In the ordinary course of events, a male dung fly copulates
with a female for a full 40 minutes, and even when he is
finished delivering what he came to deliver and disengages,
he hangs onto her for an additional 20 minutes. Otherwise,
she is likely to - how to put this? - collect sperm from
another suitor. When a male ghost crab mates, his first
move is to shoot into his beloved a bit of fluid that hardens
into an epoxylike plug. The plug blocks any rival sperm
that may have arrived earlier from swimming out of the tract
where the female stores the wiggly gifts that males deposit,
and so keeps the other guys' sperm from reaching her eggs.
Only then does the latest crab, optimistic about his paternal
chances, introduce his own sperm. The male redback spider,
seconds after inseminating a female, does a somersault into
her mouth so he becomes her postcoital meal; since matings
followed by cannibalism last twice as long as those that
don't, his ultimate sacrifice improves the chance that his
own sperm will fertilize her eggs before someone else can
have at her.
From behavior to physiology to anatomy, sex throughout the
animal kingdom has always been and will surely always be
more bizarre than the Kama Sutra lets on. But at least it's
becoming less mysterious. Such previously inexplicable facts
of life as weird genitalia and ludicrous copulatory practices
(such as the 79 straight days that stick insects remain
inflagrante delicto), biologists are now realizing, are
adaptations to something they managed to overlook for a
few millenniums: female promiscuity. "Generations of
reproductive biologists assumed females to be sexually monogamous,"
says biologist Tim Birkhead of the University of Sheffield
in his new book "Promiscuity", a masterly recounting
of scores of recent studies. "But it is now clear that
this is wrong. Females of most species ... routinely copulate
with several different males." How routinely?
Once they started looking, biologists,
found promiscuous females in some 70 percent of the species
they studied. A clutch of grasshopper eggs can have several
fathers. Thirty-five percent of baby indigo buntings, a
pretty little songbird, are sired by a male other than the
guy Mom came in with. So are 76 percent of Australian fairy
wrens. In five hours, a female Scottish Soay sheep paired
up with seven rams for a total of 163 encounters. Female
chimps copulate a total of 500 to 1,000 times for each pregnancy:
a 1997 study using DNA to run paternity tests found that
54 percent of baby chimps were fathered by males other than
Moms supposed partner. A single clutch of goshawk eggs is
inseminated some 500 times. Female monogamy is the exception
rather than the rule, contrary to the pat sociobiology argument
that only men gain an evolutionary edge by spreading their
seed widely. Surveys find that human females' - that is,
women's - "ideal number" of lifetime sexual partners
is less than men's, and that they indeed have fewer partners
than men.
Based on that, "a lot of people
want to simplify human mating and say that women are monogamous
and men are promiscuous; says psychologist David Buss of
the University of Texas. "But that's a gross oversimplification:
both sexes pursue both strategies." Female promiscuity
triggers a war beeen the sexes. If a male is to have a fighting
chance of fathering offspring and getting his genes into
the next generation (the definition of evolutionary success)
in the face of faithless females, he needs both crafty mating
habits and seemingly outlandish mating equipment. One favored
adaptation is a penis decked out with features much more
elaborate than your basic sperm-meets-ova job requires.
Hence the tools of male damselflies and dragonflies: both
are covered with horns and hooks whose sole purpose, biologists
have finally deduced, is to scoop out sperm that have arrived
in the female's genital tract before theirs. The wild variety
in testes size (relative to body size) throughout the animal
kingdom suddenly makes sense in light of female promiscuity,
too: the more promiscuous the females of a species, the
larger the equipment a male grows so his sperm have a swimming
chance at fatherhood. That's why gorillas' testes are small
(faithful females) but chimps are ... well, there's a reason
circus chimps are usually clothed.
Scientists arnt sure what influences
the females of a species (or the males, for that matter)
to remain monogamous. But in humans, female promiscuity
is a response to tough living conditions. In Bhutan, many
women practice polyandry because in the poor valleys of
the Himalayas a lone husband cannot support a family. Spouse
exchange among the Inuit improves their chance of survival
in the unforgiving Arctic because kin are morally obliged
to provide for each other; the more in-laws, the more support.
Which brings us to the central question: what do females
get out of promiscuity? (No, besides that.) One benefit
of polyandry is increased resources and protection for a
female and her brood. Most males are happy to trade food
for sex; every male a female cricket copulates with brings
her a protein-rich meal, good for her eggs. And female Adelie
penguins collect a stone, to build a nest, every time they
offer themselves to a male.
Females also gain by sowing the
seeds of confusion, paternitywise. Each Galhpagos hawk that
mates with a female helps rear her chicks, even though some
are not the sires. But by trading sex for paternal care,
the female increases the chance that her chicks will survive.
Female red-winged blackbirds who copulate with multiple
males are less likely to lose their chicks to predators:
each male, thinking he may be Dad, attacks would-be predators.
Male primates and lions commit infanticide against babies
not their own; by confusing paternity, female chimps and
lionesses keep more of their offspring out of harms way.
Female promiscuity may bring not more offspring (as male
promiscuity does) but better ones. Females of several species
seem to have a mysterious detector built into their reproductive
system that rejects "genetically incompatible"
sperm but accepts sperm whose DNA complements their egg's,
producing the most viable offspring. Several studies find
that females produce better quality offspring by mating
with several males; scientists are only starting to figure
out how females discriminate duds from winners. Some aspects
of sex, it seems, will remain a mystery. Thank goodness.
©
2000 Newsweek, Inc. 
Contraception