快猫短视频

THE SUMMIT of Mount Hakepa on Pitt Island, one of New Zealand鈥檚 Chatham
Islands, is the perfect place to be if you want to witness the first dawn of the
new millennium. As the Sun鈥檚 rays move across the dateline into the year 2000,
the island is the first inhabited land they will reach.

Crowds of tourists are expected to huddle on the mountaintop, waiting for the
great event. But although these may be the first people to see 2000, they won鈥檛
be the first living creatures. That honour will almost certainly belong to two
brown skuas鈥攑air 15B to be precise鈥攚hich nest at the summit of
Rangatira Island, 9 kilometres to the south. At dawn, they are likely to be
soaring high above the island in the updraughts that blow from the cliff just
behind their nest.

More than a hundred brown skuas live on the island, and biologist Euan Young
of the University of Auckland has been studying their family lives, relations
and breeding habits for the past 25 years. And as a symbol of the new
millennium, Rangatira鈥檚 brown skuas are just about perfect鈥攊f, that is,
you believe the next century will be the one in which females finally come out
on top.

Young and his fellow researchers have found that a sizeable minority of the
skuas practise long-term polyandry, in which an individual female may have
anything from two to seven mates who stay with her for decades on end. They work
together to build her nest, defend her breeding territory and look after the
chicks she produces, as well as fathering her young. Even the really big groups
are very stable. One female was seen living on her breeding territory with seven
males back in 1978. Twenty years later, two males are known to have died, but
the other five are still with her. Overall, on Rangatira more than 35 per cent
of birds breed in groups.

To anyone who has run into a skua, these harmonious breeding relations will
seem incredible. Skuas are big, aggressive birds which kill rivals and
fearlessly attack humans. How can this group of two to seven males suppress
their usual extreme hostility and live together sharing one mate? And to the
biologist, there is a bigger question: how can such an unusual mating system
have evolved? Guessing the benefits for a single female of being looked after by
several males isn鈥檛 too difficult, but what possible advantage can there be for
the males?

Young鈥檚 25-year struggle to tackle this question suggests a patience and
stamina rare even among fieldworkers. An answer requires detailed knowledge of
the birds鈥� relationships and how well offspring survive. That means studying
the birds for decades, as well as using sophisticated DNA analysis. Rangatira
Island is remote, and the nesting areas are covered with scrub and high grass
which makes observation difficult.

And the skua鈥檚 life cycle is complex. After fledging, the young birds fly far
off into the oceans. Two or three years later, they reappear and then live for
several more years attached to the 鈥渃lubs鈥濃�攏oisy congregations of between
10 and 30 non-breeding birds that gather at one regular spot. After a further
three years or more, they will begin to move out into breeding territories.

This month, Young has finally brought together his decades of observations
into one monograph. In it, he explains how he discovered that several males
really do live with one female and鈥攕tranger still鈥攖he males are not
related to each another. If the males were close relations, it would be much
easier to explain the evolution of a cooperative system: each male would have a
genetic stake in any of the chicks produced. But without that link, why should
one male help another male pass on his genes?

Even the benefits for females are not as obvious as they first seem.
Normally, a bird lays two eggs which are tucked into brood pouches on either
side of the breast and rest on the feet. This means there is no place to put
extra eggs, so a female can鈥檛 easily produce more young however well she may be
looked after.

It was only in Young鈥檚 long-term observations of threesomes (he hasn鈥檛 enough
detailed data for bigger groups), that he saw where the true benefits of group
breeding might lie. Females living in threesomes don鈥檛 lay more eggs in one
season, nor raise more chicks, but they do live for longer. And, unexpectedly,
their chicks are twice as likely to breed as an adult than a chick reared by
just two parents, perhaps because they are stronger at fledging. Add these
advantages together and a female living in a trio will leave more breeding
offspring behind over a lifetime than a female living in a pair.

This surprise observation might also explain why two males should share one
female. Even if they take turns fathering her offspring, they will still each
leave the same number of breeding descendants as if they lived separately with
single females.

But that still does not explain why males are prepared to be part of bigger
groups. What possible evolutionary advantage could there be if your turn to
father a chick comes only once in six years? No obvious mechanism seems to work.
It can鈥檛 be a shortage of breeding territory or available females: there is
plenty of grassland suitable for breeding on the island, and plenty of unmated
females of the right age who remain in the club.

One possibility is that polyandry evolved under different circumstances and
somehow survives today even though it is now no longer adaptive. Rangatira
Island was more heavily forested before humans came along so in earlier times
grassy breeding areas might have been in such short supply that males were
forced to breed cooperatively. Or perhaps there are some females with a special
ability to attract more than one male. In other words, certain 鈥渟uperfemales鈥�
might have taken control of the breeding system for their own benefit, leaving
other females without mates. It may be no coincidence that female brown skuas
are generally bigger than the males.

Both these possibilities could be investigated. If superfemales exist, they
are likely to behave differently when they are forming bonds during their days
in the 鈥渃lub鈥�. And if the ancestral environment is important, then there are
untouched islands nearby where skuas could be studied. The trouble is that it
could take up to four decades of fieldwork to find the answers.

Young himself has just retired. But he wishes well to anyone who鈥檇 like to
start the millennium with a project that will last a working lifetime.
Unravelling evolution is not for the fainthearted.