快猫短视频

IT鈥橲 ALMOST that time. The baby that was so much fun to make nine months ago
is poised to squeeze its way into the world. It鈥檚 all very exciting, but if
friends, parents, midwives and popular self-help books are to be believed, the
birth experience is going to be long, arduous, painful work. Especially for you,
the father. Your job these days is to be there鈥攅ncouraging, coaching,
empathising and steadfastly refusing to pass out.

Can this be natural, ask squeamish dads-to-be. Does it do any good? Or is it
just some punishment thought up by feminists? After all, the males of many
species are allowed to sire their offspring and then disappear, off to spread a
little more seed. Isn鈥檛 it enough that human fathers don鈥檛 eat their young?

Sorry, chaps. It鈥檚 true that genuinely attentive caring fathers are rare in
the animal world. But there is a select group of males that are different. They
have always made it their business to be around for the special event. And
although human dads have only recently been allowed into maternity wards, they
seem to be primed for fatherhood in much the same way as these other doting
dads. What鈥檚 more, the harder scientists look, the more paternal participation
they find in nature. If anything, human fathers could pitch in a little
more.

Richard Brown, an ethologist at Dalhousie University in Halifax, Nova Scotia,
was one of the first to realise that the father鈥檚 presence at the birth matters.
He specialises in California mice, Peromyscus californicus, a
monogamous species in which both parents share in raising the young. Everyone
knew male California mice were caring fathers. Even so, no one had suspected
they had a role in the birth. 鈥淣o one had ever looked for it,鈥� he says.

So last year Brown decided to do just that. Working with graduate student
Debora Cantoni, he used the lab鈥檚 video camera to tape a pair of California mice
on the big day鈥攐r night, as it usually is. They were not entirely sure
what to expect. Female gerbils, for example, have the habit of chasing away even
the most attentive fathers at some point during a birth.

As it turned out, the recording revealed something extraordinary: the male
California mouse was better than a midwife. In the lead-up to the birth, he
prepared his mate by grooming her and licking her anogenital region. As each of
the two or three pups was born, he helped by pulling it out, licking it off and
nesting with it to keep it warm and cosy. Throughout the birth, he groomed and
licked his mate. In the end, he joined his partner in dining on the
placenta.

Brown and Cantoni were astonished. In all the research into parturition, no
one had ever ascribed a role like this to the father. So they decided to
investigate further. They wanted to know what effect the male presence
had鈥攊f it made the birth easier, say, or if the pups were more likely to
survive. So they videotaped more births, occasionally removing the male two days
before the scheduled due date of 34 days, to see what would happen when the
female was left to cope on her own.

The researchers found that when the male was removed, the birth took almost
twice as long鈥攐n average 55 minutes, as opposed to 29 minutes when he was
around to help. More surprisingly, the father鈥檚 absence also delayed the start
of the birth. 鈥淲e took the male away and set up the camera,鈥� he says. Two days
later, when the mother was scheduled to deliver, nothing happened. The next day
she still didn鈥檛 give birth. When males were taken out on day 32, females didn鈥檛
give birth until day 36鈥攖wo days overdue, says Brown.

Birth pangs

This was all intriguing stuff. 鈥淢aybe the male is the stimulus to the female
giving birth,鈥� he ventures. But how? He tinkered with his experiment a bit more,
sometimes putting the male in a separate cage above or even inside his mate鈥檚.
If the father was giving her some kind of smell signal, she should be able to
detect it if he was nearby, Brown reasoned. But whenever the male was not right
there, sharing the cage with her, the onset of birth was delayed.

Brown and another of his graduate students, Anna Lee, are now looking into
what the mechanism might be. Their working hypothesis is that the male
stimulates the birth by licking the mother and releasing oxytocin, a hormone
known to trigger uterine contractions. Alternatively, says Brown, the delay may
be due not to lack of stimulation, but rather to the fact that the stress of
being apart inhibits birth for a few days.

Whatever the cause, Brown suspects that this effect is not something unique
to California mice. 鈥淢aybe it鈥檚 a general phenomenon and no one has noticed it
before,鈥� he says. He points out that there is a huge variation in the timing of
human pregnancy, too. 鈥淥ne variable might be the absence or presence of the
father.鈥� Do sailors鈥� wives have different gestations, he asks. What about single
mothers? No one has ever bothered to find out.

In animals of either sex, something must switch on their parental
inclinations at the appropriate time. For the mother, it has always been assumed
that hormonal changes during pregnancy bring on maternal behaviour. Prolactin
seems to be key. Evolutionarily speaking, it is an old hormone, found all the
way up the evolutionary ladder from fish through birds and rodents to humans. It
plays a role in hundreds of bodily functions but is best known for the one it is
named in honour of: promoting lactation. Studies show that if you block
prolactin, mothers stop nurturing and caring for their young; return the
prolactin and maternal behaviour resumes.

But males don鈥檛 get pregnant, so what drives them to care about their young?
The traditional view is that fathers become paternal when they have contact with
their offspring. Brown鈥檚 work shows that males may be primed for fatherhood even
before the birth. Either way, the question remains: what physiological changes
lead to paternal behaviour?

For a few years, there has been a suspicion that prolactin might be the key
here, too. Now there is some evidence to back up this hunch. Katherine
Wynne-Edwards and Catharine Reburn, both biologists at Queen鈥檚 University in
Kingston, Ontario, began looking at hormonal changes in male dwarf hamsters
around the time their pups were being born. One species, Phodopus
campbelli, is biparental: fathers and mothers both care for the young. But
a closely related species, Phodopus sungorus, is known for fatherly
neglect.

Fathers鈥� day

Wynne-Edwards and Reburn reckoned that if pup-to-father contact stimulates
the release of hormones which bring on paternal behaviour, then levels should
be starkly different in the two species. So she compared blood samples taken
from 22 P. campbelli and 16 P. sungorus males鈥攆irst
before they were paired up, and then five days after the birth of their first
litter. As she had expected, in the fatherly P. campbelli prolactin
levels increased significantly between the two tests, while levels in P.
sungorus stayed much the same.

Wynne-Edwards and another colleague, Anne Storey of Memorial University of
Newfoundland in St John鈥檚, speculate that hormones may be at least partly behind
paternal behaviour in human males as well. Men鈥檚 hormones surge and plunge
around the time their babies are born, the researchers have found in a study
that is yet to be published. 鈥淚t鈥檚 terribly exciting, terribly important. It
changes how we view the role of the father,鈥� says Sue Carter, a biologist at the
University of Maryland in College Park. 鈥淭hese findings suggest there may be a
whole shift in the male physiology that allows him to be more reactive, less
辞惫别谤谤别补肠迟颈惫别.鈥�

Many textbooks still take the line that hormones don鈥檛 turn males into caring
parents. It鈥檚 a view that is backed up by some landmark work on rats鈥攂ut
Wynne-Edwards thinks they used the wrong rodent. 鈥淭he work is beautiful in the
rat,鈥� she says. 鈥淚t鈥檚 just the wrong model.鈥� Male rats are not paternal by
nature, she says. If you give them enough exposure to pups, or 鈥渂abysitting
experience鈥�, as Brown puts it, they can be made to develop parental behaviour.
But it does not happen all on its own.

Species that are naturally biparental are different, agrees Toni Ziegler,
from the Wisconsin Regional Primate Center in Madison. In every one that has
been studied to date, hormones have been linked to parenting behaviour of males
as well as females. Ziegler鈥檚 own work even helps explain Brown鈥檚 finding, that
males may become primed for parenthood before their offspring arrive.

She has been studying cotton-top tamarins鈥攖he 鈥渄ads of all dads鈥�. Among
these monkeys, Saguinus oedipus, fathers are the main caregivers, doing
up to 80 per cent of the rearing of the infants. Males take on almost all the
tasks that don鈥檛 involve nursing, says Ziegler鈥攚hich is a lot of work,
considering cotton-tops tend to be born in twos and threes. 鈥淎lmost from the
moment of birth,鈥� she says, 鈥渢he mother will allow the father to take the
迟飞颈苍蝉.鈥�

Mum, on the other hand, spends most of her time eating and promenading. Since
she will typically ovulate within two weeks of giving birth and probably
conceive again then, she needs to preserve her strength for reproduction. The
older offspring usually hang around and help out for a few years before striking
out to start families of their own鈥攁nd it鈥檚 mainly the young males that
are keenest to lend a hand.

Ziegler measured levels of prolactin in tamarin fathers both during gestation
and after the birth of their young. In keeping with findings in other biparental
species, she found that following the birth, fathers had elevated prolactin
levels鈥攋ust as high, in fact, as their nursing mates. Interestingly,
eldest sons also had higher prolactin levels than other single males, perhaps
due to their babysitting activities.

Among males, the most experienced fathers had the highest levels of the
hormone. In fact, the mean level of prolactin for each male is highly correlated
with the number of previous births he has experienced as a father. The paternal
highs tended to stay elevated for six weeks after the birth, and never quite
dropped back to the levels of non-fathers.

Hormonal high

But what Ziegler found in cotton-top tamarins before the birth was even more
surprising. Two weeks before parturition, she detected a prolactin surge in the
fathers that was nearly as high as the one after the birth. It鈥檚 well known that
females experience hormonal changes around birth. High levels of oestrogen and
progesterone during pregnancy unleash prolactin. But what could trigger such
changes in males?

鈥淚t鈥檚 not just the infant that is promoting the prolactin,鈥� says Ziegler. She
suggests that there might also be some kind of interaction between mothers and
fathers in the lead-up to the birth. Could it be pheromones, or even
behaviour?

Indeed, her latest work suggests that whatever it is that is telling males to
expect the pitter-patter of tiny feet, experienced fathers seem to respond more
quickly than inexperienced ones. Ziegler is looking at prolactin levels in male
cotton-top tamarins throughout their mates鈥� six-month gestation period. Her
preliminary findings are intriguing. In fathers who鈥檇 already had a few batches
of twins, prolactin peaked early, at around month two or three, whereas
first-time fathers were slower to catch on; their levels did not peak until well
into the fifth or sixth month. This suggests that the response is not simply
instinctive. 鈥淲e鈥檙e wondering if experienced fathers are just perceiving
better,鈥� she says.

Wynne-Edwards is also keen to find out how mothers and fathers communicate in
the lead-up to birth. She points out that some birds send signals back and forth
during gestation.

鈥淚 think we鈥檒l find in mammals that prolonged contact and cohabitation is
really important,鈥� she says.

鈥淭he male goes through changes in pregnancy alongside his mate,鈥� says
Wynne-Edwards. So maybe those annoying men who chirp 鈥淲e鈥檙e pregnant鈥� aren鈥檛 so
far off the mark after all.

INVOLVING males in the care of young is a successful strategy for animals
that have long, complex developments鈥攖he price of having a large brain,
says John Allman, a neurobiologist at the California Institute of Technology in
Pasadena. Species like humans need some out-of-womb experience to develop fully,
and time the parents spend nurturing that growing brain is not available for
producing more offspring. 鈥淪o you鈥檙e going to have to take awfully good care of
those few babies you have,鈥� he says.

The good news is that caregivers seem to be heartily rewarded in terms of
longevity. When Allman looked at records of birth and death in eight captive
primate species, he found that the doting sex always lives longer. 鈥淚t sort of
dazzled me,鈥� he admits. 鈥淚t鈥檚 a very clear relationship.鈥�

In chimpanzees, for instance, where fathers hardly know their children,
mothers live a whopping 42 per cent longer. Similarly, female orang-utans
survive 20 per cent longer than their partners and gorillas 12 per cent longer.
But in species where males were the caregivers, the tables were turned: male
titi monkeys, for example, lived 20 per cent longer than females.

Humans fit somewhere in between. Allman studied records from 18th-century
Sweden鈥攁 time and place where there were no major wars to kill off the
men鈥攁nd found that females lived on average 5 per cent longer. And this
despite women鈥檚 high risk of death during childbirth. Allman concludes that
survival advantage in the caregiving sex is reinforced by natural selection. He
believes the increased longevity is linked to an ability to deal with stress,
which also increases the likelihood of successful parenting. 鈥淕enes that enhance
stress tolerance would be favoured in the caregiver sex.鈥�

That human males play a big role in child-rearing should come as no surprise,
Allman says. Taking brain size as an indicator, humans probably don鈥檛 fully
mature until about 40. Yet we become reproductively mature long before that. 鈥淚
would maintain that that could only come about if there are other caregivers
around to help out,鈥� he says. Other species with the big-brain/slow-development
dilemma have figured it out: recruit males. 鈥淭hat鈥檚 the way out of this box,鈥� he
says.