Some whales are unstable. You may not know how to look at them, but their skulls are really incredibly asymmetrical. This mysterious feature helps with echolocation, the way whales work where things are by making sounds and feeling how they reflect.
But this weakness is not present in all whales. My colleagues and I recently conducted research to discover why and when awkward whales began to evolve differently than their symmetrical cousins. We now know that twisted whale skulls first appeared around 30 million years ago, and that they continued to become even more asymmetrical as creatures evolved into the modern species we know today.
To understand how crooked whales became like this, we needed to see how they lived and adapted in the past. Fortunately for us, the whale fossil record is so remarkably represented that scientists have even called the whale “daughter of evolution.” Complete skulls and skeletons date back to the first whales 50 million years ago, and more fossils are dotted throughout the history of whales, down to the living animals we know today.
Ellen Coombs / UCL, Author provided
With this record, we can see that the whales’ nostrils have moved from the tip of their snouts to the top of their heads, an evolutionary tactic to facilitate breathing on the surface of the water. And toothy whale skulls (which technically include dolphins, as well as species like sperm whales) have become more uneven, with the bones on one side in different positions than the same bones on the other side.
This is due to a mass of fatty tissue called “melon” that toothed whales use for echolocation. The melon and soft tissue needed for echolocation are placed to the left over the skull in toothed whales, giving them a bulbous forehead and also causing the skull bones below to become crooked to the left. As toothed whales evolved, their skulls became stranger.
But why don’t all whales have this wonder? The first whales were called “arqueocetes” (which literally means “ancient whales”). They evolved from walking on land to being fully aquatic in about 8 million relatively short years.
We know that archeocete fossils have awkward faces (or snouts). This could be a distortion of the fossils or a feature that helped archaeocetes determine which direction the sounds from the water came from.
Ghedoghedo, CC BY-SA
Then, about 39 million years ago, the whales were divided into two groups: those with teeth in their mouths, known as “odontocetos, and those with baleen (rows of bristles that allow whales to filter food from the water) , known as “mysticetes”.
At some point, toothed whales developed crooked skulls and echolocation. However, the mysticetes, which include large baleen whales (such as blue whales), diverged on a completely different evolutionary path. They evolved the feeding of beards and filters and skulls that are more symmetrical than archaeocets and toothed whales.
We wanted to understand why, and exactly when, this happened. So, to track the skewness in the evolution of the whale’s skull, we produced 3D scans of 162 skulls, 78 of which were fossils. By mapping this unstable change in skull through the whale’s family tree, we could accurately trace when it appeared in evolutionary history and in which families it evolved.
Asymmetry appears
Based on analyzes of these skulls, the naso-facial asymmetry (instability) appears to have evolved about 30 million years ago. This was after the transition from archaeocets to modern whales, and after the division between odontocetes and mysticetes. Around the same time this weakness appeared, these first toothed whales were evolving high-frequency hearing and complex echolocation.
We also confirm that the earliest ancestors of living whales had little cranial asymmetry in the naso-facial area and were probably unable to echolocate. As such, it is likely that baleen whales have never been able to echolocate.
The most surprising thing is that this asymmetry has reached its highest levels in some specific animals, such as sperm whales and narwhals and other species that live in deep or extreme environments.
This suggests that animals living in these complex environments, including beluga whales that live in icy and crowded waters and river dolphins that live in shallow and cloudy rivers, have developed a different echolocation capacity, such as a more repertoire of sound. Diverse and discreet to help them navigate and hunt, and with it the bones around the nose and face have become more asymmetric.
This evolutionary path of toothed whales becoming increasingly asymmetric suggests that their skulls and overlying soft tissues may continue to become more bizarre as their echolocation techniques become more specialized.
These findings remind us not only of the complex evolutionary pathways that cetaceans have undergone to become the magnificently adapted iconic ocean dwellers that we know today, but also that despite living alongside some of the largest animals that have ever existed, There is still much to be done. us to learn about them.
