So that's very interesting, because you also note that, generally, the animals with smaller sizes live longer. That's true in dogs. Animals with smaller sizes live longer than animals with large sizes. But then there are some animals of similar size. So, for example, a naked mole rat that has the same size but lives nearly ten times longer as laboratory mouse. What could explain such differences in aging? Dr. Steven Austad, PhD. Dr. Anton Titov, MD. Yeah. With all the modern molecular tools we have, I think we're starting to hone in on precisely that difference. I don't think we're there. Yet. Certainly, there are intrinsic processes that [affect aging]. 

Naked mole rats, for instance, repair DNA better than mice do. Not as well as humans do. And we understand more about their cancer prevention strategies. Dr. Steven Austad, PhD. Dr. Anton Titov, MD. But I think we are not yet sure why they live ten times as long as a mouse. It is an intriguing model. And I think it's a model for what we need to do next. We can't just focus on one long-life species, though, because that's liable to tell us something that's very idiosyncratic to that species. for instance, naked mole rats live underground. 

They live in low oxygen, high carbon dioxide atmosphere. Whether that has anything for their adaptation, whether that has anything to do with it or not, we don't know. I think what we need to do is focus on a handful of really long-life species, to try to think of it as triangulating in on the best methods for working to extend human health. 

By the way, I wanted to point out something that you mentioned earlier, and I think it is a source of confusion, something that I struggle with when I'm talking about aging with people. Dr. Steven Austad, PhD. Dr. Anton Titov, MD. So if you look at individual species, there's a general pattern for large species to live longer than small species. If you look within a species, there's a general pattern for smaller individuals to live longer than larger individuals, for smaller dog breeds live longer than larger dog breeds. 

Smaller mice live longer than larger mice. Smaller horses live longer than larger horses. So the within-species pattern is the exact opposite of the between-species pattern. Dr. Steven Austad, PhD. Dr. Anton Titov, MD. So that's fascinating. What do you think can explain this sort of interspecies versus within-species differences in lifespan? Because the metabolic rates are supposed to be more similar within the same species, is it not? 

Yeah, I've spent a lot of my career trying to kill this idea that metabolism is inevitably linked to longevity. But in thinking about it, it's not running the show. It doesn't determine longevity, but certainly metabolism is a player in longevity. And so I think when we think about species that we can learn something from that, we really need to think about species that have a high metabolic rate because they're suffering more intracellular damage potentially than species with a low metabolic rate. So, for instance, we know that giant tortoises live 175 to 180 years, much longer than people, but they have an incredibly slow metabolism. And so my thought is in slow cell turnover. 

But I thought we're unlikely to learn something about extending the human health from studying something like a giant tortoise. Whereas we are studying something like a naked mole-rat because, for its metabolism, it lives longer than humans do. Whereas for its metabolism, giant tortoises are shorter-lived than humans.