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Assessing biomarkers of ageing as measures of cumulative animal welfare 2019-09-27T08:00:22.716Z · score: 59 (25 votes)

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Comment by willbradshaw on Assessing biomarkers of ageing as measures of cumulative animal welfare · 2019-10-08T12:44:20.265Z · score: 4 (2 votes) · EA · GW

Maybe it would be helpful if I try to lay out my rough model of why fasting responses to limited food availability exist in the wild, and we can see if there's actually a disagreement here.

I certainly agree that "we should doubt that animals eat (and our ancestors ate) more than is best for their fitness". If they were eating more than was good for their fitness, you'd expect them to evolve to eat less. However, wild animals exist in a state of severe food insecurity, in which food may be abundantly available one day and scarce for weeks thereafter. It probably is quite difficult to have offspring while food is scarce, and probably not very valuable anyway since those offspring will be food-deprived during crucial developmental periods. So it makes sense to use what energy you have to maintain a healthy body, and wait for better times.

The response to DR would therefore be a "making the best of a bad situation" sort of thing: from a fitness perspective it would be better to eat lots of food, have lots of offspring, and die young, but since that option is unavailable due to food scarcity it is better to activate an energy-conserving fasting response that will keep you in better shape until the good times return.

Importantly, the claim is not that DR improves fitness. It is that it increases lifespan. Natural selection doesn't care about increased lifespan, or even increased healthspan, except insofar as it increases the number of descendents you have. And food deprivation is certainly very costly: DR mice show dramatically reduced fertility relative to AL (=eat-as-much-as-you-want) mice. However, they also show less age-related decline in fertility, so if you later put them back on an AL diet they are more fertile than mice of the same age that have been on AL the whole time. I think that summarises the evolutionary point of a fasting response pretty well.

Comment by willbradshaw on Assessing biomarkers of ageing as measures of cumulative animal welfare · 2019-10-05T11:47:49.675Z · score: 1 (1 votes) · EA · GW

Okay, so from this I think you mean the metabolic response to dietary restriction, not the actual restriction of diet.

If that's dangerous in expectation, why would it have evolved?

Comment by willbradshaw on Assessing biomarkers of ageing as measures of cumulative animal welfare · 2019-10-03T15:50:20.366Z · score: 1 (1 votes) · EA · GW

I find this comment a bit confusing, I'm afraid. Do you mean dietary restriction would be dangerous or that the metabolic response to DR would be dangerous? If the latter, why? Given that it seems to be an evolved fasting response I'd be somewhat surprised if it was bad for fitness overall.

Comment by willbradshaw on Assessing biomarkers of ageing as measures of cumulative animal welfare · 2019-10-02T12:29:58.947Z · score: 2 (2 votes) · EA · GW

Given all that...

Obviously, it's highly speculative at this point, but what would you guess the correlation coefficient is between cumulative welfare and biological ageing? How large does the correlation need to be before it's useful?

We'd need to weight the different exposures by how widespread and frequent they are: some potential exceptions (e.g. food or reproduction) would be much more important than others (e.g. addictive drugs). Given some sort of weighted measure of this kind, I'd guess a moderate negative correlation, with a pretty wide uncertainty. I'd be quite surprised if it turned out to be near-zero or positive, though.

I think a moderate correlation like this should definitely be enough to be useful in many or most cases, given a sufficiently large sample. However, it also depends what exposure you're actually interested in studying; if it turns out to be one of the exceptions then it doesn't really matter how rare those exceptions are. So I think a better idea of where exactly exceptions to the rule might lie in the space of potential experiences would be more useful than estimating the overall correlation.

Comment by willbradshaw on Assessing biomarkers of ageing as measures of cumulative animal welfare · 2019-10-02T12:08:56.938Z · score: 4 (3 votes) · EA · GW

These are good questions, thanks for commenting. I'll start with your more general question then address the specific examples raised. Please be aware that everything I'm saying here is based on theory

Let's summarise the overall hypothesis as being that cumulative welfare and rate of biological ageing are generally negatively correlated. In general, I'd expect counterexamples to this general idea to be fairly rare among exposures whose general form has been present in the environment long enough for the species to evolve in response to it: a species should evolve to become averse to damaging (and generally ageing-causing) exposures and attracted to protective ones.

Pro-ageing exposures might be hedonically positive on-net if (a) they provide a gain to short-term reproductive fitness that is large enough to outweigh the longer-term costs of accumulating damage, or (b) the exposure is new and misaligned with the animal's evolved incentives. An important potential example of the former that is discussed in the 2019 Bateson & Poirier review is reproduction, which is quite stressful and costly but obviously vital to reproductive fitness. Potential examples of the latter might be various addictive-but-damaging superstimuli, such as addictive drugs or wireheading. I'd expect examples of the latter group to be fairly rare in nature, but they could potentially be important for some captive populations.

The same general classification would apply to exposures that are anti-ageing but hedonically negative: they might be good for the condition of the body but bad for reproductive success, or they might be new exposures that are misaligned with the animal's evolved drives. It's not as easy for me to come up with examples for these. There's also potentially the issue of hormetic effects: damaging or aversive stimuli that provoke a response that is on-net beneficial to health and longevity. The pro-lifespan effect of dietary restriction and fasting is the most well-studied and important of these, and is probably the biggest issue with this idea I've thought about that didn't make it into the report.

So there are probably some exceptions to the overall hypothesis the sorts of methods proposed here would rest on. However, I expect these exceptions to be fairly rare for animals in their natural environment, and I'm actually sceptical of most of the candidates raised here:

  • Reproduction is obviously crucial to fitness, but while sex is clearly hedonically positive in many species, it's far from clear to me that reproduction as a whole is. It certainly doesn't seem to be clear in humans that having children is good for your happiness on-net, for example.
  • I'd be at least mildly surprised to learn that smoking is on-net hedonically positive, even in the first couple of decades of regular use. My impression is that many of the health effects kick in pretty quickly, and a lot (though not all) of the perceived pleasure of smoking is actually lifting of negative feelings that wouldn't be present if you weren't addicted to nicotine. So I suspect many or most smokers significantly overestimate how much net pleasure they get from smoking, though I'm not confident about this. The same applies to many other addictive drugs.
  • Insofar as castration is negative in humans I think that largely arises from psychological/social factors I'd expect to be largely missing from most animals. There's the pain of the actual procedure, of course, and some lost pleasure from sex, but apart from that it's not obvious to be that it's bad on-net. I don't actually know if we have any data on what the lives of castrated wild animals are like, though.
  • Dietary restriction is probably the potential exception I'm most worried about in the wild (though I'd guess it's not much of a problem when applying the technique in captive populations), but even here I'm not sure how severe it is. DR typically involves mild to moderate caloric deprivation, and AFAIK more serious starvation does not generally extend lifespan. I'm not sure the level of deprivation required to extend lifespan is all that hedonically negative after you've got used to it: this seems to be the reported experience of people on intermittent fasting, for example, though it might be different if it's out of your control. So while mild food deprivation might be mildly hedonically negative, it might not be strongly so, and so might not actually be net-negative when the hedonic gains of improved health etc. are taken into account. (Also, AFAIK there's not much evidence that DR increases longevity in humans.)

My level of uncertainty is high for most of these cases, and I'm largely falling back on the classic academic cop-out of "more research is needed". But suffice it to say that there aren't any obviously true and important exceptions I'm aware of. The main potential exceptions I'm worried about that might be important in the wild are reproduction and food deprivation, which I'd say definitely need to be looked into further.