Any high school student can explain the most obvious difference between toothed and baleen whales: the first are hunting carnivores, the latter are filter feeders.
Most, however, aren’t quite so aware of the other great difference between the two. Baleen whales have hearing in the infrasonic – ultra-low – range, while toothed species hear right at the other end of the scale, at very high, or ultrasonic, frequencies.
Just when this very stark difference evolved is unknown, but new research from palaeontologist Maeva Orliac of the Université de Montpellier in France indicates that it is a much more recent occurrence than previously thought.
Orliac and colleague Mickaël Mourlam studied the fossilised ear canals of an extinct group of very early whales – perhaps “proto-whales” is a more accurate description – called protocetes to determine their hearing range.
They found that they had a hearing range that did not encompass the extremes of those of modern whales. Indeed, protocetes probably heard in much the way as hippos and pigs – to which they are distantly related.
The protocetes – named after the first fossil of their type, discovered in 1904 – were around during the middle Eocene, about 45 million years ago. They represent a transitional from between even-toed land dwelling ungulates and today’s whales.
A protocete had a long, toothed jaw with nostrils set far back. Its front and back legs ended in webbed toes, the rear ones well on the way to becoming vestigial. It is likely the species still spent some time on land, if only intertidal zones.
To reach their conclusions, Orliac and Mourlam studied fossil examples encased in marine deposits in Togo, West Africa.
Using micro computed tomography (micro-CT) they were able to peer inside the fossilised skulls and model the internal layout of the petrosal bone, which harbours organs for hearing and balance.
This was not an easy task, the scientists report.
“This process was long and difficult because this cavity was filled with sediments and partly recrystallized and because the petrosal bone in cetaceans is particularly thick and dense, which lowers the quality of the images and sometimes impedes analyzing them,” says Orliac.
Nevertheless, once complete, the interior layout of the bone cavity allowed an accurate deduction of the shape and dimensions of the cochlea – and thus the range of frequencies it was capable of detecting.
“We found that the cochlea of protocetes was distinct from that of extant whales and dolphins and that they had hearing capacities close to those of their terrestrial relatives,” explains Orliac.
The findings suggest that the specialised ultrasonic and infrasonic hearing abilities of modern whales only developed after their ancestors had adapted to a complete marine life cycle.