Eva Sawyer still remembers the awe she felt as a child when she saw a lobster for the first time. Raised in New York City, she saw the creature as utterly alien.
Sawyer, who is now a doctoral student in neuroscience at Vanderbilt University, likened that childhood awe to her first time seeing a sea lion brain in person.
“I am someone who has seen brains from many different animals, and I had seen pictures or diagrams of a sea lion brain before,” Sawyer told R&D Magazine. “But seeing it myself for the first time was shocking to me. It was larger than I had imagined, and none of the cortical folds seemed to be where I would expect them to be.”
Sawyer and her colleagues, fellow doctoral student Emily Turner and Prof. Jon Kaas, are mapping the sea lion brain in the flesh for the first time. Whereas previous researchers have created 3D maps of the sea lion brain with magnetic resonance imaging (MRI), Sawyer and colleagues are cutting into the brain.
Recently, the team published a study in The Journal of Comparative Neurology, in which they reported their findings from mapping the California sea lion’s somatosensory system that is responsible for touch, pressure, pain, temperature, position, and vibration.
Sea lions belong to a group of semiaquatic marine mammals known as pinnipeds, which also include walruses and seals. These creatures are capable of some pretty incredible underwater feats, particularly with their whiskers. Previous studies have shown that sea lions can employ their whiskers to track hydrodynamic trails left by fishes when their eyes are covered and noise is controlled. Robyn Grant of Manchester Metropolitan University has even said their whisker control is akin to human hand movement, and can be used for balance and to discriminate between different objects.
“We know seals and sea lions from watching them lay around and move awkwardly on the beach, but so much of their life is underwater and nearly invisible to us,” Sawyer said.
Like the aforementioned studies, Sawyer was also interested in the sea lions’ whiskers. Sawyer, familiar with the whisker systems of lab rats and lab mice, wanted to see if those systems were analogous for other animals.
“Sea lions are a good animal to ask this question because they, like rodents, have very sensitive and robust whiskers,” she said.
For her work, Sawyer needed very fresh specimens. But like all marine mammals, sea lions are protected by the federal government under the Mammal Protection Act of 1972.
At the time though, there was a large uptick in California sea lion strandings in the central and southern regions of the state.
“To use the kind of methods I used, the researcher has to be present at the time of death and the death must happen naturally. This is such a rare occurrence that it is hard to get well-preserved specimens,” she said. “I was able to visit the Marine Mammal Center, a large marine mammal veterinary clinic, during a time when there was a huge influx in their patients. The staff there kindly let me wait in their waiting room while they did everything they could to save the animals that were brought in.”
If the animal could not be saved, they allowed Sawyer to use the remains for her research.
“It is sad that some of the animals cannot be healed at the clinic, but it is nice that if they do die, then can we can contribute to something positive,” she added.
Using histochemical methods, the researchers were able to discern which parts of the brain belonged to the somatosensory system. Like mice and rats, sea lions have specific areas of their brain devoted to processing information from their whiskers. Each whisker, the researchers discovered, has an area devoted to it in the brainstem, according to Vanderbilt University.
“It is interesting that we see these similarities because pinnipeds and rodents are not that closely related (we are more closely related to rodents than pinnipeds are), live in very different habitats, and have such a great size difference,” Sawyer said. “It suggests that these traits will be very widely shared among mammals.”
Additionally, the researchers found that the sea lion’s brain cortex was intensely folded. Other animals that have such cortices are cetaceans, elephants, and humans.
Most surprising to Sawyer was the prominence of the hind and fore flippers in the somatosensory system. “I had made an assumption that without obvious individual digits these animals would not care too much about their limbs; I was thinking about flippers as arms without hands,” she said. “But there was a robust representation of the flippers in the brainstem, and it was organized in such a complicated way that I still don’t fully understand it at all.”
In the future, the researchers hope to map other parts of the brain, and perhaps learn how and why the sea lion’s brain is so big.
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