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Дуглас Эмлен - биолог из Университета Монтаны (США) - интересно рассуждает об эволюции вооружения самцов жуков-навозников и проводит злободневные параллели с гонкой вооружения у людей.
Текст и иллюстрации заимствованы на сайте NYTIMES
Дуглас - автор недавно вышедшей в издательстве "Henry Holt & Company" книги "Animal Weapons: The Evolution of Battle".
Douglas Emlen is a biologist at the University of Montana |
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Illustration by Paul Sahre |
By DOUGLAS EMLEN
On a still-sweltering evening in Tanzania more than a decade ago, my colleagues and I crouched around a fresh pile of elephant dung to witness an epic struggle of wills. Immediately upon our arrival, beetles began plonking into the grass around us. Soon dozens were raining down from the sky at once, and, not being the most agile fliers, they tumbled into our hair and bounced down the backs of our necks. Within a few short minutes, as many as 50,000 dung beetles landed, all racing to carve out a meal for themselves and their young.
The most astonishing thing about these creatures - aside from their diet, perhaps - is their weaponry. Scores of dung-beetle species on the Serengeti have no horns at all. But as many as five different horns jut from the bodies of other species, and many of these protrusions are proportionally huge; in Onthophagus raffrayi, for example, the male has a horn that arcs all the way over its head, extending more than twice the length of its body.
Observing the beetles' behavior, it became clear why only some species have big weapons. The same simple trigger leads to evolutionary arms races all across the animal kingdom. From the fiddler crab's prodigious claw on up to the elephant's crescent tusks, extreme weaponry has the same root cause: violent dueling, in which the lesser-armed combatant loses out. But investment in weapons suited only to a single task - matched contests with similarly armed rivals - leaves the bearer vulnerable to attack from new enemies. In that regard, animal arms races are not so dissimilar from the ones in which we humans, sometimes foolhardily, engage.
Males of all dung-beetle species face intense battles for opportunities to feed and mate, but the context of these battles differs. A male Scarabaeus pius, for example, carves a hunk from a dung pile, sculpts it into a smooth orb and rolls it away from the competition. If he's lucky, a female will join him as he leaves, often clinging to the ball and somersaulting along for the ride. When the pair reach a suitable patch of moist soil, they'll bury the ball, then mate and place eggs beside the hidden dung, as fodder for their unborn offspring.
Before he gets there, however, an S. pius male may have to fend off a dozen or more challengers. Rival males lunge from all sides, trying to steal his dung ball and the female traveling with it. Vigorous battles are commonplace in ball-rolling species, as males pile up in chaotic scrums of combatants, but none of these species have evolved horns. Their fights occur out in the open, where the dung balls are mobile and malleable. Big weapons would not settle these contests, for there is no purchase from which to push and no way to wield a horn against multiple foes simultaneously. For species like S. pius, dexterity matters more than bulk or strength.
Other species like Onthophagus nigriventris fight inside burrows, where big weapons make all the difference. Females of these species excavate tunnels directly beneath piles of dung, pulling their food down into these tunnels and stashing it for their young. Meanwhile the males battle for tunnel ownership. Planting himself near the entrance, a male will fight to keep rival males from pushing past. Only one rival can enter the tunnel at a time, however, so his fights unfold face to face in a string of successive duels. Unlike the disorderly scrambles of ball rollers, where outcomes are often serendipitous, duels inside tunnels are consistent and predictable, and in these fights, the male with the bigger horn wins.
A simple change in the arena of the battle creates the conditions for an unending arms race. The evolution of weapons in these species surges forward unencumbered. Chance mutations lead to increasingly grotesque or extreme armaments, which beat opponents with smaller weapons. And in doing so, these beetles secure opportunities to breed. Their progeny, as impressive in their weapons as their parents, quickly replace earlier forms and advance the population another notch in weapon size. As this process repeats itself, it pushes the population still further on the path to the extreme.
Much as in geopolitics, animal arms races rack up staggering costs. For example, these beetles' horns can make up 30 percent of a male's total weight, and because nutrients are redirected to horn growth, males often have stunted eye and reproductive-organ growth. Soon horns become so pricey only a select few can afford them, and once this happens, the sole option left for the rest of the males is to cheat. And so nearly every heavily armed species has small males who break the rules.
Not all O. nigriventris males have horns. Those that don't grow past a certain size - because they were born smaller or were malnourished - never hit the genetic trigger that leads to horn growth, forcing them to circumvent the whole system of duels in order to breed. Instead of dueling (and losing), small O. nigriventris males dig their own tunnels, bypassing the guarding males in order to mate with the female, and slip back out again undetected. They waste no resources on weapon development at all, leaving them nimbler and even more virile - instead of growing horns, they grow big testes that produce extra sperm. They may not mate with as many females as the larger males of their species, but they make the most of every opportunity.
Over time, this end run around the logic of the arms race can completely upend it, pushing the armed animals out of the gene pool. Overburdened and outmatched, animals with weapons eventually die off. Biologists were baffled when they first encountered this trend - it seemed to fly in the face of sexual-selection theory, which is the notion that the best-armed males will be the most genetically prolific. But much as duels necessarily create arms races, arms races necessarily create cheaters - and cheaters can win, bringing an end to the race.
Human arms races don't just start in the same fashion as animal arms races; they seem to proceed through the same sequence of stages too. Take, for example, the oared galleys of the early Mediterranean. For centuries, these slender-hulled ships shuttled warriors around the ancient world, rowed by teams of 50. Then in the eighth century B.C., shipbuilders added cast-bronze rams to the bows, allowing ships to strike and sink other ships. Ramming took place at close range, in duels. Soon the galleys grew by leaps and bounds, as the need for bigger rams drove the need for more oars and more men, until they became nautical absurdities. This arms race culminated in the third century B.C. with a double-hulled behemoth, built by Ptolemy IV. It carried 3,000 sailors and was rowed by some 4,000 men.
More recently, the Cold War - in a sense, a duel - led to an arms race that only the best-conditioned could afford. Since then, the United States has fought not another superpower but a series of enemies that have circumvented our defenses by thinking creatively or sneakily. As our weapons become more intelligent, these attacks don't even have to take place in the physical realm: Computer systems involved with the most expensive weapons program ever, the F-35 joint-strike fighter, were infiltrated by Chinese hackers in 2007. Increasingly, the most powerful military in the world faces threats from inexpensive tricks that erode our defenses.
All across the natural world, arms races - even our own - seem to start for the same exact reason. If there's a lesson to be learned from these remarkable creatures, it's that such races tend to end the same way too.