When dolphins swim in the ocean, it looks effortless. By flicking their tails up and down, the graceful marine mammals propel themselves forward in a continuous glide that could make any human swimmer jealous. But this up-and-down tail motion puts a lot of pressure on a dolphin’s body, compressing its organs and sending pulses of blood pressure to its brain.
Now researchers in Canada have a theory about how cetaceans (dolphins, whales and porpoises)manage to protect your brain of these swimming-induced blood pressure pulses. As described in a new article published in Sciences It’s all thanks to specialized networks of blood vessels known as “retia mirabilia.”
Scientists have long known that many animals have retia mirabilia. The Greek physician Galen described the structures in the 2nd century CE and gave them his name, which translates as “wonderful webs.” In fact, retia mirabilia resembles complex fibrous networks made up of fine veins and large arteries. They can be found in a variety of mammals, birds, and fish, but rarely in humans.
In most animals that have them, the retia mirabilia serve as a temperature regulation mechanism and have a unique structure. “You can almost imagine drawing a flower with a really big center, like a sunflower, for example, and thinking of it as a big central tube surrounded by several smaller tubes around that circle,” he says. sarah kienle, a biologist at Baylor University, who was not involved in the recent study. “That’s essentially what we’re talking about.”
That large central artery carries warm blood from the body’s heart to its extremities, while surrounding veins carry cold blood in the opposite direction, Kienle explains. And because they are situated next to each other, heat is transferred between the artery and the veins to ensure neither end up too hot or too cold.
Flamingos are a classic example of animals that benefit from retia mirabilia, says Kienle. “Because they stay in the water overnight, [retia mirabilia] on your lower legs help keep all the cold water from causing your body temperature to get too cold,” she adds. Similar retia mirabilia have been found in marine mammals, helping to regulate the temperature of their fins, tongue, and testicles.
Dolphins and other cetaceans have additional retia mirabilia that snake around their lungs, up their spine, and into their brains. These particular networks are quite different from those found in other animals. For one thing, the blood vessels involved are much larger and resemble a mass of writhing worms. On the other hand, they don’t seem to work as temperature regulators.
“This area, this region of the chest cavity that leads to the brain, is much less studied and identified among mammals and especially marine mammals,” says Kienle. She adds that there have been a number of hypotheses about the function of the structures in this area, but no explanation has been well tested or widely accepted. The authors of the new Sciences paper believe they have found the answer.
The researchers looked at the internal biological structure of 11 different species of cetaceans, 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. “They were all animals that had already died”, most of them because they were stranded, he says Robert Shadwickbiomechanics researcher at the University of British Columbia, co-author of the article.
Analyzing the entrails of all these cetaceans took some time. “This study took about 10 years to complete, more than 10 years, actually,” he says. Wayne Vogelbiologist at the University of British Columbia, who was also involved in the study.
Based on their analysis, the researchers now believe that one of these previously puzzling retia mirabilia that is present around the cetacean brain likely evolved as an adaptation to protect against the physical demands of swimming.
Whales, dolphins, and porpoises evolved from mammals that once lived on land. Tens of millions of years ago, the ancestors of cetaceans rejected life on land in favor of the open ocean. The transition to an aquatic existence was no small thing for these mammals; requires a series of specialized adaptations.
One challenge these creatures had to overcome was the stress that swimming creates. in the body. As noted above, dolphins propel themselves forward by pushing their large tail up and down, which causes them a lot of stress. This is the case for other cetaceans today as well. “The entire body cavity is below the spine, so on the downstroke, everything below the spine gets squeezed,” says Shadwick. “And on the upstroke, it’s decompressing.”
That constriction and relaxation, Shadwick explains, is the source of an enormous amount of pressure, not only on the cetacean’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, compares this process to sit-ups. “When we do sit-ups or sit-ups, we compress our abdominal cavity,” he says. “We breathe in, and then when we sit down, we breathe out, and that takes some of the pressure off.”
But marine mammals don’t have the luxury of exhaling. With the exception of times when they surface for air, cetaceans have to hold their breath while swimming. So how do cetaceans handle the internal pressures caused by their tail whips? In particular, how do they ensure that the blood pressure pulses generated by each downward stroke do not cause brain damage when they reach the skull?
That’s where the retia mirabilia comes into play. Shadwick and his colleagues hypothesize that one of these spongy webs that sits next to the cetacean brain mitigates pressure pulses as blood passes through. Specifically, the researchers propose that this rete mirabile (the singular form of “retia mirabilia”) transfers pulses 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 transfer system worked: It could protect the animals’ brains from 97 percent of pressure pulses. Now they are sure they have found the long-sought secret purpose of the cetaceans’ “wonderful webs.”
Vogl also points out that seals, which belong to a different group of marine mammals, do not have a mirabile network around their brains. This further supports the team’s hypothesis about network function. While cetaceans move their tails up and down, pressing their organs against the spine, seals move their tails from left to right, which does not cause the same internal pressure. Seals don’t need to regulate swimming-related blood pulses, and if that’s what a cranial rete mirabile is for, he explains why seals don’t have one.
Vogl speculates that cetacean ancestors probably had retia mirabilia leading to the brain before they reached the oceans, but that this network served a different purpose on land. “I suspect that at one point it was probably thermoregulatory and that the function changed,” says Vogl.
But Ekdale, who studies the evolutionary transition from mammals to the ocean, isn’t sure about that. He suspects that the terrestrial ancestors of cetaceans did not have retia mirabilia running up the spine to the brain and that this network only developed after those mammals moved into the oceans and had to adapt to swimming breathlessly. “It’s probably a novel structure, a novel adaptation for life in water,” he says. But he admits that it is impossible to know exactly when this structure developed because soft tissues, such as blood vessels, are not preserved in the fossil record.
Despite taking a different stance on its origins, Ekdale says he finds the new paper a plausible explanation for the function of the once-mysterious and undeniably wondrous network of blood vessels around the brains of whales and dolphins. “I think it’s kind of a neat solution to the specific problem of a completely aquatic mammal,” says Ekdale.