Adventures among Ants. Mark W. Moffett
Читать онлайн книгу.
when struggling with prey. Like a lion, an ant is easiest to approach and photograph when it is preoccupied.
In my six months in India, my photography budget was tight, but I took an occasional picture of marauders swarming, collecting seeds, and being harassed by hairy Meranoplus workers. Before I flew to Singapore to continue my work in Southeast Asia, I wrote the Committee of Research and Exploration at the National Geographic Society, which had given me a grant, to ask if they could develop my film. The committee’s chairman, Barry Bishop—a member of the first American team to climb Mount Everest—kindly agreed. I put six rolls of Kodachrome 64 film in an express package and sent it off to him. Two weeks later, I was surprised by a Telex announcing that a writer from National Geographic was flying to India to meet me—about what, it didn’t say.
A few weeks later, I left Sullia and traveled to Bangalore, where I was to meet the writer, Rick Gore, for breakfast at his hotel, Bengaluru, the finest in the city. By then, I had been living in rural villages so long that the hotel gave me culture shock. The corn flakes and coffee, though everyday American foods, were pricy by Indian standards, costing more than I spent in a week in Sullia.
Rick told me my photographs had gone to Mary Smith at “the magazine,” who wanted to support my efforts, maybe even have me write a story for the magazine. I didn’t know it at the time, but Mary is legendary for her work with such iconic scientists as the paleontologists Louis and Mary Leakey, the undersea explorer Jacques Cousteau, and the ape experts Dian Fossey and Jane Goodall. Why did she want to work with me? “She likes what you are discovering,” Rick told me. “She also has no idea how you are making the ants look so glamorous.”
I had no idea either. Up to that time the only photos of mine I’d seen were test shots I had taken of dead specimens back in Massachusetts, and they weren’t anything to crow about. So a month later, when I arrived in Singapore, where Mary had sent the developed slide images, I was stunned. The ants that had been half visible to me through my camera in dim light were clear and crisp on film. Here were marauders confronting furry Meranoplus, sleek Leptogenys hunting termites, eagle-eyed Harpegnathos seizing crickets in mid-jump. Two years later, after my return to the States, when I met Mary, she compared my images to the visuals in the film The Terminator. “For you ants are huge, so they become huge for the rest of us,” she told me. The photographs became part of my first article for National Geographic magazine.1
THE PLAN OF ATTACK
In Singapore, I splurged on flash attachments that did not shock me. To take in the mass-foraging pattern, I stepped back each day to observe the raids as a whole. But like a physiologist who examines muscle fibers to find out how humans move their fingers, I also came in close with my camera “microscope” to record the individual ants in action and learn the details of how they made their kills and harvested the victims.
These observations came as a welcome relief after months at Harvard measuring ants in museum drawers and categorizing them as minor, media, or major based on their frequency and size.2 What I discovered in the field was that the slender minor workers form 98 to 99 percent of the population. Tiny, with heads about 0.6 millimeter wide, they are distinct. There are no intermediates between them and the other ants, which range widely and continuously in size. Within this continuum, there is a distinct peak in the numbers of ants at just over 2 millimeters’ head width, and so these I called “media workers,” and another peak at just over 3 millimeters’ head width, for the majors. A few of the majors are substantially larger, with heads 5 millimeters wide or more—the size category I informally called the giants. The queen, who ordinarily stayed in the nest, had a smaller head than a giant, but a much larger body: she could be about 2 centimeters long.
Among different kinds of ants, I learned, work is divided up in two ways. In some species the workers are similar in appearance but flexible in their job skills, temporarily taking on any tasks as they arise, but the colonies of other species can also develop workers of different sizes to do different jobs on a more permanent basis. The former method allows colonies to adjust more rapidly to changing conditions, but it has its limitations: since the workers are identical and interchangeable, duties that require a specialized skill set may be poorly executed. Polymorphism—variation in size and shape, along with physiology and brain development—is an indicator of a more permanent specialization, and is the primary determinant of division of labor in the marauder colony. Because the workers of differing size are suited to a narrower set of tasks, they expand their activities, if at all, only under stress; in some ant species, for example, soldiers who ordinarily do little except fight will help tend the brood if other workers are taken away by a meddling researcher.3
From this, it has been determined that an extremely polymorphic species like the marauder ant is likely to have predictable labor needs, because the number of members in each physical caste, or size group, changes slowly, if it can be changed at all, based on the colony’s requirements.4 In fact, the size frequency distribution reveals something about how many ants of each caste a society requires, somewhat equivalent to the distribution of people in different job descriptions in a city.5
To pursue again the earlier metaphor, a colony can be seen as a “superorganism” that functions like the body of an organism, with the number of castes and the frequency of each being analogous to the number of types of cells and tissues and the size of organs. Ant species with small colonies are like the cells in simple organisms in that they have few labor specialists, but marauder ants are intricately specialized. Add the arrangement of the workers in space and their interactions with each other to the numbers and frequencies of the various workers, and one has the “scaffolding” of the superorganism, much as a body is built upon the number, location, and interactions of cells. The parallels are all the more remarkable since both the ant workers in a colony and the cells in a body communicate largely by chemical cues (hormones being a prime example for cells), the biggest difference being that workers are mobile and accumulate dynamically when and where they are needed, while most cells are fixed in place within the body.
Essentially all the participants in the raid front are the little minors. With my photographs, I was able to disentangle the blur of action as these ants brought down a nightcrawler or grasshopper thousands of times their weight. A single minor worker has no more chance of catching such a behemoth on her own than would an equally small worker of a solitary-foraging ant species. But she shares the front with other minors that contact prey at about the same time, and they pile on like tacklers in a game of American football. With this strategy, the chances of capture improve markedly: as in Swift’s tale of Gulliver toppled by the Lilliputians, strength in numbers can’t trump size.
It makes sense for a colony to produce a lot of minor workers and concentrate them at the front. If the prey were confronted by a single media ant instead, even one weighing as much as all those smaller tacklers combined, the larger worker would be less effective at subduing the worm or grasshopper. Though individually weak, minors working together simultaneously grab their quarry at different places and angles, making it hard for a victim to move. The prey is also more likely to slip by a single big worker than by a barricade of spread-out small ones.
Countless times I’ve watched a nightcrawler inching over the ground or a grasshopper resting on its green blade, minding its own business, as a swarm moves toward it with a whisper like a snake in the grass. If it doesn’t respond by reflex, death is certain. At the touch of the first worker, the worm flips back and forth; the grasshopper makes its leap. But out of view in the vegetation, more ants are swarming in. About half the directions the flipping worm or leaping grasshopper could choose will land it deeper among the ants, while the other half will allow it to evade the ants by getting ahead of the raid. Blundering deeper is like colliding with a dragnet with a mesh of the width and strength approximated by the closeness and size of the ants; the more the worm or grasshopper struggles, the more the masses converge on it, as other ants are alerted and drawn into the fray. Soon all the little ant jaws hold their prey taut.
Avoiding the ants by moving ahead of the raid provides a temporary respite. The best hope for any creature is to dash to freedom to the left or right of the raid, and so carry itself out of the ants’ path; but the distribution of ants must be difficult for prey to determine down