Chapter 12. Sex: Evolutionary, Hormonal, and Neural Bases
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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
By BENJAMIN EWEN-CAMPEN This may seem obvious. But in evolutionary terms, the benefits of sexual reproduction are not immediately clear. Male rhinoceros beetles grow huge, unwieldy horns half the length of their body that they use to fight for females. Ribbon-tailed birds of paradise produce outlandish plumage to attract a mate. Darwin was bothered by such traits, since his theory of evolution couldn’t completely explain them (“The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me feel sick!” he wrote to a friend). Moreover, sex allows an unrelated, possibly inferior partner to insert half a genome into the next generation. So why is sex nearly universal across animals, plants and fungi? Shouldn’t natural selection favor animals that forgo draining displays and genetic roulette and simply clone themselves? Yes and no. Many animals do clone themselves; certain sea anemones can bud identical twins from the sides of their bodies. Aphids, bees and ants can reproduce asexually. Virgin births sometimes occur among hammerhead sharks, turkeys, boa constrictors and komodo dragons. But nearly all animals engage in sex at some point in their lives. Biologists say that the benefits of sex come from the genetic rearrangements that occur during meiosis, the special cell division that produces eggs and sperm. During meiosis, combinations of the parents’ genes are broken up and reconfigured into novel arrangements in the resulting sperm and egg cells, creating new gene combinations that might be advantageous. © 2013 Salon Media Group, Inc.
By Felicity Muth Humans love their victory displays. You only have to watch a game of football (or soccer to US-readers) to see some victory displays of the most ridiculous kind. Why do people do such things? If there was no crowd there, it is unlikely that they would perform such displays. But is it for the sake of the sex they are wishing to attract, or perhaps to put people they are competing with in no doubt of their accomplishment? Other animals, of course, also compete with each other, for food, resources and mates. And, like humans, how they behave once they win or lose a competition may depend on who’s around to see it. Male spring crickets fight with each other for territories and females Male spring field crickets fight with other males. The winners tend to do a lot better with the lady crickets, as the winners may gain the best territory, and because females of this species prefer dominant males. Now for the part that may surprise you: the males that win these fights will perform a victory display just like humans – after beating another male, the male winner performs an aggressive song and jerks his body in a particular way to show off that he’s won this fight. But, like with humans, the question arises: why do males do these victory displays? Is it to show the loser male that he has lost, or to show other males and females that he’s won? © 2013 Scientific American
By Jessica Shugart Sometimes it pays to be mediocre. A new study shows that sheep with a 50/50 blend of genes for small and big horns pass along more of their genes over a lifetime than their purely big-horned brethren, who mate more often. The finding offers rare insight into an enduring evolutionary paradox—why some traits persist despite creating a reproductive disadvantage. The results, published online August 21 in Nature, reveal that while big-horned sheep mated most successfully each season, small-horned sheep survived longer. Rams who inherited one of each type of gene from their parents got the best of both worlds: they lived longer than bigger-horned sheep and mated more successfully than those with the smallest horns. As a result, middle-of-the-road sheep passed on more of their genes over time. “They’re the fittest of them all,” says Jon Slate of the University of Sheffield in Scotland, who led the study. “This is a marvelous combination of using the most modern tools available to confirm classic older views of sexual selection,” says evolutionary geneticist Allen Moore of the University of Georgia in Athens, who was not involved in the study. Traits such as bold peacock feathers and giant antlers evolved to garner the attention of prospective females and boost reproductive success. Yet if each generation of females continues to pick the most stellar males, Charles Darwin wondered, how do sub-par versions of a trait continue to persist? “It’s something that has preoccupied evolutionary biologists ever since,” Slate says. © Society for Science & the Public 2000 - 2013
By Patrick Cooney The Disney film Finding Nemo lied to your kids! The people at Disney would simply argue that they altered reality to create a more entertaining storyline, but read below for the true story, and you tell me which you think is more entertaining. How Finding Nemo started: Father and mother clownfish are tending to their clutch of eggs at their sea anemone when the mother is eaten by a barracuda. Nemo is the only surviving egg, and he grows up in his father’s anemone before getting lost on a crazy adventure! How Finding Nemo should have started if it were biologically accurate: Father and mother clownfish are tending to their clutch of eggs at their sea anemone when the mother is eaten by a barracuda. Nemo hatches as an undifferentiated hermaphrodite (as all clownfish are born) while his father transforms into a female clownfish now that his female mate is dead. Since Nemo is the only other clownfish around, he becomes male and mates with his father (who is now female). Should his father die, Nemo would change into a female clownfish and mate with another male. Although a much different storyline, it still sounds like a crazy adventure! As you can see, the first minute of Finding Nemo, outside of the talking fish part, is the only biologically accurate part of the movie. Considering that Disney demonstrated reproduction and the killing of the mother in the first minute of the movie, how did it decide that a natural sex change is outside the bubble of viewable material? © 2013 The Slate Group, LLC
Keyword: Sexual Behavior
Link ID: 18549 - Posted: 08.22.2013
by Ed Yong In the image above, all the eggs in the top row are laid by cuckoos and those in the bottom row belong to their victims. These uncanny similarities help cuckoos to fob off their parental duties by laying their eggs in the nests of other species. If the hosts can’t tell the difference between their eggs and the foreign ones, they’ll end up raising the cuckoo chick as their own. And they pay a hefty price for their gullibility, since cuckoo chicks often kill or outcompete their foster siblings. The relationship between cuckoos and their hosts is a classic example of an evolutionary arms race. Cuckoos, should evolve eggs that more closely match those of their hosts, while the hosts should evolve keener senses to discriminate between their own eggs and a cuckoo’s. The greater honeyguide isn’t a cuckoo but uses the same tactics—it parasitises the nests of little bee-eaters by laying eggs of the same size and shape. But this mimicry doesn’t help it to fool the bee-eaters, which seem to accept any old egg no matter how different it looks. Instead, Claire Spottiswoode from the University of Cambridge has found that the parasitic honeyguides are fighting an evolutionary arms race against… each other. Bee-eaters build their nests underground, usually within abandoned aardvark burrows. When honeyguides invade, they’ll puncture the bee-eater’s eggs before laying their own. This kills some of the eggs outright and weakens others. If any chicks survive to hatching, they’re finished off by the honeyguide chick, which stabs its foster siblings to death with a vicious hooked bill.
Louis Herman The mournful, curiously repetitious yet ever-changing songs of male humpback whales have long puzzled scientists. The tunes are part of the males’ mating displays, but researchers don’t know their exact function, or which males in a population are doing the singing. Now, scientists who’ve been studying the giant marine mammals in Hawaii for almost 40 years report that even sexually immature males join older males in singing, apparently as a way to learn the music and to amplify the song. The beefed-up, all-male choruses may attract more females to the areas where the songsters hang out. Scientists generally thought that only adult male humpbacks (Megaptera novaeangliae) sing, says Louis Herman, a marine mammal biologist emeritus at the University of Hawaii, Manoa, and the lead author of the new study. “But that’s just because you can’t easily tell which ones are mature and which ones are immature,” he says. “We know that mature males are larger than immature ones, so we had to figure out an unobtrusive way to measure them in the open ocean.” Herman and his team hit on a technique by looking at 20th century whaling records. Biologists with whaling operations in the Southern Ocean had the opportunity to measure many humpbacks killed during the commercial hunts. They determined, based on the weight of males’ testes, that the whales reached sexual maturity at a body length of 11.2 meters. Working independently, whaling biologists in Japan, who also measured killed whales, reached a similar conclusion; they described 11.3 meters as the break point between adolescents and adults. © 2012 American Association for the Advancement of Science
Karen Ravn The authors of a new study write that this plant bug, Coridromius tahitiensis, “lacks precopulatory courtship, and males instead pounce on nearby females, with whom they struggle violently in their attempt to mate.” If you lived on an exotic island where unsafe sex was all too common, you'd find ways to ward off unwanted attention. On Tahiti, the females of two related insect species have had to move their genitals to different sides of their bodies and even impersonate the opposite sex — all to avoid getting pierced in the abdomen by the sexual organs of the wrong males, biologists report. The two insects, which live side by side on the Pacific island of Tahiti and feed on the same plants, are known as Coridromius tahitiensis and Coridromius taravao. Both species follow the aptly named practice of traumatic insemination. With his genital organ reminiscent of a hypodermic needle, the male stabs a female in the side and shoots sperm into her abdomen. The ritual — shared by a number of other invertebrates, including bedbugs (Cimex lectularius) — can cause injury or infection for the female. Not only that, but insects that use this type of reproductive method are not particularly persnickety about partners, so a male of one species may try to mate with another male — or even with a member of another species. Such interspecies mating can be costly to both species in terms of wasted time, energy and sperm, says Nikolai Tatarnic, a biologist who is now curator of insects at the Western Australian Museum in Welshpool. © 2013 Nature Publishing Group,
by Laura Poppick, LiveScience Researchers have widely examined homosexual behavior in mammals and birds, but have addressed it less frequently in insects and spiders. To assess the range of evolutionary explanations for same-sex intercourse in the invertebrate world, a team of biologists from Tel Aviv University in Israel and the Swiss Federal Institute of Technology in Zurich, Switzerland examined roughly 100 existing studies on the topic and compiled the first comprehensive review of homosexuality in invertebrates. The review was published earlier this month in the journal Behavioral Ecology and Sociobiology. The team focused on male-male interactions to simplify the analysis, and found that most of these encounters occurred as accidents. Whereas larger animals have developed more complicated homosexual motivations — like maintaining alliances, which has been found in certain primate and seagull species — insects seem to mistakenly partake in it in a hasty attempt to secure mates. [Gay Animals: Alternate Lifestyles in the Wild] "They have evolved to mate quick and dirty," said study co-author Inon Scharf, an evolutionary ecologist at Tel Aviv University. "They grab every opportunity to mate that they have because, if they become slow, they may give up an opportunity to mate." In some cases, males carry around the scent of females they have just mated with, sending confusing signals to other perusing males. In other cases, males and females look so similar to one another that males cannot tell if a potential mate is a female until he mounts "her" and prepares for the act, Scharf said. © 2013 Discovery Communications, LLC.
by Simon Makin Autism may shape the brains of women differently to those of men. The condition seems to cause female, but not male, brains to look more masculine, suggesting that one controversial view of autism – as an extreme version of the male brain – may need rethinking. Simon Baron-Cohen at the Autism Research Centre in Cambridge, UK, has previously found that men tend to be better at systematising tasks and females better at ones involving empathising. As people with autism tend to be good systematisers and below-average empathisers, he has argued that autism may be an extreme version of the male brain, or EMB. However, the theory is contentious. "The jury is still out," says autism researcher Uta Frith of University College London. That's partly because of the difficulty in pinning down the source of the gender differences. "It's far from clear which male-female differences are biological and which are cultural," says developmental psychologist Caspar Addyman, of the Birkbeck Babylab at the University of London. In their latest study, Baron-Cohen's team used MRI scans to look for differences in the volume of brain regions in 120 adults, split into four equal groups – men and women, with and without autism. The researchers first compared the brains of males with and without autism, then did the same for female brains. They then compared these two differences. "If autism manifests the same in both genders, these two differences should be alike," says Baron-Cohen's colleague at the Autism Research Centre Meng-Chuan Lai, "but if not, they should be different – and this is what we found." © Copyright Reed Business Information Ltd.
Roger Dobson Older male nightingales have perfected an art that would be the envy of men having a mid-life crisis: a trick that makes them more attractive to females than their younger male competitors. Their mastery of successful courtship is achieved with a dazzling array of up to 100 trills a second, far more than their younger competitors can manage, and more than any other investigated bird, according to new research. That ability, backed up by a sophisticated playlist of about 200 songs, means that they are probably seen as better mates by young trill-seeking females. Singing so many trills at peak frequency requires a lot of physical effort and, as a result, it has evolved as a sign on fitness, say the researchers. "Females could assess the age of the male singer by the trill rate, and mate preferably with older ones," says the zoologist Dr Valentin Amrhein, who led the study at the University of Basel, Switzerland. "This makes sense for the females because older males have more experience with defending their territory or with raising young, and therefore have a better reproductive performance." The research, being published in the Journal of Avian Biology, shows that older birds can come up with 100 trills a second, making them the fastest singers. They also performed about 200 different song types, but the researchers think it is the immediate impact of the trills that is attracting the females. It would take more than an hour for the male to go through his whole song list. "Since the performance of these sounds is very demanding, the rate at which they can be repeated is limited. Trying to sing rapidly increasing sounds in fast repetition is very hard for us humans as well," says Dr Amrhein. "Singing rapid broadband trills comes at a certain price for the male nightingale, so trilling is a good indicator for mate quality." © independent.co.uk
Autism affects male and female brains differently, a study has suggested. UK experts studied brain scans of 120 men and women, with half of those studied having autism. The differences found in the research, published in journal Brain, show more work is needed to understand how autism affects girls, the scientists say. Experts said girls with the condition could be more stigmatised than boys - and it could be harder for them to be diagnosed at all. Autism affects 1% of the population and is more prevalent in boys, so most research has focused on them. In this study, scientists from the Autism Research Centre at the University of Cambridge used magnetic resonance imaging (MRI) to examine how autism affects the brain of males and females. The study looked at the difference between the brains of typical males and those with autism - and then females with and without autism. They found the brains of females with autism "look" more like - but still not the same as - healthy males, when compared with healthy females. But the same kind of difference was not seen in males with autism - so their brains did not show "extreme" male characteristics. Dr Meng-Chuan Lai, who worked on the study said: "What we have known about autism to date is mainly male-biased. BBC © 2013
// by Jennifer Viegas Single parenting takes on new extremes for certain starfish that are hermaphrodites -- male and female at the same time and, in some cases, self-fertilizing. The species faces high risk of extinction, according to new research. The dire situation faced by the non-mating starfish, Parvulastra parvivipara and Parvulastra vivipara, helps to explain why so many organisms, including humans, have sex. Genetic diversity and the dispersal of youngsters support population growth. The plight of the starfish, documented in the latest issue of the journal Biology Letters, reveals how a life without sex but with self-fertilization could result in eventual oblivion. “There are quite a few reasons why these species are vulnerable,” senior author Michael Hart of Simon Fraser University’s Department of Biological Sciences told Discovery News. “The whole species could be wiped out.” Hart and his team studied the starfish, which are restricted to high intertidal pools of South Australia and Tasmania. These starfish also go by the nickname "sea cushions," since they look a bit more like a cushion than a star when viewed from the side. Most adult starfish of other species do reproduce via a separate male and female. Females usually produce eggs that males fertilize in the seawater. At that point, the fertilized eggs develop and grow before becoming little starfish that will attach themselves to the substrate and start the whole process over again. © 2013 Discovery Communications, LLC.
A gene that affects the brain's dopamine system appears to have influenced mothers' behavior during a recent economic downturn, researchers say. At the beginning of the recession that began in 2007, mothers with the "sensitive" version of a gene called DRD2 became more likely to strike or scream at their children, the researchers say. Mothers with the other "insensitive" version of the gene didn't change their behavior. But once it appeared that the recession would not become a full-fledged depression, the "sensitive" mothers became less likely than "insensitive" mothers to engage in harsh parenting. "You have the same genes, and with a different environment it's a completely different story," says , a professor of contemporary urban problems at Columbia University. "I think that's the most amazing part of what we found." Garfinkel and four other researchers in the Proceedings of the National Academy of Sciences. The surprising finding came about because Garfinkel and the other researchers happened to be studying "fragile" families in 20 large cities when the 2007 recession began. One of the things they were tracking was reports of harsh parenting, including spanking, hitting or screaming at a child, he says. Previous research had found that harsh parenting is more common during economic hard times, so Garfinkel says that's what researchers expected to see during the 2007-2009 period, often called the Great Recession. ©2013 NPR
By Luciana Gravotta Small gains now or big rewards later? The conundrum plagues every decision we make, whether we are investing or dieting. Now researchers find that men and women use different strategies to make such choices. Researchers use gambling games to understand what we do when immediate rewards are pitted against long-term gains. Most of these games find no major differences in how men and women play. An experimental setup called the Iowa Gambling Task (IGT), however, finds consistent—and large—differences between the behavior of men and women: men are better at figuring out the strategy that reaps the bigger payoff. Players are given four decks of cards, and they choose one card at a time from any deck they want. Each card has a win or loss amount on it, and each deck has its own unique payout pattern. Two of the decks contain cards that dole out large or frequent rewards, but consistently choosing cards from these decks leads to losses in the long run. The other two decks provide a modest amount of cash per win but less loss over time, so they offer long-term gains for players who pick from them most frequently. These patterns are carefully obscured so that the winning strategy is not obvious. A review published in February in Behavioural Brain Research finds that men focus on the big picture, watching their total earnings and quickly homing in on which of the decks will lead to gains in the long run. Women focus on details such as the frequencies of wins and losses for each deck, missing the overall impact each deck has on their total balance. Sensitive to losses, women tend to switch to a different deck as soon as they are pinged with a setback, making it more difficult for them to identify the prize deck. © 2013 Scientific American
By CARL ZIMMER “Monogamy is a problem,” said Dieter Lukas of the University of Cambridge in a telephone news conference this week. As Dr. Lukas explained to reporters, he and other biologists consider monogamy an evolutionary puzzle. In 9 percent of all mammal species, males and females will share a common territory for more than one breeding season, and in some cases bond for life. This is a problem — a scientific one — because male mammals could theoretically have more offspring by giving up on monogamy and mating with lots of females. In a new study, Dr. Lukas and his colleague Tim Clutton-Brock suggest that monogamy evolves when females spread out, making it hard for a male to travel around and fend off competing males. On the same day, Kit Opie of University College London and his colleagues published a similar study on primates, which are especially monogamous — males and females bond in over a quarter of primate species. The London scientists came to a different conclusion: that the threat of infanticide leads males to stick with only one female, protecting her from other males. Even with the scientific problem far from resolved, research like this inevitably turns us into narcissists. It’s all well and good to understand why the gray-handed night monkey became monogamous. But we want to know: What does this say about men and women? As with all things concerning the human heart, it’s complicated. © 2013 The New York Times Company
Elizabeth Pollitzer Transplanting muscle-derived stem cells into diseased muscle regenerates it — a phenomenon that holds major potential for human therapies. But for years, researchers were puzzled by the unpredictability of these cells — sometimes they would promote fast regeneration, at other times none at all. Then, in 2007, a group led by Johnny Huard, a stem-cell researcher at the University of Pittsburgh in Pennsylvania, hit on the rather surprising explanation — sex1. Muscle stem cells taken from female mice regenerate new muscle much faster than those from male mice when transplanted into diseased muscle of mice of either sex. Researchers have also found that cells taken from male and female mice respond differently to stress2, and that human cells exhibit wildly different concentrations of many metabolites across the sexes3. Evidence is mounting that cells differ according to sex, irrespective of their history of exposure to sex hormones. These differences could have major implications for the susceptibility to and course of many diseases, their diagnosis and treatment. However, most cell biologists do not note whether the cells they are using come from males or females4. Between 1997 and 2001, ten prescription drugs were withdrawn from the market by the US Food and Drug Administration (FDA), eight of which were more dangerous to women than to men (see go.nature.com/ksindo). The ingredients used in non-prescription drugs can also pose greater health risks to women. In 2000, for instance, the FDA took steps to remove phenylpropanolamine, a component of many over-the-counter medications, from all drug products because of a reported increased risk of bleeding into the brain or into tissue around the brain in women but not in men. Such drug therapies are developed through basic research — but what if sex-related differences in studied cells contribute in a significant way to the observed effects? © 2013 Nature Publishing Group
By Melissa Hogenboom Science reporter, BBC News Male zebra finches that fail to socialise with females during adolescence are less successful at courtship later in life, a study says. This effect mimics the "loser effect" where, after a defeat, an animal is more likely to lose a subsequent fight. Social friendships at a young age were also found to be more important than physical and social attractiveness. The findings are reported in Proceedings of the Royal Society B: Biological Sciences. The loser effect has been well demonstrated in many species, from spiders and fish to birds. After a fight hormonal levels change which negatively affects performance in further fights. Now scientists have found a similar effect for mating. Adolescent males who failed to pair with a juvenile female were later unsuccessful with females they encountered in adulthood. Scientists also paired young males as a control in the experiment. They found that if males failed to pair with another male, it had no effect on their later success. Mylene Mariette, from the University de Saint-Etienne, France, and lead author of the study said: "We know that social interaction is important for some aspects of development, like the role of males to teach youngsters to sing, but so far no study has looked at the effect of how interaction between juveniles affects their behaviour as adults." BBC © 2013
By CARL ZIMMER The golden lion tamarin, a one-pound primate that lives in Brazil, is a stunningly monogamous creature. A male will typically pair with a female and they will stay close for the rest of their lives, mating only with each other and then working together to care for their young. To biologists, this deeply monogamous way of life — found in 9 percent of mammal species — is puzzling. A seemingly better evolutionary strategy for male mammals would be to spend their time looking for other females with which to mate. “Monogamy is a problem,” said Dieter Lukas of the University of Cambridge in a telephone news conference on Monday. “Why should the male keep to one female?” The evolution of monogamy has inspired many different ideas. “These hypotheses have been suggested for the past 40 years, and there’s been no resolution of the debate,” said Kit Opie of the University College London in an interview. On Monday, Dr. Opie and Dr. Lukas each published a large-scale study of monogamy that they hoped would resolve the debate. But they ended up coming to opposing conclusions, which means the debate over monogamy continues. Dr. Lukas, co-author of a paper in the journal Science with Tim Clutton-Brock of Cambridge, looked at 2,545 species of mammals, tracing their mating evolution from their common ancestor some 170 million years ago. The scientists found that mammals shifted from solitary living to monogamy 61 times over their evolution. They then searched for any factors that these mammals had in common. They concluded that monogamy evolves when females become hostile with one another and live in ranges that do not overlap. When females live this way, they set up so much distance between one another that a single male cannot prevent other males from mating with them. Staying close to one female became a better strategy. Once males began doing so, they sometimes evolved to provide care to their offspring as well. © 2013 The New York Times Company