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Jawbones reveal how evolution has shaped the face of whales
The lower jaws of toothed whales are revealing an evolutionary story that played out over 50 million years.
Analysis of the bones of 100 living and extinct species of whales has shown how the twin demands of diet and echolocation have affected the evolution of the most diverse group of marine mammals.
It’s said that you are what you eat – and that’s certainly true of toothed whales.
New research links the development of the lower jaw of dolphins, porpoises and sperm whales with their diet. While species feeding on invertebrates on the seafloor tend to have the fastest evolving jaws, the mandibles of those feeding on squid and their relatives change much more slowly.
At the same time, toothed whales are under pressure to maintain their ability to echolocate, making their jaw bones part of a complex evolutionary balancing act.
Dr Ellen Coombs, the lead author of the new research and scientific associate at the Natural History Museum, says, ‘Toothed whales are the most diverse group of marine mammals, with a lot of variation in morphology, diet and behaviour.’
‘Echolocation is an important part of this, with the toothed whales’ highly specialised mandible [lower jaw] allowing sounds to be transmitted to the inner ear. However, little research has focused on the evolution of the jaw regarding this unique adaptation.’
‘Due to the uniqueness of aquatic echolocation and its development, toothed whales provide a rare opportunity to investigate the evolution of a key innovation in a complex environment and this really excited me as an evolutionary biologist.’
The findings of the study were published in the journal Current Biology.
The shape of the sperm whale's jaw is normally heavily conserved, but in the mandible of this whale became twisted during development. Image © The Trustees of the Natural History Museum, London, All Rights Reserved.
From the land to the sea
The ancestors of the first whales and dolphins (collectively called cetaceans) evolved around 50 million years ago. At the time, these animals were semi-aquatic creatures living in, and around the edges of, lakes and rivers.
Over time, their descendants spent increasingly more time in the water, until eventually they became fully aquatic and never looked back. Using scans of around 100 living and extinct cetacean species, the team found that the jaw rapidly adapted at this time as the animals evolved a variety of different underwater lifestyles.
These changes would have allowed early whales to hear underwater, as well as give them the jaw strength needed to eat fish, invertebrates and later other mammals. These were among the changes that allowed cetaceans to become fully aquatic in as little as eight million years, which is extraordinarily fast in evolutionary terms.
Around 38 million years ago, the cetaceans experienced a split which still defines them to this day.
One group became the baleen whales. These are species with large keratin plates which are used in filter feeding to catch prey. The others became the toothed whales. Whilst both groups use vocalisations, only the toothed whales evolved complex echolocation.
Investigating how this took place and the effects it had on the animals is important in understanding the evolution of these marine mammals. However, while there have been many studies of the skulls of these animals relatively few have focused on the mandible.
Dr Agnese Lanzetti, a Natural History Museum researcher who co-authored the new paper, says, ‘In most mammal groups, there are enough similarities between the jawbones of different species that you can use landmarks such as teeth or ridges to compare them.’
‘The problem with cetacean mandibles is that they are flat, single bones with little in the way of surface structure, while the number of teeth can vary a lot between species.’
To account for this, the team had to study how the general outline of the jaw changed, rather than adaptation in the specific parts of the bone that make it up. Their analysis revealed the changes were mostly being driven by two factors – the hunt for food, and then eating it.
Beluga whales, along with their relatives, specialise in suction feeding. The demands of this limit how their head can develop. Image © Danny Ye/Shutterstock.
Eating and echolocation
When looking for food in muddy estuaries or in the deepest parts of the ocean, eyesight alone isn’t enough to find prey. Instead, toothed whales make use of echolocation to find their food.
Ellen and Agnese have already shown that toothed whale development as babies has changed to make them better at echolocating, and so it came as no surprise to them that the demands of this ability have also defined the shape of cetacean jawbones.
‘The mandibles of cetaceans are very different to those in terrestrial mammals, which are generally just for feeding,’ Agnese says. ‘In living whales, there is a triangular hollow inside the bone at the back of the jaw where a fat body is found, which helps to focus sound so that the animal’s brain can interpret it.’
This structure evolved early in the evolution of toothed whales as they honed their ability to find objects with sound. Evolution at the rear of the jaw has since slowed down significantly to maintain this structure, as any major changes might limit their ability to echolocate.
The same limitations, however, don’t apply to the front of the jaws. As toothed whales began to specialise for different prey, the researchers found cetacean jaws began to evolve in a wide variety of different directions.
Whales which have biting or snapping styles of feeding, like orcas, tend to evolve faster than species like porpoises, which mainly use suction feeding to draw prey into their mouth.
‘Although all toothed whales feed by suction to some degree, it is predominantly seen in beaked whales, porpoises, belugas and sperm whales,’ Ellen explains. ‘It allows these mammals to hunt in incredibly deep water and fill an otherwise vacant niche.’
‘With few other species able to occupy this role in the environment, there’s not much pressure for them to evolve rapidly. It’s likely that suction feeding evolved several times as a strategy for capturing squid and other cephalopods.’
Having looked into the toothed whales, the researchers now hope to investigate the baleen whales to see how the rest of the cetacean family have adapted for dining at depth.
‘Baleen is such an effective way of consuming calories that other than increasing in size, baleen whale skulls and jaw bones haven't changed a huge amount in over 20 million years,’ Ellen says.
‘I think that the evolution of their jaw would have slowed down significantly to preserve its shape, which is so effective at consuming tonnes of krill and fish in one mouthful. I would love to investigate this in the future and put my hypothesis to the test.’
