Learning under the influence

Natural History, Sept, 1997 by Bennett G. Jr. Galef, Catherine Rankin

Some years ago, German applied ecologist Fritz Steiniger tried to reduce the cost of rodent control by placing large amounts of poisoned bait in rat-infested areas in Berlin. He hoped that such relatively permanent baiting stations would both eliminate resident populations of Norway rats and prevent immigrant rats from moving into the empty territories, thereby saving a great deal of effort and money.

Steiniger faded, and the rat colonies flourished (after an initial, short-term reduction in rat numbers) for two reasons. First, despite Steiniger's best efforts, a few members of each rat colony almost always managed to survive their first contact with the poisoned bait. They sampled a small amount, fell ill for a while, recovered, and then never touched the bait again. Second, and even more unfortunate for Steiniger, young rats raised by poison-bait survivors never so much as tasted the bait left for them. They had apparently learned from their companions what to eat and what to avoid, although just how was unknown.

This sort of "social learning" takes place among laboratory rats as well, enabling researchers to investigate the process more closely. Some learning apparently starts before weaning: given a choice, juvenile rats prefer foods their mother ate during lactation. As a result, when rat pups make the switch from mother's milk to solid food, they prefer to eat foods with the same flavors they imbibed while nursing.

Even when fully weaned, a young rat still turns to adults in its search for food. A weanling may snatch food directly from its mother's mouth or approach another adult, crawl under its belly, emerge between its front legs just under the chin, and start feeding on whatever the generally unprotesting adult is eating. Hungry young rats also find food by following scent trails laid down by adults. Another helpful habit of the adults is urinating and defecating where they eat; pups find such soiled locations attractive dining spots.

Adult Norway rats also appear to choose food largely on the basis of what their companions are eating. They normally find cayenne pepper unpalatable, for example, but after spending time with other rats that have been trained to eat pepper-flavored food, they overcome their reluctance and begin eating it too. In fact, they often come to prefer pepper-flavored to unadulterated food. Similarly, a rat that has learned to avoid a food that made it sick will begin to eat the food again when placed in the company of rats that have no such unhappy association.

The expression on a rat's face during these kinds of experiments suggests that interacting with other rats affects not just what it is willing to eat but also its very perception of the food's palatability. Offered food it likes, a rat eats with what we in the business of watching rats call a "yummy" face. Given a food that tastes bad or makes it sick, it reacts very differently, turning down both corners of its gaping mouth -- a clear sign of disgust. If, however, this same rat is put with other rats that recently ate the offensive food, it no longer makes the "disgust" face.

We now know that a crucial key to one rat's influence over another's eating habits is its breath. In the course of normal digestion, a rat's gastrointestinal tract produces sulfur compounds. For one rat to develop a taste for food eaten by another, it has to simultaneously smell the food -- generally from bits of food clinging to the other rat's fur and whiskers -- and sulfur compounds on the rat's breath.

Many mammals learn where and what to eat from others or their kind (the list includes mice, gerbils, rabbits, cats, goats, sheep, bats, hyenas, naked mole rats, and a variety of monkeys and apes). In some species, social interaction also plays a role in learning how to eat. In the pine forests of Israel, roof rats fill an ecological niche occupied by squirrels elsewhere in the world. These rats feed on the seeds concealed beneath the scales of pine cones. To get at the seeds, an experienced rat first strips scales from the base of the cone, then -- following the spiral of scales that leads to the cone's apex -- pulls off one scale after another, eating each seed as it is revealed.

Joseph Terkel and his students at the University of Tel Aviv have shown that only rats reared by adults proficient at stripping pine cones become adept themselves. No active teaching is involved; young rats learn by taking partly opened cones from adult rats (or from human experimenters working with a pair of pliers) and finishing the job.

Social learning is not limited to feeding or, for that matter, to manuals. Animals can also learn fear from one another. Monkeys reared in captivity are not afraid of snakes until they witness the fearful grimacing and vocalizing of wild-reared monkeys responding to one. Like monkeys, some birds can learn to identify potential predators by listening to their more knowledgeable fellows. In the laboratory, European blackbirds were shown an unfamiliar object (a bleach bottle) while they listened to an audio tape of a wild-caught blackbird giving an alarm call in the presence of an owl. The listening birds soon came to respond to bleach bottles as though they were dangerous; birds watching their reaction soon came to fear the bottles as wen.