Chapter 12. Sex: Evolutionary, Hormonal, and Neural Bases
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By JAN HOFFMAN There are activities common to most humans that we enjoy immensely, without much thought, and as frequently as opportunity and instinct provide. On occasion, researchers feel they need to know why. Recently, experimental psychologists at Oxford University explored the function of kissing in romantic relationships. Surprise! It’s complicated. After conducting an online survey with 308 men and 594 women, mostly from North America and Europe, who ranged in age from 18 to 63, the researchers have concluded that kissing may help people assess potential mates and then maintain those relationships. “The repurposing of the behavior is very efficient,” said Rafael Wlodarski, a doctoral candidate and lead author of the study, published in Archives of Sexual Behavior. But another hypothesis about kissing — that its function is to elevate sexual arousal and ready a couple for coitus — didn’t hold up. While that might be an outcome, researchers did not find sexual arousal to be the primary driver for kissing. Participants in the survey were asked about their attitudes toward kissing in different phases of romantic relationships. They were then asked about their sexual history: for example, whether they had been more inclined toward casual encounters or long-term, committed relationships. They also had to define their “mate value” by assessing their own attractiveness. Later, during data analysis, the researchers looked at how individual differences affected a person’s thoughts on kissing. Copyright 2013 The New York Times Company
Keyword: Sexual Behavior
Link ID: 18845 - Posted: 10.29.2013
by Bethany Brookshire There are many animal species out there that exhibit same-sex mating behavior. This can take the form of courtship behaviors, solicitation, all the way through to mounting and trading off sperm). In some species, it’s clear that some of this behavior is because the animals involved have pair bonded. But what about insects? Many insects mate quickly, a one and done approach, with very little bonding involved beyond what’s needed to protect against other potential suitors. When it comes to bugs, is it intentional same-sex behavior? Or is it all a mistake? Hypotheses are out there, but in the end, we need science. A new study in the November Behavioral Ecology and Sociobiology wants to answer these questions. The authors did a meta-analysis of papers looking at same-sex sexual activity in male insects and arachnids. They tried to tease out why same-sex sexual behavior might occur in insects. What are the benefits? The potential downsides? And from that, to hypothesize why it might occur. Some of it, it turns out, could be due to context. A lot of observed same-sex mating behavior in insects is observed, for example, when the males are all housed together, away from the females. Partially because of this (but possibly for other reasons as well), same-sex sexual behavior in insects tends to occur much more frequently in the lab than in the wild. But it’s still often documented in the field. Why does it happen? Some say that by mating with a “passive” male and transferring sperm, that sperm then gets passed over to the female when the passive male mates. Sneaky. But does it really happen? And if it does, is it effective? So far, it doesn’t appear that it is; less than 0.5% of the offspring resulted from the transfer of sperm when these cases were documented. © Society for Science & the Public 2000 - 2013
by Susan Milius Your calamari, it turns out, may have come from a temporary transvestite with rainbows in its armpits. Well, not armpits, but spots just below where the fins flare out. “Finpits,” cell biologist Daniel DeMartini nicknamed them. He and his colleagues have documented unusual color-change displays in female California market squid, popular in restaurants. Squids, octopuses and cuttlefishes are nature’s iPads, changing their living pixels at will. DeMartini, of the University of California, Santa Barbara, saw so many sunset shimmers, blink-of-an-eye blackouts and other marvels in California’s Doryteuthis opalescens that it took him a while to notice that only females shimmered the finpit stripe. It shows up now and then during life, and reliably for about 24 hours after decapitation, DeMartini found. The squid are color-blind, and what prompts their display is known only to them. But the researchers have figured out how it works. The squid make rainbows when color-change cells called iridocytes lose water. Other kinds of color-change cells work their magic via pigments, but not iridocytes. “If you take a bunch of iridocyte cells in red, blue, green or yellow and you grind them up, then you wouldn’t see any color,” DeMartini says. Instead, little stacks of protein plates inside the cells turn colorful only when water rushes out of the stack. How closely the plates snug together determines whether the stack looks blue, scarlet or anything in between. © Society for Science & the Public 2000 - 2013
By ELISABETH ROSENTHAL The barrage of advertisements targets older men. “Have you noticed a recent deterioration of your ability to play sports?” “Do you have a decrease in sex drive?” “Do you have a lack of energy?” If so, the ads warn, you should “talk to your doctor about whether you have low testosterone” — “Low T,” as they put it. In the view of many physicians, that is in large part an invented condition. Last year, drug makers in the United States spent $3.47 billion on advertising directly to consumers, according to FiercePharma.com. And while ever-present ads like those from AbbVie Pharmaceuticals have buoyed sales of testosterone gels, that may be bad for patients as well as the United States’ $2.7 trillion annual health care bill, experts say. Sales of prescription testosterone gels that are absorbed through the skin generated over $2 billion in American sales last year, a number that is expected to more than double by 2017. Abbott Laboratories — which owned AbbVie until Jan. 1 — spent $80 million advertising its version, AndroGel, last year. Once a niche treatment for people suffering from hormonal deficiencies caused by medical problems like endocrine tumors or the disruptive effects of chemotherapy, the prescription gels are increasingly being sold as lifestyle products, to raise dipping levels of the male sex hormone as men age. “The market for testosterone gels evolved because there is an appetite among men and because there is advertising,” said Dr. Joel Finkelstein, an associate professor at Harvard Medical School who is studying male hormone changes with aging. “The problem is that no one has proved that it works and we don’t know the risks.” © 2013 The New York Times Company
By Scicurious When most of us hear birds twittering away in the trees, we hear it as background noise. It’s often hard to separate out one bird from another. But when you can, you begin to hear just how complex birdsong can be, a complex way of male signaling to a female how THEY are the best, and THEY are the one they should clearly pick. You hear ups and downs and trills and repeating themes. We used to think that birdsong was a relatively simple gene by environment interaction. The big males with the big songs get the best females, and then it’s a matter of also getting the best food, and the then healthy bird teaches its offspring to sing, and the health offspring goes on to display the best song. The song is therefore an “honest signal” of the bird’s fitness, it’s got good genes and good food and it is ready to MATE, baby! But how much of it is really training and how much is genetic? To find out, we go to what may possibly be the cutest of research subjects…the zebra finch. To look at the relationship between genes and environment in song learning, the authors turned to the zebra finch. Many other studies have also looked at the zebra finch and how it learns song, and how environmental pressures (like say, not enough food) change the way the song is displayed. But those experiments usually bred the birds and looked at the environment…they didn’t look at the teachers. The father birds, who were “teaching” their offspring to sing. © 2013 Scientific American
by Colin Barras Male marsupial mice just don't know when to stop. For Antechinus stuartii, their debut breeding season is so frenetic and stressful that they drop dead at the end of it from exhaustion or disease. It may be the females of the species that are driving this self-destructive behaviour. Suicidal breeding, known as semelparity, is seen in several marsupials. This is likely linked to short breeding seasons and the fact that the marsupial mice only breed once a year. It is not clear why this is, but it may be that females can only breed when the population of their insect prey reaches its peak. A year is a long and dangerous time for a small animal, so under these circumstances males might do best to pump all their resources into a single breeding season. To test this idea, Diana Fisher of the University of Queensland in St Lucia, Australia, and her colleagues tracked how insect abundance changed with the seasons in the marsupials' home forests. Sure enough, they found that the marsupials' breeding seasons were shortest where insect abundance followed a predictable annual pattern. But the insects are not the whole explanation. It turns out that females do sometimes survive the year and breed again. So why do the males always die? The key factor is that the females are highly promiscuous, says Fisher. Coupled with the short breeding season, this leads to intense competition between males. "Males that exert extreme effort in this short time are at an advantage." © Copyright Reed Business Information Ltd.
By Cat Bohannon Halos, auras, flashes of light, pins and needles running down your arms, the sudden scent of sulfur—many symptoms of a migraine have vaguely mystical qualities, and experts remain puzzled by the debilitating headaches' cause. Researchers at Harvard University, however, have come at least one step closer to figuring out why women are twice as likely to suffer from chronic migraines as men. The brain of a female migraineur looks so unlike the brain of a male migraineur, asserts Harvard scientist Nasim Maleki, that we should think of migraines in men and women as “different diseases altogether.” Maleki is known for looking at pain and motor regions in the brain, which are known to be unusually excitable in migraine sufferers. In one notable study published in the journal Brain last year, she and her colleagues exposed male and female migraineurs to painful heat on the backs of their hands while imaging their brains with functional MRI. She found that the women had a greater response in areas of the brain associated with emotional processing, such as the amygdala, than did the men. Furthermore, she found that in these women, the posterior insula and the precuneus—areas of the brain responsible for motor processing, pain perception and visuospatial imagery—were significantly thicker and more connected to each other than in male migraineurs or in those without migraines. In Maleki's most recent work, presented in June at the International Headache Congress, her team imaged the brains of migraineurs and healthy people between the ages of 20 and 65, and it made a discovery that she characterizes as “very, very weird.” In women with chronic migraines, the posterior insula does not seem to thin with age, as it does for everyone else, including male migraineurs and people who do not have migraines. The region starts thick and stays thick. © 2013 Scientific American
Alice Roberts It's the rutting season. From Richmond Park to the Isle of Rum, red deer hinds will be gathering, and the stags that have spent the past 10 months minding their own business in bachelor groups are back in town, with one thing on their minds. A mature male that has netted himself a harem is very dedicated. He practically stops eating, focusing instead on keeping his hinds near and his competitors at bay. If you're a red deer stag, one of the ways you make sure that your adversaries know you mean business – and that you're big – is roaring. And you don't let up. You can keep roaring all day, and through the night too, twice a minute, if necessary. While female red deer prefer the deeper roars of larger stags, roaring also appears to be part of how stags size one another up, before deciding whether or not to get engaged in a full-on physical fight. Most confrontations are settled without locking antlers. In male red and fallow deer, the voicebox or larynx is very low in the throat – and gets even lower when they roar. Strap-like muscles that attach to the larynx contract to drag it down towards the breastbone – lengthening the vocal tract and deepening the stag's roar. Deepening the voice exaggerates body size. Over generations, stags with deeper roars presumably had more reproductive success, so the position of the larynx moved lower and lower in the neck. When a red deer stag roars his larynx is pulled down so far that it contacts the front of his breastbone – it couldn't get any lower. In human evolution, much is made of the low position of the larynx in the neck. So much, in fact, that it has been considered to be a uniquely human trait, and intrinsically linked to that other uniquely human trait: spoken language. But if red and fallow deer also have low larynges, that means, first, that we're not as unusual as we like to think we are, and second, that there could be other reasons – that are nothing to do with speaking – for having a descended larynx. © 2013 Guardian News and Media Limited
By EILEEN POLLACK Last summer, researchers at Yale published a study proving that physicists, chemists and biologists are likely to view a young male scientist more favorably than a woman with the same qualifications. Presented with identical summaries of the accomplishments of two imaginary applicants, professors at six major research institutions were significantly more willing to offer the man a job. If they did hire the woman, they set her salary, on average, nearly $4,000 lower than the man’s. Surprisingly, female scientists were as biased as their male counterparts. The new study goes a long way toward providing hard evidence of a continuing bias against women in the sciences. Only one-fifth of physics Ph.D.’s in this country are awarded to women, and only about half of those women are American; of all the physics professors in the United States, only 14 percent are women. The numbers of black and Hispanic scientists are even lower; in a typical year, 13 African-Americans and 20 Latinos of either sex receive Ph.D.’s in physics. The reasons for those shortages are hardly mysterious — many minority students attend secondary schools that leave them too far behind to catch up in science, and the effects of prejudice at every stage of their education are well documented. But what could still be keeping women out of the STEM fields (“STEM” being the current shorthand for “science, technology, engineering and mathematics”), which offer so much in the way of job prospects, prestige, intellectual stimulation and income? As one of the first two women to earn a bachelor of science degree in physics from Yale — I graduated in 1978 — this question concerns me deeply. I attended a rural public school whose few accelerated courses in physics and calculus I wasn’t allowed to take because, as my principal put it, “girls never go on in science and math.” Angry and bored, I began reading about space and time and teaching myself calculus from a book. When I arrived at Yale, I was woefully unprepared. The boys in my introductory physics class, who had taken far more rigorous math and science classes in high school, yawned as our professor sped through the material, while I grew panicked at how little I understood. The only woman in the room, I debated whether to raise my hand and expose myself to ridicule, thereby losing track of the lecture and falling further behind. © 2013 The New York Times Company
Keyword: Sexual Behavior
Link ID: 18749 - Posted: 10.05.2013
by Laura Sanders When I started to get out and about with Baby V, I occasionally experienced a strange phenomenon. Women would approach and coo some pleasant little noises. After an appropriate amount of time had passed, these strangers would lean in close and ask to smell my baby. I’m the first to admit that this sounds creepy. Truth be told, it is a little creepy. But now I completely get it. The joy from a single whiff of newborn far outweighs any trifling social conventions about personal space and body odors. So when women approach looking for a little hit of eau de bebe, I get sharey. By all means, ladies, lean in and smell away. Tiny babies smell very, very good. So good that I’m getting a little high from just thinking about how good babies smell. So good that people attempt to bottle and sell this scent (like this baby-head-scented spray— pleasant, but pales in comparison). So good that scientists really want to know why some women find this smell irresistible. Scientists recently studied the brains of women as they sniffed new baby scent. Two-day-old babies delivered the good stuff by wearing the same pajamas for two nights. Women then sniffed the odor extracted from the outfit while brain scans assessed neural activity. Overall, the 30 women in the study (who weren’t told what they were sniffing, by the way) rated the scent as mildly pleasant. As the intoxicating scent of newborn wafted into their brains, neural activity increased in areas of the brain linked to good feelings, called neostriate areas. In the brains of the 15 women who also happened to be mothers, the brain activity seemed stronger. (No word yet on what new baby smell does to dads’ brains.) © Society for Science & the Public 2000 - 2013.
Sarah C. P. Williams A person might be caught off-guard without an umbrella in a sudden downpour, but rain doesn’t catch insects by surprise. Moths, beetles, and aphids predict storms by sensing changes in air pressure and then alter their behavior, researchers have discovered. In particular, the new study finds that insects change their mating behaviors when the air pressure drops, which often precedes rain, or when the air pressure rises, which can signal strong winds. “People have observed before that birds, bats, and even fish respond to changes in [air] pressure,” says entomologist Maria Fernanda Peñaflor of the University of São Paulo in Brazil, a co-author of the new study. “This is the first time such behavior has been studied in insects.” Peñaflor and her colleagues knew that insect behavior was mediated by temperature, wind, and rainfall and wondered whether air pressure played a role as well. They first correlated air pressure data from a local meteorology station with the behavior of male cucurbit beetles (Diabrotica speciosa), green and yellow beetles about 6 millimeters long that feed on cucurbit vegetables, such as cucumbers, pumpkins, and squashes, in South America. They discovered that on days when the pressure was falling—indicating impending rain—the male beetles were less likely to walk in the direction of female pheromones, which they normally follow to pursue mates. To find out more, Peñaflor’s group collaborated with researchers at the University of Western Ontario in Canada who had a controlled pressure chamber in which they could perform experiments. © 2013 American Association for the Advancement of Science.
Keyword: Sexual Behavior
Link ID: 18738 - Posted: 10.03.2013
By WILLIAM J. BROAD SCIENCE has looked into some strange things over the centuries — reports of gargantuan sea monsters, purported images of Jesus, sightings of alien spaceships and so on. When I first heard of spontaneous orgasm, while researching a book on yoga, including its libidinal cousin, tantra, I figured it was more allegory than reality and in any event would prove beyond the reach of even the boldest investigators. Well, I was wrong. It turns out science has tiptoed around the subject for more than a century and of late has made considerable progress in determining not only the neurophysiological basis of the phenomenon but also its prevalence. Men are mentioned occasionally. But sex researchers have found that the novel type of autoerotism shows up mainly in women. Ground zero for the research is Rutgers University, where scientists have repeatedly had female volunteers put their heads into giant machines and focus their attention on erotic fantasies — the scans reveal that the pleasure centers of their brains light up in ways indistinguishable from everyday orgasms. The lab atmosphere is no-nonsense, with plenty of lights and white coats and computer monitors. Subjects often thrash about so forcefully that obtaining clear images of their brains can be difficult. “Head movement is a huge issue,” Nan Wise, a doctoral candidate at Rutgers who helps run the project, said in an interview. “It’s hard to get a decent signal.” She said a volunteer’s moving her head more than two millimeters — less than a 10th of an inch — can make for a bad day in the lab. It is easy to dismiss this as a new kind of narcissism in search of scientific respectability, a kinky pleasure coming out of the shadows. Many YouTube videos now purport to show people using controlled breathing and erotic introspection to achieve what they describe as “thinking off” and “energy orgasms.” © 2013 The New York Times Company
by Megan Gannon, Live Science Deep in the cloud forests of Central America, two species of singing mice put on a high-pitched opera to mark their territory and stave off clashes, researchers discovered. Alston's singing mouse (Scotinomys teguina) and the Chiriqui singing mouse (S. xerampelinus) have overlapping lifestyles in the cloud forests of Costa Rica and Panama. But the tawny cousins seem to establish geographic boundaries so they can avoid competing with each other. "A long-standing question in biology is why some animals are found in particular places and not others," study researcher Bret Pasch, a postdoctoral fellow at the the University of Texas at Austin, said in a statement. "What factors govern the distribution of species across space?" As it turns out, a little communication between individuals affects the spread of both species as a whole. Both species of singing mice produce vocalizations that are barely audible to humans. As video footage of the mouse-y opera from the foggy forest floor shows, the creatures throw their heads back and belt out songs in the form of rapidly repeated notes, known as trills. The Alston's mouse in the clip even looks likes it's taking a bow after its solo. © 2013 Discovery Communications, LLC
Mark Peplow Hormone-disrupting chemicals may be far more prevalent in lakes and rivers than previously thought. Environmental scientists have discovered that although these compounds are often broken down by sunlight, they can regenerate at night, returning to life like zombies. “The assumption is that if it’s gone, we don’t have to worry about it,” says environmental engineer Edward Kolodziej of the University of Nevada in Reno, joint leader of the study. “But we’re under-predicting their environmental persistence.” “Risk assessments have been built on the basis that light exposure is enough to break down these products,” adds Laura Vandenberg, an endocrinologist at the University of Massachusetts in Amherst who was not involved in the study. “This work undermines that idea completely.” Endocrine disruptors — pollutants that unbalance hormone systems — are known to harm fish, and there is growing evidence linking them to health problems in humans, including infertility and various cancers1. But pinpointing specific culprits from the vast array of trace chemicals in the environment has proved difficult. Indeed, concentrations of known endocrine disruptors in rivers often seem to be too low to explain harmful effects in aquatic wildlife, says Kolodziej. He and his colleague David Cwiertny, an environmental engineer at the University of Iowa in Iowa City, decided to find out whether the breakdown products of endocrine disruptors could be boosting their environmental impact. Their team focused on trenbolone acetate, a synthetic anabolic steroid used as a growth promoter in more than 20 million cattle in the United States each year (this practice is banned in the European Union). © 2013 Nature Publishing Group
By Bruce Bower Cartoon ghosts have scared up evidence that the ability to visualize objects in one’s mind materializes between ages 3 and 5. When asked to pick which of two mirror-image ghost cutouts or drawings fit in a ghost-shaped hole, few 3-year-olds, a substantial minority of 4-year-olds and most 5-year-olds regularly succeeded, say psychologist Andrea Frick of the University of Bern, Switzerland, and her colleagues. Girls performed as well as boys on the task, suggesting that men’s much-studied advantage over women in mental rotation doesn’t emerge until after age 5, the researchers report Sept. 17 in Cognitive Development. Mental rotation is a spatial skill regarded as essential for science and math achievement. Most tasks that researchers use to assess mental rotation skills involve pressing keys to indicate whether block patterns oriented at different angles are the same or different. That challenge overwhelms most preschoolers. Babies apparently distinguish block patterns from mirror images of those patterns (SN: 12/20/08, p. 8), but it’s unclear whether that ability enables mental rotation later in life. Frick’s team studied 20 children at each of three ages, with equal numbers of girls and boys. Youngsters saw two ghosts cut out of foam, each a mirror image of the other. Kids were asked to turn the ghosts in their heads and choose the one that would fit like a puzzle piece into a ghost’s outline on a board. Over seven trials, the ghosts were tilted at angles varying from the position of the outline. The researchers used three pairs of ghost cutouts, for a total of 21 trials. © Society for Science & the Public 2000 - 2013
By Neuroskeptic The comparative anatomy of male and female brains is an incredibly popular topic. From teachers to cartoonists, everyone’s interested in it. One supposed dude-dame dimorphism is the width of the corpus callosum, the white matter bridge that connects the brain’s left and right hemispheres. Some studies suggest that women have a larger corpus callosum, relative to overall brain size, than men. This has led to a lot of speculation about how females, with their more ‘interconnected’ brains, are therefore better at things like multitasking: The corpus callosum is 30 percent more highly developed in the female brain… allowing information to flow more easily from one side of the brain to the other, which allows a woman to focus on more than one thing at a time. However, according to Eileen Luders and colleagues, that’s all a wash, because: Differences in Brain Volume Account for Apparent Sex Differences in Callosal Anatomy It’s been argued that women’s relatively larger corpus callosa may reflect the fact that men have larger brains, on average, and that the corpus callosum is relatively smaller in larger brains. In other words, the corpus callosum difference might be a side-effect of the true gender difference (perhaps the only one) – bigger male brains overall. Luders et al confirmed this with a clever technique: they looked in a large online brain database to find some extremely small male brains, and extremely large female ones. This, the two genders were matched on total size.
By Felicity Muth In most animals, females are generally the ones that choose the males. This is a massive generalisation (for example, it doesn’t apply in this case), but I hope people who work on this topic will forgive me for it. Generally speaking, it’s the females that get to size up the males, check out whatever trait it is that’s attractive to them (be it weight, head feather colour, ability to sing, or muscle size) and then choose who they want to mate with. However, how animals (even insects) behave when choosing mates is by no means governed by fixed rules, and is influenced by many different things. I’ve previously written about fish that will change how they court females depending on who’s watching and male crickets that will change their victory displays after fighting with another male depending on their audience. Similarly, what a female chooses in a male mate isn’t totally free from influences outside the quality of the male in question. In some species, such as the field cricket, wolf spider and cowbirds, females with more experience choose differently to naïve females. But what other things might affect what females choose? Pretty much all animals come into contact and may be infected by parasites at some point in their life. Amazingly, parasites seem to affect the mating behaviour of animals in some unusual and unexpected ways. Some parasites castrate their hosts, or change who the host wants to mate with. Others can even cause sex-role reversals, such as in the bush cricket. © 2013 Scientific American
By PAULINE W. CHEN, M.D. One afternoon at a school not far from the hospital where I was working, a teacher opened a utility closet and found a staff member passed out on the floor. He was clutching a small bloody mass in one hand, a sharp knife in the other, she reported, a red stain spreading rapidly at his middle. He had amputated his genitals. Once he’d been brought to our emergency room and resuscitated, the man refused further treatment. Doctors and nurses, concerned that if they waited any longer to reattach the severed part the surgery might not work, took the necessary steps to deem him mentally incompetent to make such decisions. “The guy was seriously nuts,” I remember one of the doctors saying afterward. “He kept screaming that he didn’t want ‘it’ back.” For days after the successful operation, the gruesome story was all anyone at the hospital could discuss. Most of us chalked it up to his being “certifiable,” and several wondered if maybe they should have skipped the surgery. “After all,” said one clinician, “isn’t that what he wanted?” But in all the chatter none of us mentioned a key part of the patient’s story: the unbearable suffering that must have pushed him to commit so brutal an act. In fact, anyone overhearing our conversations might have been hard pressed to find any of the warmth and sensitivity we routinely displayed toward patients with cancer, AIDS or heart disease. I remembered the man and our reactions this past week while reading “Falling Into the Fire: A Psychiatrist’s Encounters With the Mind in Crisis,” a thought-provoking new book by Dr. Christine Montross. Of all the afflictions that fall upon us, few remain as misunderstood and stigmatized as those that affect the mind. Copyright 2013 The New York Times Company
By GINA KOLATA It is the scourge of many a middle-aged man: he starts getting a pot belly, using lighter weights at the gym and somehow just doesn’t have the sexual desire of his younger years. The obvious culprit is testosterone, since men gradually make less of the male sex hormone as years go by. But a surprising new answer is emerging, one that doctors say could reinvigorate the study of how men’s bodies age. Estrogen, the female sex hormone, turns out to play a much bigger role in men’s bodies than previously thought, and falling levels contribute to their expanding waistlines just as they do in women’s. The discovery of the role of estrogen in men is “a major advance,” said Dr. Peter J. Snyder, a professor of medicine at the University of Pennsylvania, who is leading a big new research project on hormone therapy for men 65 and over. Until recently, testosterone deficiency was considered nearly the sole reason that men undergo the familiar physical complaints of midlife. The new frontier of research involves figuring out which hormone does what in men, and how body functions are affected at different hormone levels. While dwindling testosterone levels are to blame for middle-aged men’s smaller muscles, falling levels of estrogen regulate fat accumulation, according to a study published Wednesday in The New England Journal of Medicine, which provided the most conclusive evidence to date that estrogen is a major factor in male midlife woes. And both hormones are needed for libido. “Some of the symptoms routinely attributed to testosterone deficiency are actually partially or almost exclusively caused by the decline in estrogens,” said Dr. Joel Finkelstein, an endocrinologist at Harvard Medical School and the study’s lead author, in a news release on Wednesday. © 2013 The New York Times Company
Sarah Zhang Fathers with smaller testes are more involved in child care, and their brains are also more responsive when looking at photos of their own children, according to research published online today in the Proceedings of the National Academy of Sciences1. Evolutionary biologists have long observed a trade-off in male primates between mating efforts to produce more offspring and the time males spend caring for their progeny. For instance, male chimpanzees, which are especially promiscuous, sport testes that are twice as big as those of humans, make a lot of sperm and generally do not provide paternal care. By contrast, male gorillas have relatively small testes and protect their young. The latest study suggests that humans, whose paternal care varies widely, show evidence of both approaches. The analysis1 incorporates measures of testicular volume, brain activity and paternal behaviour, notes Peter Gray, an anthropologist at the University of Nevada, Las Vegas, who was not involved in the study. “We’ve got something that pulls those strands together, and it does so in a really interesting way.” The research team — led by James Rilling, an anthropologist at Emory University in Atlanta, Georgia — set out to investigate why some fathers are more involved in child care than others. The researchers recruited 70 fathers of children aged between one and two years, and scanned the men’s brains and testes in a magnetic resonance imaging (MRI) machine. The fathers and the children's mothers also filled out surveys rating the fathers' commitment to child care. © 2013 Nature Publishing Group