Scientists have mapped the sea lion brain for the first time, learning much more about the animal’s amazing sense of touch.
“It was shocking to see something so large and so different from any other brain I had ever worked with.”
Sea lions—along with seals and walruses—are one of only four groups of animals that have evolved extremely large brains weighing more than 1.5 pounds. The other large-brained groups are humans, elephants and cetaceans (whales and dolphins).
But, despite considerable evidence of their cerebral skills, very little is known about pinniped brains. Now a team, of neuroscientists has taken an important step toward rectifying this lack of knowledge by conducting the first comprehensive study of the California sea lion’s central nervous system, concentrating on the somatosensory system, which is concerned with conscious perception of touch, pressure, pain, temperature, position, and vibration.
Last year, the dramatic upsurge of juvenile California sea lion deaths due to strandings throughout central and southern California made it possible for researchers to obtain two juvenile sea lion brains for study. (Sea lions, like all marine mammals, are protected by the federal government under the Marine Mammal Protection Act of 1972.)
“It was amazing to see the sea lion brain for the first time because, after spending years studying brains, it was shocking to see something so large and so different from any other brain I had ever worked with,” says Eva Sawyer, a doctoral student in neuroscience at Vanderbilt University and first author of the study that is published in the Journal of Comparative Neurology.
“We know a lot about how some brains are organized and function, mice and primate brains for example, but mammals are highly varied, and we know almost nothing about the brains of most mammals, including pinnipeds,” says Jon Kaas, professor of social and natural sciences.
A large, convoluted brain
Humans, elephants, cetaceans, and pinnipeds each evolved large brains independently, so the way that their brains are organized is quite different, particularly the cortex. Due to our inherent interest in our own evolution and the importance of the large human brain to our identity, it is worthwhile looking at these independent examples of how brains become large, Sawyer says.
Sea lion brains are about the same size as chimpanzee brains. Although they have some features in common with the brains of their closest relatives—dogs, cats, bears, and weasels—their brains are also intensely folded in a fashion similar to that of whales and dolphins.
“It is striking that both cetaceans and pinnipeds tend to have large and convoluted brains, but there is no single accepted explanation for this observation,” Sawyer says. “It is at least partially explained by their large body size. Animals with larger bodies are expected to have larger brains. But it may also be related to other factors, such as the weightlessness of the marine environment or coping with cold water temperatures. Or it could just be a random outcome of evolution.”
The researchers focused on the sea lion’s somatosensory system because they were particularly curious about the animal’s sense of touch. “We have long wondered about how the somatosensory systems of sea lions and seals are organized, as they have such remarkable abilities to use touch and their whiskers to explore and feed in deep water,” Kaas says.
Using modern histochemical methods, the researchers were able to identify and characterize the major parts of the sea lion brain that are used in processing touch information from the whiskers and the skin. These include areas in the brainstem, thalamus, and cortex.
They discovered that the marine mammal’s brain has specific areas for processing information from its whiskers that are strikingly similar to those found in mice and rats, who are considered the whisker specialists in the animal kingdom.
Specifically, they found that each whisker on the sea lion’s nose has a specific, corresponding area in the brainstem devoted to it. These are comparable to specific areas on the human brain that correspond to individual fingers. Their existence confirms that these “mystacial ” whiskers play an important role in sea lion sensation and behavior.
In addition, the researchers identified the brain areas devoted to processing touch sensations from the sea lion’s flippers and tail. One of the surprises was discovery of a well-developed region, called Bischoff’s nucleus, which is found in animals with prominent tails ranging from raccoons to kangaroos to whales. It is surprising because the sea lion’s tail is not prominent at all: It is small and tucked between its hind flippers.
Once mapping of the sea lion’s somatosensory region is complete, the researchers plan on mapping the parts of its brain devoted to other functions. “It will be interesting to compare its overall structure to that of primate brains,” Sawyer says.
The research was funded by National Institutes of Health.
Source: Vanderbilt University
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