Old dolphins with signs of Alzheimer’s show the danger of living beyond our reproductive years

A wild animal—with signs of dementia .
A wild animal—with signs of dementia .
Image: Reuters/Gary Cameron
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The kind of neurodegeneration that leads to Alzheimer’s in humans may be more prevalent across the animal kingdom than researchers once thought.

Earlier this year, researchers discovered that chimpanzees living in captivity developed the same neurological markers for the devastating form of dementia. It turns out wild dolphins may as well. New research, published (paywall) in the journal Alzheimer’s & Dementia late in Sept. 2017, found that three different species of dolphins (bottlenose, striped, and Risso’s) had developed amyloid plaques and tangled clumps of fibers in their brains—both telltale signs of the disease in humans.

The study looked at eight dolphins in total, all of which had already died and washed up on Spanish beaches, so it’s impossible to say whether they were also showing signs of cognitive impairment. Nevertheless, the findings suggest that Alzheimer’s may develop in animals that share certain physiological traits with humans.

Like us, dolphins tend to live far beyond their reproductive years. It’s a relatively rare trait in the animal kingdom, and it comes with unique consequences. The longer we’re alive, the less sensitive we become to insulin, the hormone that tells the cells in our bodies to suck up extra sugar floating around in our blood. (Excess sugar is bad for your blood vessels and heart.) Insulin resistance forces our bodies to produce extra insulin, which taxes the circulatory system. It can also eventually lead to type 2 diabetes.

Poor responses to insulin have been closely associated with Alzheimer’s disease in humans, and previous research has shown that bottlenose dolphins can also develop insulin resistance. That, combined with the recent discovery of amyloid plaques and tangled clumps of fibers in bottlenose dolphin brains, led the researchers to believe that dolphins, like us, may develop signs of Alzheimer’s disease as a result of high levels of blood sugar and insulin.

This research doesn’t say for sure whether or not dolphins have the same types of cognitive problems as humans with Alzheimer’s. It also can’t say for sure how widely spread this type of brain damage is in the dolphin population. But it does add credence to the idea that insulin resistance may be a good place to start looking for Alzheimer’s triggers. “The observations recorded in this paper linking longevity, insulin signaling, and naturally occurring [Alzheimer’s] pathology in dolphins might guide us to how to create better experimental models,” Simon Lovestone, a University of Oxford psychiatrist and lead author of the paper, wrote in an email.

Alzheimer’s research won’t be focusing on dolphins in the future. It’s much easier to model the disease in other animals like fruit flies, mice, and monkeys, and it’s pretty difficult to to study wild dolphins in general because they can only live in large bodies of water. But because these animals show similar signs of the disease in their brains, scientists may start looking for the same signs in other long-living creatures, including other marine mammals like orcas, to solidify the link between insulin responses and Alzheimer’s.

If researchers can show that this is a definite link, it may be easier to intervene in people before they develop full-blown Alzheimer’s-related dementia. By the time Alzheimer’s symptoms like memory loss and confusion show up, the plaques and fiber tangles have been building up for years. At that point, it’s impossible to reverse the damage, although some drugs can slow the disease’s progression down.

Insulin resistance, though, is identifiable early on—and there are already drugs available that are highly effective at stabilizing glucose levels. Which means, if scientists could confirm that insulin resistance leads to Alzheimer’s, they could help patients forestall the devastating symptoms of disease with a relatively simple treatment plan.