Male and Female Probablities Effected by Partner Age New Scientist 97
HERE is some advice for women who want sons: cultivate a taste for older, well-heeled men. But if you'd rather have a daughter, find yourself a toy boy. John Manning and his colleagues at the University of Liverpool have found that women whose partners are 5 to 15 years their senior are nearly twice as likely to have a boy as their first child as they are a girl. For women with partners a year or more younger than themselves, the ratio is reversed. The researchers studied 301 couples from Liverpool, and report their findings in this week's Nature (vol 389, p 344). Biases in sex ratio have emerged in studies of populations as disparate as the Havasupai native Americans of Arizona, and the families of clergymen and test pilots. Attempts to explain the biases are "problematic", says Manning. However, he doubts that the apparent relationship between sex ratio and parental age differences is due to sampling errors, because it ties in with another observation: during and immediately after the two World Wars, the proportion of boys born in England and Wales increased. Marriage records from the period show a corresponding trend for women to marry older men. But Manning believes that age is a secondary factor to social status. He says middle-aged men who attract young women are likely to owe their allure to money and status. If these men really do sire more boys than girls, it supports an evolutionary theory which says that it pays well-fed, high-status animals to have male offspring and poor, starving animals to have daughters. Other evidence backs Manning's theory. American presidents have had 50 per cent more sons than daughters ("Why presidents have more sons", New Scientist, 3 December 1994, p 28). And a male-biased sex ratio has been found among the children of men listed in the Who's Who guides of Britain, Germany and the US. David Concar
Sex Genes and Gender Wars New Scientist 97 (extract from a larger article)
While fruit flies contain genes which effect mating to the detriment of the female by the male evolving to include seminal toxins, some even related to poisonous spiders, which relax the ovipostor to encourage egg laying and which in large doses may have a paralytic effect detrimental to the survival of the female, the evidence in vertebrates is less clear.
Human brains contain roughly 30,000 active genes-far more than in the fruit fly-making it even more difficult to find any major-effect behaviour gene. "This is very tough to do in flies, and I can just imagine how hard it will be in humans," says Sokolowski. Her group discovered the strong behavioural role of the fruit fly foraging gene only after 20 years of research, having investigated seven other behaviours without finding a single major-effect gene. But there are others who think they are closing in on major-effect genes for human behaviour. David Skuse, a child psychiatrist at the Institute of Child Health in London, reported in Nature this summer (vol 387, p 705) that he has evidence for the existence on the X chromosome of a single gene, or possibly a small cluster of genes, that plays a major role in normal social skills, such as awareness of other people's feelings and the ability to chat and make friends. Skuse got his first hint that the gene existed from girls with Turner's syndrome, who have onlv one X chromosome. Girls who inherit their mother's X chromosome lack social skills, whereas girls who inherit their father's have normal skills. Next, Skuse showed that normal girls without the syndrome, all of whom have a copy of their father's X chromosome, generally have rather better social skills than boys, who only get an X from their mother. Unlike disease linkage studies, Skuse's investigations show that the completely routine absence or presence of an active gene, caused by a mechanism called imprinting, underpins two very different-but entirely normal-personality types. Skuse speculates that the gene or genes may alter the wiring or levels of chemicals in the brain's prefrontal cortex, where tasks involving memory, reasoning and judgment are mediated. "I suggest that girls are genetically preprogrammed to learn almost by instinct to interpret social cues," he says. "Boys on the other hand do not have this advantage and have to work harder to get to the same point."
This has yet to be proven. But even if human geneticists never manage to pin down a major-effect behaviour gene, it may one day be possible to predict a fair bit about someone's personality from clusters of "small-effect" genes, which are becoming easier and easier to find. Geneticists have already spotted genes that account for tiny variations in normal human behaviour. For instance, Hamer has discovered that variations in a gene for a protein that affects the amount of serotonin in the brain accounts for just 4 per cent of the difference in the amount of anxiety normal people experience. "The technology is getting more powerful," he says. "Resolution will continue to increase to the point that we will detect genes of such small effect that they will no longer be interesting." There are several scientific developments spurring this progress. First, there is the sheer number of new gene sequences being churned out by the Human Genome Project. Combine that with an increasingly sophisticated understanding of what genes with a particular sequence may or may not do, and researchers have plenty of candidate genes on which to focus. Then there are "personality DNA databases". All around the world, behavioural geneticists are asking volunteers to submit blood or cheek swabs for DNA testing, and having them fill out psychological questionnaires that gauge personality. Finally, there are improved computer programs and statistical techniques that allow geneticists to sift through vast amounts of data, picking out the subtle interactions between genes, the environment and personality traits. With these tools, geneticists are able to rely more on "association studies" to track the relationship between candidate genes and a particular trait across whole populations. This is a far more powerful way of pinning down behaviour genes than any that have gone before. Plomin suggests that researchers may one day be able to announce that, say, 10 or 20 genes account for half of the normal variation seen in a particular human behaviour. "But that," he says, "is a long way down the line." In the meantime, animal studies continue to give researchers new insights into the murky relationship between genes and behaviour. Insel, for his part, wants to prove conclusively that the oxytocin and vasopressin receptor genes can explain the differences in bonding behaviour in prairie and montane voles. He plans to create transgenic voles in which the genes for the receptors have been switched between the two species. If he can make a montane vole faithful and family-oriented, and a prairie vole promiscuous, he will surely have revealed the awesome power of some genes over behaviour.
Karen Schmidt is a freelance writer based in Greenville, North Carolina
Further reading: "Natural behavior polymorphism due to a cGMP-dependent protein kinase of Drosophila" by K. A. Osborne and others, Science, vol 277, p 834 (1997) "A neurobiological basis of social attachment," by T R. Insel, American Joumal of Psychiatry, vol 154, p 726 (1997)
Why Some Males Choose Monogamy New Scientist 1 Nov 97
IN LIFE, as in the soaps, monogamy is a calculated safe option for males, while females stick with the same mate because they can't get along with other females. So say biologists who have overturned ideas about why some mammals are faithful to their mates.
Monogamy occurs in only about 5 per cent of mammal species. There have been two main explanations for this lifestyle. Monogamy may evolve when the offspring cannot easily be raised by the mother alone, or when females are solitary and too widely dispersed for a male to monopolise more than one.
To test these ideas, Petr Komers of the Calgary based consultancy AGRA Earth and Environmental and Peter Brotherton of the University of Cambridge compiled information on the evolutionary histories of 174 species of mammal, 63 of them monogamous. Surprisingly, the researchers found that monogamy usually evolved in the absence of paternal care. They also showed that the female dispersion argument didn't stand up; females from monogamous species had smaller territories than promiscuous ones. Monogamy most commonly evolved where solitary females occupied small, mutually exclusive ranges (Proceedings of the Royal Society B, vol 264, p 126 1). But why would a male stick with one female, when there is another close by.? "Males don't necessarily know where other females are," Komers speculates. "And the predation risk in seeking another mate can be high." Komers admits that some puzzling species did not fit the pattern - aardwolves, for instance. "They form a monogamous pair, but the female has a huge range while the male sits at home," says Komers. "During her travels she'll mate with other males, so the offspring her mate is tending may not even be his.