Opossums, Hydras And Hummingbirds: What We're Learning About Aging From Animals

Enlarge this image toggle caption Leonardo Santamaria for NPR Leonardo Santamaria for NPR

A stint as lion tamer in Hollywood got Steven Austad interested in animal biology. And soon he turned from training animals to studying them. He's now chair of the biology department at the University of Alabama at Birmingham, where his research focuses on aging.

He's learned that aging happens at different rates in different animals, without following any clear rules. Austad says it's not heart rate that predicts lifespan. An animal's size has something to do with it, but some animals defy that pattern. And even more perplexing are animals that don't seem to age at all — like a tiny creature called a hydra.

Listen to the related Invisibilia episode For more on hydras, aging and the dream of human immortality, listen here to Invisibilia's Episode 5, Season 6.

Austad spoke with Invisibilia's Lulu Miller to discuss what science has uncovered about animal aging processes, and how researchers might be able to use what they've learned to extend human lifespans. There's no immortality on the horizon — or anything close to it — but it's likely science can eventually lengthen our lives by at least a little, Austad says.

This interview has been edited for clarity and length.

How did you go from lion taming for the movies to studying aging?

I was a reporter for the Oregonian newspaper in Portland. And a friend of mine had a couple of African lions for pets, because he was crazy. He got an offer to use them in a movie, and he needed somebody to help him transport them from Portland to Hollywood. And he talked me into helping out. When I got down there, the movie producer offered me a job and I said, "You understand I don't know anything at all about this, right?" And he said, "that's okay." It awakened my interest in animals and what makes animals tick. After I got fairly seriously injured one time, I thought maybe this is not really what I want to do the rest of my life. So I decided to study animals in graduate school.

What did you like about training lions?

What I liked the most about lions is because they live in social groups, they like contact. They're almost like dogs, more like dogs than cats, except they sometimes will try to kill you. But I just love the intimate contact with them. For the first year, I never took a day off. I worked seven days a week.

How did opossums' short life span get you interested in longevity?

We were working on some animals in South America — opossums. I discovered that they age really quickly, almost like mice. And that was so puzzling to me that I completely abandoned what I was working on. It was the size and the longevity combination. I think we all have this kind of intuitive feel from being around animals that smaller animals are going to [have] shorter lives. So you know, a dog has a longer life than a mouse, and a horse has a longer life than a dog, and an elephant has a longer life than a horse. And this just seemed to grossly violate that. I had to recapture them every month, and I would come upon one that was in prime physical health, and two months later it would have cataracts, and it would have lost muscles, and had parasites all over it, and arthritis. It all happened so abruptly.

So, are size and lifespan linked in animals or not?

Yeah, it's a very general pattern. It's true of mammals. It's true of birds. It's true of reptiles. It's true of almost every group of animals. We know that smaller ones are shorter-lived and bigger ones are longer-lived. But there are exceptions, and actually I think the exceptions are the ones that are most interesting from a scientific perspective.

What is the billion beats hypothesis and why do you question it?

I've spent a good deal of my career trying to kill it, but obviously, I haven't been able to. The [idea] is that life is inherently destructive and that burning energy is inherently destructive. Let's say all mammals have a kind of a fixed amount of energy that they can burn over the course of a lifetime. And if they burn it fast, they'll be short-lived, like mice. And if they burn it slow like an elephant, they can live much longer than that. If you actually look at a whole bunch of animals, it turns out that smaller ones actually have more heartbeats and use more energy over the course of a lifetime than large ones.

And then there are these massive exceptions to it. Hummingbirds have a heart rate of over 1,200 beats per minute, which is kind of like a machine gun, but yet they can live in the wild into their teens.

And there's a very small animal that's actually one of the longest lived creatures, right?

Hydras were discovered actually in the early 1700s by Van Leeuwenhoek, who invented the first decent microscope. They're freshwater animals, maybe a quarter to a half inch in length. They almost look like a sea anemone, they're just smaller and skinnier. They really started to be studied in earnest a few years later by a Swiss biologist named Trembley who discovered if he cut them in half across the middle, the bottom would grow a new top, and the top would grow a new bottom. It turns out that you can even treat them with chemicals that basically dissolve all the things that make their cells stick together. You'd make a pile of cells and they will eventually reassemble into a hydra. He started chopping them up in all kinds of ways to see exactly what you needed to regenerate. He eventually created a hydra that had multiple heads. That's how it really came to be [called a] hydra, because a hydra in Greek mythology was this monster that had many heads.

And what did we learn about aging from the hydra? How is it even possible for them to have this kind of longevity?

Hydras have stem cells in them. When they divide, one half of it remains a stem cell, but the other half will eventually turn into part of the tentacle or part of the mouth or part of the body wall. It changed the way we thought about animal development at that point in time. We didn't really know how animals develop [in the 1700s], and one idea was that animals were just very, very tiny replicas of themselves when they were in an embryonic stage, and that pre-formed thing just grew. At that point they thought, maybe inside of a human egg there's a little tiny human and it hatches out into a baby and then it just grows and grows and grows. The hydra pretty much killed that idea because we could take just part of it, which clearly did not contain a whole hydra, and grow a whole new hydra out of it.

Are hydras really immortal?

Rumors really started to accumulate in the 1950s. People had followed individual hydras for a few years, and they didn't seem to die at any higher rates. So there was a rumor that they might be potentially immortal. Daniel Martínez in the late 1990s actually reported that they didn't age. Few people believed him.

At least for as long as anybody's had the patience to follow individual hydras — that has been about seven years at the most — there's no indication that they age at all. It is possible that if we followed them long enough, we would discover that they aged, but no one has had the patience to do it. Certainly it would be a very, very long time. They're not the only animal that doesn't age, but they're one of the few, and the others that don't appear to age are really close relatives — the various kinds of jellyfish, for instance.

What has been unlocked in the science of aging by looking at hydras?

So the idea that if you manipulate single genes, it can have a dramatic effect on aging was really discovered in the late 1980s I would say. And then through the 90s it was confirmed and other genes were discovered.

One of those genes directly interacted with this gene FOXO. Finding this in everything from little worms to people [with long lifespans] suggested that the activity of FOXO might be a key to understanding slow aging. So the hydra work really confirmed what had been seen in a number of other animals.

How has research on slowing aging progressed?

Starting about 30 years ago, people discovered that there were genes that if you either knocked down their activity or souped up their activity could really have a major impact on aging. We started to look at drugs that could affect aging, and we now have at least half a dozen drugs that we know affect aging in a lot of different animals. Some of those things will turn out not to work in humans, but I'm quite confident that we will develop ways to improve human health either by injections, by transfusions, by taking certain pills every day. And that's what the biotech industry is going nuts with right now.

You often hear people fantasize that we're going to live 500 or 1,000 years in the future, and I don't buy that at all. We haven't been able to do that with different species. What we can do is we can increase the longevity of mice, worms and flies — let's say by 20% — many, many ways. And so I think that's a reasonable idea. What's unclear is how much of that will be healthy life.

Are there drawbacks to potentially extending lifespan?

Let's imagine that we discover a gene mutation that doubles lifespan. If this is so great, why didn't nature do this a long time ago? If it has an effect on reproduction or the [time] to sexual maturity, it may turn out — from an evolutionary standpoint — not to be a good gene, but to be a bad gene. For all of the benefits that we get in terms of health, there may be some downsides to some of these treatments. We need to be careful.

Knowing everything you do about aging, do you live any differently?

I don't take anything. I don't do any weird diets. I do a lot of sensible stuff. I exercise a lot. I eat right. I don't smoke. Once there's enough evidence, I may try some other stuff. I don't think there's evidence enough in humans to be doing anything else right now.