What really causes tradeoffs between longevity and reproduction?
Now the New York Times is reporting on the two aging studies I mentioned yesterday. It's a good article, except for this part:
Dietary restriction seems to set off an ancient strategy written into all animal genomes, that when food is scarce resources [calories?] should be switched to tissue maintenance from breeding.This is the "disposable soma" hypothesis of Kirkwood, and I don't think it applies to the monkey experiment. The monkeys in the study aren't reproducing anyway, so those on a low calorie diet should have fewer resources available for maintenance, yet they have lower rates of death from aging-related causes.
More generally, if the only way reproduction shortened lifespan were by consuming resources, then eating more (enough to outweigh the metabolic cost of reproduction) should increase longevity. It doesn't.
There is plenty of evidence for a tradeoff between reproduction and longevity, but I don't think it's mainly due to competition between reproduction and maintenance for calories. It's more likely that blood pressure or levels of insulin and testosterone have different optima for reproduction and longevity. Even in males for whom reproduction has negligible energy costs -- I know this is not true of males of all species -- testosterone levels that maximize reproduction have a long-term cost, reducing lifespan.
I suspect that many aging researchers would agree that there's more to the reproduction-vs.-longevity tradeoff than calories, so this isn't the really novel part of the hypothesis we published last week.
What's new in our paper is the reason that delaying reproduction increases fitness. The key point is that Darwinian fitness is the relative contribution to the next generation, not the absolute number of offspring produced. So, if population is decreasing, delaying reproduction can increase fitness, simply because each offspring makes a bigger splash in the smaller gene pool.
I think we're the first to link the fitness benefits of delaying reproduction in a declining population to environmental cues that predict such decreases: calorie restriction, crowding, or consumption of "famine foods" with toxins like resveratrol, asprin, glucosinolates, or alcohol.