When dolphins swim through the ocean, it looks effortless. Whipping their tails up and down, the slender sea mammals propel themselves forward in a seamless glide that would make any human swimmer envious. But this up-down tail motion puts a lot of strain on a dolphin’s body, compressing its organs and sending blood pressure pulses to its brain.
Now researchers in Canada have a theory on how cetaceans — dolphins, cetaceans and porpoises — manage to protect their brains from these swimming-induced blood pressure pulses. As described in a new article published in Science, It’s all thanks to specialized networks of blood vessels known as “retia mirabilia.”
Scientists have long known that many animals have retia mirabilia. Greek physician Galen described the structures in the second century AD and gave them their name, which translates to “wonderful webs.” In fact, retia mirabilia resemble complex filamentous networks made up of thin veins and thick arteries. They are found in a wide range of mammals, birds and fish – but rarely in humans.
In most animals that have them, Retia mirabilia serve as a temperature regulation mechanism and they have a unique structure. “You can almost imagine drawing a flower with a really big center – like a sunflower for example – and imagining it as a big central tube surrounded by several smaller tubes around that circle,” says Sarah Kienle, Biologist at Baylor University. who was not involved in the recent study. “That’s essentially what we’re talking about.”
This large central artery carries warm blood from the body’s heart to its extremities, while the surrounding veins carry cold blood in the opposite direction, Kienle explains. And because they’re right next to each other, heat is transferred between the arteries and veins to keep them neither too cold nor too hot.
Flamingos are a classic example of animals that benefit from Retia mirabilia, says Kienle. “Because they stand in the water overnight, [retia mirabilia] in her lower legs keep all that cool water from getting her body temperature too cold,” she adds. Similar retia mirabilia have been found in marine mammals, which help regulate the temperature of their fins, tongue and testicles.
Dolphins and other whales have extra retia mirabilia that snake around their lungs, up their spine, and into their brains. These special networks are very different from those of other animals. For one, the blood vessels involved are much larger, resembling a squirming mass of worms. Second, they don’t seem to act as temperature controllers.
“This area — this thoracic cavity region that leads to the brain — is much less studied and identified in mammals, and particularly marine mammals,” says Kienle. She adds that there have been a number of hypotheses about the function of structures in this area, but no explanation has been well tested or widely accepted. The authors of the new Science Paper believe they have found the answer.
Researchers studied the internal biological structure of 11 different cetacean species, including fin whales and bottlenose dolphins. Some of the animals were dissected by these scientists, while others had been analyzed by other biologists as part of previous research. “All were animals that had already died,” most by beaching themselves, says Robert Shadwick, a biomechanist at the University of British Columbia who co-authored the paper.
Analyzing the innards of all these whales took some time. “It took about 10 years for this study to come to fruition — actually more than 10 years,” says Wayne Vogl, a biologist at the University of British Columbia who also participated in the study.
Based on their analysis, the researchers now believe that one of these previously confounding retia mirabilia present around whales’ brains likely evolved as an adaptation to protect themselves from the physical demands of swimming.
Whales, dolphins and porpoises have evolved from mammals that once lived on land. Tens of millions of years ago, the ancestors of whales rejected terrestrial life in favor of the open ocean. The transition to an aquatic existence was no easy task for these mammals; it required a number of special adjustments.
One challenge these creatures had to overcome was the stress caused by swimming on the body. As previously mentioned, dolphins propel themselves by wagging their large tail up and down, which causes such stress. This is also the case with other whales today. “The body cavity is completely below the spine, so everything below the spine is compressed on the downstroke,” says Shadwick. “And it doesn’t get crushed on the upstroke.”
This constriction and relaxation, Shadwick explains, is the source of tremendous pressure — not only on the whale’s organs, but also on the surrounding blood vessels. Eric Ekdale, a biologist and paleontologist at San Diego State University who was not involved in the study, likens this to sit-ups. “When we do crunches or sit-ups, we compress our abdominal cavity,” he says. “We breathe in, and when we do sit-ups we breathe out, and that relieves some of the pressure.”
But marine mammals don’t have the luxury of exhaling. Except for the moments when they surface for air, whales must hold their breath when swimming. Then how do whales deal with the internal pressure caused by their tail whips? In particular, how do they ensure that the blood pressure pulses generated by each downward blow do not cause brain damage when they reach the skull?
This is where the retia mirabilia comes into play. Shadwick and his colleagues hypothesize that one of these spongy networks, located next to the whale’s brain, dampens pressure pulses as blood flows through it. In particular, the researchers propose that this rete mirabile (the singular form of “retia mirabilia”) transmits impulses from veins to adjacent arteries in a way that protects the brain from damage.
To test this claim, the researchers developed a computer model based on the internal biological structures of the 11 species they observed. And indeed, they found that their hypothetical pressure transmission system worked: it was able to protect the animals’ brains from 97 percent of the pressure pulses. They are now confident that they have found the long-sought secret purpose of the whales’ “wonderful webs”.
Vogl also points out that seals — which belong to a different group of marine mammals — don’t have a rete mirabile around their brains. This further supports the team’s hypothesis about the function of the network. While whales swing their tails up and down, pressing their organs against their spines, seals swing their tails left and right, which doesn’t cause the same internal pressure. Seals don’t need to regulate their swimming-related blood pulse – and if that’s the point of a cranial rete mirabile, that explains why seals don’t have one.
Vogl speculates that whales’ ancestors likely had a retia mirabilia leading to the brain before they even ventured out into the oceans — but that this network served a different purpose on land. “I suspect that it used to be thermoregulatory and the function has changed,” says Vogl.
But Ekdale, who studies the evolutionary transition of mammals into the ocean, isn’t sure. He suspects that whales’ terrestrial ancestors did not have retia mirabilia running up the spine to the brain, and that this network only evolved after these mammals migrated to the oceans and had to adapt to breathless swimming. “It’s probably a novel structure — a novel adaptation to aquatic life,” he says. But he admits that it’s impossible to know exactly when this structure evolved because soft tissues like blood vessels are not preserved in the fossil record.
Though Ekdale takes a different stance on its origins, he thinks the new paper offers a plausible explanation for the workings of the once-mysterious, undeniably miraculous network of blood vessels around the brains of whales and dolphins. “I think it’s a nice solution to the specific problem of an all-aquatic mammal,” says Ekdale.