Wonders of Life. Andrew Cohen
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Название: Wonders of Life
Автор: Andrew Cohen
Издательство: HarperCollins
Жанр: Прочая образовательная литература
isbn: 9780007452682
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This thought may have been adrift somewhere in Frank Borman’s consciousness, catalysed by his feelings of isolation 400,000 km from home, when he ended the 1968 Christmas broadcast with a phrase I have always found overpowering in its simplicity and depth of meaning. To me, it was an instinctive plea to all of us to value our home – the absolutely necessary platform for the continued existence of, just possibly, the only living civilisation in the Universe:
‘And, from the crew of Apollo 8, we close with good night, good luck, a Merry Christmas – and God bless all of you, all of you on the good Earth.’
‘Earth rise’, first observed from Apollo 11 in 1969, gave us a totally new perspective on the planet we call home.
In early September each year, monarch butterflies gather in their millions east of the Rocky Mountains before migrating south to the evergreen forests of central Mexico.
With its vivid orange colour and beautiful markings, the monarch butterfly (Danaus plexippus) is a striking example of the simple aesthetic beauty of life. But as is so often the case in the natural world, the superficial beauty of these butterflies is immeasurably enhanced by a deeper scientific understanding of their life cycle and biochemistry, and the reasons for their form and function.
Each year, as autumn approaches across Canada and the northern United States, millions of monarch butterflies begin preparations for an arduous expedition. To survive the harsh northern winter, they embark on one of nature’s great migrations, travelling up to 4,000 km to warmer domains in the south. It is a vast distance for such a small and seemingly fragile creature to travel, and requires the birth of a special generation of butterflies. An average adult monarch has a life span of little more than four weeks, but, when faced with the journey south, a ‘methuselah generation’ emerges; a generation that lives nearly ten times longer than its parents and grandparents.
Living for up to eight months, these butterflies carry with them the privilege of a longer life and the responsibility of carrying their genes through to the following year. As autumn begins in the forests, fields and meadows of the north, preparation starts for travel. The fading of the northern Sun, a result of Earth’s journey around the Sun coupled with the 23-degree tilt of its axis, causes temperatures to fall and food to become scarce. By early September, the young butterflies sense the shortening days and begin to gorge themselves on nectar, laying down extra layers of fat to increase their resilience. When the temperature approaches the very limits of their tolerance, they take flight. This is no random journey south. Covering up to 100 km a day, half a billion monarchs head towards a very specific location. None of them has travelled the route before, yet their destination has remained the same for thousands of years.
Of the many possible solutions to this annual challenge, the monarch butterflies have evolved into skilled navigators, using time and a star as their guide. From their starting point east of the Rocky Mountains, they journey across the great plains of the central United States into the damp humidity of the south. Along the way they face the same dangers as all long-distance travellers; illness and infection, bad weather and storms are a constant danger, and predatory birds will pick off thousands before they come close to completing their annual voyage.
But every year, despite the daunting distance and difficulties, millions of monarchs arrive in a single small area of evergreen forest in the heart of central Mexico. Populations of monarchs that were living west of the Rockies will have made a similar, though shorter, voyage to safety in southern California.
USING SPECTRAL GRADIENTS TO FIND THE POSITION OF THE SUN: Long wavelengths (green light) dominate the solar hemisphere, and shorter wavelengths (violet) dominate the anti-solar hemisphere.
The monarchs navigate like eighteenth-century explorers, using the position of the Sun in the sky and an internal clock to guide them. Taking a southerly bearing using the Sun is simple if you know the time. At noon in the northern hemisphere, the Sun will always be due south. This can be taken as a definition of noon. You can take a southerly bearing at other times of day if you have a watch. Point the hour hand at the Sun, and the line halfway between the hour hand and the 12 o’clock mark will point due south. The monarchs use a sophisticated version of this technique – known as a time-compensated Sun compass – to maintain their southerly orientation during their migration.
This magnified image of the head of a butterfly clearly shows its long, segmented antennae, its two segmented eyes, and its tightly coiled proboscis – the three most important sensory organs.
The butterflies measure the position of the Sun using their sophisticated eyes, which can detect the polarisation of sunlight, enabling them to ‘see’ the position of the Sun, even through cloud. They are also thought to use ‘spectral gradients’, whereby the precise mixture of colours in any given patch of sky depends on how close it is to the Sun. This is due to the way that different wavelengths of sunlight scatter in the atmosphere, an effect that is most familiar in the reddening of the sky at sunset and sunrise.
The nature of the monarch’s clock is more elusive. Biological clocks are ubiquitous in nature and thought to be a very ancient evolutionary invention. Circadian rhythms, which require the beating of an internal biological clock, are found in every corner of the biosphere, from the most complex of mammals to the simplest of bacteria. It is possible that biological clocks could have emerged as a form of protection against the destructive effects of the Sun’s radiation. An organism’s DNA is most exposed to damage at the point of replication, so restricting cell division to the hours of darkness would have been advantageous. This requires a clock that is synchronised to the rotation of the Earth.
Until recently it was assumed that, in common with other animals, the monarch’s clock must reside in the brain. But an experiment conducted by neurobiologists at the University of Massachusetts Medical School in 2009 revealed that it is instead located in the delicate structure of the antennae. The reason for this unusual location is not known. Timing information from the antenna clock is combined with information on solar position from the eyes in dedicated regions deep within the butterflies’ tiny brains, and this allows them to maintain a southerly bearing on their journey to central Mexico.
For the next five months, a handful of Mexican valleys are home to a billion butterflies, clustering on the firs in such numbers that the forests are painted with a magnificent orange glow. The monarch migration is a powerful example of the way that an organism’s home is not a fixed place, but rather a set of conditions that enable it to survive. If those conditions change, it may be necessary to move.
The monarch is an evocative example of a deep truth in biology. The form and function of an animal cannot be understood in isolation. The СКАЧАТЬ