The Purpose of Dolphins’ Mysterious Brain Net May Finally Be Understood

The Purpose of Dolphins’ Mysterious Brain Net May Finally Be Understood

Dolphins swimming through the ocean seem effortless. The sleek marine mammals glide forward effortlessly by whipping their tails up and down. However, this up-and-down tail motion puts a lot on a dolphin’s bodies. It compresses its organs and sends pulses of blood pressure to the brain.

Now researchers in Canada have a theory as to how cetaceans–dolphins, whales and porpoises–manage to protect their brain from these swimming-induced blood pressure pulses. As described in a new paper published in Science, it’s all thanks to specialized networks of blood vessels known as “retia mirabilia.”

Scientists know for a long time that many animals have retia Mirabilia. Galen, a Greek physician, described the structures in the second-century C.E. Galen, a Greek physician, described the structures in the second century C.E. and gave them their name, which means “wonderful webs”. Indeed, retia Mirabilia look like complex, stringy nets made of thin veins. They are found in a variety mammals, birds, and fish. But they rarely occur in humans.

Retia mirabilia are a mechanism for temperature regulation in most animals. They have a unique structure. “You can almost picture a flower with an enormous center, such as a sunflower–and imagine it as a large tube surrounded by many smaller tubes around that circle,” says Sarah Kienle , a Baylor University biologist. “That’s basically what we’re referring to .”

The big central artery transports warm blood from the heart to the extremities of the body, while the surrounding veins carry the cold blood in the opposite direction. Because they are located next to each other heat transfers between the artery, and the veins to ensure neither end is too hot or cold.

Kienle states that

Flamingos is a classic example animal that can benefit from retia mirabilia. She adds that retia mirabilia in the lower legs of flomingos prevents cool water from freezing their bodies. Similar retia mirabilia were found in marine mammals. They regulate the temperature of their tongue, testicles, and flippers.

Dolphins, and other cetaceans, have additional retia mirabilia that runs around their lungs and up their spines. It also runs into their brains. These networks are very different from those in other animals. The blood vessels involved are larger than usual, resembling a mass of worms. They don’t seem able to regulate the temperature.

Resin cast showing aorta and arteries in the retia of a beluga whale.
Resin cast showing aorta and arteries in the retia of a beluga whale. Credit: Wayne Vogl

” This area, the thoracic cavity that leads to the brain, is much less studied and recognized among mammals than it is among marine mammals, Kienle states. She says that although there have been many hypotheses about the structure of this area, none of them has been proven to be correct. The new Science paper authors believe they have the answer.

The researchers looked at the internal biological structure of 11 different cetacean species, including fin whales and bottlenose dolphins. These scientists dissected some of the animals, while others were analyzed by biologists as part a prior research. “All were animals that had already died,” most by beaching themselves, says Robert Shadwick, a biomechanics researcher at the University of British Columbia, who co-authored the paper.

It took some time to analyze the innards all these cetaceans. “It’s taken about 10 years for this study to come to fruition–more than 10 years, actually,” says Wayne Vogl, a biologist at the University of British Columbia, who also took part in the study.

Based on their analysis, researchers believe that one of the previously perplexingretia mirabilia found around the brains cetaceans may have evolved as an adaptation to swimming’s physical demands.

Whales, porpoises and dolphins evolved from mammals that once lived in the land. Cetaceans’ ancestors abandoned terrestrial life for the open ocean tens of millions of year ago. These mammals had to adapt to aquatic life.

One challenge these creatures had was the stress swimming causes on the body. As we have seen, dolphins propel themselves forward using their large tails up and down. This causes stress. This is also true for other cetaceans. Shadwick explains, “The body cavity is all beneath the spine, so on downstroke, everything below is being squeezed.” “And on the upward stroke, it’s being unsqueezed .”

This combination of relaxation and constriction, Shadwick says, is responsible for a lot of pressure, not only on cetaceans’ organs, but also on the blood vessels around them. Eric Ekdale, a biologist and paleontologist at San Diego State University, who was not involved in the study, compares this process to sit-ups. He says, “When we do crunches or sit ups, we compress our abdominal cavity.” “We take a deep breath and then do the sit-up. This relieves some pressure .”

But marine mammals don’t have the luxury to exhale. Cetaceans must hold their breaths during swimming, with the exception of moments when they are able to breathe in air. How can cetaceans manage internal pressures created by their tail whips, then? How can they ensure that the pulses generated by each downward stroke of blood pressure don’t cause brain damage when they reach their brains?

This is where the retia Mirabilia come in. Shadwick and his collaborators hypothesize that cetaceans’ blood pressure pulses are mitigated by one of these spongy networks. The researchers suggest that the rete mirabile (the singular version of “retia mirabilia”) transfers pulses between veins and adjacent arteries in a way that protects 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 was able to protect the animals’ brain from 97 percent of pressure pulses. They now feel confident that they have discovered the long-sought-after secret purpose for the cetaceans “wonderful webs “.

Vogl also pointed out that seals, which belong to a different group of marine mammals, don’t have a “rete mirabile” around their brain. This supports the team’s hypothesis regarding the network’s function. Seals, on the other hand, swing their tails upwards and downwards, compressing their spines and organs. However, they swing their tails left and right, which causes less internal pressure. Seals don’t need to regulate swimming-related blood pressures. If that’s what a Cranial rete Mirabile is for, then it explains why seals do not have one.

Vogl speculates the cetacean ancestors had retia mirabilia that led to the brain before they took to the oceans, but that this network served a different purpose at land. Vogl suggests that it was once thermoregulatory, and that the function has changed.

But Ekdale who studies mammals’ evolutionary journey to the ocean isn’t certain about that. He believes that cetaceans’ terrestrial ancestors did not have retia mirabilia leading to the brain. This network developed only after mammals moved to the oceans and learned to breatheless swimming. He says that it is likely a new structure, a novel adaptation for living in water. He admits that it is impossible to determine when this structure was created because soft tissues, such as blood vessels, aren’t preserved within the fossil record.

Ekdale believes the new paper is a plausible explanation for the function and origin of the blood vessels that surround the brains of dolphins and whales. Ekdale states that he thinks it is a neat solution for the problem of an aquatic mammal.

ABOUT THE AUTHOR(S)

    Daniel Leonard is a freelance science journalist and current S

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