Boy vs. Girl : Some Animals Naturally Skew the Sex of Offspring, Succeeding Where Humans Fail

Lemming mothers prefer daughters, while South American coypus prefer sons, and if you ever asked a turtle whether she was expecting pink or blue for her little reptiles, she’d probably say that depends on whether the eggs hatch in sunlight or shade.

Scientists charting a path through the kingdom of creatures from insects to mammals are finding a variety with an unusual ability to skew the odds in favor of producing one sex over the other.

While humans have tried abortions, infanticide and a new chromosome-sorting technique developed by scientists in Tokyo to achieve the same end, studies of lower orders raise the question of whether there exists an innate drive to favor sons over daughters or daughters over sons.

Some creatures actually appear to have preferences. Others are genetically equipped to ambush targeted chromosomes during fertilization, causing certain genetic material to instantly wither and disappear, ensuring that all offspring will be of one sex.

Such preselective phenomena in some organisms may sometimes appear as warfare between the sexes, with females of some species tilting odds in favor of their own sex, but scientists are finding the mechanisms are more subtle than that.

Often, this skewing of ratios in favor of one sex over the other is hidden in the quiet world of the genes, unknown to the creatures who carry them. Such molecules can, in rare cases, automatically alter the sex of the creatures’ offspring before they ever see the light of day.

Consider the Wasp

Nowhere in nature is this phenomenon more evident than in Nasonia vitripennis, the species of wasp in which certain males are endowed with what scientists call the most selfish DNA known.

When eggs of these wasps are fertilized with sperm containing a piece of DNA governing paternal sex ratio, all the eggs originally destined to be females are instantly transformed into males.

The sex ratio DNA tilts the number of offspring in favor of males, wresting control from female wasps. The females always have say over when and how many eggs are fertilized, which usually ensures that the majority of offspring are females.

Female wasps, like bees, store sperm from males and fertilize eggs at will, always biasing ratios of their populations toward their own sex by leaving only a small number of eggs unfertilized. Unfertilized eggs, lacking the extra chromosome, automatically hatch as males.

But when the paternal sex ratio is involved, the entire population hatches as single chromosome males because the ratio material causes one chromosome to wither and disappear.

Different Viewpoints

The wasps offer provocative evidence that some creatures naturally favor their own sex and that this conclusion could hold true for all--but many scientists disagree.

“I don’t believe that there’s a basic drive (in nature) to prefer one sex over another,” said Sarah B. Hrdy, a UC Davis anthropologist.

But she emphasized that, as in most phenomena in nature, pragmatic reasons underlie apparent sex pre-selection in virtually all organisms capable of doing it, reasons related to transmission of certain genetic traits that ensure survival of the species.

“Only humans impose value systems on matters of birth,” said the researcher, who developed an interest in sex and survival among animals as an offshoot of her primary focus on the preponderance of patriarchal societies in human cultures.

With humans, she said, “there is a preference for sons expressed around the world. Soon after a new wife conceives (in parts of India), mantras are chanted so that if by some mischance the fetus is a female she can be magically transformed into a son.”

Male infants are not inherently stronger than females, nor are their numbers fewer, a factor that might make them more prized. Worldwide, she said, the two sexes are born in approximately the same numbers, but concerns with lineage, property and money have influenced humans to prefer sons over daughters in many cultures.

Citing a recent University of Colorado study, Hrdy said researchers have determined “that sex-based infanticide has characterized some 9% of the world’s cultures, and more often than not, the unwanted sex has been female.”

Geneticist Uzi Nur of the University of Rochester in New York said nature operates on rules governed by natural selection.

That concept, he said, promotes temperature, environment, quality of food and availability of water as the primary factors determining which species will survive and how they will manage that survival.

So organisms that better adapt to their environment, those possessing more favorable variations--even if that means greater numbers of one sex over the other--are most likely to continue.

The concept of natural selection is the centerpiece in Darwin’s theory of evolution and may help explain why a preponderance of male offspring are born to Scotland’s red deer and a South American rodent called the coypu.

Large, dominant females of these species give birth only to large males.

It is generally believed that because these males are born to strong mothers, they inherit traits of strength and large size and will sire a larger than average number of offspring, Hrdy said.

“The lower-ranking females usually produce weak males that rarely breed,” she said.

This phenomenon also has been documented in opossums and the spider monkey, a lower primate, but is not found in such higher primates as gorillas or chimpanzees, Hrdy said.

But recent studies of the coypu conducted by zoologist Morris Gosling of the Ministry of Agriculture in Britain found that the small, furry creatures tend to abort litters of predominantly female offspring. This was particularly true in cases in which the mother was large and healthy.

Not So With Lemmings

In lemmings, by comparison, the skew is always toward daughters “with three to four times as many females as males born in the population, some individual mothers producing no sons at all,” Hrdy said.

C. J. McCoy, a herpetologist at the Carnegie Museum in Pittsburgh, said scientists just learned a decade ago of “temperature-dependent sex determination” in lizards and turtles.

“We know that nests in warmer and cooler sites yield different sex ratios of the hatchlings,” said McCoy, who directs the museum’s section on reptiles and amphibians.

In turtles, he said, eggs laid in sunny spots produce females and those laid in shade produce males in virtually all 200 species of the reptile, which is native to much of the temperate and tropical part of the world.

“Some turtles will lay all of their eggs in the sun or all in the shade,” McCoy said. “Individuals don’t try to even out the ratios.

“But I don’t think female turtles are that intelligent about where they put their nests. They’re not trying to produce one sex over the other.”

Opposite for Lizards

He noted that the situation is reversed for lizards, a species in which males are produced from nests in sunny locations, females in cooler positions.

McCoy believes that any skewing of male-female ratios in the reptile world is purely unintentional, much as similar findings in the plant kingdom show.

Orchids grown in full sunlight, botanists have found, usually produce female flowers, while male flowers grow in shade.

Geneticist Nur says that even DNA situations like that of the N. vitripennis wasp, which ensures that all offspring fertilized with it will be male, seems geared more toward its own survival as a genetic entity than switching ratios in favor of male wasps.

“It’s like a virus. (The DNA material) is selfish and parasitic,” he said.

While the DNA, which is transmitted with sperm, causes one chromosome to condense and disappear, it ensures greater percentages of males in successive generations because all males inheriting that DNA transmit the male-producing trait.

This pushes up ratios of males in each new population of eggs fertilized with sperm that carry the DNA element.

Working With Chromosomes

Scientists at the University of Tokyo recently developed a chromosome-sifting technique for people. It can separate sperm-bearing X chromosomes from those chromosomes carrying a Y.

Women’s egg cells carry only X-bearing chromosomes. An X chromosome from both parents results in a girl, an X and Y, a boy.

The Japanese technique allows a skewing of the sex odds by having the mother inseminated only with chromosomes of the parents’ choice.

But Hrdy said such techniques are expensive and beyond the means of most people, adding that the Japanese procedure borrows on the same idea behind Gender Choice Child Selection Kits, marketed in the United States for a brief period last year.

For $49.95, couples, in the comfort of their homes, could attempt to weight the odds in favor of sons or daughters by determining the best moment to conceive a specific sex.

The kits included directions, thermometers and other materials required to assess the best time for conceiving one sex over the other. But the federal Food and Drug Administration pulled the kits off the market.

“The implied claims of the packages, some of which were pink, some blue, had not been substantiated,” Hrdy said.

Continuing Efforts

Efforts to skew sexual ratios after birth continue to be reported. In ancient times, infant girls in some cultures were left in the wilderness to starve. Recently, statistics were published in the British science journal Nature from a social worker in Bombay, India, involving 8,000 abortions at clinics throughout the country.

The abortions were all selective procedures after amniocentesis that revealed the sex of the child. Of the abortions performed, 7,997 involved female fetuses, according to the journal.

But despite the dismal outlook for female fetuses in India and elsewhere in the world where similar practices occur, “nowhere is sex-selective abortion sanctioned. In fact, Maharashtra has just become the the first state in India to ban prenatal sex determination precisely for this reason,” Hrdy said.

Apart from abortion and infanticide, socioeconomic status has been a factor in some cultures of the world governing ratios of men and women available for marriage, consequently affecting the number of births, she said.

In medieval Portugal, Hrdy said, sons in the highest-ranking families were more likely to marry than daughters. She said this was because as many as 40% of the elite women were in convents and as a result were unable to marry and have children.