Life Is Short...
Added: Tue, 19 Aug 2008 23:00:00 UTC
Sure, Michael Phelps may have snapped a string of Olympic records like so many Rice Krispies in milk, but what was this child of Poseidon up against, anyway? Elite human athletes from 250 countries.
A small, speckled, asparagus-green chameleon of Madagascar, by contrast, holds a world speed record among just about all of the nearly 30,000 different animals equipped with four limbs and a backbone.
Admittedly, it's not a record many of us would aspire to best. As researchers recently reported in The Proceedings of the National Academy of Sciences, the entire life span of the Furcifer labordi chameleon — from the moment of conception to development in the egg, hatching, maturation, breeding and right through to its last little lizardly thud to the ground — clocks in at barely a year.
That hypercondensed biography, the scientists said, may well make the chameleon the shortest-lived tetrapod on Earth, a creature chronologically more like a butterfly or a sea squirt than like the other reptiles, frogs, birds and mammals with which it is taxonomically bundled.
Equally bizarre, said Christopher J. Raxworthy, an author of the new report, the chameleon spends some two-thirds of its abbreviated existence as an egg buried in sand, with a mere 16 to 20 weeks allocated to all post-shellular affairs.
Moreover, the chameleons operate by a synchronized schedule, hatching, growing, mating and dying at more or less the same times and at the same pace throughout the year. As a result, said Dr. Raxworthy, associate curator of herpetology at the American Museum of Natural History, "if you go into a forest during the dry season, the whole population of chameleons there will be represented by eggs."
Counterintuitive though it may seem, the extremity of F. labordi's schedule could prove valuable for tracking down genes and other biological factors that promote longevity. The researchers observed that the chameleon is not merely short-lived as a matter of averages. It is an obligate annual species, destined for death after a single spin around the sun, and that stated fate differs markedly from the varying degrees of perenniality found throughout the tetrapod clan.
"There are about a dozen lizard species known to be short-lived, in which a good proportion of individuals die off by a year," said Kristopher B. Karsten of the zoology department at Oklahoma State University, another author of the report. "But there are always some that make it to the next year, so the species' maximal longevity is greater than one year."
No such luck for our bug-eyed Malagasy friends, which live in the arid, scrubby southwestern region of the giant island. "Once they reach the end of the season," Dr. Karsten said, "they're done," and they will drop from the trees with the papery grace of autumn leaves.
Assuming the execution orders are somehow part of the chameleon's program, researchers might be able to identify the specific genetic or hormonal assassins in lizard cells, find their analogues in human cells and put a cap in them.
The new work also underscores the growing use of so-called life history theory to trace the history and contours of life on Earth.
Scientists have determined that many essential features of an animal's portfolio are linked, among them whether at birth it looks fetal and helpless like a newborn kitten or precocious and competent like a neonatal giraffe; how big the average litter is; the speed with which the animal reaches sexual maturity; the length of time between births; and the pace at which an adult ages.
Try to improve or optimize one of these parameters and you end up paying somewhere else along the line. "One of the most robust things to come out of life history theory is that trade-offs exist," said Steven N. Austad, the author of "Why We Age" and a professor of cellular and structural biology at the University of Texas Health Science Center.
"If you increase the number of young, the cost is often accelerated aging. If you get something that lives longer, you get costs early in life, with lower fertility and even sterility."
Selective pressures in the environment push species toward one life history course or another. One example is that if you're a species in which the great majority of adults end up being killed by predators or disease, it's best to invest your resources in breeding early and often and not to bother worrying about long-term needs like a robust DNA repair system. And so it is that rodents beloved by carnivores everywhere have high fecundity and relatively poor longevity.
If you're a species in which infant and juvenile mortality is comparatively great, as it is with giant tortoises, for example, the emphasis is often on making the best of adulthood, with delayed maturity and extended life spans.
Catastrophic extrinsic changes may quickly rewrite a species' game plan. In another new report from the National Academies journal, researchers presented evidence that Tasmanian devils, the largest of all carnivorous marsupials, have responded to an epidemic of fatal transmissible tumors among adults with a 16-fold increase in precocious puberty among the young. If you're likely to be gone tomorrow, you'd better start begetting today.
Furcifer labordi's extreme life history likewise seems born of extreme adversity and volatility. The chameleon is one of the smallest members of its genus, and adults are readily, avidly snacked on by birds and snakes. The local climate is harsh and unpredictable, lowering the odds of survival beyond a single rainy season.
In addition, the rainy season, which begins in November, when the chameleons hatch en masse, is brief and must be frantically exploited. The young coil-tongued lizards immediately start lassoing insects, and they eat so much, Dr. Raxworthy said, "that they practically grow in front of your very eyes."
By January the chameleons are ready to mate, a nasty, often violent business of males fighting males, females fighting males, and all of them wishing they were somewhere else. Despite their cuteness, Dr. Raxworthy said, "chameleons can be very antisocial, and if you crowd them, they'll happily fight to the death."
Dr. Karsten suspects that Furcifer labordi's compressed breeding season fosters such high levels of aggression that the chameleons die, in part, of hormone overdose.
Another athletic career cut tragically short by steroids.
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