Pick from the Past
Natural History, November 1959

The Wild Rat

This animal’s behavior has given it a reputation for cleverness.

N AMERICAN FARMER, according to Sir James Fraser in The Golden Bough, wrote a polite letter to his rats, to say crops were short, and he could not afford to keep rats during the winter: he had been very kind to them, but for their own good they should go to some of his neighbors who had more grain. “This document he pinned to a post in his barn for the rats to read.” The farmer was in good company in treating wild rats as human, or more than human. Five hundred years ago, the Bishop of Autun, in France, put rats under a formal curse; and there are plenty of other examples of resorting to magic, charms, prayers, and curses in the hope that thereby rats would be driven away or killed.

Two main species of the
wild rat, Rattus rattus, left, and
Rattus norvegicus, are represented
here by male specimens.

Even some twentieth-century biologists, usually rather blasé about the capabilities of species other than their own, have been impressed by Rattus norvegicus: one referred to attempts to exterminate rats as “a veritable battle of wits,” and still another has described rats as “diabolically clever animals.”

Certainly, whether clever or not, rats are found in almost every human community, and in some they probably outnumber the human population. They undoubtedly do so in many laboratories, but in laboratories, as a rule, they belong to one or more of the tame varieties and most typically they are white, although they still belong to the species R. norvegicus (also known as Mus decumanus). These domesticated creatures have been so much used in so many kinds of research that the psychologist N. L. Munn has written a vast and valuable volume entitled Handbook of Psychological Research on the Rat, in which wild rats are hardly mentioned. Munn’s treatise contains, inadvertently, a curious comment on the studies of rat behavior: it is in a chapter oddly entitled “Abnormal and Social Behavior,” and is to the effect that little research has been done on social behavior in rats because “. . . rats are not especially influenced by each others’ actions.” In what follows, I hope to show that the Bishop of Autun and his followers, the believers in the intellectual powers of rats, and Dr. Munn are all mistaken. The Bishop and the rest overestimate the murine status, Dr. Munn underestimates it.

There are two species of rats of world-wide importance. The so-called “black,” “roof,” or “Alexandrine” rats (Rattus rattus) range from black to tawny with a pale belly. This was the plague rat of the Middle Ages in Europe. The other species (already mentioned) is the common brown rat often called the Norway rat for no good reason; it is sometimes black, even in the wild state. Unlike R. rattus, which is a climbing species, R. norvegicus is a burrower. All laboratory rats are of this species,

Investigating food, two rats sniff liver, wheat, flour and sugar. Rats’
feeding behavior is part of this animal’s great curiosity: rats will
sample everything they encounter, but have the ability to choose
the most nutritious of several foods.
although they seldom get the opportunity to burrow. Matings between wild and white are fully fertile; the offspring are all wild-type, but their offspring come in variety of colors. R. norvegicus can, and does, carry plague, too.

There are other species of economic importance in the East and in Australia. They have been serious pests of sugar plantations, and in Java a small species upset Europeans by nesting inside their bamboo furniture.

Apart from “cleverness,” two things are likely to impress the casual observer of rats. One is their catholic choice of foods; the other is their capacity for multiplying. Rats will eat anything that men will, and some things we refuse. Apart from house mice, few animals are so versatile. The study of the feeding behavior of rats has been put on a rigorous basis during the past fifteen years or so, and has shown that their success owes much to a very delicate system of checks and balances, which had not been suspected by naturalists lacking a technique of controlled experiment.

To make this clear, we must first consider the exploratory behavior of rats. As everyone knows, if wild rats are disturbed in a familiar environment, they at once take to cover. But, if wild rats are released into an unfamiliar area, their response, despite the disturbance, is likely to be exploration. Even if they enter a place of concealment, they soon emerge to sniff about further.

The motivation behind such exploratory behavior is so powerful that it may have priority over both fear and hunger. Hungry laboratory rats will ignore food until they have investigated a new environment. The opportunity to explore can, indeed, be used as a “reward” in a learning experiment: if a rat is repeatedly put in a situation in which it can take one of two pathways, and if one path leads to a dead end, but the other to a larger area, then the rat tends to learn to go to the latter, just as it would learn to go to food or water.

Newcomer to colony, left, is groomed by
resident of same species—“amicable”
response, avoiding fight over territory.
An important difference, however, is that a rat learns to go to food or water only if it is hungry or thirsty, that is, suitably deprived; while exploration can be evoked without first imprisoning the animal in any way.

What exactly makes a rat go exploring? Ingenious experiments on laboratory rats have shown that this behavior is closely analogous to what we call curiosity in ourselves, and that rats often behave as if they wished to avoid boredom. The effect of the behavior is that it puts them in as wide a variety of situations as they can reach. In the wild state, there is no doubt that, within a certain area around the nest, every accessible point is regularly investigated; other things being equal, the more recently a place has been investigated, the less likely it is to be revisited.

Rats do not confine themselves to topographical exploration: they sniff, taste, and sample everything potable or edible they encounter. Here, too, the “boredom effect” is evident; if a variety of foods is available, all will at least be nibbled; and when a quantity of one has been eaten, the rat is likely to move over and take a second and third course off the alternatives.

This behavior is an essential element in the feeding behavior of rats. Just as general exploration gives the rat experience of every feature of the environment, so sampling of food informs it what sorts of nourishment are available. Through this sampling, a rat learns not only where food can be found, but also—in some instances—what the nutritional value of the food is. This ability to choose the better of two alternative foods has been very fully studied in laboratory rats. Of course, like most animals, they adjust the total amount of food eaten to their energy needs; but they do more than this. If they are made salt-deficient, and are offered a choice between plain and salty water, they choose the latter, even though before the deficiency existed they would have drunk the plain water. The same sort of effect has been observed with some, though not all, vitamins.

The picture so far presented is of insatiably restless, “inquisitive” animals, always poking their noses into something, especially something new, and sampling everything they can get hold of. This is true for laboratory rats, but if it applied also to wild rats they would be quite unable to survive the most casual efforts of men to poison them.

Subservient attitude is adopted by newcomer,
left, to resident. Newcomer is Rattus norvegicus;
resident, rattus.
Yet, in fact, they are decidedly difficult to eliminate by poisoning—and still more difficult to get rid of by other means. This is due to another feature of their behavior, one which is opposite to their exploratory activity and, so to speak, protects them from it. Wild, but not laboratory, rats tend to avoid anything new if it is encountered in a familiar environment. This superficially simple fact accounts not only for much of the difficulty we have in ridding ourselves of rats, but also for the widespread belief in their wiliness.

This behavior was first clearly described during the Second World War, at Oxford, England, as a result of research on the problems of protecting food stocks from pests. Wild rats in a settled colony develop regular habits of movement: they use fixed paths between nest or cover and food and, although they also explore elsewhere, one can be confident that they will use these paths each night. However, if an object—however harmless—is placed on such a path, the result may be a complete avoidance of that part of the home range. The avoidance may last only a few hours or it may go on for days. If the object is food, eventually it will be approached and sampled, at first only in the smallest quantities. What happens after this depends on the physiological effects, and the flavor, of the food. If both are satisfactory, the food will be eaten. If, however, the food contains poison, the ill effect may have time to make itself felt before a second sample is taken. In this case all feeding stops for a time, and eating the food comes to be associated with illness. This sequence of events has undoubtedly been responsible for a great many failures to kill rats by poison baiting. Repeatedly putting down the same poison bait merely makes things worse, since learning to avoid the bait is then regularly “reinforced,”

Poison shyness is, in fact, another aspect of an ability we have already mentioned—to choose the more nutritious of two foods. This, in practice, turns out to be an extraordinarily efficient protection against poisoning. It might be thought, for example, that if the poison were very highly toxic this would overcome the difficulty of coping with the initial, tentative sampling. But when this was tested with sodium fluoroacetate, which is several times more toxic than strychnine, it was found that some rat populations developed poison shyness to baits containing it. Yet this poison was so dangerous to man (several children have been killed by it) that it was clearly at or beyond the level of toxicity acceptable for general use.

Boxing rats, both of
norvegicus type, are
drawn here. The new-
comer, on left, seems
to be getting worst of
the fight.

During and after the War, this difficulty was solved in Britain and many other countries by the method of “prebaiting”—a technique we owe to the Oxford ecologists already mentioned. The rats are first trained to accept bait by distributing only a harmless mixture. After a few days, during which the rats may even come to adjust their movements to the visits of those laying the bait, poison is added and is then, as a rule, readily eaten by the animals.

This is a good example of a fairly simple technique developed from matter-of-fact, careful observation of what a pest actually does in typical environments. A short account, however, is inevitably oversimplified, and cannot do justice to the scale or difficulties of the research required. Also, we must add that there is a substantial objection to prebaiting—one which may account for the fact that it was never generally adopted in North America. It is a prolonged job, and it is difficult to train a lot of low-salaried people to do it properly. (Fortunately, there is now a way of getting round the need for prebaiting.)

In most of the scientific studies of poisoning rats, much stress has been laid on the importance of killing a large proportion of the total population. The reason for this has been especially clearly shown by workers in Baltimore. Consider how a population grows. If it starts with a small number, perhaps only with a litter of six from a single pregnant female, its growth is slow at first. However, if the environment is favorable, numbers increase geometrically, and there comes a stage of exceedingly rapid growth. This is sooner or later checked: the rate of increase declines until a maximum is reached, and this maximum is then maintained, provided the environment remains constant. Something like this “ideal” demographic sequence has been actually observed in a number of rat populations.

Slowing the increase in numbers and reaching a stable plateau depend on the action of controlling influences, which act with increasing force as population density increases. Such factors can include predation by dogs, cats, hawks, and men; a shortage of undisturbed nesting places; a shortage of food; and conflict among the rats themselves. The most important implication of all this, for practical purposes, is that it is futile to kill, say, around half a rat population by poisoning or trapping: the result of doing so is that the survivors then have greatly improved conditions (owing to the reduced competition for food and nests), and so breed much more rapidly; the population is thus soon restored to its original numbers—as has been directly observed, in Baltimore and other places,

The aim, therefore, must be either to kill a very high proportion of the total number, or to alter the environment so that it will not support so many rats. The latter has a more lasting effect, but is often far more difficult to achieve; it can rarely be accomplished quickly.

If a human community does succeed in reducing the shelter and other amenities available for rats, its efforts are substantially aided by the rats themselves. This is because in certain circumstances rats fight each other, with a consequent increase in the death rate and decrease in fertility of those “defeated.” The exact conditions of this fighting have been studied in experimental colonies of wild rats kept in large cages. The rats included both species we mentioned; they were either trapped as adults in the docks of Glasgow, Scotland, or were descendants of rats so trapped.

Two early experiments helped to clarify the sort of questions that would arise from a study of rat societies. In one, a number of males were put together in a cage; all were strangers to one another, and none had previously experienced the cage. They settled down, grew well, (rats grow throughout life) kept excellent health and slept together in groups in the nest boxes. This observation was especially surprising because of the twelve rats present, six were R. norvegicus and six R. rattus,
Female not in heat, at right, rejects
advances of a male—who,
aroused but frustrated,
may then fight
another male.

and the rivalry between these two species has long been known. In the other experiment, six males and six females, all R. norvegicus, formed the colony. After a few weeks, only one male remained—the largest; the females were still in excellent shape, and some were pregnant. Later, other comparable experiments gave results similar to those two.

An obvious question to ask, then, is: What are the causes of fighting among rats? But observation showed that it was equally profitable to ask: Are there features in the behavior of rats which tend to prevent fighting? We often think of rats as very combative creatures and ignore the fact that they can live in large thriving colonies, so we must ask a third question: What makes rats assemble?

Close observation and filming of rats in artificial colonies showed that wild rats, like tame ones, are intimately gregarious. They not only huddle together while asleep or resting, but they have a system of actions that promotes close contact. The most distinctive of these acts consists of crawling under the belly of another rat; it is seen especially among males in situations that might lead to conflict—as in the encounter of strangers described below. Rats deliberately walk over each other, too—an act that has nothing in common with one human being trampling on another; this is seen most often in peaceful conditions. Rats also groom each other; that is, they nibble gently another’s fur. This, like crawling under, is prominent in situations of conflict, but is not confined to them.

These activities all come in the category of “social signals,” of the type which tend to prevent conflict and to encourage herd behavior. Undoubtedly, odors are other “social signals,” though these are difficult to study. A female is recognized by a male as being in heat as a consequence of glandular secretions with a typical odor. It has been shown, in fact, that laboratory rats can recognize a receptive female by odor alone. Wild rats of both sexes leave odor trails as they move about their living space. Where they regularly run in contact with light-colored surfaces, they leave clear evidence of their passage in the form of a dark smear, and such trails are followed by other rats.

The conditions in which sociability breaks down and fighting breaks out are quite narrowly limited; and they can be defined with some precision from observations of rats kept in artificial cages and enclosures. If one wishes to establish a stable, healthy colony of wild rats in artificial conditions, the best method is to begin with a group of sexually immature individuals. They need not be litter mates. Alternatively, a colony may be started with a single adult male and several females. In either case conflict is exceedingly unlikely.

By contrast, there is one type of situation in which fighting is highly probable. This is when an adult male enters a region in which another adult male is already established. In one series of experiments twenty males were introduced into established colonies: all were attacked, and eighteen died.

Resident leaps at a newcomer. Fights sometimes follow “amicable” reaction when newcomer initially enters colony.
The addition of females rarely resulted in injury or death; and of young, never. Male R. norvegicus, when already settled in a cage, did, however, attack R. rattus of both sexes.

The attacks by residents on strangers are typical examples of territorial behavior. The conventional definition of a “territory” is a “defended area” In wild rats, territorial behavior is, with one exception, confined to males. The exception is the behavior of a female with young nestlings, and she defends not a large area, but only the nest itself—and even that not invariably. Granted that male rats indulge in territorial fighting, there is still the question whether they also fight for females. At first, during the study of experimental colonies, it seemed certain that they did; but, when detailed observations were made, it was found that there was no fighting for females at all. When a female was in heat, one or more males concentrated on copulating with her—an act which in rats can be repeated at intervals of a minute or two over long periods; there was no competition, and the female was quite indiscriminating.

Then what, it may be asked, are we to make of the high mortality among the males in mixed colonies, when in all-male colonies there are few or no deaths? A likely explanation is that it is due to a kind of “overflow” fighting, resulting from excitation evoked by females not in heat. In this case, the males, stimulated but frustrated, turned to fighting among themselves—although ordinarily, in this particular colony, there was little conflict.

The fighting in the colony just mentioned was harmless: the males concerned thrived throughout the ten weeks of the experiment. This was because the way in which the colony had been set up was exceptional: it had begun as an all-male group—a peaceful one as usual, and females had been added later. Evidently, by the time their subversive influence appeared, the males resembled one big, happy family, and not any number of females could evoke really serious fighting among them. They were behaving, in fact, like the members of a family group— among whom fighting never, as far as present knowledge goes, occurs.

How is this inhibition against fighting a member of the colony maintained? It seems that a group odor is responsible for it, but we have little detailed evidence. By analogy with other species, we might also expect that in stable colonies there would be some form of dominance hierarchy, or “peck order,” among the males. In its simplest form, there would be a dominant or number one rat, with prior access to females, food, nesting places, a second rat that would give way only to the first, and so on. In fact, nothing quite like this was found. In colonies of adults who had previously been strangers to each other, three types of male could be distinguished. The most successful, which have been called alphas, were always large compared with other members of the colony; they moved about confidently, and if there were any fighting, it was the alphas that initiated it.

Rolling on ground, newcomer norvegicus and resident,
on right, continues battle. Fights occur most com-
monly when a male enters area in which
adult male is already established. New-
comers died in eighteen of twenty
such cases studied.

A second type, the omegas, reached their status as the result of defeat by one or more alphas. In the experimental colonies, the omegas were marked by drooping posture and bedraggled appearance. They lost weight and died if not removed from the group. The third category comprises rats that, after defeat, adapted themselves to an inferior role: they have been called betas. They endured defeat without severe “shock” and fed with enough freedom to enable them to gain in weight. Betas and omegas associated together without conflict: there was no “hierarchy” among them.

There was no uniformity in the numbers of alphas, omegas, and betas in the small colonies studied, but one regularity was observed: in the conditions of intense conflict, which existed in the mixed colonies, there were fewer alphas than in the more peaceful, all-male colonies; often there was only one. In unconfined colonies, it seems likely that the adult males vary in status from alpha to beta; any rat with omega status would soon die or emigrate from the colony.

But in the last sentence, a question is implied: What do rats die of, in these conditions? This is one of those questions, obvious enough, that tend to go, not only unanswered, but also unasked. It is mentioned now because it would be wrong to give an account of the life of the wild rat--or of any other species—that implied that there were no more major problems to solve. It has been possible to show conclusively that rats under attack may die though quite unwounded; and that sometimes they die so quickly that there is no question of starvation through being barred from food (though that may be a factor on occasion). The rats die from “shock.” To say this is, however, hardly more meaningful than to say that they die from fright or from humiliation. Some of the physiological changes that these rats undergo are the same as those produced by forms of “stress” such as acute infection, exposure to severe cold, and so on, and the expression “social stress” has been used to describe this condition, but it is still only a descriptive term.

Rats, in fact, sometimes die in circumstances seemingly as mysterious as those of the so-called “voodoo death” in man—an expression used by the great physiologist W. B. Cannon, in a discussion of death resulting from no evident cause except emotional disturbance. Though we may not conclude from this that the Bishop of Autun was justified in putting rats under a curse, this unsolved puzzle does suggest some interesting possibilities for future research.

A member of the zoology department at Glasgow University, in Scotland, Anthony Barnett is also author of The Human Species, a biology of man.

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