"....Among green-veined white butterflies, for example, a virgin male ejaculates a sperm packet roughly 15 percent of his weight that also contains nutritious substances. Females that have sex with several virgins lay more and bigger eggs than those that do it with only one or with males that have lost their virginity and consequently make sperm packets only half the size of their virgin glory...."
This is not your father's birds and bees
Recent research challenges notion of female monogamy
Carol Cruzan Morton, Special to The Chronicle Monday, February 17, 2003
Bar the doors and break out the chastity belts, boys, because girls of most species sleep around, and it's for their own good, if not yours.
For generations, biologists had assumed females to be naturally chaste, while males were renowned for their promiscuity. Even Charles Darwin, who invented the idea of sexual selection, didn't dare challenge the Victorian morals of his day. Man evolved from ape, fine. But an immodest and lustful Mother Nature? Heaven forbid!
Now, hundreds of studies and a spate of books are challenging that conventional wisdom. Females of many species, it turns out, have evolved strategies for passing on their genes that involve copulating with multiple males -- and recognition of that fact is literally changing our view of the birds and the bees.
"Natural selection, it seems, often smiles on strumpets," says evolutionary biologist Olivia Judson, author of "Dr. Tatiana's Sex Advice to All Creation," the most recent and entertaining book exploring the variety of female harlotry.
"As a rule, loose females have more and healthier children."
To be sure, biologists are examining these questions in the dispassionate light of scientific inquiry. In describing their theories, they prefer the more neutral term "polyandry," meaning many males, instead of "promiscuity." And they caution laypeople not to look to nature's own apparent infidelities for any justification of their own behavior.
The misbegotten idea that males evolved to make love and females to demur gained scientific currency in the late 1940s in fruit fly experiments by Angus Bateman, a British scientist who reached his erroneous conclusions in part because his experiments lasted only three or four days.
Had he run his experiments longer, he might have discovered that male black- bellied fruit flies secrete an anti-aphrodisiac in their semen that's relatively short lived. As soon as it runs out, females become interested in copulating again.
On the surface, the conventional view made sense. Sperm seemed to come cheap to males, while eggs were expensive to females, which have to invest the time to raise offspring. Scientists could not fathom any possible benefit of multiple partners of females, and they could come up with plenty of potential costs, such as sexually transmitted diseases.
BIRDS DO IT
Then came DNA paternity testing. In one species after another, it turned out that biologists were as cuckolded as the males they had been observing. The first and most extensive examples of polyandry were found among avian species, which was quite a shock to scientists because birds had appeared to be paragons of traditional family values.
"The way the male and female rush back and forth to their demanding brood of chicks seems like nature's model of good parenting," says Marlene Zuk, biology professor at UC Riverside and author of "Sexual Selections: What We Can and Can't Learn About Sex From Animals."
"Now, we find that they're actually in the same situation as millions of modern-day husbands and wives, eyeing a child warily and making uneasy jokes about the milkman," she says.
DNA testing in chicks of seemingly monogamous females showed a wide range of extra mates. In one study, for example, as much as 90 percent of the offspring of the brilliantly colored Australian fairy wren were from mates other than the presumed father.
Biologists have struggled to come up with broad theories for why females benefit from playing the field, but so far the reasons seem to vary widely according to species. A lot of complex theory boils down to this: A gal's got to do what's necessary to ensure the survival of her genes.
In some cases, females may get more help around the home. Among bronze- winged jacana, for example, harems of up to four males do all the child care, enabling a female to have four times as many broods. Male greater rheas, flightless South American birds that resemble ostriches, receive eggs from several females, incubate them and rear all the chicks, while females go off to mate and lay other clutches.
In other cases, females swap sex for food -- the more sex, the more food and the healthier their offspring.
Among green-veined white butterflies, for example, a virgin male ejaculates a sperm packet roughly 15 percent of his weight that also contains nutritious substances. Females that have sex with several virgins lay more and bigger eggs than those that do it with only one or with males that have lost their virginity and consequently make sperm packets only half the size of their virgin glory.
SURVIVAL OF THE LOOSEST
In other cases, promiscuity is simply a matter of survival. Male chimpanzees, for example, have been known to kill infants not their own. Frequent sex with several males -- in one 15-minute period, a female was observed having sex with eight males -- can heroically confuse paternity and act as insurance against harm to her offspring.
But while females are busy ensuring their genetic survival by sleeping around, males have not been idle. After all, female promiscuity puts the genes of males at risk. It's no good being Don Juan, seducing all the females in sight, if none of them uses your sperm, Judson says. So males have developed counterstrategies to ensure their genetic survival.
"This is perhaps the most significant discovery of the past two decades, that male and female attributes coevolve," writes Tim Birkhead, professor of behavioral ecology at the University of Sheffield in Britain and author of "Promiscuity: An Evolutionary History of Sperm Competition."
SOME MALES' WEAPONS LETHAL
In the arms race between the sexes, males of some species have developed penises that are more than sperm delivery devices.
Damselflies, close relatives of dragonflies, have penises with inflatable balloonlike bulbs, two horns at the tip and long bristles down the sides. In one species, males use this to scour sperm from inside a female before depositing his own. In another, males use it for extra stimulation, inducing her to eject sperm from previous lovers.
Male honeybees, on the other hand, sacrifice themselves on the altar of love. Upon climax with the queen, he explodes, and his genitals rip from his body, leaving the mutilated member as a kind of chastity belt.
"You might imagine that male honeybees would have evolved some way of removing the chastity belt. You'd be right," Judson says. "If you look closely,
you'll see that each male honeybee sports, on the tip of his phallus, a hairy structure that can dislodge the severed genitalia of his predecessor."
Other species resort to guarding their mates. A possessive postcoital male Idaho ground squirrel, for example, won't let his partner out of his sight and follows her everywhere, stationing himself at the entrance to her burrow and picking fights with other males that happen to come near.
When it comes to Homo sapiens, scientists urge us not to read too much into all this. Depending on their point of view, people may be horrified or intrigued by the infidelity of the birds and the bees, but in truth birds aren't cheating, they're just doing what they do.
"If we try and use their behavior as a model or justification for our own," says Zuk, the UC Riverside biologist, "we not only run the risk of making decisions about our morals on very shaky grounds, we miss what is interesting and vital about the animals' own behavior."
NEW LOOK AT REPRODUCTION
Females of many species have sex with multiple partners. Males, in turn, have adapted ways to ensure that their genes, and not those of competitors, are passed on. Understanding this co-evolution is changing our view of male and female sex roles.
Among reed buntings, small brown songbirds, every male a female has mated will come flying to her defense. But her infidelity can backfire if the female's main squeeze suspects her of cheating.
Male honeybees sacrifice themselves on the altar of love. Upon climax with the queen, he explodes and his genitals rip from his body, leaving the mutilated member as a kind of chastity belt.
Male chimpanzees sometimes murder infants not their own. So females have adopted a strategy of confusing paternity by having frequent sex with several males.