Cetaceans, the class of marine mammals including whales, porpoises and dolphins, are some of the smartest creatures on earth. Bottlenose dolphins, for example, have slightly bigger brains than humans (1600 grams versus 1300 grams), as well as developed highly complex language abilities. Some dolphins have even developed the use of tools, considered a marker of animal intelligence.
But sometimes dolphins do some pretty unintelligent stuff, at least from a human perspective. Specifically, some dolphins will beach themselves, getting stranded on sand in a way that often results in death. Entire pods of dolphins may end up doing this at once.
Occasionally they do this in pursuit of food, a behavior known as "strand feeding," while other times it's because of pollution or industrialization. In other instances, it could be due to a "sick-leader syndrome," in which dolphins unwittingly follow an older, confused and feeble-minded dolphin to their death.
A new study in the European Journal of Neuroscience suggests that some of this stranding behavior could be due to dolphins developing Alzheimer's disease, a progressive brain disorder that erodes the capacity for memory, often manifesting as dementia. By analyzing the brains of 22 dolphins, researchers at the universities of St Andrews and Edinburgh, the University of Glasgow and the Moredun Research Institute in Scotland, present some of the most detailed evidence yet of this disease occurring in dolphins. The notion that Alzheimer's could affect animals outside of humans poses profound implications for neurology research into the disease.
Notably, the study notes that this isn't definitive proof that dolphins can get Alzheimer's, although previous research has also pointed to the possibility. Scientists won't know for sure unless they are able to test these animals for cognitive deficits. Moreover, this beaching behavior could have resulted from some other factor, including sonar from navy ships.
"These are significant findings that show, for the first time, that the brain pathology in stranded odontocetes [another word for toothed whales and dolphins] is similar to the brains of humans affected by clinical Alzheimer's disease," lead researcher, Dr. Mark Dagleish from the University of Glasgow, said in a statement. "While it is tempting at this stage to speculate that the presence of these brain lesions in odontocetes indicates that they may also suffer with the cognitive deficits associated with human Alzheimer's disease, more research must be done to better understand what is happening to these animals."
When Dagleish and his colleagues dissected the brains of 22 dolphins that had died and washed up on the shores of Scotland, they found three that had the same proteins and brain scarring that occurs in human Alzheimer's patients.
Alzheimer's is a frankly terrifying disease, the exact cause of which is not fully understood. The condition is characterized by overall mental decline, difficulty thinking, concentrating and understanding things, disorientation, forgetfulness, confusion and inability to create new memories. It affects 6 million Americans, sometimes leading to death, yet experts still aren't sure what causes it or how to best treat it.
A prevailing theory has to do with the buildup of proteins in the brain call beta-amyloids, which play an essential role in the growth and repair of neurons. When too many of these proteins accumulate in the brain, they can clump together, forming amyloid plaques that disrupt communication between neurons. Think of it as the hard plaque that grows on teeth, but instead gunking up the brain.
Though this is the dominant theory of what causes Alzheimer's, medical science hasn't been able to develop any drugs that work against amyloid plaques, suggesting that some other mechanism might be at play.
Nonetheless, when Dagleish and his colleagues dissected the brains of 22 dolphins that had died and washed up on the shores of Scotland, they found three that had the same proteins and brain scarring that occurs in human Alzheimer's patients.
The three dolphin species were the common bottlenose dolphin (Tursiops truncates), whitebeaked dolphin (Lagenorhynchus albirostris) and a long-finned pilot whale (Globicephala melas) — which, despite its name, is a species of dolphin. Each of these specimens were older, as evidenced by worn or missing teeth, while the younger dolphins from the sample had no signs of Alzheimer's.
In the three with amyloid plaques, they also found another hallmark of Alzheimer's called tau tangles. Tau proteins play an important role in stabilizing neurons, but with age, they can start to clod together, forming tangles that are neurotoxic and can jam themselves into the entire intracellular space of a neuron. This makes communication between neurons very difficult, creating cognitive impairment.
Finding both tau tangles and amyloid plaques in dolphins are strong indicators that these marine mammals can develop Alzheimer's. But it's not definitively proven yet. Human and dolphin brains are quite different and there could be other explanations for why these proteins were present in the deceased cetaceans.
Nonetheless, this research has interesting implications for the future of Alzheimer's research, as well as protecting marine animals. Part of the reason we don't fully understand Alzheimer's is because we don't have very good animal models on which to do experiments, at least not anything close enough to humans. While certain animals may exhibit similar brain damage, it may not translate into the same exact cognitive deficits.
Don't worry, humans aren't going to start testing this in living dolphins, as that would most likely be considered unethical. However, since dolphins are much closer to humans in intelligence than rodents, additional research in more dolphins and other marine mammals such as baleen whales, would be helpful for understanding how Alzheimer's manifests.
The researchers also suggested studying the lives of captive dolphins, such as those at Sea World, which have detailed known life-histories that "might provide greater understanding of the pathogenesis, risk factors and underlying mechanisms of [Alzheimer's disease]," the authors wrote.
"We were fascinated to see brain changes in aged dolphins similar to those in human ageing and Alzheimer's disease," Professor Tara Spires-Jones at the University of Edinburgh said in a statement. "Whether these pathological changes contribute to these animals stranding is an interesting and important question for future work."