Size Matters

by

Mark Hager

A tale of whale scale

Twenty-five meters long. Some 150 tons in weight. Her heart the size of a Suzuki four-door, her arteries wide enough for children to crawl through. Her vocalizations louder than a screaming jet engine if you could hear them, but you can’t because they’re too long in frequency for human ears. That precise quality may enable her to communicate through hundreds of ocean miles. She can gulp down 9000 pounds a day and her surfacing spout could sprinkle the roof of a three-story building if it happened to be alongside.

The biggest creature ever to live abounds in Lankan waters. The elusive blue whale, twice the weight of the hugest known dinosaur, seems to like Serendib (and who doesn’t?). The past decade has established both Mirissa and Trincomalee among the world’s best spots for blue whale viewing excursions. Continental shelf edges pinching in close to shore bring blue cruising routes within easy range of shore-based boats. Ocean floor canyons—remnants of extended rivers when sea levels were lower—facilitate ‘upwellings’ of organic nutrients from the deep. These support plankton blooms, essential food for krill, the mainstay of blue whale diets. As naturalist Howard Martenstyn points out, these canyons also feature ‘turbidity currents’ of sediment-heavy river flow outward to the continental shelf edge, bringing run-off nutrients from land into surrounding seas. Nice for blues and many other aquatic creatures. (In periods of especially heavy rainfall as in 2011, however, heavy river flows from the island may render nearby seas too murky and insufficiently saline for phytoplankton/krill blooms. Blues and other feeders then go elsewhere.)

Well then, but why is the blue whale so huge? How did it get that way? We can shrug off such questions as imponderable (some things are big and something has to be the biggest, right?) but certain facts and principles illuminate the matter. Earth’s largest animal must necessarily be aquatic and must eat low on the food chain.

The heaviest animals need to be aquatic because water provides buoyancy helping hold up their weight. Animals as heavy as blues would collapse as land-dwellers or be too sluggish to move. The largest terrestrial dinosaur was roughly the same length as blue whales but only half their weight. The demands on legs to hold up and move an animal on land place constraints on maximum weight. Weight increases with the cube of animal length but support strength increases only with the square of the length, as in the area of a leg bone cross-section. Exceed the size of dreadnoughtus shrani and bones holding up a land-walker simply cannot support the weight or must become so thick as to be unmovable.

No such constraint operates in water. Water’s buoyant force presses upward to counteract gravity, keeping aquatic creatures from simply falling to the bottom of the sea. No legs required.

The next point to ponder is that on land the largest plant-eaters are invariably bigger than the largest carnivores. This was true in the age of dinosaurs—veggie Dreadnoughtus shrani outweighed meat-shredding Spinosaurus by eightfold--and it remains true today: elephants are way heftier than grizzly bears. Large size can of course be an advantage for predators: strength to subdue prey. Why then don’t they reach the size of the largest herbivores? The reason is that top predators face constraints on size that herbivores avoid. Those constraints follow what’s call the Second Law of Thermodynamics as it operates in food chains (what eats what).

Sunlight furnishes essentially all energy available to life on earth for biomass construction and metabolism. Plants convert solar energy into plant stuff, which herbivores eat and convert into herbivore stuff. Primary carnivores convert herbivore stuff into carnivore stuff and top predators do likewise with both large herbivores and lower-level carnivores. Food chains are therefore sequences of converting energy to mass, mass to energy again, energy again back to mass and so on. The Second Law tells us that with each such conversion or transformation, some of the input energy will be lost or dissipated into what could be considered non-usable waste (actually heat). This means that at successively higher food chain levels (‘trophic levels’ biologists call them) the total energy available to support biomass and metabolism progressively dwindles. Estimates hold that only 10% of the energy biologically embodied at any trophic level makes it through to get embodied at the next level upward. This means that the total energy available to species at the top of a typical food chain may be only 1/10,000th of that for herbivores grazing on plants at the base of the chain.

This in turn means that top predator species operate within far tighter energy budgets than herbivores do. Their constricted energy budget effectively limits carnivore size. If they grew larger, they would have to shrink their population numbers to stay within their available energy budget. The blue whale is a carnivore, to be sure, but not a top predator in the sense of hunting and eating large animals. Instead, it gobbles krill—pinkie-sized, orange, shrimplike creatures—which themselves eat mainly phytoplankton: those microscopic photosynthesizers that make up the vast bulk of ocean plant biomass. Blue are grazers—honorary herbivores, you might call them—rather than hunters like typical large carnivores. They are only two steps up the food chain, as opposed to five or so for top predators. As one observer puts it, they come close to dining directly on sunlight. Lots of it.

Subsisting on phytoplankton, krill constitute more aggregate biomass than any other ocean creatures. Fueled in turn by massive ingestions of krill, blue whale energy budgets can sustain gigantic size without undue curtailment in population numbers or activity levels. The sheer size of blue whales allows them to consume vast quantities, aided by jaws opening to nearly 90 degrees and by ventral pouches holding huge gulps of krill-laden seawater. Gargantuan musculature help blues swim long distances in search of krill blooms while tons of stored fat let

them go long times between meals and stay warm in icy waters where plankton and krill populations swarm.

A typical blue feeding dive is a marvel in itself. Strokes from powerful flukes power the whale downward against her own buoyancy through the first 25 meters. As she descends, pressure from the water above forces her flexible rib cage inward, decreasing her volume and increasing her density so that her buoyancy dissipates and she begins to fall rapidly with gravity toward the sea bed. She stops fluking to save energy as she plummets. She may descend as far as 300 meters. Her size indicates oxygen capacity enough to stay down for 30 minutes but her standard dive is more like eight. The reason lies in the astonishing athletics she performs below.

She turns and heaves herself upward in a strenuous ‘lunge’ through krill blooms, fighting not only gravity but also the huge hydrodynamic drag created by her own gaping jaws. After a few seconds, she shudders to a halt, having gulped maybe sixty tons of seawater into her ventral pouch, nearly doubling her weight. Wielding a gelatinous tongue the weight of an elephant, she spews the water out through her baleen—cartiligenous venetian-blind-like sieves that line her mouth instead of teeth—retaining thousands upon thousands of krill then to swallow. She does this all again and then again in a handful of successive lunges back toward the surface. She regains buoyancy while she depletes her oxygen and wears herself out, which is probably why she feeds in this fashion--first way down and then back up. After her final upward feeding lunge, air and energy dwindling fast, she sprints up to the surface, where she huffs out CO2 and lies gasping in the waves. Then maybe, after long minutes of recuperation, does it all again.

Her enormous krill binges furnish massive energy but also require huge energy outlays. Marine biologist Asha de Vos cites studies suggesting that increasing blue size would actually decrease her energy yield per kilogram from lunging. Increased krill consumption would yield insufficient energy gains to offset the added energy cost of pushing her expanded body weight around to find and gulp down extra biomass. Considering factors described above, this means that blue is not only the largest animal ever to live on our beautiful planet but the largest that ever could.

A graduate of Harvard Law School, Mark Hager lives in Pelawatte with his family, consulting on legal, writing and NGO problems.