If you’ve ever watched a dolphin swimming, you might have wondered why they move their bodies up and down, unlike fish, which swim by moving side to side.
Even though dolphins have a fishlike appearance, they belong to a group of mammals known as cetaceans (which includes whales, dolphins, and porpoises). These animals are descendants of land-dwelling ancestors, much like cats, dogs, elephants, and humans.
Transition from land to water
Over time, cetaceans evolved and made profound changes to their bodies and skeletons to adapt to life in the water. Their hindlimbs reduced, they developed flippers and tail flukes, and their bodies streamlined for efficient swimming.
Despite these significant changes, cetaceans still retain certain characteristics from their land-dwelling days, such as having lungs and nursing their young with milk.
The up-and-down motion of their swimming also reflects the vertical movements that allow land mammals to run fast. But how their backbone adapted during the transition from land to water, about 53 million years ago, remained a mystery - until now.
Dolphins developed new backbones
A recent study sheds new light on how cetaceans’ backbones were reorganized as their ancestors adapted to aquatic life. This research, conducted by an international team of scientists, challenges previous beliefs about the structure of these marine mammals' backbones .
The study found that, contrary to earlier assumptions, the backbone of cetaceans is highly regionalized, even though its shape is more uniform along its length compared to land mammals. However, the way this regionalization happens is drastically different from how it occurs in terrestrial mammals.
“When their ancestor went back into the water, whales and dolphins lost their hind legs and developed a fish-like body,” said lead author Amandine Gillet, a Marie Curie Fellow at Harvard University ’s Department of Organismic and Evolutionary Biology and the University of Manchester ’s Department of Earth and Environmental Sciences.
“But that morphological change also means the vertebral column is now the main part of the skeleton driving locomotion in an aquatic environment.”
Streamlined movement for swimming
In land mammals , the vertebral column helps support the body, with the legs bearing the body’s weight. But when cetaceans transitioned to water, the forces acting on their bodies changed - gravity shifted to buoyancy, meaning their bodies no longer had to support their own weight on land.
This new environment required the backbones of dolphins and whales to adapt to allow for more fluid, streamlined movement in water. Previous studies focused on the shape of the vertebrae, looking for changes in their structure.
Evolutionary history of backbones
In 2018, co-authors Stephanie Pierce and Katrina Jones used a statistical method to study the backbone’s evolutionary history in terrestrial mammals. They demonstrated that, compared to amphibians and reptiles, land mammals have a much more regionally diverse vertebral column.
“It’s a challenge to understand how the regions of a terrestrial mammal can be found in whales and dolphins, and one reason is because their backbone looks very different in terms of morphology, even though they evolved from them,” said Pierce, senior author of the study and a professor of organismic and evolutionary biology at Harvard.
“They lost the sacrum, a fused string of vertebrae bracing the hind legs and a critical landmark needed to distinguish the tail from the rest of the body.”
Dolphin and porpoise backbones
Complicating matters, cetaceans' vertebrae became more homogeneous in appearance, making it harder to distinguish transitions between regions compared to land mammals, which have more distinct vertebrae changes.
“Not only do they have very similar vertebrae, but certain species, in particular porpoises and dolphins, have many more vertebrae than terrestrial mammals, with some species having close to 100 vertebrae,” said Jones, a presidential fellow in earth and environmental sciences at the University of Manchester.
“This makes it really challenging to translate regions found in terrestrial mammals to the backbones in whales and dolphins.”
Identifying patterns in the backbone
To overcome this, the team used a statistical method called Regions, which can identify patterns in the backbone despite the varying number of vertebrae in each species.
While this worked well for land mammals, cetaceans’ complex vertebrae counts presented computational challenges. Gillet, working with the Data Science Services team at Harvard, re-coded the program to handle the high vertebrae counts of cetaceans, resulting in the new software, MorphoRegions, which is now available for other researchers to use.
“This is definitely one of the biggest advances of our study,” said Pierce. “Amandine spent months refining the program so that it could analyze a system of high repeating units without crashing the computer.”
Analyzing an extensive dataset
Using the MorphoRegions method, Gillet analyzed data she collected during her PhD, examining 7,500 vertebrae from 139 specimens representing 62 cetacean species.
This extensive dataset allowed the team to see that the organization of the cetacean backbone differs from that of terrestrial mammals. Moreover, they found variations within cetaceans, identifying between six and nine regions depending on the species.
“We then worked from there to find commonalities across regions and identified a pattern common to all cetaceans, which is summarized by our Nested Regions Hypothesis,” said Gillet.
Movement in specific tail regions
This hypothesis suggests a hierarchical organization of the backbone, where the precaudal and caudal segments are divided into several modules, such as cervical, thoracic, lumbar, and fluke. Depending on the species, each module is further subdivided into regions, with some species having as many as nine post-cervical regions.
“Surprisingly, this showed us that, compared to terrestrial mammals, the precaudal segment has fewer regions, whereas the caudal area has more,” Pierce said.
“Terrestrial mammals use their tails for various functions, but not usually for generating propulsive forces, like cetaceans do. Having more regions in the tail may allow for movement in very specific regions of the tail.”
Backbones, habitats, and swimming speed
The team also explored how these backbone regions correlate with habitat and swimming speed. Species that live offshore, further from the coast, tend to have more vertebrae and regions, which likely helps them swim faster.
Meanwhile, species living closer to shore, such as in rivers and bays, tend to have fewer vertebrae and regions, allowing for more maneuverability.
“It’s a beautiful study,” Pierce said. “You go from dusty skeletons in a museum to showing how the backbone of one of the most charismatic groups of mammals was repatterned due to their aquatic environment.”
Future research directions
Next, the researchers plan to study how these regions relate to function, using data on vertebral flexibility to understand how the backbone evolved from supporting land movement to driving swimming.
These findings could even help scientists infer the swimming abilities of ancient whales, shedding light on how their backbones evolved from supporting land animals to propelling marine mammals through water.
The research is published in the journal Nature Communications .
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