Unfaithful reproduction. (sexual behavior in birds and people)(excerpt from 'The Red Queen: Sex and the Evolution of Human Nature') New Statesman & Society v7, n286 (Jan 21, 1994): 30
Аннотация: Выдержка из книги "Красная королева: Секс и эволюция человеческой природы" содержащая любопытную информацию о нелигитимном размножении (включающем "внебрачное" зачатие) среди птиц. Проводится параллель с человеческим кодом поведения.
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Ridley, Matt Unfaithful
reproduction. (sexual behavior in birds and people)(excerpt from
'The Red Queen: Sex and the Evolution of Human Nature') New
Statesman & Society v7, n286 (Jan 21, 1994): 30 (2 pages). [Abstract][Long
Display]
COPYRIGHT Statesman and Nation Publishing Company Ltd. (UK) 1994
In the 1980s, a number of women scientists, led by Sarah Hrdy,
now of the University of California at Davis, began to notice that
the promiscuous behaviour of female chimpanzees and monkeys sat
awkwardly alongside theories that heavily female-biased parental
investment leads directly to female sexual choosiness. Far from
being choosy, female primates seemed to be initiators of much
promiscuity. Hrdy began to suggest that there was something wrong
with the theories.
The solution to Hrdy's concern lay in her own work. In her study
of the langurs of Abu, in Rajasthan, Hrdy had discovered a grisly
fact; the murder of baby monkeys by adult male monkeys was routine.
Every time a male takes over a troop of females, he kills all the
infants in the group. Exactly the same phenomenon had been
discovered in lions and is common in rodents, carnivores and
primates. Even our closest relatives, the chimpanzees, are guilty.
Infanticide, said Hrdy and her colleagues, was an "adaptation"--an
evolved strategy. By killing their stepchildren, the males would
halt the females' milk production and so bring forward the date on
which the mother could conceive again. Hrdy suggested that female
promiscuity in monkeys and apes can be explained by the need to
share paternity among many males to prevent infanticide. But does it
apply to humanity?
The short answer is no. It is a fact that stepchildren are 65
times more likely to die than children living with their true
parents, and it is inescapable that young children often have a
terror of new stepfathers that is hard to overcome. But neither of
these facts is of much relevance, for both apply to older children,
not to suckling infants. Their deaths do not free the mother to
breed again.
Moreover, the fact that we are apes can be misleading. Our sex
lives are very different from those of our cousins. Compared to our
ape cousins, we, the commonest of the great apes, have pulled off a
surprising trick. We have somehow reinvented monogamy and paternal
care without losing the habit of living in large multi-male groups.
Like gibbons, men marry women singly and help them to rear their
young, confident of paternity, but, like chimpanzees, those women
live in societies where they have continual contact with other men.
There is no parallel for this among apes. However, there is a close
parallel among birds. Many birds live in colonies, but mate
monogamously within the colony. And the bird parallel brings an
altogether different explanation for females to be interested in
sexual variety.
A female human being does not have to share her sexual favours
with many males to prevent infanticide, but she may have a good
reason to share them with one well-chosen male apart from her
husband. This is because her husband is, almost by definition,
usually not the best male there is---else how would he have ended up
married to her? His value is that he is monogamous and will not
divide his child-rearing effort among several families. But why
accept his genes? Why not have his parental care and another's
genes?
In the 1980s, it became possible for the first time to do genetic
blood-testing of birds, and an enormous surprise was in store for
zoologists. They discovered that many of the baby birds in the
average nest were not their ostensible fathers' sons. Male birds
were cuckolding each other at a tremendous rate.
What's in it for the birds? For the males, it is obvious enough:
adulterers father more young. But for the female, it is not at all
clear why she is so often unfaithful. Zoologists Tim Birkhead and
Anders Moller have suggested that female birds benefit by being
promiscuous because it enables them to have their genetic cake and
eat it--to follow the Emma Bovary strategy of adultery from within
marriage. A female bird needs a husband who will look after her
young, but by the time she arrives at the breeding site she might
find all the best husbands taken. Her best lactic is therefore to
mate with a mediocre husband or a husband with a good nest site and
have an affair with a genetically superior neighbour.
This theory is supported by the facts: females always choose
males that are more dominant, older, or more "attractive" (for
example, ornamented with longer tail feathers) lovers than their
husbands; they do not have affairs with bachelors (meaning,
presumably, those rejected by other females), but with other
females' husbands; and they sometimes incite competitions between
potential lovers and choose the winners. In Moller's study, male
swallows with artificially lengthened tails acquired a mate ten days
sooner, were eight times as likely to have a second brood, and had
twice as high a chance of seducing a neighbour's wife as ordinary
male swallows.
In short, the reason adultery is so common in colonial birds is
that it enables a male bird to have more young and enables a female
bird to have better young.
The work on birds has been conducted by people who knew little of
human anthropology. In just the same way, a pair of British
zoologists had been studying human beings in the late 1980s, largely
in isolation from the bird work. Robin Baker and Mark Bellis of
Liverpool University were curious to know if sperm competition
happened inside women, and, if it did, whether women had any control
over it. Their results have led to an astonishing explanation for
female orgasm.
They discovered that the amount of sperm retained in the vagina
varies according to the woman's tendency to have an orgasm. If she
has no orgasm, or if she has an orgasm more than a minute before the
male ejaculates, very little sperm remains in the vagina at all. If
she has an orgasm less than a minute before him or up to 45 minutes
after him, then most of the sperm stays in.
Baker and Bellis also asked their subjects about their
extramarital affairs. They found that in faithful women about 55 per
cent of the orgasms were of the high-retention (most fertile) type.
In unfaithful women, only 40 per cent of the copulations with the
partner were of this kind, but 70 per cent of the copulations with
the lover were of this fertile type. Moreover, whether deliberately
or not, the unfaithful women were having sex with their lovers at
times of the month when they were most fertile. This meant that an
unfaithful woman in their sample could have sex twice as often with
her husband as with her lover, but was still slightly more likely to
conceive a child by the lover than the husband.
Baker and Bellis interpret their results as evidence of an
evolutionary arms race between males and females, one in which the
female sex is one evolutionary step ahead. The male is trying to
increase his chances of being the father in every way. Many of his
sperm do not even try to fertilise her eggs, but instead either
attack other sperm or block their passage. By these, and other
means, the male's sexual behaviour is designed to maximise his
chances of fertilising an egg. But the female has evolved a
sophisticated set of techniques for preventing conception except on
her own terms. In particular, by judicious orgasm she can virtually
decide by which of two lovers she chooses to be impregnated. Of
course, women did not know this before now and do not set out to
achieve it. But the astonishing thing is that, if the study by Baker
and Bellis proves to be right, they are doing it anyway, perhaps
quite unconsciously.
Baker and Bellis do not claim to have found more than a
tantalising hint that this is so, but they have tried to measure the
extent of cuckoldry in human beings. In a block of flats in
Liverpool, they found by genetic tests that fewer than four in every
five people were the sons of their ostensible fathers. The rest were
apparently fathered by somebody else. In case this was something to
do with Liverpool they did the same tests in southern England and
got the same result. We know from their earlier work that a small
degree of adultery can lead to a larger degree of cuckoldry, through
the orgasm effect. Just like birds,. women may be--quite
unconsciously--having it both ways by conducting affairs with
genetically more valuable men while not leaving their husbands.
What about the men? Baker and Bellis did an experiment on rats
and discovered that a male rat ejaculates twice as much sperm when
he knows that the female he is mating with has been near another
male recently. The intrepid scientists promptly set out to test
whether human beings do the same. Sure enough, they do. Men whose
wives have been with them all day ejaculate much smaller amounts
than men whose wives have been absent all day. It is as if the males
are subconsciously compensating for any opportunities for female
infidelity that might be present. But, in this particular battle of
the sexes, the women have the upper hand because even if a
man--again unconsciously--begins to associate his wife's lack of
late orgasms with a desire not to conceive his child, she can always
respond by faking them.