Chapter 8. Hormones and Sex
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By Joshua A. Krisch When Kathleen Gardiner first encountered female mice with Down syndrome, she was surprised to find that the rodents’ brains showed unexpected abnormalities. Gardiner, a neuroscientist at the University of Colorado School of Medicine in Aurora, knew that trisomic male mice typically had perturbed protein levels in their hippocampuses. But these trisomic female mice showed the most serious changes in their cerebellums. “Right away, there’s a brain region sex difference,” Gardiner said. “It’s very interesting to ponder the fact that this could lead to sex differences in the learning, memory, or behavioral abnormalities associated with [Down syndrome].” Although Gardiner recognized that differences between mouse sexes would not necessarily translate into sex differences in humans, she considered the potential implications for clinical studies on Down syndrome therapies. “If we find that males or females are differing not only in their baseline impairment, but in their response to drugs, we need to know that,” she said. “We could be missing a big piece of information that could lead to better or different clinical trials.” Indeed, sex differences in model organisms are becoming increasingly apparent. Studies have shown sex differences in mice can affect cardiovascular health, liver disease, and cancer risk. Many of these studies are now published in Biology of Sex Differences, where Gardiner’s own work on the trisomic female mice appeared. © 1986-2017 The Scientist
By Carolyn Gramling Trilobites—three-sectioned, crablike critters that dominated the early Paleozoic—are so abundant that they have become the gateway fossil for most collectors. But paleontologists have found little evidence of how the extinct arthropods reproduced—until now. Researchers studying a fossil specimen of the trilobite Triarthrus eatoni spotted something odd just next to the animal’s head: a collection of small (about 200 micrometers across), round objects (in light blue, above). Those, they determined, are actually eggs—the first time anyone had observed fossil trilobite eggs right next to the critters themselves. The structures were exceptionally well preserved, the eggs and exoskeletons of the trilobites replaced with an iron sulfide ore called pyrite. They came from the Lorraine Group, a rock formation that spans much of the northeastern United States and dates to the Ordovician period (about 485 million to 444 million years ago); it has long been a mecca for trilobite hunters because of the pyritization. The placement of the eggs is suggestive, the researchers report in the March issue of Geology: They hypothesize that trilobites released their eggs and sperm through a genital pore somewhere in the head—much like modern horseshoe crabs do today. One possible reason for the rarity of the find may be that the brooding behavior of T. eatoni was relatively unusual in the trilobite world: The species tended to prefer a harsh, low-oxygen environment, and may have kept a closer eye on their eggs than other trilobite species. But, the authors note, one idea this finding does lay to rest is that trilobites might reproduce via copulation—a titillating but little-regarded hypothesis based on the fact that trilobites are sometimes found clustered on top of one another. Instead, trilobites were most likely spawners—and, in fact, that clustering behavior may be another parallel to horseshoe crabs, which can climb on top of one another in competition to fertilize released eggs. © 2017 American Association for the Advancement of Science
By Claudia Wallis Dinosaurs, Star Wars, train schedules, Disney princesses, maps, LEGO—subjects such as these can become all-consuming passions for children on the autism spectrum. What therapists and educators often call “circumscribed” or “restricted” interests (or, more generously, “special” interests) make up a characteristic symptom of autism spectrum disorder (ASD). The current edition of psychiatry’s Diagnostic and Statistical Manual of Mental Disorders describes them as “highly restricted, fixated interests that are abnormal in intensity or focus.” Roughly 90 percent of high-functioning kids with ASD display at least one such interest during their elementary school years, according to a 2007 survey conducted at the Yale University Child Study Center, one of the few studies to have examined the topic. For a family with an affected child, this kind of narrow preoccupation can be tedious and exhausting. Imagine a kid who will talk about nothing but the exits on the New Jersey Turnpike or the Captain Underpants book series. (Both actual examples.) Therapists and educators have traditionally tried to suppress or modulate a child’s special interest, or use it as a tool for behavior modification: Keep your hands still and stop flapping, and you will get to watch a Star Wars clip; complete your homework or no Harry Potter. But what if these obsessions themselves can be turned into pathways to growth? What if these intellectual cul-de-sacs can open up worlds? That is the idea explored in the film Life, Animated, a contender for the Academy Award for Best Documentary this Sunday night. © 2017 Scientific American
Link ID: 23277 - Posted: 02.24.2017
Jon Brock What if I told you that we can now identify babies who are going to develop autism based on a simple brain scan? This, in essence, is the seductive pitch for a study published last week in the journal Nature, and making headlines around the world. Early identification and diagnosis is one of the major goals of autism research. By definition, people with autism have difficulties with social interaction and communication. But these skills take many years to develop, even in typically developing (i.e., non-autistic) children. Potential early signs of autism are extremely difficult to pick out amidst the natural variation in behaviour and temperament that exists between all babies. A brain scan for autism would be a major step forward. But is the hype justified? Are we really on the brink of a new era in autism diagnostics? Without wishing to detract from the efforts of everyone involved in the study, it’s important to look at the results critically, both in terms of the scientific findings and their potential implications for clinical practice. The study, led by Heather Cody Hazlett at the University of North Carolina, was part of a larger research program investigating the development of babies who have an older sibling with autism. Because autism runs in families, these babies are much more likely to develop autism than babies from the general population.
Patricia Neighmond Many men over 65 with low testosterone levels say their sense of well-being, not to mention sexual function, isn't what it used to be. That's why some doctors prescribe testosterone replacement. But the effectiveness of testosterone has been controversial. Studies of the risks and benefits have been mixed, and the Food and Drug Administration beefed up its warnings about cardiac side effects of testosterone supplementation in 2015. And the findings of five studies released Tuesday aren't likely to clear up the confusion. They appear in JAMA, the journal of the American Medical Association and JAMA Internal Medicine. The studies are collectively called the Testosterone Trials (TTrials) and they compared a testosterone gel, AndroGel, against a placebo. The results are based on 788 men with below normal levels of testosterone studied at 12 sites across the country over a year. Overall, researchers saw improvements in bone density and bone strength in men who used a testosterone gel, which raised their testosterone to levels seen in younger men. In men with unexplained anemia, testosterone also improved iron levels in the blood. (A reviewer of the study raised questions about whether it was done ethically.) But in men using testosterone who had been reporting memory problems at the start of the study, there were no improvements in memory or cognition. And there were worrisome signs of an increase in the risk of cardiovascular problems. © 2017 npr
By Greta Keenan The ocean might seem like a quiet place, but listen carefully and you might just hear the sounds of the fish choir. Most of this underwater music comes from soloist fish, repeating the same calls over and over. But when the calls of different fish overlap, they form a chorus. Robert McCauley and colleagues at Curtin University in Perth, Australia, recorded vocal fish in the coastal waters off Port Headland in Western Australia over an 18-month period, and identified seven distinct fish choruses, happening at dawn and at dusk. You can listen to three of them here: The low “foghorn” call is made by the Black Jewfish (Protonibea diacanthus) while the grunting call that researcher Miles Parsons compares to the “buzzer in the Operation board game” comes from a species of Terapontid. The third chorus is a quieter batfish that makes a “ba-ba-ba” call. “I’ve been listening to fish squawks, burble and pops for nearly 30 years now, and they still amaze me with their variety,” says McCauley, who led the research. Sound plays an important role in various fish behaviours such as reproduction, feeding and territorial disputes. Nocturnal predatory fish use calls to stay together to hunt, while fish that are active during the day use sound to defend their territory. “You get the dusk and dawn choruses like you would with the birds in the forest,” says Steve Simpson, a marine biologist at the University of Exeter, UK. © Copyright Reed Business Information Ltd.
Ewen Callaway Researchers have no way to tell whether young babies may later be diagnosed with autism. But brain scans could help, a small study suggests. By scanning the brains of babies whose siblings have autism, researchers say they have been able to make reasonably accurate forecasts about which of these high-risk infants will later develop autism themselves. The findings raise the prospect of diagnosing autism spectrum disorder (ASD) months before children develop symptoms, a goal that has proved elusive. Nature looks at the new study and its implications. Why has it been so tough to diagnose autism in infants? Children typically show symptoms of ASD, such as difficulty making eye contact, after the age of 2. Researchers believe that the brain changes underlying ASD begin much earlier — possibly even in the womb. But behavioural assessments haven't been helpful in predicting who will get autism, says Joseph Piven, a psychiatrist at the University of North Carolina (UNC) in Chapel Hill, who co-led the study, published online in Nature1. “Children who end up with autism at 2 or 3, they don’t look like they have autism in the first year," he says. Certain rare mutations are linked to ASD, but the vast majority of cases cannot be pinned to a single or even a handful of genetic risk factors. Beginning in the 1990s, Piven and other researchers noticed that children with autism tended to have larger brains than developmentally normal children, suggesting that brain growth could be a biomarker for ASD. But Piven and colleague Heather Cody Hazlett, a psychologist at UNC-Chapel Hill, say it had not been clear when overgrowth occurred. What did their latest study look at? © 2017 Macmillan Publishers Limited,
By Jesse Singal Those who advocate for sound, evidence-based research about autism are extremely alarmed about Donald Trump, and for good reason: In addition to Trump’s ties to Andrew Wakefield, the disgraced British doctor whose debunked research helped fuel the false idea of links between childhood vaccines and autism, Robert F. Kennedy Jr., a notorious anti-vaxxer himself, told reporters back in January that Trump planned to tap him as chair of a commission on “vaccine safety.” There is no question at this point that Trump has significant connections to a pseudoscientific medical movement that spreads dangerous, disproven ideas. Today, Trump gave nervous observers yet more reason to worry. It occurred at a White House event in which Trump and Secretary of Education Betsy DeVos met with a bunch of educators. Trump seemed to fixate, for a moment, on one educator named Jane (her last name is hard to make out) after she explained that she is the principal of a special education center in Virginia. The sequence starts at about 5:38 in this video: After Jane noted that many of her students have autism, Trump asked, “Have you seen a big increase in the autism, with the children?” Jane replied in the affirmative, but seemed to couch her response as being more about an increase in demand for services — she didn’t explicitly agree there’s been a big increase in the overall rate. Trump continued: “So what’s going on with autism? When you look at the tremendous increase, it’s really — it’s such an incredible — it’s really a horrible thing to watch, the tremendous amount of increase. Do you have any idea? And you’re seeing it in the school?” Jane replied — again, in a way that seems a bit noncommittal vis-à-vis Trump’s claim — that the rate of autism is something like 1-in-66 or 1-in-68 children. To which Trump responds: “Well now, it’s gotta be even lower [presumably meaning higher, rate-wise] than that, which is just amazing — well, maybe we can do something.” (Jane had the rate right, and Trump is incorrect that it has crept higher.) © 2017, New York Media LLC.
Link ID: 23233 - Posted: 02.15.2017
Elizabeth Eaton A prehistoric marine reptile may have given birth to its young alive. A fossil from South China may be the first evidence of live birth in the animal group Archosauromorpha, scientists report February 14 in Nature Communications. Today Archosauromorpha is represented by birds and crocodiles — which both lay eggs. Whether this fossil really is the first evidence of live birth in Archosauromorpha depends on how another group of semiaquatic animals is classified, says Michael Caldwell, a vertebrate paleontologist with the University of Alberta in Canada. Placement of Choristodera, a now-extinct group that included a freshwater reptile that gave live birth, remains murky, with some researchers putting them with Archosauromorpha and others with a group that includes snakes and lizards. “Our discovery is the first of live birth in reptiles with undoubted archosauromorph affinity,” says Jun Liu, a paleontologist at Hefei University of Technology in China. Researchers have speculated that the biology of archosauromorphs prevented their reproductive traits from evolving, says study coauthor Chris Organ, an evolutionary biologist with Montana State University in Bozeman. This find may disprove that view. “Ancestrally, the science suggests that live birth is not absolutely prohibited,” Organ says. Even though birds and crocodiles haven’t yet evolved to give life birth, this discovery suggests that it’s possible. |© Society for Science & the Public 2000 - 2016
Moises Velasquez-Manoff This Valentine’s Day, as you bask in the beauty of your beloved, don’t just thank his or her genes and your good fortune; thank microbes. Research on the microbes that inhabit our bodies has progressed rapidly in recent years. Scientists think that these communities, most of which live in the gut, shape our health in myriad ways, affecting our vulnerability to allergic diseases like hay fever, how much weight we put on, our susceptibility to infection and maybe even our moods. They can also, it seems, make us sexy. Susan Erdman, a microbiologist at M.I.T., calls it the “glow of health.” The microbes you harbor, she argues, can make your skin smooth and your hair shiny; they may even put a spring in your step. She stumbled on the possibility some years ago when, after feeding mice a probiotic microbe originally isolated from human breast milk, a technician in her lab noticed that the animals grew unusually lustrous fur. Further observation of males revealed thick skin bristling with active follicles, elevated testosterone levels and oversize testicles, which the animals liked showing off. Microbes had transformed these animals into rodent heartthrobs. When given to females, the probiotic also prompted deeper changes. Levels of a protein called interleukin 10, which helps to prevent inflammatory disease and ensure successful pregnancy, went up, as did an important hormone called oxytocin. Oxytocin, often called the love hormone, helps mammals bond with one another. Our bodies may release it when we kiss (and mean it), when women breast-feed, even when people hang out with good friends. And the elevated oxytocin Dr. Erdman saw had important effects during motherhood. Some of the mice in her studies were eating a high-fat, high-sugar diet — junk-foody fare that’s known to shift the microbiome into an unhealthy state. Not surprisingly perhaps, mothers that didn’t imbibe the probiotics were less caring and tended to neglect their pups. But mothers that had high oxytocin thanks to the probiotic were nurturing and reared their pups more successfully. © 2017 The New York Times Company
Patti Neighmond It's tough to be a teenager. Hormones kick in, peer pressures escalate and academic expectations loom large. Kids become more aware of their environment in the teen years — down the block and online. The whole mix of changes can increase stress, anxiety and the risk of depression among all teens, research has long shown. But a recent study published in the journal Pediatrics suggests many more teenage girls in the U.S. may be experiencing major depressive episodes at this age than boys. And the numbers of teens affected took a particularly big jump after 2011, the scientists note, suggesting that the increasing dependence on social media by this age group may be exacerbating the problem. Psychiatrist Ramin Mojtabai and colleagues at Johns Hopkins University Bloomberg School of Public Health wanted to know whether rates of depression among teens had increased over the past decade. They analyzed federal data from interviews with more than 172,000 adolescents. Between 2005 and 2014, the scientists found, rates of depression went up significantly — if extrapolated to all U.S. teens it would work out to about a half million more depressed teens. What's more, three-fourths of those depressed teens in the study were girls. The findings are just the latest in a steady stream of research showing that women of all ages experience higher rates of depression compared to men, says psychologist and author Catherine Steiner-Adair. And no wonder, she says — despite gains in employment, education and salary, women and girls are still "continually bombarded by media messages, dominant culture, humor and even political figures about how they look — no matter how smart, gifted, or passionate they are." © 2017 npr
By STEPH YIN If you’re reading this at home, pause and put on a song you can’t resist dancing to. Go on, bop your head to the beat. Let yourself wiggle a bit. Throw in some arms and legs. If you’re reading this at work, maybe imagine these things at your desk. As you’re dancing, pay attention to where and how you’re moving. How much are you swaying your hips? Are your legs moving together or independently of each other? How vigorously are you moving your torso? You should note those movements, because very specific patterns may make some people appear to be better dancers than others. That’s the conclusion of a study published on Thursday in Scientific Reports, in which researchers asked 200 people to rate 39 female dancers. A few features stood out as contributing to higher-quality dance: big hip swings, and the right and left limbs moving independently of one another (which the researchers describe as asymmetric arm and thigh movements). The researchers speculate that those moves serve two purposes for heterosexual women. “One is, they’re showing off their reproductive quality, perhaps their hormonal status, to males,” said Nick Neave, an associate professor of psychology at Northumbria University in England and an author of the paper. “Another is, they’re showing off how good they are to female rivals.” In 2011, the same researchers reported that women preferred certain dance moves by men, especially exaggerated movements in the upper body. In other studies, Dr. Neave and his colleagues have found links between male dance attractiveness and risk-taking, as well as handgrip strength, a marker for overall body strength. “We know that dance moves are signaling strength and vigor in males,” Dr. Neave said. “Now we’re beginning to do the same research with females.” In the study, his team asked 39 female university students in Britain to dance alone to a drum beat. The researchers used a motion-capture system to track the women’s moves. They animated each dancer as an avatar to try to make sure that only the dance movements — and no other physical features — would affect ratings. Then they recruited 57 men and 143 women to watch 15-second clips of the avatars and rate them each on a numeric scale. © 2017 The New York Times Company
Having a thicker outer layer of the brain is linked to an increased likelihood of having autism. The cerebral cortex is the wrinkled outer layer of the brain that is responsible for many of our most human traits, including thought, language and consciousness. This layer is typically thicker in men than in women, and its structure has been linked to differences in personality. Now brain scans have shown that women who have a more male-like brain structure are three times more likely to have been diagnosed with autism. The study compared the brains of 98 men and women with high functioning autism with those of 98 people who don’t have autism. These findings provide new insights into the brain’s role in sex differences in autism, according to the team that did the study. Autism is thought to be two to five times more common in men than in women, and some think the condition is caused by having an “extreme male brain”. Journal reference: JAMA Psychiatry, DOI: 10.1001/jamapsychiatry.2016.3990 © Copyright Reed Business Information Ltd.
by Bethany Brookshire Gender bias works in subtle ways, even in the scientific process. The latest illustration of that: Scientists recommend women less often than men as reviewers for scientific papers, a new analysis shows. That seemingly minor oversight is yet another missed opportunity for women that might end up having an impact on hiring, promotions and more. Peer review is one of the bricks in the foundation supporting science. A researcher’s results don’t get published in a journal until they successfully pass through a gauntlet of scientific peers, who scrutinize the paper for faulty findings, gaps in logic or less-than-meticulous methods. The scientist submitting the paper gets to suggest names for those potential reviewers. Scientific journal editors may contact some of the recommended scientists, and then reach out to a few more. But peer review isn’t just about the paper (and scientist) being examined. Being the one doing the reviewing “has a number of really positive benefits,” says Brooks Hanson, an earth scientist and director of publications at the American Geophysical Union in Washington, D.C. “You read papers differently as a reviewer than you do as a reader or author. You look at issues differently. It’s a learning experience in how to write papers and how to present research.” Serving as a peer reviewer can also be a networking tool for scientific collaborations, as reviewers seek out authors whose work they admired. And of course, scientists put the journals they review for on their resumes when they apply for faculty positions, research grants and awards. |© Society for Science & the Public 2000 - 2017.
By Chelsea Whyte It was a gruesome scene. The body had severe wounds and was still bleeding despite having been lying for a few hours in the hot Senegalese savanna. The murder victim, a West African chimpanzee called Foudouko, had been beaten with rocks and sticks, stomped on and then cannibalised by his own community. This is one of just nine known cases where a group of chimpanzees has killed one of their own adult males, as opposed to killing a member of a neighbouring tribe. These intragroup killings are rare, but Michael Wilson at the University of Minnesota says they are a valuable insight into chimp behaviour such as male coalition building. “Why do these coalitions sometimes succeed, but not very often? It’s at the heart of this tension between conflict and cooperation, which is central to the lives of chimpanzees and even to our own,” he says. Chimps usually live in groups with more adult females than males, but in the group with the murder it was the other way round. “When you reverse that and have almost two males per every female — that really intensifies the competition for reproduction. That seems to be a key factor here,” says Wilson. Jill Pruetz at Iowa State University, who has been studying this group of chimpanzees in south-eastern Senegal since 2001, agrees. She suggests that human influence may have caused this skewed gender ratio that is likely to have been behind this attack. In Senegal, female chimpanzees are poached to provide infants for the pet trade. Fall from power Thirteen years ago, Foudouko reigned over one of the chimp clans at the Fongoli study site, part of the Fongoli Savanna Chimpanzee Project. As alpha male, he was “somewhat of a tyrant”, Pruetz says. © Copyright Reed Business Information Ltd.
Nicola Davis Girls as young as six years old believe that brilliance is a male trait, according research into gender stereotypes. The US-based study also found that, unlike boys, girls do not believe that achieving good grades in school is related to innate abilities. Andrei Cimpian, a co-author of the research from New York University, said that the work highlights how even young children can absorb and be influenced by gender stereotypes – such as the idea that brilliance or giftedness is more common in men. Are gendered toys harming childhood development? Read more “Because these ideas are present at such an early age, they have so much time to affect the educational trajectories of boys and girls,” he said. Writing in the journal Science, researchers from three US universities describe how they carried out a range of tests with 400 children, half of whom were girls, to probe the influence of gender stereotypes on children’s notions of intelligence and ability. In the first test, a group of 96 boys and girls of ages five, six and seven, were read a story about a highly intelligent person, and were asked to guess the person’s gender. They were then presented with a series of pictures showing pairs of adults, some same-sex, some opposite sex, and were asked to pick which they thought was highly intelligent. Finally, the children were asked to match certain objects and traits, such as “being smart”, to pictures of men and women. © 2017 Guardian News and Media Limited
By NATALIE ANGIER Whether personally or professionally, Daniel Kronauer of Rockefeller University is the sort of biologist who leaves no stone unturned. Passionate about ants and other insects since kindergarten, Dr. Kronauer says he still loves flipping over rocks “just to see what’s crawling around underneath.” In an amply windowed fourth-floor laboratory on the east side of Manhattan, he and his colleagues are assaying the biology, brain, genetics and behavior of a single species of ant in ambitious, uncompromising detail. The researchers have painstakingly hand-decorated thousands of clonal raider ants, Cerapachys biroi, with bright dots of pink, blue, red and lime-green paint, a color-coded system that allows computers to track the ants’ movements 24 hours a day — and makes them look like walking jelly beans. The scientists have manipulated the DNA of these ants, creating what Dr. Kronauer says are the world’s first transgenic ants. Among the surprising results is a line of Greta Garbo types that defy the standard ant preference for hypersociality and instead just want to be left alone. The researchers also have identified the molecular and neural cues that spur ants to act like nurses and feed the young, or to act like queens and breed more young, or to serve as brutal police officers, capturing upstart nestmates, spread-eagling them on the ground and reducing them to so many chitinous splinters. Dr. Kronauer, who was born and raised in Germany and just turned 40, is tall, sandy-haired, blue-eyed and married to a dentist. He is amiable and direct, and his lab’s ambitions are both lofty and pragmatic. “Our ultimate goal is to have a fundamental understanding of how a complex biological system works,” Dr. Kronauer said. “I use ants as a model to do this.” As he sees it, ants in a colony are like cells in a multicellular organism, or like neurons in the brain: their fates joined, their labor synchronized, the whole an emergent force to be reckoned with. © 2017 The New York Times Company
written by Claire Lehmann I learned about Debra through reading her LA Times op-ed on the futility of gender neutral parenting. I got in touch with Debra because I wanted to learn more about her field of sex neuroscience, her own research and her thoughts on studying sex differences in the brain. Because the study of sex and sex differences is often fraught with political roadblocks, I also wanted to get a picture of how a neuroscientist-sex researcher approaches some of these contentious issues. Hi Debra, thanks for chatting to Quillette. Can you briefly tell us who you are — where you studied, who was your supervisor and what made you interested in neuroscience, in particular sex neuroscience? I am a sex researcher at York University in Toronto and I write about the science of sex for several media outlets, including Playboy. For my PhD, which I just defended, I worked with Dr. Keith Schneider, who has pioneered new methods in high-resolution fMRI and is the Director of the University of Delaware’s Center for Biomedical and Brain Imaging, and Dr. James Cantor at the University of Toronto, who is a world expert in the brain imaging of pedophilia. I remember opening up a textbook during my first neuroscience course as an undergraduate student, seeing images from an fMRI study, and thinking it was incredible. I decided to pursue neuroscience in grad school and had the opportunity to do a placement in sexology as part of my Master’s degree. That’s how I got hooked! And I haven’t looked back. © 2017 Quillette
In the mood? Feeling sexy and romantic has been linked to a hormone named kisspeptin. Researchers hope the chemical may help treat people with some sexual problems. Kisspeptin occurs naturally in the body, where it stimulates the release of other signalling chemicals that have been linked to reproduction. Now a study of 29 heterosexual young men has found that injections of the hormone enhance the brain’s response to sexual and romantic pictures of couples. After injection, MRI scans showed increased activity in the regions of the brain that are usually stimulated by sexual arousal and romance. But this activity was only prompted by arousing pictures – non-sexy images did not have the same effect. “Our findings indicate that kisspeptin could play a role in stimulating some of the emotions and responses that lead to sex and reproduction,” says Waljit Dhillo, at Imperial College London. “Ultimately, we are keen to look into whether kisspeptin could be an effective treatment for psychosexual disorders.” The team now plans to study the effects of the hormone in a larger group of people, including women as well as men. Journal reference: Journal of Clinical Investigation © Copyright Reed Business Information Ltd.
By NANCY L. SEGAL and SATOSHI KANAZAWA In 1973, the biologist Robert Trivers and the computer scientist Dan Willard made a striking prediction about parents and their offspring. According to the principles of evolutionary theory, they argued, the male-to-female ratio of offspring should not be 50-50 (as chance would dictate), but rather should vary as a function of how good (or bad) the conditions are in which the parents find themselves. Are the parents’ resources plentiful — or scarce? The Trivers-Willard hypothesis holds that when their conditions are good, parents will have more male offspring: Males with more resources are likely to gain access to more females, thereby increasing the frequency with which their genes (and thus their parents’ genes) are preserved in future generations. Conversely, male offspring that lack resources are likely to lose out to males that have more resources, so in bad conditions it pays for parents to “invest” more in daughters, which will have more opportunities to mate. It follows, as a kind of corollary, that when parents have plentiful resources they will devote those resources more to their sons, whereas when resources are scarce, parents will devote them more to their daughters. In short: If things are good, you have more boys, and give them more stuff. If things are bad, you have more girls, and give more of your stuff to them. Is this hypothesis correct? In new research of ours, to be published in the April issue of The Journal of Experimental Child Psychology, we suggest that in the case of breast-feeding, at least, it appears to be. In recent years, evidence has emerged suggesting that in various mammalian species, breast milk — which is, of course, a resource that can be given to children — is tailored for the sex of each offspring. For example, macaque monkey mothers produce richer milk (with higher gross energy and fat content) for sons than for daughters, but also provide greater quantities of milk and higher concentrations of calcium for daughters than for sons. © 2017 The New York Times Company