Changes in food distribution and not falling ocean temperatures could hold key to shift towards giant lengths
The blue whale has a body the length of a jet airliner, a heart the size of a car, and a tongue the same weight as an elephant.
Now researchers say they might have solved the mystery of why baleen whales a group that includes these blue beasts, the largest animals on the planet became so large.
Scientists say the shift towards sizes of more than 10 metres in length probably cropped up in baleen whales just 2m-3m years ago, and was driven by changes in the distribution of their food in the ocean.
We think of [baleen whales] as being giants but if you consider this in the context of their 36m-year evolutionary history, they have only been giants for a 10th [of it], said Graham Slater, an evolutionary biologist and co-author of the research from the University of Chicago.
Writing in the Proceedings of the Royal Society B journal, Slater and colleagues reveal how they unpicked the driver behind the boom in size by exploring the lengths of 63 extinct baleen whales, as estimated from measurements of fossilised skulls, as well as the lengths of 13 living species.
The lengths, as well as the position of each species on the baleen whale family tree, were then fed into a series of computer models to examine how and when gigantism cropped up.
The results reveal gigantism emerged independently in several branches of the family tree, including the bowhead and the right whales. The team also found that it was not just that the largest whales became bigger over time the smallest found today are also much larger than the smallest extinct species.
Further analysis revealed that although baleen whales had been gradually diversifying in size for 30m years, a step change began to take place between 4.5m and a few hundred thousand years ago.
The recent timing , the authors say, rules out the possibility that the emergence of gigantism was caused by the evolution of bulk filter feeding or large predators, such as giant sharks. They also rule out that the gigantism was a response to falling global ocean temperatures.
Instead, they say, it is probably down to systems driving cold, nutrient-rich water upwards in regions around the continental shelf, caused at least in part by the onset of glaciation in the northern hemisphere.
These systems emerged about 3m years ago and resulted in dense pockets of prey rather than an even spread. That, says Slater, drove the evolution of gigantism in baleen whales.
The bigger your mouth, the more you can take in and the less energy it costs you to do it, he said. Being large also helps when it comes to moving to the next pocket of prey.
If you are big you can just store more energy you have got a bigger fuel tank to get you where you want to go, said Slater. If you are big your cost of transport is also lower so you get more miles to the gallon.
Olivier Lambert, a palaeontologist from the Royal Belgian Institute of Natural Sciences, who was not involved in the study, said the research provided a convincing scenario for one of the last stages in the evolution of size among baleen whales.
However, he said it was likely that other factors, such as the evolution of large predators and other climatic events, could also have played a role in the development of the animals size, particularly earlier in their evolution.
But Lambert believes the latest research has implications for modern ecosystems. Such a work further emphasises the precarious situation of todays baleen whale populations, currently facing major threats considering the short-term effects of climate change on oceanic circulation and nutrient transport, he said.
Slater agrees. If we do shut down that cold water supply, if climate change does go the route we think it is going, it is going to spell real bad news for these big baleen whales, he said.