how did blue whales get so big?
Cetaceans came into being when a group of mammals left the land some 55 million years ago, to return to the oceans (creatures first left the oceans for the land some 375 million years ago). The closest land species to whales are the artiodactyls or even-toed ungulates, a large group which includes sheep, goats, cattle, giraffes, camels, llamas, pigs and deer, but another artiodactyl species, the hippo, is most closely related to cetaceans. But, of course, since returning to the oceans, the creatures who finally evolved into cetaceans were able to become ‘super-sized’. The blue whale, likely the largest creature ever to exist on this planet, can tip the scales at over 170 tonnes, and can measure well over 30 metres. The largest dinosaur unearthed so far, Argentinosaurus, a titanosaur sauropod (that’s to say a really effing big dino – named for the ancient mythical titans – with a long neck and tail and a comparatively small head, like the brontosaurus of my youth, now sadly out of favour) weighed around 75 tonnes.
Cetaceans have managed to fill a diverse range of ecological niches. Some of the best-known are the blue whale (a filter-feeding baleen whale or rorqual), the orca (often called a killer whale, but in fact it’s the largest species of dolphin) and the sperm whale, the largest of the toothed whales. Their success, and especially that of rorquals, may owe much to the abundance of krill in the oceans. Some researchers have also attributed the great growth spurt of the blue whale over the past few million years to this ready supply of food. It’s been estimated that, in the southern oceans alone, the krill biomass may be as much as 500 million tonnes, twice the biomass of humans on the planet.
Of course the behaviour of humans has had a massive impact on blue whales, especially in the century of so before 1966, when they came under international protection. The Antarctic population before whaling has been estimated at between 200,000 and 300,000, possibly as much as ten times the current population, though numbers are difficult to determine. You can’t help but wonder what would have happened to whale – and krill – populations without human depredations.
Researchers and analysts point to two main and perhaps complementary reasons for whale ginormity; the abundance of food, and the lack of restraint on size in an oceanic medium. I’ll focus on the second reason first. This presumably has to do with physics, my weakest subject, so I want to get it straight in my mind.
Allometry is the study of the size of organisms and what it means in terms of growth, behaviour, environment and other constraints and factors. Allometry helps explain how a large oxygen-breathing mammal can survive in and transport itself through its chosen medium. Whales are ‘neutrally buoyant’ – that’s to say, their body’s density is equal to the density of the water around them. This means that they don’t have to expend the energy that land animals have to in counteracting the effects of gravity – scuba divers have to learn the correct breathing underwater to achieve this neutral buoyancy. Every step we landlubbers take involves a lifting up of our bodies against the gravitational force pinning us to the earth. The endless gentle push of gravity is what makes us wrinkle and sag over our lifetime. Okay, let’s not think about that anymore. Locomotion in the water has much to do with allometric scaling, because the rate of oxygen consumption per gram body size decreases consistently with increasing body size. Other factors include shape and type of movement, which influence the laminar or turbulent flow around the organism. All of this is very complicated and can be worked out with equations – the Reynolds equation, which relates turbulence to velocity, being of prime importance, though hard to work out in nature, especially with cetaceans, who seem to break all the rules. That’s to say, there’s much about their physiology and how it’s adapted to water that we still don’t know.
Of course, aquatic mammals have to pump blood around their bodies and get air into their lungs just like land mammals. Interestingly, mammals have much the same heart-body mass ratio, whether they’re mice or elephants, land or aquatic. That of course means that the blue whale has the biggest heart of any mammal, and that also goes for a number of other organs. Scaling is much the same, for example, for lungs, and for lung capacity, and for blood, which represents around 5.5% of body mass. So, for mammals of similar form, larger ones can travel more quickly, because it requires the same expenditure of energy to move a body length. The large body length of a blue whale enables it to move great distances in search of food or for other purposes at less metabolic expense. It also enables them to dive for much longer than other cetaceans. Whales have a lower heart rate and can carry more oxygen through their bloodstream than smaller marine mammals. These are just some of the advantages of size in the oceans.
Of course, greater mass requires greater volumes of food to sustain it, but krill seems to have provided just about all a blue whale needs in that department, though it’s also partial to a class of small crustaceans called copepods, and it’s happy, too, to consume any other stray crustaceans and little fishes it catches up in its lunge dives through the krill – described recently as ‘the largest biomechenical event on earth’. Its feeding system and technique is adapted to these small but vastly numerous life forms. For all its size, a blue whale’s throat opening won’t allow it to swallow anything larger than a beach ball, yet it can eat up to 40 million krill a day. It’s jawline is huge, extending over halfway down its body, and the jaws can open to almost a ninety degree angle during lunge diving, allowing it to scoop up about 100 tonnes of krill-infested water in about ten seconds. The water is then squeezed out through the baleen with the help of its ventral pouch and massive tongue.
So it’s understandable why the blue whale has grown to this size, which raises the question – has it ended its growth spurt? There’s a bit of an argument going on about this. Obviously the present moment is but a snapshot, and we can never be certain about where evolution is heading, but often growth spurts in species occur at a rapid clip, and then things stabilize. The blue whales are relatively recent, judged as having split from an ancestor at around 10-15 million years ago, but it may be that they grew to their present size quickly after the split. We have no way of knowing as yet, unless we find a massive blue whale fossil dating back more than 10 million years, which is unlikely. However, other ways of knowing might crop up. There’s also an argument that these rorquals have reached their limit due to feeding limitations and oxygen supply limitations. Lots of interesting research questions to ponder over.