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The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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Horizons Social Mores of Ants and ApesWhen it comes to accurate portrayals of ants, Hollywood has a lot to learn. The recent animated movies "Antz" and "A Bug's Life" both depicted dictatorial queens ruling colonies where workers plug away at the same task, day in and day out. Scientists once held similar views of ant society, but most have cast aside metaphors of rigid castes and assembly lines in the wake of discovering that ant behavior can be dynamic. "It turns out that the queen doesn't have any authority, and the workers are flexible," says biologist Deborah Gordon of Stanford University. In the longest continuous study of any social insect, Gordon has led a 17-year project scrutinizing the activity of some 300 ant colonies on 25 acres of a cattle ranch in the southeastern Arizona desert to learn how at least one of the 15,000-odd ant species, the red harvester ant (Pogonomyrmex barbatus), maintains order in the absence of orders. Gordon's new book, Ants at Work: How an Insect Society is Organized, presents what she's learned so far. Rather than assembly-line automatons, suggests Gordon, individual ants may be to a colony what neurons are to a brain: a multitude of simple units that respond to local cues and in sum comprise something much more complex. On a clear afternoon near the onset of summer rains, mature harvester ant colonies release their alates (virgin queens and males) for an annual mating flight. Alates from several colonies somehow converge on a single site, where mating occurs. Males die soon afterward, but the females fly off and settle elsewhere. Most have a fatal encounter with predators or other ants, but a few queens manage to dig out a shallow nest, from which they will never emerge. Of some 1,000 new nests that might be scattered over Gordon's study site in a given summer, only 20 to 50 of them survive as young colonies a year later. In that make-or-break first year, the queen uses sperm stored from her brief bout of mating to produce several hundred workers. A successful colony rapidly increases in size and gradually grows more secure. After about five years, the queen needs to crank out at least 10,000 workers a year for the colony to remain stable and able to reproduce by sending out its own alates. A colony may last for 15 years, or as long as the queen lives. She is not replaced after her death, so the colony withers away as surviving workers die. A colony's success depends in part on its neighbors, who influence whether it grows enough to reproduce. Colonies tend to be spaced about 30 feet apart, but as Gordon has found with a densely crammed cluster that she dubs "the inner city," overcrowding can keep otherwise mature colonies from becoming large enough, and from finding enough food, to reproduce. The ants can't go anywhere without bumping into neighbors, which restricts their foraging forays. Gordon has witnessed intriguing changes in ant behavior during the lifetime of a colony. While ants from the youngest, smallest colonies will go out of their way to avoid potentially combative contact with neighbors, those from three- or four-year-old colonies tend to be persistent and aggressive, ready to contest for resources. These fast-growing adolescent colonies, literally young and hungry, have thousands of larval mouths to feed, so workers may more willingly engage in conflict or lay claim to new foraging turf. After age five, colonies revert to a more prudent and diplomatic stance toward neighbor ants, perhaps because they have amassed sufficient food to sustain a larger group. This is all the more amazing and puzzling since all the ants, except for the queen, live only a year. There are no senior ants to impart their wisdom to the group, yet somehow the ants collectively seem to adjust their behavior. By watching ants marked with drops of model airplane paint (applied after the ants have been cooled down inside an ice cream maker), Gordon has also documented flexibility among individuals. During their brief lives, workers may switch "professions"several times, often shifting from duties inside the nest to external service. At any time, about a quarter of a colony's workers engage in outside tasks: foraging for seeds, patrolling, and maintaining the nest entrance and refuse midden. It's unclear how changes in individual behavior become transmitted throughout the colony, but from observing captive ant colonies at her Stanford lab, Gordon concludes that the rate at which workers of different tasks briefly touch their antennae as they enter and leave the nest may provide key information for them to determine that it's time to change tasks, from nest repair, for instance, to foraging in order to exploit a new food supply. Since signature chemicals on their exoskeletons indicate an ant's current job, Gordon plans to experiment with chemically coated glass blocks to see if scent alone causes ants to switch tasks. And she will keep gathering evidence for her hypothesis that increasing colony size, by raising the rate of interactions between individuals, directs the changes in group behavior. Even after nearly two decades studying harvester ants in the field, lab, and with computer models, Gordon has many mysteries still to ponder and unravel about just how ants manage to create collective complexity from their singular simplicity. For despite liking ants, Gordon says, she can't help feeling sorry about how much time and energy her subjects fritter away heading off in wrong directions or seemingly doing nothing at all. "They're certainly not models of industry and efficiency," she says, "like it says in the Bible." While long-term observation illuminates the inner workings of ant colonies, a recent survey of more than 150 years' worth of field studies of the common chimpanzee may have edged our closest living relative even closer. In the June 17 issue of Nature, nine prominent primatologists, including Jane Goodall, pooled data for the most complete report yet about culture in chimpanzees. Describing behavior in two subspecies found at six sites across Africa, from Guinea to Tanzania, the survey provides a provocative answer to the question of whether apes actually ape, concludes Frans de Waal of Emory University. In an accompanying commentary, he writes, "All in all, the evidence is overwhelming that chimpanzees have a remarkable ability to invent new customs and technologies, and that they pass these on socially rather than genetically." Chimp watchers have known for years that their subjects display a striking range of behavior, and that populations of Pan troglodytes possess distinct local customs of tool use, grooming, and other activities. But the extent of these behaviors surprised even the experts, says one of the study's co-authors, William McGrew of Miami University in Ohio. Of 65 separate behaviors documented at the six sites, 39 appear to be cultural variants--differing in their presence and in the degree to which they have become habitual or even customary. Some customs occur in just one community, but more often they are shared by several, with slight differences. The other documented behaviors were either too common at all sites or too rare at any one to be considered the product of cultural transmission. And three behaviors were best explained by ecological rather than social factors. The findings suggest that chimp communities display a human hallmark: a specific signature repertoire of group behavior. "Chimps show astonishingly variable behavior," says Harvard University's Richard Wrangham, who contributed to the Nature paper and was co-editor of the book Chimpanzee Cultures. Wrangham has also noted many simian similarities during studies at Gombe and in Uganda's Kibale Forest, but he believes that chimps "may be the most variable species in the world, except for humans" Changeable behaviors that the scientists deem cultural include: using a stone or wood hammer to bash open nuts on a stone or wood anvil, a custom limited to the West African subspecies P.t. verus; fishing for ants and termites with a stick or the midrib of a leaf, a foraging strategy that varies between eastern and western groups; and performing a "rain dance," a multi-male display of hooting and pounding on trees or roots at the onset of a storm that has been witnessed at all but one of the sampled sites. Striking differences can be found even between near neighbors. In the Mahale Mountains of Tanzania, chimpanzee pairs commence a grooming session with the more active groomer, usually a socially submissive adult or adolescent, clasping the hand or wrist of the other and extending their arms overhead, like a referee crowning a victorious prizefighter. But among members of the same subspecies (P.t. schweinfurthii) roughly 100 miles away, at Goodall's site of Gombe, the hand-clasp gesture has never been seen in four decades of chimp-watching. Rather than genetics predisposing the entire subspecies to perform this particular behavior, it appears among only a subset. And nut-cracking seems to stop east of the Sassandra-N'Zo River in Ivory Coast, despite the same kind of chimps, the same nuts, and plenty of perfectly good rocks there. Behavioral differences can be more subtle than the complete absence or regular presence of some activity. Take dining habits, for instance. At Gombe, chimps dip a two-foot-long twig into an anthill, collect a swarm of insects, and swipe them off with the opposite hand to eat them. More sedate chimps at two West African sites, however, use shorter sticks to gather far fewer ants and place the probes straight into their mouths. A sticking point in all this concerns what cuts the mustard as "culture." Most experts agree that some sort of social learning is key, but it's not certain whether the chimpanzee skills get transmitted by teaching or by imitation--essential aspects of human culture which have been very rarely seen in wild apes. Is language likewise an essential element of culture, without which chimps must reinvent their inventions? McGrew and Caroline Tutin, another of the Nature paper's authors, have previously suggested eight verifiable criteria for recognizing culture in other animals: innovation, dissemination, standardization, durability, diffusion, tradition, non-subsistence (not tied to environmental factors), and natural (unaffected by human interference). Longer and more focused studies, McGrew says, are needed to determine which of the 39 chimp customs meet some or all of these criteria. But he's optimistic that as more data pour in from the six populations and others being observed, a fuller picture of chimpanzees' cultural predilections will emerge. Blake Edgar is an associate editor at California Wild. |
Fall 1999
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