How to tell if your aphid is done reproducing.

If you want to know if a parthenogenetic pea aphid is all done reproducing, look at her abdomen. If there are eyespots, she still has embryos in her. If not, she's done. If she is post-reproductive, she's likely to move to the edge of the colony, where risk of predator attack is highest. Details are here, in a paper written with some very talented undergraduates at Bates College.

Old pea aphids

You are a post-reproductive pea aphid. You have spent a long and happy life sucking the juices out of a pea plant. As the plant has grown, so has your large and clonal family, who you love as you love yourself, because genetically they are self. You were born with all your daughters already developing in your ovaries, and now the last one is out, and already reproducing, and what are you going to do with yourself? You may be only halfway thorough your lifespan. What to do with the remaining weeks? Pompano Beach is out, too many insecticides.

The obvious answer if you are a natural selection minded aphid is you'd like to help all the clones of yourself you've created to grow fast, live long and reproduce a lot. But how? Reproductive adults contribute more to the growth of the colony than do the young'uns, so throughout your reproductive lifespan, you've tried hard to stay at the center of the colony, where there is a touch of protection from predators. So maybe now you should move to the edge? If a hungry predator comes along, you can martyr yourself for the good of the clone. You don't have any chemical defenses or strong sharp pokey bits, and your kick is frankly rather unimpressive, but maybe if the predator eats you, it will allow time for your great-great-grandkids to escape, or at least make the predator full enough that it eats fewer of them. And maybe, just maybe, when that predator comes, you will be brave enough to just stay put and get eaten for the team.

Or perhaps rather than just sitting around waiting to get eaten, you can help to feed the family? Aphids suck sap, so if you could either put some chemical into the plant, or create enough suction, you could stimulates flow to that part of the plant where your family resides. Your young might grow faster or start reproducing sooner.

I mean, I don't really know. No post-reproductive aphid has ever sought my advice before. I'll do some experiments and get back to you.

Who has an adaptive post-reproductive life-stage?

To establish a case for an adaptive post-reproductive life-stage, one needs to show (at least) the following things:

(1) Prevalence: Across environments, but especially in a wild or non-protected environment, the population experiences more post-reproductive lifespan than is expected due solely to demographic stochasticity.

(2) Utility: Post-reproductive individuals do something selectively advantageous, such as helping younger kin to survive or reproduce.

In many cases, one would like to also address

(3) Advantage: Those individuals who become post-reproductive have a selective advantage over same-age individuals who simply continuing reproducing indefinitely.

However, this third is more complicated, because in many cases there are no individuals who fail to stop reproducing to compare to. For example, 55 year old women giving birth are rare and not easily compared to those who stopped at a more usual time. So testing (3) requires extrapolation and counter-factuals. This assumes that if continuing to reproduce past the current age of cessation were selectively advantageous, that the species' reproductive physiology would allow for it. In many cases, theoretically advantageous traits simply don't exist in the population, and therefore cannot be selected for. If the choice is not between ceasing reproducing or continuing, but rather between ceasing and being useful or ceasing and not being useful, useful wins.

So we are left with basically two fairly simple tests to make decent case an adaptive post-reproductive life-stage. And after some decades of interest in the evolution of post-reproductive lifespan, for how many species has this case been convincingly made? By my count, three. Humans, orcas and a gall-forming social aphid, Quadrartus yoshinomiyai. There are several other likely candidates. Short-finned pilot whales, and possibly other social cetaceans. African and Asian elephants. But I've become very interested in a much more accessible and experimentally tractable species. It lives in multi-generational groups of (very) closely related individuals.  Individuals play subtly different roles in the group throughout their lives. Older individuals stop reproducing and can (assuming no one comes along and kills them) live for extended periods post-reproductively (on the time scale these things live). You may have it in your garden.

Any guesses? Later this week I'll give you the answer.