Human Oestrus

Iris and I are reading Jared Diamond's book on the evolution of human sexuality, "Why is sex fun?" An interesting read, and in many ways a good introduction to the science of behavioral ecology, except that Diamond falls into his usual habit of making the same point many times and many ways, as if to beat you into agreeing with him. Since I generally start out agreeing with him, this gets tiring. The chapter we were reading last night was primarily his speculations on the evolutionary basis of the lack of obvious oestrus in humans. I was therefore very interested when this evening, browsing the table of contents of ProcRoySoc B, I cam across an article titled "Human Oestrus"

Here is the abstract:

For several decades, scholars of human sexuality have almost uniformly assumed that women evolutionarily lost oestrus—a phase of female sexuality occurring near ovulation and distinct from other phases of the ovarian cycle in terms of female sexual motivations and attractivity. In fact, we argue, this long-standing assumption is wrong. We review evidence that women's fertile-phase sexuality differs in a variety of ways from their sexuality during infertile phases of their cycles. In particular, when fertile in their cycles, women are particularly sexually attracted to a variety of features that likely are (or, ancestrally, were) indicators of genetic quality. As women's fertile-phase sexuality shares with other vertebrate females' fertile-phase sexuality a variety of functional and physiological features, we propose that the term oestrus appropriately applies to this phase in women. We discuss the function of women's non-fertile or extended sexuality and, based on empirical findings, suggest ways that fertile-phase sexuality in women has been shaped to partly function in the context of extra-pair mating. Men are particularly attracted to some features of fertile-phase women, but probably based on by-products of physiological changes males have been selected to detect, not because women signal their cycle-based fertility status.

Art of Science!

My department every year has an Art and Science show (This year it is the 4th Annual IB Art & Science Show, April 25, 2008. 5PM in VLSB. Open to the public, food and drink provided.)

Submissions are supposed to be artworks inspired by, related to, or otherwise modified by science.

I was thinking about what to submit when someone (alias Plastic Devil Ducky) posted a bag of Legos on Freecycle. So my submission will be a Lego rotifer (with egg). The difficult part was finding pieces from a pirate ship and a space station that could be made to look like cilia.

Dream of Science!

I dreamed there was a news paper article with the headline, "The Nutroavailabitizer!"

Below the headline, there was a picture of a long skinny fish with a big weird bumpy appendage, almost as massive as the main fish, sticking up from its tail. The article was quoting a scientist who had concluded that the function of this big appendage was to take nutrients in the water in the stream where it lived, and convert them to a form that would be easier for the trees around the stream to use. In the dream, Iris asked me what I thought of that, and I said that it didn't make very much sense from an evolutionary perspective, because the fish would be putting enormous energy and resources into building and operating the filter thing, and it would make the fish much less streamlined, and in the end the only benefit the fish gets is healthier trees around the stream. I argued that if there was a mutation for not having that appendage, the mutant fish would still get the benefit of the healthier trees, because of the other fish still having the filters, but would not have to support the costs of having a filter itself. Likewise, if there were no fish with that filter, and one had a mutation that caused it, or an earlier version of it, only it would bear the costs but all would get the benefits, so it would be at a disadvantage again. Iris asked why I thought the fish did have it, and then my cellphone rang with a 7AM automated sales call.

Public warned that lead bullets make meat toxic

In an extremely important, despite being obvious, revelation, the government of North Dakota has ordered the destruction of tons of venison, after being presented with evidence that deer shot with high velocity lead bullets are not fit for eating because of microscopic lead fragments spread throughout the body of the deer.

While the science is not new, the difference this time is that the study was done by a hunter from North Dakota, rather than by the Peregrine Fund.

Note to hunters: If you shot a deer with a lead bullet and you are planning on feeding it to your family, you are poisoning them. Throw that meat out, buy some copper bullets, go shoot another deer.

Residual life

Most of us take for granted that we will live for a long time after we stop having kids (for those of us who will have kids). The standard picture for Americans these days is to stop at 40 or so, hope to live to twice that age. And this doesn't seem unrealistic. But when viewed evolutionarily, it seems a bit bizarre. Most organisms do not have a post-reproductive lifespan, human females very clearly do. I've previously written about the most common hypothesis to explain this fact, is the "Grandmother Hypothesis."

On Tuesday I was giving a talk on my research to Berkeley's Primate Research Group, including my experimental examination of the Grandmother Hypothesis. After the talk, I got a lot of good feedback, including an interesting question. How certain are we that females of other species of primates wouldn't live as long post-menopause as humans if they lived as cushy lives as we do?

The only answer I could give them is that I haven't seen any data suggesting otherwise. But then, on my way home, it occurred to me that I already have probably the world's best data set for answering exactly that question.

Primates in well run zoos tend to greatly outlive their wild cousins. Medical care, reliable food supplies, no predators and few pathogens. Not to say the life of a captive primate is perfect, or that there isn't significant variation in the quality of care, but for many species maximum longevity in captivity is much greater than in the wild. And it just so happens that I have life tables, including age specific reproductive rates and mortality rates, for 120 species of primates. These come from ISIS data, meaning data from relatively well run zoos, and I will need permission from ISIS to use them in this way, but I doubt they will have any major objections.

The idea of writing a paper based on data I already have is exciting to me. Usually I spend years between having an idea and having assembled the data to address it. I have almost all the data I need to address this question safely on several computers. I'll get the hang of this science thing yet.

Scrounge of Science

I don't have a grant for my current rotifer experiments. The professor who pays for my Graduate Student Research stipend pays for minor research expenses out of the same grant. Even so, I am not lacking in equipment. In my lab space I have 5 (soon to be six) microscopes of various descriptions, a several thousand dollar refrigerated incubator, a refrigerator, an 8-channel multipipettor and a panoply of smaller bits. I have my own wireless network, desks, chairs, office supplies and so-forth. In all, probably $15K worth of equipment, almost all of it borrowed, traded, scrounged, temporarily absconded with or pulled out of the trash.

The incubator is a perfect example. The temperature in the room varied too much for my rotifers. So I decided I needed an incubator. I went to the incredibly helpful people in my department's instructional support office and asked if they had an incubator they weren't using. They did, but they weren't sure it worked. We pulled it out and I cleaned it up and got it working, so they lent it to me. But there was a problem. This incubator would heat up if the room got cold, but it didn't cool when the room got too hot. I could set a lower bound, but not an upper bound, and I need both. I went wandering the various labs I have access to, and observed that the MVZ molecular genetics lab had a big fancy refrigerated incubator with all the bells and whistles, and it had a piece of paper taped to it that said "always keep at 37C."

I guessed that in order to stay at 37C, it didn't need the refrigeration function. So I talked the super helpful lab manager into letting me temporarily trade the incubator I had borrowed from instructional support for the fancy refrigerated one.

My latest quest is to borrow a compound microscope with digital camera attachment, so I can take known magnification pictures of, and thereby measure, rotifer eggs. It took several days and emailing various friends within the department, but I now know where there is an unused microscope with camera fitting, and I'm working on getting permission to borrow it. I've already borrowed a camera that will work with it.

The thing that is amazing to me, once I stop patting myself on the back for my scrounging prowess, is how much scroungable stuff there is around here. Chairs and desks get thrown away because the professor who used them moved to another university. There are cabinets full of rarely used microscopes everywhere. People have 40 years of equipment piled in their labs and are eager to see it put to use.

When my rotifer experiments are done, I'm going to have an awful lot of borrowed stuff to give back. I hope I remember where I got everything.

Arthur C. Clarke passes into the Monolith

Aides report Clarke planned post-life tour of whole solar system, except Europa.

Gender Biased applicants

In the time I have worked on the rotifers, I have had the following assistants:

Lauren
Myfanwy
Rose
Laurel
Chris
Phoebe
Polina
Christina
Nicole
Deanna
Willard
Vincent
Catherine

That makes 3 males and 10 females. Hardly 50/50.

I was wondering why this was, if I was in some way unconsciously discriminating against male applicants, or practicing affirmative action. So I counted how many male and female applicants I have had in the last two rounds of hiring. The most recent I had 2 male and 8 female applicants. The previous time, I had 3 male and 13 female applicants. So if anything, I'm over-hiring males as compared to their representation in the applicant pool.

What I am wondering about now is why so many more females than males apply. The biology classes in my department have a slight majority of females, but not nearly that big. My guess is that because the project specifically investigates the evolutionary basis of menopause, and women tend to be more interested in (and comfortable with the topic of) menopause, fewer males are interested.

Rotifers, sex and locomotion: fast males, slow females

A while back I saw a male rotifer and for the first time knew that it was a male rotifer I was seeing. Three things immediately struck me:
1. Oh! so that's what I've been seeing all this time.
2. Damn they're so tiny compared to the females
3. Golly-gee-willackers they move fast.

To give you a sense of this, observe the following Youtube video I came across. The little bizarrely fast ones are the males.



The males are short-lived, have no digestive system or foot (meaning they can't eat or anchor in one place). They hatch from an unfertilized egg, carry their mothers' genes to other females, and die.

I was discussing this with a friend of mine, who asked, "what good are the males anyway?"

"They're just swimming sperm packets." I replied. But then I thought about it more, and realized the question could be viewed another way. There are plenty of invertebrates that are hermaphroditic. A single individual has both ovaries and testes. I fertelize you while you fertilize me. No need to build a whole separate individual to deliver the sperm. So why go to all the expense of pumping out fleets of males?

Maybe, I thought, it was that speed. The smaller a rotifer is, the faster it can swim. This is the result of the fluid dynamics of how they swim. I don't know a thing about fluid dynamics, so I won't try to explain that, but the data show that swimming speed is predicted with great accuracy by size.

Having fast moving sperm deliverers could have two benefits that immediately occur to me. First, one can spread one's genes much further by producing small, fast males and sending them off in all directions, than by having one big slow female swim around. Especially considering that the female's immediate neighbors have a good chance of being clones of herself, to make sexual reproduction worthwhile, she needs to get get her sperm far away. That may require speed.

Second, maybe being fast is useful in the competition for mating. If the females are not just willing to mate with every rotifer that wanders along, perhaps being fast increases the chance of fertilizing her eggs.

These are all just hypotheses, but they are testable ones, and perhaps some day I will get to testing them.

Research website

I've put up a somewhat expanded research website: dlevitis.org
In addition to an abbreviated CV, it has:

A page on my teaching philosophy and student feedback.

A section on undergraduate mentoring which includes pages on useful resources for my students to use in finding and tracking scientific literature, and advice for students applying for research positions.
I'd particularly appreciate feedback on this advice from others who have interviewed many undergrads for research positions.

There is also, of course, the obligate links list.

Nesting Season

The time of year has come when it is hard for bird lovers to concentrate on almost anything. Sit down and try to type, and there are ravens making absurd hooting noises in the pine tree over the house. Go out in the backyard and there are Red-shouldered Hawks screaming at each other up the hill, scrub-jays carrying twigs and sparrows pulling grass. The male wrens are dancing on the fencetops and the male hummingbirds are doing their shooting-star routine. Everyone is nesting, breeding displaying. And of course it isn't just the birds. Frogs incessantly sing their one-line resumes. Squirrels chitter with more vim than usual. Young women prance around campus in too little clothing for the still chilly air. Plum and cherries trees engage in shock-and-awe advertising war against a billion other flowers in the fight for the affection of bees.

It is hard to sit inside and type.

phylogenetic inertia and cooperative breeding

I spent yesterday evening reading the literature on the extent to which there is phylogenetic inertia (the tendency for related species to have similar traits because they both inherited those traits from the common ancestral population) in cooperative breeding (the habit of having more than two individuals caring for the young).

My general conclusions are:
1. A species whose relatives are cooperative breeders are often cooperative breeders themselves.
2. It is not entirely clear if cooperative breeding itself is phylogenetically conserved in many groups, or if the traits that make it a useful strategy are conserved, leading to the impression of inertia in the evolution of cooperative breeding.
3. Phylogenetists spend a lot of time and ink poodling on about the flaws in each other's methods, but always end by saying that the conclusion about the trait is probably robust to minor variations in the shape of the tree.
4. Cooperative breeding is a blanket term for several different phenomena, and papers that deal with this explicitly are more convincing than those that only pay it lipservice.
5. The data I am already putting together on who provides how much care in 120 primate species could probably also be used for a very useful paper on phylogenetic inertia in cooperative breeding.

Biofuels won't fix it.

Pretty much every issue of Trends in Ecology and Evolution has a paper or two worth the reading. I thought this one was worth drawing your attention to:

Christopher B. Field, J. Elliott Campbell and David B. Lobell. 2008. Biomass energy: the scale of the potential resource. Trends in Ecology & Evolution. Volume 23, Issue 2, Pages 65-72

Abstract:
Increased production of biomass for energy has the potential to offset substantial use of fossil fuels, but it also has the potential to threaten conservation areas, pollute water resources and decrease food security. The net effect of biomass energy agriculture on climate could be either cooling or warming, depending on the crop, the technology for converting biomass into useable energy, and the difference in carbon stocks and reflectance of solar radiation between the biomass crop and the pre-existing vegetation. The area with the greatest potential for yielding biomass energy that reduces net warming and avoids competition with food production is land that was previously used for agriculture or pasture but that has been abandoned and not converted to forest or urban areas. At the global scale, potential above-ground plant growth on these abandoned lands has an energy content representing ~5% of world primary energy consumption in 2006. The global potential for biomass energy production is large in absolute terms, but it is not enough to replace more than a few percent of current fossil fuel usage. Increasing biomass energy production beyond this level would probably reduce food security and exacerbate forcing of climate change.

Global Amphibian Declines call for frog costumes.

There are few, if any, taxa losing species diversity as quickly and thoroughly as the amphibians.

From amphibiaweb.org:

"Amphibians, a unique group of vertebrates containing over 6,200 known species, are threatened worldwide. A recent assessment of the entire group (globalamphibians.org) found that nearly one-third (32%) of the world’s amphibian species are threatened, representing 1,856 species. Amphibians have existed on earth for over 300 million years, yet in just the last two decades there have been an alarming number of extinctions, nearly 168 species are believed to have gone extinct and at least 2,469 (43%) more have populations that are declining. This indicates that the number of extinct and threatened species will probably continue to rise (Stuart et al. 2004)."

And this is almost surely an overly optimistic view. The thousands of species for which we have no data are not considered threatened, and an unknown number of others, likely in the hundreds, has gone extinct without ever being discovered.

Amphibian lineages that have existed for tens of millions of years are ending shockingly quickly. The factors that are killing off these ancient groups are all the works of humanity: pesticides, industrial pollutants, introduced species and diseases, habitat destruction and alteration and rapid climate change. There is no reasonable doubt but that we have driven the rate of amphibian extinction to several thousand times its natural rate. We could barely kill them off more quickly if we tried. Genocide is not too strong a term.

Amphibians are widely seen as indicator species. Healthy ecosystems have healthy amphibians. Indications are poor.

So to help raise (the currently abysmally low) awareness of the scope of amphibian declines, Iris and I have decided to walk Bay to Breakers wearing homemade from costumes. And we hope to organize a group of friends and colleagues to walk with us, similarly garbed. A costume recognizable as an amphibian, or even a shirt with amphibian pictures on it. Perhaps we will hand out informational pamphlets to people who ask what we are about. If you are interested in joining us, please email me.

Sloppy

I've written here before about what a poor job journalists generally do in describing science to the public. I was just reading this Op-Ed piece in the NYTimes and wincing over phrases such as "sharks are very primitive living fish" and "ensure the survival of a species," which reveal a very out of date and human-centric misunderstanding of evolution. The piece includes at least half a dozen points contradictory to modern evolutionary theory. I was just considering writing yet another response letter explaining to the author that natural selection does not favor traits because they help ensure the survival of the species, and so forth, when I looked at who the author is.

The attribution:
"Neil Shubin, an associate dean at the University of Chicago and the provost of the Field Museum, is the author of 'Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body.'"

Neil Shubin knows that evolution doesn't work that way. So why is he writing like a journalist? I can only guess it is because he wants to sell more books, and someone (the NYTimes, his publicist or him) felt that one has to adopt the public's misunderstanding in order to interest them. It is sad when scientists start writing like science journalists.

Field Notes

FEB. 23 2008
D. Levitis
Hillside Natural Area (North Unit) El Cerrito, Contra Costa County, CA

Sijie Mao, Nichole Winters and I went out from 1300 to 1530. It was quite windy with very strong gusts, temp in the lower 50s and varying between slight drizzle and significant rain. Our fist stop was the detritus field around the storm drain at the lowest point in the park, just behind my house, under the Eucalyptus. A lot of good cover items, but nothing under there. From there we went just up the hill to the small pile of shipping pallets on the grass. Under the first one I lifted was an Aneides lugubris (Arboreal Salamander)! The first I'd seen here. We took this as a sign of good salamandering to come.

We followed a route approximated by that marked on this map, but with many side excursions to check under log, rocks and other cover objects:


Much of this part of the Nature Area has many of the cover objects removed for yearly mowing, so much of the available cover is in piles of cut log pieces or small rocks piled at the base of trees and in gullies.

Sijie and Nichole are extremely contentious about putting every cover item back exactly as it was, and are energetic and enthusiastic herpers. We found almost nothing under rocks, but the logs were very productive today. We checked under approximately 400 wood bits and 100 rocks/cement bits and found 71 Batrachoseps attenuatus (California Slender Salamanders) 3 Aneides and one Elgaria multicarinata (Southern Alligator Lizard).

The Aneides were all near oaks, as Dave Wake told me to expect, while the Batrachoseps were under logs and a few rocks in every habitat we checked. The Elgaria was under a pine log in grassy habitat.

In addition to the herps, invertebrates were extremely numerous under cover objects, including Jerusalem crickets, cave crickets, ants, termites, ground beetles, several kinds of spiders including a couple of Black Widows, copious pill-bugs, snails, slugs, worms and sundry.

Many fresh gopher diggings were evident in the soft wet earth, and many of the logs and rocks we lifted had smaller rodent tunnels, quite possibly vole. Deer prints and fox scat were in abundance.

One bird species to add to my list for the nature area, Wild Turkey. One was seen at the northwest corner of our route, just north of Snowdon Ave. and another was heard responding to its calls. Nichole took pictures of the turkey, which was tame enough to allow her to approach to within 6 meters.

Our explorations revealed no standing water in which frogs could breed, which may explain their absence from our observations.

Birding today wasn't great, due to wind and rain, but saw/heard following birds today:

Western Scrub-Jay (2)
American Crow (10)
Northern Raven (2)
Black Phoebe (1)
Yellow-rumped Warbler (25)
Chestnut-backed Chickadee (4)
House Sparrow (15)
Ruby-Crowned Kinglet (2)
American Robin (40)
California Towhee (3)
Dark-eyed Junco (4)
Wild Turkey (2)
Red-shouldered Hawk (1)
Anna's Hummingbird (3)
Mourning Dove (6)

Mammals:
Eastern Fox Squirrel (Sciurus niger) (2)
Mule Dear (Odocoileus hemionus) (tracks)
Voles (Microtus californicus? runways and burrows)
Pocket Gophers (burrows)
Fox (Grey fox? scat)

Herps:
Southern Alligator Lizard (Elgaria multicarinata) (1)
California Slender Salamander (Batrachoseps attenuatus) (71)
Arboreal Salamander (Aneides lugubris) (3)

"Science" "Journalism"

Even taking into account how poor the state of science journalism is in this country, and what a sorry rag the San Fransisco Chronicle is, this piece on the danger posed to San Francisco by invasive pythons is well below the curve.
To summarize, there is an introduced population of Burmese pythons in the Everglades, 3100 miles away. A USGS lab used an off the shelf climate model to predict where in the US Burmese Pythons could deal with the climate, and San Francisco (along with pretty much the southern half of the US) showed up as one potential habitat.

The idiot who wrote the article then follows this train of thought to show that pythons will be eating pedestrians on Market Street by 2020:
•One python showed up at Lake Okeechobee, 100 miles north of the Everglades, and in the same direction it would travel if its single-minded goal was to reach Fisherman's Wharf.
•Pythons can travel up to 20 miles a month, and the road to SF is 3000 miles, so it will be here in 150 months, or about 12 years.
•Pythons eat mammals.
•"Human beings - like rodents, beavers and deer - are mammals, government scientists confirmed."
•Giant snakes falling from office buildings and eating you.
•QED

The sad thing is, reading the article, I am not sure if he is joking, but I am sure there are some people who will think this is a real threat.

Chemicals of Science

A very kind scientist at McGill sent me samples of four different strains of rotifers, along with the formula for the medium he keeps them in. The rotifers look so lively and healthful that I decided to buy the chemicals and mix up some of his medium. The problem is that while I need a few milligrams of this and a couple of grams of that, nobody sells the stuff in quantities smaller than 500 grams of this and 1kg of that. So at the end of the project, I have almost the entire container left over, and because it came in a form that is so concentrated as to be toxic if swallowed, it all has to be treated as toxic waste. Iron-chloride is not very toxic, but if you swallow 500g you would be very unhappy. So what I will probably end up doing is mixing lots of extra solution, basically making artificial pond water, because that does not need to be disposed of as toxic waste.
I was complaining about all this to some colleagues, who replied that this is a perennial problem, and that almost any time one does lab work, one has to budget for disposal of the left over chemicals, because one can't buy them in small enough quantities. The College of Chemistry on campus has a chemical reuse library for this purpose (drop off unused chemicals with out paying for disposal, pick up extra chemicals without buying a whole container) but they don't allow people from other departments to participate, even if we ask real nice.
I looked through the various suppliers and ordered the smallest quantities possible, even when it cost more.
How thoroughly wasteful and silly this whole system is.

Deep thoughts

It is striking to think that over the billions of years there has been life on earth, there have been countless billions of individual organisms that have died before reproducing successfully, and billions of organisms who would end up being the ancestors of the living things alive on earth today, but there has been absolutely no overlap between these two groups.

Why no "grandfather effect"?

Much of my time at present is consumed by setting up an experimental test of what is known as the grandmother hypothesis. The grandmother hypothesis, in short, is the best guess we have as to why the females of humans and a few other species can live well past the age of reproductive cessation. In most species, and indeed in human males, there is no significant post-reproductive lifespan. Individuals are physiologically capable of reproducing for as long as they live. But the females of humans, a few other primates, a couple of cetations and maybe elephants go through menopause and then can live a significant portion of their lifespan after that. Our closest relatives, the chimps and bonobos, apparently go through menopause at the same age as our females, but live at most a few years after that.

Evolutionarily, this makes sense. If one is no longer increasing one's lifetime reproductive success, staying alive offers no obvious selective advantage. No point in investing in physiologies and structures that will last 100 years if one is only going to reproduce for 50 years. Better to put those resources into having more kids now.

But under a certain set of circumstances, reproduction does not end with, or shortly after, childbirth. If your young aren't really able to take care of themselves for a decade or two, you aren't done reproducing until they don't need you any more. In most hunter gatherer societies, the survival rate of five year olds whose mothers die is quite low. So for human women, having a kid in the last several years of life was likely a waste of time.

Worse, the kid who didn't make it took time and resources that could have been put into other kids, and childbirth, particularly late in life, is dangerous. Plus, elder human females are important for helping their daughters raise their own young, and learn how to do so. It has been shown that young mothers in several societies have a higher success rate raising kids if their mothers are around. The women who stopped having kids and focussed on the kids and grandkids they already had, and avoided the risk of late life childbirth, are thought to have ended up getting more of their genes into future generations than women who kept giving birth as long as they lived. If so, and if this variation in life history was heritable, which seems likely, this differential reproductive success would inevitably lead to a population with more and more women stopping early and fewer and fewer giving birth late in life. This is, we think, why we ended up with this "grandmother effect" of women living well past reproductive age.

The benefit of having a grandmother around seems to be restricted to maternal grandmothers. And this observation, that paternal grandmothers don't seem to make as much of a difference (at least in the societies studied) to the survival of their grandkids, points to at least two possible reasons why we don't see a "grandfather effect" to go along with this "grandmother effect."

First, in most societies, at least those studied in this context, males are providing less in the way of vital care. So if a women has a son who has kids, perhaps she is less involved in care, or in teaching how to care, because her son is not as involved as his mate in that care, and the daughter-in-law is not nearly as likely to look for advice and help from her husband's mother than her own mother. And perhaps this same logic applies to grandfathers on both sides. If they are not who the primary caregiver can go to for help and advice, the advantage of having them around to help is smaller.

Second, paternal grandmothers are less certain of which is really their genetic grandchild. If a woman gives birth to a daughter, and watches that daughter give birth to babies, she can be very confident that those are her descendants. If a woman gives birth to a son, and then watches that husband's mate give birth, there is a significant chance (and we have the genetic data to substantiate this) that the baby was fathered by some other man, and those babies aren't her genetic kin. So investing in them heavily may not be doing her any good. This argument is doubly true for grandfathers. The daughter who is giving birth may not even be his. A couple of generations removed, and who can be sure?

A final reason males may not have evolved to have a post reproductive period comes back to that risk in late life childbirth. Men don't give birth, so the risk to late life survival posed by late life reproduction may be greatly reduced, or completely absent. Without that trade-off, why not keep on breeding as long as possible?