Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan

Tuljapurkar et al. present an interesting model for why human lifespan continues well past the age of menopause in women.. even though women no longer reproduce, older men still mate with younger women, and natural selection favors survival for as long as men reproduce (that is, there is a selective force against deleterious autosomal mutations at ages well past menopause). Here is their abstract, you should check the figures in the paper....

Evolutionary theory predicts that senescence, a decline in survival rates with age, is the consequence of stronger selection on alleles that affect fertility or mortality earlier rather than later in life. Hamilton quantified this argument by showing that a rare mutation reducing survival is opposed by a selective force that declines with age over reproductive life. He used a female-only demographic model, predicting that female menopause at age ca. 50 yrs should be followed by a sharp increase in mortality, a “wall of death.” Human lives obviously do not display such a wall. Explanations of the evolution of lifespan beyond the age of female menopause have proven difficult to describe as explicit genetic models. Here we argue that the inclusion of males and mating patterns extends Hamilton's theory and predicts the pattern of human senescence. We analyze a general two-sex model to show that selection favors survival for as long as men reproduce. Male fertility can only result from matings with fertile females, and we present a range of data showing that males much older than 50 yrs have substantial realized fertility through matings with younger females, a pattern that was likely typical among early humans. Thus old-age male fertility provides a selective force against autosomal deleterious mutations at ages far past female menopause with no sharp upper age limit, eliminating the wall of death. Our findings illustrate the evolutionary importance of males and mating preferences, and show that one-sex demographic models are insufficient to describe the forces that shape human senescence.

Male fertility in 1980 France (black), Pakistan 1984 (blue dots) and Cameroon 1964 (red dashes). Cameroon's distribution is common of high-fertility polygynous societies. The Y-axis shows age-specific fertility rates as a fraction of the total fertility rate.

Sexual orientation in women - brain correlates

Ponseti et al. suggest that they can observe a brain correlate of prenatal androgenization in homosexual women - they have less grey matter in their perirhinal cortex than heterosexual women. The article starts with a useful review of the controversial literature on previously suggested brain differences between homosexual and heterosexual individuals of the same sex. Here is their abstract and two figures:

Is sexual orientation associated with structural differences in the brain? To address this question, 80 homosexual and heterosexual men and women (16 homosexual men and 15 homosexual women) underwent structural MRI. We used voxel-based morphometry to test for differences in grey matter concentration associated with gender and sexual orientation. Compared with heterosexual women, homosexual women displayed less grey matter bilaterally in the temporo-basal cortex, ventral cerebellum, and left ventral premotor cortex. The relative decrease in grey matter was most prominent in the left perirhinal cortex. The left perirhinal area also showed less grey matter in heterosexual men than in heterosexual women. Thus, in homosexual women, the perirhinal cortex grey matter displayed a more male-like structural pattern. This is in accordance with previous research that revealed signs of sex-atypical prenatal androgenization in homosexual women, but not in homosexual men. The relevance of the perirhinal area for high order multimodal (olfactory and visual) object, social, and sexual processing is discussed.

Figure 1. Areas of increased GM concentration in heterosexual women compared to homosexual women. Coronal sections from y = 8 to y = −6 (p<0.05;>homosexual) was implicit masked with the contrast heterosexual (women>men). That way the intersection of significant voxels of both contrasts was gathered. As a result we found one cluster that matches both comparisons. That is, this brain area showed both, a lower GM concentration in heterosexual men compared to heterosexual women and a lower GM concentration in homosexual women relative to heterosexual women.

Figure 2. Heterosexual men and homosexual women compared to heterosexual women.
Areas of decreased GM concentration in heterosexual men are shown in blue and areas of decreased GM concentration in homosexual women are shown in yellow. Reduced GM concentration of homosexual women (relative to heterosexual women) is located within a sex dimorphic brain area.

Male sexual behavior circuits in female brains...

A fascinating bit of work by Kimchi and Dulac shows that female mice missing a gene involved in pheromone detection show the same sexual behaviour as males. Their abstract:

In mice, pheromone detection is mediated by the vomeronasal organ and the main olfactory epithelium. Male mice that are deficient for Trpc2, an ion channel specifically expressed in VNO neurons and essential for VNO sensory transduction, are impaired in sex discrimination and male–male aggression. We report here that Trpc2-/- female mice show a reduction in female-specific behaviour, including maternal aggression and lactating behaviour. Strikingly, mutant females display unique characteristics of male sexual and courtship behaviours such as mounting, pelvic thrust, solicitation, anogenital olfactory investigation, and emission of complex ultrasonic vocalizations towards male and female conspecific mice. The same behavioural phenotype is observed after VNO surgical removal in adult animals, and is not accompanied by disruption of the oestrous cycle and sex hormone levels. These findings suggest that VNO-mediated pheromone inputs act in wild-type females to repress male behaviour and activate female behaviours. Moreover, they imply that functional neuronal circuits underlying male-specific behaviours exist in the normal female mouse brain.

There is some controversy over whether this result is unique to inbred laboratory mouse strains, so Dulac is now breeding wild mice with the Trpc2-mutant mice, to experiment with a 'wilder' version.

Followup on male promiscuity...

Gina Kolata follows up here original article which I mentioned earlier, getting responses concerning means, medians, averages, etc.:

The Median, the Math and the Sex

By GINA KOLATA

David Gale, eminent emeritus mathematics professor at the University of California, Berkeley, member of the National Academy of Sciences, household name in certain rarefied circles, wanted to sound off about sex surveys. Over and over again, he said, they report results that logic dictates can’t be right. The total number of sex partners for men, he said, must equal the total number for women.

So when I said I’d write about that for The New York Times, he was delighted. Do you think I’ll get a response, he asked?

Little did he know. The moment the article hit The Times’s Web site last weekend, the e-mail messages flew and a discussion ensued that might be bewildering to the nonmathematician.

Gotcha, readers said: Data in one of the surveys cited, they said, reported the median numbers of sexual partners — and Dr. Gale’s argument, they said, only holds for the average number of partners.

Some were polite. Andrew Odlyzko, a mathematician at the University of Minnesota, wrote, “David Gale’s argument is unimpeachable” but there is this problem of medians.

Others were blunt: “Theorem: Professor Gale never went to prom. Proof: His High School Prom Theorem fatally assumes that everyone has a partner for every dance and no one ever sits out. The alternative is he has confused medians (at issue in the article) with means (averages, not middle-points in data), a rather elementary mistake for an emeritus professor of mathematics at Berkeley.”

The problem, Dr. Gale says, is that people leapt to conclusions. He had looked at the actual data from the survey citing medians and found that it could not possibly be correct. Of course he knew the difference between a median and a mean.

He wrote an explanation to send out to his numerous correspondents. But the e-mail messages kept coming.

A few correspondents, at least, were gracious. James Smith, an economist at the RAND Corporation, replied: “Yes, my point was only that of a nerd,” he wrote. “I have no doubt that the puzzle you pointed out is indeed true and cannot be explained away by medians and means.”

For the unconvinced, Dr. Gale’s response:

What I did was to get a copy of the C.D.C. report and use the data in its tables. The C.D.C. groups people into four groups and gives percentage of men and women in each group


From these figures you can estimate the total partners claimed by each sex. I got between 40 percent and 75 percent more male than female partners depending on how you guess the average on each interval. Thus, the raw data is inconsistent (so it doesn’t matter whether you take averages or medians or any other statistic).

I hope this clarifies.

The male promiscuity myth

Perhaps the most common evidence offered in support of males and females having evolved different sexual psychologies is from numerous surveys that show, across cultures, that men report having many more sexual partners than women. This is argued to reflect a basic underlying genetic fact: the best investment for a male to generate the maximum number of offspring with his genes is to impregnate as many females as possible. A woman has to invest much more in her offspring and is better served by selected fewer male partners who are more likely to provide support for her and her children.

Gina Kolata, in the Aug. 12 N.Y. Times, reports an interesting slant on this story. Simple math shows that the numbers don't add up... she quotes David Gale of U.C. Berkeley:

Surveys and studies to the contrary notwithstanding, the conclusion that men have substantially more sex partners than women is not and cannot be true for purely logical reasons...By way of dramatization, we change the context slightly and will prove what will be called the High School Prom Theorem. We suppose that on the day after the prom, each girl is asked to give the number of boys she danced with. These numbers are then added up giving a number G. The same information is then obtained from the boys, giving a number B...Theorem: G=B...Proof: Both G and B are equal to C, the number of couples who danced together at the prom. Q.E.D.

Sex researchers know that this is correct. Men and women in a population must have roughly equal number of partners. So why do men report many more than women? They exaggerate? They go to prostitutes who don't appear in the survey? (The latter would not explain the huge difference in reporting.) The most likely explanation:

...the survey data themselves may be part of the problem. If asked, a man, believing that he should have a lot of partners, may feel compelled to exaggerate, and a woman, believing that she should have few partners, may minimize her past...In this way the false conclusions people draw from these surveys may have a sort of self-fulfilling prophecy.

237 reasons to have sex....

These evolutionary psychology entrepreneurs know how to get their press: John Tierney describes a study from David Buss and collaborators asking 2,000 people why they'd had sex. You can see some of the answers, which includes "to get rid of a headache," here.

The Science of Gaydar

This is the title of an excellent article in New York Magazine, mentioned also in MindHacks.

Horniness of male mice - up and down with estrogen

Here is the abstract from Taziaux et al. Might this tempt pharmaceutical companies to go for a human male equivalent?

Estrogens are classically viewed as hormones that bind to intracellular receptors, which then act as transcription factors to modulate gene expression; however, they also affect many aspects of neuronal functioning by rapid nongenomic actions. Brain estrogen production can be regulated within minutes by changes in aromatase (estrogen synthase) activity as a result of calcium-dependent phosphorylations of the enzyme. To determine the effects of rapid changes in estrogen availability on male copulatory behavior, we mimicked in male mice the rapid upregulation and downregulation of brain estrogen concentration that should occur after inactivation or activation of aromatase activity. A single injection of different aromatase inhibitors [Vorozole, 1,4,6-androstatrien-3,17-dione (ATD), or its metabolite 17-OH-ATD (1,4,6-androstatrien-17ß-ol-3-one)] almost completely suppressed male sexual behavior (mounts and intromissions) expressed 10–20 min later by C57BL/6J mice but did not affect behavior in aromatase knock-out (ArKO) mice, activated by daily injections of estradiol benzoate, thereby confirming the specificity of the behavioral inhibition observed in wild-type mice. The rapid ATD-induced inhibition was reversed by the simultaneous injection of a large dose of estradiol. A single injection of estradiol to ArKO mice also activated male sexual behavior within 15 min. Thus, rapid increases or decreases in brain estrogen concentrations are followed within minutes by corresponding changes in male sexual behavior. Sexual behavior can thus be used to monitor changes in local estrogen concentrations and analyze the mechanisms mediating the rapid decline in estrogen signaling that takes place after inhibition of estrogen synthesis.

Gay animals II - from National Geographic

Gay animals I

El comportamiento homosexual en animales como el carnero, el búfalo y el bonobo...

Anheuser-Busch would never do this

..comments the friend who emailed me this video of a Guiness commercial: Married to a Man

Sexual Desire

Natalie Angier writes an engaging review of research into factors regulating sexual desire and sexual arousal (PDF here). The Science section of the April 10 New York Times contains a number of other interesting articles on sex and desire.


One brief edited clip from Angier:

According to the sequence put forward in the mid-20th century by the pioneering sex researchers William H. Masters, Virginia E. Johnson and Helen Singer Kaplan, a sexual encounter begins with desire, a craving for sex that arises of its own accord and prods a person to seek a partner. That encounter then leads to sexual arousal, followed by sexual excitement, a desperate fumbling with buttons and related clothing fasteners, a lot of funny noises, climax and resolution...A plethora of new findings, however, suggest that the experience of desire may be less a forerunner to sex than an afterthought, the cognitive overlay that the brain gives to the sensation of already having been aroused by some sort of physical or subliminal stimulus — a brush on the back of the neck, say, or the sight of a ripe apple, or wearing a hard hat on a construction site and being surrounded by other men in similar haberdashery.

In a series of studies at the University of Amsterdam, Ellen Laan, Stephanie Both and Mark Spiering demonstrated that the body’s entire motor system is activated almost instantly by exposure to sexual images, and that the more intensely sexual the visuals, the stronger the electric signals emitted by the participants’ so-called spinal tendious reflexes. By the looks of it, Dr. Laan said, the body is primed for sex before the mind has had a moment to leer.... arousal is not necessarily a conscious process. In other experiments... when college students were exposed to sexual images too fleetingly for the subjects to report having noticed them, the participants were nevertheless much quicker to identify subsequent sexual images than were the control students who had been flashed with neutral images.

By reordering the sexual timeline and placing desire after arousal, rather than vice versa, the new research fits into the pattern that neurobiologists have lately observed for other areas of life. Before we are conscious of wanting to do anything — wave at a friend, open a book — the brain regions needed to perform the activity are already ablaze. The notion that any of us is the Decider, the proactive plotter of our most lubricious desires, scientists say, may simply be a happy and perhaps necessary illusion.

This is precisely the message of my I-Illusion essay on this website.

More on "gay" male fruit flies...

Ed Kravitz is an amazing guy who has made numerous major shifts in his research approach. I was a postdoctoral student with him in the 1960's in the Neurobiology Dept. at Harvard Med. Then we were working on the lobster nervous system and its GABA and glutamate neurotransmitters. Now the Kravitz laboratory is studying the genetics of aggressive and sexual behavior in Drosophila fruit flies. From his recent PNAS abstract:
legend: Drosophila courtship dance

The reproductive and defensive behaviors that are initiated in response to specific sensory cues can provide insight into how choices are made between different social behaviors. We manipulated both the activity and sex of a subset of neurons and found significant changes in male social behavior. Results from aggression assays indicate that the neuromodulator octopamine (OCT) is necessary for Drosophila males to coordinate sensory cue information presented by a second male and respond with the appropriate behavior: aggression rather than courtship. In competitive male courtship assays, males with no OCT or with low OCT levels do not adapt to changing sensory cues and court both males and females. We identified a small subset of neurons in the suboesophageal ganglion region of the adult male brain that coexpress OCT and male forms of the neural sex determination factor, Fruitless (FruM).

A single FruM-positive OCT neuron sends extensive bilateral arborizations to the suboesophageal ganglion [Su oes g in the image, click on image to enlarge], the lateral accessory lobe, and possibly the posterior antennal lobe, suggesting a mechanism for integrating multiple sensory modalities. Furthermore, eliminating the expression of FruM by transformer expression in OCT/tyramine neurons changes the aggression versus courtship response behavior. These results provide insight into how complex social behaviors are coordinated in the nervous system and suggest a role for neuromodulators in the functioning of male-specific circuitry relating to behavioral choice.

Are ideas like food and sex?

"Does human creativity stem from a process that turns arbitrary ideas into goals like food and sex?"

..asks Andy Clark, in his review (Nature 445, 711-712, 15 February 2007) of "Why Choose This Book? How We Make Decisions," by Read Montague (Dutton: 2006):

"The most obvious rewards are the basic biological achievements of life maintenance (such as the ingestion of a tasty and nourishing morsel) and reproduction (or rather its precursor, sexual intercourse). Montague is motivated, however, by a strong desire to unravel the mechanistic underpinnings of what he describes as a uniquely human 'superpower': the capacity to make choices that seem to value biologically arbitrary objects, achievements and actions. Examples of such biologically arbitrary goal states mentioned in the text include solving Fermat's last theorem and committing group suicide in the belief that a spaceship hidden in a comet's tail will then take you to 'the next level'. What makes all this possible, in Montague's model, is the capacity of ideas themselves to act as reward signals, hijacking the prediction-error systems implemented by dopamine neurons in the brain. When this happens, the dopamine outputs start to act as error signals that encourage the rest of the brain to learn and to make decisions in ways that increase the chances of acquiring some biologically arbitrary reward."

"Given the potentially biologically catastrophic consequences of such re-tooling of mere thoughts as rewards, Montague suggests that powerful filtering processes control what gets into the reward slot. But such processes can be fooled — in ways that the book describes in compelling and often sinister detail — by damage, by drug abuse, and perhaps even by some forms of advertising and branding (brands are just cues that predict rewards). Montague's proposal is that biologically arbitrary goals can somehow plug into a kind of 'special status reward socket', and thus become a basic, primary reward, like food or sex. He does not claim that these ideas become associated, either directly or indirectly, with food or sex; rather, they plug directly into the 'socket' normally occupied only by the most basic high-status rewards. If we humans have indeed learnt such a powerful trick, it is no surprise that it fuels so much that is both good (creative and expansive) and ill (pathological and restrictive) in our species. Montague begins by laying out this possibility, then follows it deep into the fascinating territories of creative thought, addiction, obsessive–compulsive disorder, Parkinson's disease, and then on to the psychosocial realms of trust and regret."

"The book spans several seldom-bridged worlds, from neuroscience to psychiatry, economics and social psychology, and does so with wit, precision and elegance. It succeeds in many of its goals. Above all, it left me feeling I had actually learnt something about myself: a thinking, feeling, choosing, yet painfully vulnerable chemically modulated learning machine."

Smell of male sweat raises cortisol levels in women.

The use of chemosignals in other mammals has been well documented, especially in rats. Wyart et al. have now found that if women simply smell pure androstadienone (4,16-androstadien-3-one), which is present in the sweat of men, they maintain higher levels of the stress hormone cortisol.

Evaporation of the Mystique of Religion

Dennett suggests that as young people are exposed to the worldwide spread of information technology, they quietly walk away from the faith of their parents and grandparents. (Side note: I thought I would show you, as an example of shifting fundamental attitudes, a figure from this Andrew Sullilvan post on the increase in the percentage of people who consider homosexuality acceptable, by year and by age group).

Switch brain sexuality with steroid hormones!

OK, so it is actually a fish doing it. But, Remage-Healey and Bass suggest that their findings may help explain the widespread distribution of rapidly induced intrasexual behavioral phenotypes among vertebrates in general. They observe that the male acoustic sexual behavior of the teleost type I male midshipman can be rapidly (~ 5 min) induced in type II males (that normally show female like behaviors) by the action of the androgen 11-ketotestosterone on receptors in the brain's vocal pattern generator. (11-ketotestosterone is the dominant circulating androgen in type I males, and testosterone is the dominant androgen in type II males and females). This suggests that steroid-dependent expression of "maleness" and "femaleness" may now include rapid steroid actions on the neurophysiological patterning of behavior, uncoupled from gonadal phenotype.

The perils of studying gay sheep

Pity the poor researcher, Charles Roselli at Oregon health and Science University, who set out to discover what physiological factors might explain why about 8% of rams seek sex exclusively with other rams instead of ewes. An article in today's New York Times describes the firestorm of controversy that ensued when PETA (People for the Ethical Treatment of Animals) started a campaign against the research. The research has used humane treatment guidelines in sacrificing about 18 animals to examine their brains (over 4 million sheep are killed annually for food and clothing in this county). The Sunday Times of London amplified the misrepresention of the work as having the goal of controlling sexuality in humans rather than simply understanding its basic science, and Andrew Sullivan and other popular blogs passed on the mis-information. The New York Times article describes how in this case the researchers fought back, striving to make an accurate description of the research to the critics. Several blogs, including Sullivan's, corrected themselves.

Sporno

I can't resist passing on David Haskell's piece in the NYTimes magazine "Ideas" issue in its entirety.

When approached to write an article about the homoerotic subtext of sports for Out magazine this spring, the journalist Mark Simpson feared the subject was old news. “There wasn’t much of a point,” he says, “since sport was already the new gay porn.” Indeed, with the publication of a Dolce & Gabbana underwear campaign featuring Italian soccer players glistening in a dank locker room, the phenomenon had gone well beyond subtext. All that was lacking was a name, and it didn’t take Simpson very long to invent it: sporno.

Sports, of course, have always celebrated physical form. What has changed, Simpson argues, is how we look at men. Thanks to what might be called the Abercrombie effect, the male body has become increasingly aestheticized — or “metrosexualized,” as Simpson would have it (he invented that term too) — and male imagery, particularly in fashion advertising, has become more overtly sensual. Considering that sports is visual, masculine and (like porn) geared mostly to men, Simpson was not surprised to find male athletes — even, or perhaps especially, heterosexual ones — grooming their physical image and “fetishizing themselves.”

The arrival of what Simpson calls “equal opportunity flirts” like the soccer players David Beckham and Freddie Ljungberg, who dabble in gay iconography and openly embrace their gay fans, lends sporno a celebrity cachet. (Simpson calls sporno stars like Ljungberg, whose physical assets are abundantly featured in Calvin Klein underwear ads, “young hustlers.”) Among professional sports organizations, the French national rugby team has pursued its sporno status most aggressively. The team’s annual calendars, Simpson notes, include photo shoots in which “there is no pretense that this is anything but hyperhomoerotic.” Indeed, some images are but a few soap bubbles away from pure pornography.

Gender and sexual orientation regulate the effect of invisible images.

As we are contantly bombarded with vast amounts of information, selective attention mechanisms help us quickly attend to what is important while ignoring what is irrelevant. It makes ecological and evolutionary sense if important events can influence our spatial attention even before we become aware of the event. Recent studies have shown that subliminal presentation of emotional stimuli can modulate activity of the amygdala. Jiang et al ask whether activation of the emotional system directs observers' attention to the stimulus in the absence of awareness.

They show that information that has not entered our consciousness, such as interocularly suppressed (invisible) erotic pictures, can direct the distribution of spatial attention. They further show that invisible erotic information can either attract or repel observers' spatial attention depending on their gender and sexual orientation. While unaware of the suppressed pictures, heterosexual males' attention was attracted to invisible female nudes, heterosexual females' attention was attracted to invisible male nudes, gay males behaved similarly to heterosexual females, and gay/bisexual females performed in-between heterosexual males and females.

Here is their description of how information is presented so that it does not enter consciousness. "In the interocular suppression paradigm, a pair of high-contrast dynamic noise patches are presented to both sides of a fixation point in one eye, and a test picture and its scrambled control are presented to the fellow eye in spatial locations corresponding to the noise patches. Because of strong interocular suppression, the intact meaningful image and its scrambled control remain invisible for the period they are presented. If the suppressed images exert a location-specific effect on the attentional system, these images could potentially act as attentional cues that would influence the distribution of spatial attention and thus performance on a subsequent detection task."

(click on image to enlarge it)
Figure: Schematic representation of the experimental paradigm for the invisible condition. For the visible condition, the noise patches were replaced with the same pair of intact and scrambled pictures presented to the other eye. In each trial, observers pressed one of two buttons to indicate the perceived orientation (CW or CCW) of a Gabor patch (the circle with lines in the right frame) briefly presented on either side of fixation. In the invisible condition, as shown, if observers detected any difference between the two sides of the fixation, they pressed another button to abort that trial.

Heterosexual male observers were more accurate at the orientation discrimination task when the Gabor targets followed the site of the invisible nude female pictures (attentional benefit) and were less accurate when the Gabor patches were at the site of invisible nude male pictures (attentional cost). Thus, heterosexual male observers' attention was attracted to nude female images and was repelled from nude male images, even though the images were not consciously perceived by the observers. Similarly, heterosexual female participants showed an attentional benefit (attraction) to invisible nude male pictures (positive attentional effect, although they did not show a significant attentional effect to invisible nude female pictures. Gay males had a similar pattern to female participants in that invisible female nude pictures did not attract their attention while male erotic images enhanced performance. Gay/bisexual females fell in-between the heterosexual male group and the heterosexual female group.