming large; sea mammals don’t have to support their great weight on legs. With no cosy dens to retreat into when conditions got rough, their only option was to become as big as possible. It wasn’t a choice but a necessity.

So have sea-creatures carte-blanche to be as big as they like? Can they let it all hang out when it comes to size? No, say the researchers, there are serious constraints.

In fact, the range of viable sizes at sea is much narrower than it is on land. Getting enough fuel to keep the show on the road is a major challenge. The bigger you are, the more you must eat to stay alive and, with increasing size, getting enough food to pay the fuel bill is demanding.

Dolphins and orcas illustrate the point; they must remain relatively small, and sufficiently manoeuvrable, to catch their elusive prey. The sperm whale, the largest a member of the toothed tribe can get, specialises in catching giant squid.

The baleen whales on the other hand, use less energy feeding. Filtering out vast quantities of plankton and squid in cold well-oxygenated waters, it pays to be enormous. The blue whale, which passes off our west coast, is the largest animal ever to have lived, the supreme example of “ocean gigantism”.

Otters might seem to break the size rule. However, these amphibious mammals are land dwellers, making sorties into the water to hunt. Being only partially aquatic, they can afford to remain small.

“Bigger is better for aquatic life,” the authors say, “but only up to a point.”

William Geary et al. Energetic tradeoffs control the size distribution of aquatic mammals. Proceedings of the National Academy of Sciences. 2018.

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