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.

Jon Asks: 2

In the Laura Ingalls Wilder book The Long Winter, Laura's father says that you can predict the severity of a winter by observing the thickness of muskrat nests in the summer. Muskrats, he says, will build thicker nests during the summer if the following winter is going to be relatively colder, and vice versa with thinner nests and relatively warmer winters. Has this folk wisdom been investigated? Is it true? And if so, where can I get my own muskrat colony?

I can't find anything on Google Scholar or Web of Science indicating that anyone has published anything about muskrats and weather prediction, other than this:

Man's natural craving for advance knowledge of coming weather extends thousands of years back of any attempts at scientific weather forecasting. Realizing that he has not the necessary foresight himself, he has imagined animals to be endowed with some peculiar sense which enables them to know, weeks or months ahead, what the weather will be. Thus a large group of animal weather proverbs has come into existence. Millions of people believe that the thickness of fur on a muskrat, or the number of nuts stored by a squirrel, or a supposedly early migration of certain birds, indicates a severe winter. Yet it is certain that animals have no such foresight.

from: Robert DeC. Ward. 1926. The Present Status of Long-Range Weather Forecasting
Proceedings of the American Philosophical Society,Vol. 65, No. 1, pp. 1-14

He provides no evidence to show that they don't, he is just certain. Note that the version you mention has to do with the thickness of the wall of the house, while the version Ward mentions has to do with the thicknesss of their fur. The fur hypothesis would be easier to test, if you were a muskrat hunter. I am frankly doubtful whether he or anyone else has done the work that would be needed to convincingly either story. You would have to measure the wall thickness of bunches of muskrat houses (or the pelt of many muskrats) in the summer. You would have to do this every year for quite a few years in order to make a convincing analysis of the relationship between wall thickness and hardness of winter. You would probably also want to measure various features of the microclimate, the muskrats behavior and physiology, and the local ecology, in order to get some sense of what the mechanism was. You probably would want to measure the pelts and the houses, just to make sure you were measuring the right thing. This is one limitation to testing folk-wisdom. There are often several versions, and it is hard to know if you are testing the right one unless you test all of them, and then you increase your chances of finding a strong correlation just by chance. My best guess is that there is some, but not a lot of, truth to either version of the story. Certainly they could pick up on whatever cues are available that the winter is going to be hard. But like most weather prediction, they probably aren't very accurate, at least not months in advance.

EvoDemo: "Senescence rates are determined by ranking on the fast-slow life-history continuum"

Jones, O.R., Gaillard, J.M., Tuljapurkar, S., Alho, J.S., Armitage, K.B.,
Becker, P.H., Bize, P., Brommer, J., Charmantier, A., Charpentier, M.,
Clutton-Brock, T., Dobson, F.S., Festa-Bianchet, M., Gustafsson, L.,
Jensen, H., Jones, C.J., Lillandt, G., McCleery, R., Merila, J., Neuhaus, P.,
Nicoll, M.A.C., Norris, K., Oli, M.K., Pemberton, J., Pietiainen, H.,
Ringsby, T.H., Roulin, A., Saether, B.E., Setchell, J.M., Sheldon, B.C.,
Thompson, P.M., Weimerskirch, H., Wickings, E.J. & Coulson, T. 2008.
Senescence rates are determined by ranking on the fast-slow life-history continuum.
Ecology Letters.

Abstract:

Comparative analyses of survival senescence by using life tables have identified generalizations including the observation that mammals senesce faster than similar-sized birds. These generalizations have been challenged because of limitations of life-table approaches and the growing appreciation that senescence is more than an increasing probability of death. Without using life tables, we examine senescence rates in annual individual fitness using 20 individual-based data sets of terrestrial vertebrates with contrasting life histories and body size. We find that senescence is widespread in the wild and equally likely to occur in survival and reproduction. Additionally, mammals senesce faster than birds because they have a faster life history for a given body size. By allowing us to disentangle the effects of two major fitness components our methods allow an assessment of the robustness of the prevalent life-table approach. Focusing on one aspect of life history - survival or recruitment - can provide reliable information on overall senescence.

Keywords: Aging; comparative analysis; demography; generation time; metabolic rate; senescence

Comments: One of the authors is applying for a position here in the next few days.

Nuts with no squirrels

At my parent's home in New York State, the acorns don't last long. The deer, squirrels, chipmunks and turkeys quickly gobble up, hoard or bury the best nuts, leaving only the small and wormy nuts for the insects and mice. A forest ecologist I once worked for told me that oaks are reproducing poorly in the northeastern US, in part because the unnaturally high deer populations consume so many acorns.

Walking through the woods around Rostock is a stark contrast, despite the similar mix of trees. Here, it is nearly impossible to avoid stepping on piles of big, healthy nuts. Acorns, chestnuts, beechnuts, walnuts. All the trees seem to be dropping fantastic numbers of nuts, and nothing but insects and a few birds seems to be eating them. Here neither deer nor squirrels seem to be common in urban parks as they are in the US. In fact the only wild mammals we have seen around Rostock are Fledermäuse (bats) flying around at dusk and a jackrabbit or two. The ground in the parks is also full of mole tunnels (must be moles, as there are no gophers or other rodent tunnelers here). There are of course some mice and rats, but we haven't seen them and their numbers don't seem up to the task of disposing of all those nuts.

In North America, deer and gray squirrels have increased their numbers and expanded their ranges as humans have removed predators and made food available year-round. In western Europe, where the native animals have a far longer history of being persecuted by humans, where large wilderness areas are rare, and where there seem to be fewer native mammal species anyway (perhaps because of extinctions?) there just doesn't seem to be anyone to fill that urban nut-eater niche. There aren't even any turkeys or other birds big enough to eat nuts whole. Under such circumstances, I wonder if the nut trees do well because their nuts aren't all eaten, or do poorly because their nuts don't get buried, passed though germination-inducing digestive systems and dispersed.

Adaptive male lactation?

It has long been known that male mammals, including male humans, are physiologically capable of lactating. Screw with a man's physiology by giving him the right hormones, or the wrong series of starvation and then plenty, and he may start to produce milk. No one, as far as I know, has ever suggested that this is adaptive, that this capacity exists because men gain some reproductive advantage through lactation. Rather, it is usually seen as a result of the fact that we share almost all of our genetic material with females, who do make good use of their lactational prowess. Male lactation across the mammalian world is largely thought to be a side effect of intersexual correlation, the tendency for the two sexes of a species to have similar traits.

I am therefore skeptically excited to read that males of two species of fruit bats, one in Malaysia, and one in Papua New Guinea, are said to have "well-developed lacteriforus ducts and underlying mammary tissue similar to that found in lactating females" and that milk has been "expressed" from a large number of male bats.

It is not actually known whether these males are feeding young, and if so how commonly and to what effect, but this is the closest suggestion I have yet seen of the possibility of adaptive male lactation.

Garden of Science!

Most anyone in the Northern Hemisphere who has put out a bird feeder has had the problem of dealing with squirrels raiding their feeder, spilling the seed and chasing off the birds. There are a huge range of products designed to deter, exclude or punish the offending rodents, many of which work only marginally. I am currently using a product which is highly effective and nearly free: chili-pepper seed mixed into the bird seed. As any mammal who has bit into a chili-pepper knows, chilies make mammalian mucous membranes burn. What most mammals don't know is that to birds, the active compounds in chilies, capsaicin, does not cause pain to birds. In fact, to them is is an analgesic, meaning a pain killer. 

Wild chilies make their seeds and fruits to propagate themselves. But chili seeds eaten by mammals tend to be chewed up, and even if they are not, they don't germinate successfully after passing through our digestive systems, and even if they did, mammals don't tend to transport seeds as far as birds would. So it is well worth their while to produce a compound that dissuades mammalian seed predators and simultaneously encourages avian seed dispersers. Chili seeds pass though the guts of most seed eating birds perfectly primed to germinate, and get a ride far from their parent plant.

But in my bird feeder, the seeds are an enticement to the birds I want to attract, and a strong discouragement to the squirrels. And because it is whole seed, it won't wash away as chili powder would, and pouring whole seed it does not induce as much coughing. 

I bought a very cheep bag of over-ripe Thai chilies from my local grocery store, dried them in the sun and then banged them around in the food processor until all the seeds had come loose and settled to the bottom. I then mixed them with commercial bird seed before filling the feeder. The squirrel, to my knowledge, visited the feeder only once.