Imagine trying to train wild sea lions—without them ever seeing you. That was Peter Cook's challenge 8 years ago when he was trying to figure out whether poisonous algae were irrevocably damaging the animals’ brains. With a lot of patience and some luck, the comparative neuroscientist from Emory University in Atlanta has succeeded, and the news isn't good. Toxins from the algae mangle a key memory center, likely making it difficult for sick animals to hunt or navigate effectively, Cook and his colleagues report today.
"Sea lions can be seen as sentinels of human health," says Kathi Lefebvre, a research biologist at the Northwest Fisheries Science Center in Seattle, Washington, who was not involved with the work. As oceans warm, toxic algae proliferate and cause so-called red tides because the water looks reddish. So "understanding these toxins in wild animals is going to become more important," she says.
Red tides are produced by algae called diatoms. They make a toxin called domoic acid, which is consumed by other plankton that in turn become food for fish and other organisms. Predators such as anchovies, sardines, and other schooling fish accumulate this toxin in their bodies. So when algal populations explode, say, because of warming water, domoic acid concentrations increase in these animals to a point that they affect the sea lions that feast on them.
Scientists first recognized this problem in 1998, after hundreds of sea lions were found stranded or disoriented along California's coast. Since then, researchers have studied sick and dead sea lions and documented that the toxin causes seizures and damages the brain, sometimes killing the animal.
Red tides often occur in fall and spring off both the East Coast and West Coast of the United States, especially during El Niño years. A large one in June affected the West Coast from California to Alaska, causing hundreds of sea lion strandings. Scientists have no strategies to manage or mitigate the effects of the red tides on these marine mammals, making it all the more important to better understand the toxin's effects, Lefebvre says.
To clarify the link between the brain damage and the animals' erratic behavior, Cook teamed up with researchers at the Marine Mammal Center in Sausalito, California. From MRI brain scans of dead sea lions, they knew the toxin caused shrinkage in the hippocampus, which is involved in memory. Cook sought advice from neuroscientists who study the hippocampus in mice and people to come up with a memory test for the sea lions. Then he worked for a year perfecting the test on wild sea lions being treated at the Marine Mammal Center for red tide exposure or other problems. As with other animals, training involved food rewards. However, because these animals were destined to be released into the wild again, it was important that they not learn to associate humans with food. So Cook could not let the animals know he was dishing out the fish.
In one experiment, Cook’s team tested 30 sea lions, each of which had undergone an MRI scan to assess brain damage because of suspected toxin poisoning or other health problems. Just outside their swimming tank, the animals had to learn to alternate between going through a door on the right and one on the left to get a fish (see video below). Once a sea lion mastered this routine, it would do it fluidly without stopping or thinking about where to go next. Then Cook would put a gate down in front of the doors for 7 seconds to interrupt the routine. The delay "will totally disrupt the performance of sea lions with the hippocampus damage," he says.
Cook and his colleagues also tested longer term memory—which is critical to helping these marine mammals recall and locate good hunting spots. He hung several buckets around their enclosures, one of which always had a fish in it. Then he let in the sea lion being tested and timed how long it took for each animal to find the fish from one day to the next. Healthy animals make a beeline for the fish after just 1 or 2 days, but sea lions with damage to their right hippocampus tended not to improve, Cook and his colleagues report today online in Science and at the 21st Biennial Conference on the Biology of Marine Mammals in San Francisco, California. "It's [damage to] the right hippocampus that's driving the behavioral deficit," he says.
"This paper provides novel evidence for memory impairment in large animals that have brains similar to humans," says Paul Buckmaster, a neurobiologist at Stanford University in Palo Alto, California, whom Cook consulted about how to do memory tests. Because the poisoned sea lions also have seizures, neuroscientists can learn more about epilepsy and memory loss in humans by studying these marine mammals, he says. Sea lions may even prove useful for testing to treat or prevent epilepsy, he says: "I'm impressed by how the investigators contended successfully with the limitations and constraints of experimenting with wild animals."