The higher they go, the faster baboons age

For quite a few years, Duke University’s Jenny Tung has been studying baboons in Kenya. Recently she’s become particularly interested in how they age, and why some animals appear to be much older, biochemically speaking, than others that have lived the same number of years. As she explains to Chris Smith, it turns out that fighting your way to the top comes at a high biological price…

Jenny - We've been watching the same individuals, or their descendants actually for up to 50 years now - that's the Royal we! Just like all of us, chronological age goes at the same rate, but we were interested in why some of them seem to biologically age faster than others.

Chris - The phrase we use in the hospital is that "a person has a well lived in body", but it amounts to much the same thing. Doesn't it? Biochemically, a person is creaking at the seams a bit more at the same chronological age. And it's what lifestyle factors or other factors might've have risen that caused them to find themselves in that position?

Jenny - That's right. And we were particularly interested in whether aspects of their social environments, or their early life, might be the sorts of things that explain either sort of faster or slower biological aging in our paper.

Chris - How did you do it?

Jenny - Well, we looked at a measure of biological age that involves molecular changes on DNA. Most people are familiar with the idea that our DNA stays the same throughout our lives, but there are actual little chemical marks on the DNA that can change over time. And it turns out that, if you track some of those very specific marks, they're a really good predictor of both chronological age and biological age.

Chris - These are what we dub "epigenetic changes", aren't they?

Jenny - That's right. And in our paper, we are particularly focused on one type of epigenetic change known as a DNA methylation mark. So we have baboons data in the sense that we have blood samples and therefore DNA samples from animals who we've watched their whole life. So we know when they're born, we know what their chronological age is. We know how many actual years they've been on earth by the time we sampled them. What we did with the DNA is look very carefully at the level of these particular DNA methylation marks. We knew predicted age really well. And then we said, okay, if this is pretty predictive of age; if you're an eight year old baboon, we get a predicted output based on the marks on your DNA that you're about eight years old. But some of those eight year olds actually were predicted to be a little bit old for age. That is, they looked biochemically like they were maybe nine years old. So we took that difference, and we asked whether individuals that were either say high or low status or more socially integrated versus more socially isolated or who had more challenging early life environments, consistently looked old or young for age. So, we're taking social predictors that we know are important to the lives of these baboons and asking whether that translates into accelerated or decelerated biological or epigenetic aging.

Chris - And does it?

Jenny - Well, we were surprised to find that in some cases it does, and some cases it doesn't. The main thing that we found that did was actually the social status of males in our population. And this was a really sex-specific phenomenon. So that males who were high status, near the top of the social hierarchy in the groups who were studying, tended to look older than their actual age.

Chris - What about the females?

Jenny - We actually didn't observe any particular relationship between social status in female baboons and aging, but we think we might understand why, because females basically adopt the social status of their mothers. There's a really nepotistic pattern of status inheritance. Whereas males have to fight their way to the top. And so regardless of whether they were born to a high status mum or a low status mum, when they become adults, they have to move to new social groups and physically compete with other males to reach the top and then to stay there for as long as they can.

Chris - That's quite surprising, because when one looks at us humans, there've been a lot of sociological studies - I'm thinking of things like the Whitehall study - where people who are the underdog end up with more chronic stress, accelerated aging and a higher disease risk than the people who are perceived to be high-octane, high stress, top-of-the-pile people, but nevertheless seemed to, to come off better. So how do you reconcile the two?

Jenny - I think one of the things our study emphasises is that we use the same kind of terminology: we talk about social status in human socioeconomic status or, or rank in non-human animals. That doesn't always mean the same thing because of the differences in the way that you have to reach high rank, or if you even can reach high rank. The Whitehall study, of course, is this landmark classic examination of social gradients in British civil servants and its relationship with health outcomes like cardiovascular disease. I think one of the really key things that we have to keep in mind is that, as far as I know, in the British civil service, there's actually not physical fighting to reach the top, right?

Chris - I wouldn't assume that! You never know!

Jenny - Fair enough! Okay! I think that social status in humans often looks a lot more like social status in female baboons, where you have a lot of, uh, nepotism, right? You have a lot of social continuity between generations. You have other individuals who are helping to reinforce and maintain your rank and social hierarchies can be really stable over time because of it than they do in male baboons, where everything depends on you and your ability to fight off other, to literally fight off other males and then stay in that state. You know, nobody can do that. No baboons can do that for years and years on end. So you have very dynamic rank hierarchies where males are very unlikely to stay at the top of their whole lives.

Chris - Nevertheless, I suppose that if they can stay there for as long as possible, then that argues that they are endowed with some kind of genetic fitness or resilience, which wouldn't or would, which would only afford or lesser able individual, uh, less time at the top.

Jenny - You know? So I think that this is a really key point because it tells us something about the difference between health as an outcome, or even lifespan as an outcome and Darwinian fitness, right? So, uh, there's evidence from other work in the same baboons population that high ranking males may even experience a little bit higher risk of mortality, at least during the time that they maintain that rank. And of course, as humans, we think about elevated mortality risk as sort of pretty universally negative, but the only way that a male baboons is likely to leave offspring is to get to that high rank. So from an evolutionary perspective, there's a very, very strong motivation to fight for and attain high rank. Even if there might be costs to aging or health or longevity, um, as an exchange.