 The rat is a social animal, communal burrows, from which highway systems are constructed and along which they meet their associates. They seek food in company and quench their thirst in company and reproduce in aggregates. Our familiarity with the rat is usually based upon its living in close association with humans. We shall, however, be interested in the rat's own home life, the behavioral maturation of a young rat until it becomes an adult integrated into the rat's society. This film was compiled over a six-week period. It was taken at Towson, Maryland on a quarter-acre tract of land surrounded by a rat-proof fence. Within this pen, the several colonies shown in this film were observed for 27 months. Individual rats were marked with numbered aluminum tags inserted in their ears. They were also marked foresight identification by removing patches of fur. Rats construct burrows which form their major locus of orientation. Excavated dirt forms arc-like mounds. Tails connect neighboring burrows. As the underground tunnel system enlarges, the surface becomes one fused mound. Only one-third of the mound of this burrow is shown here. Entrances about which there is no excavated dirt occur toward the periphery of the burrows. Note the rat entering such a bolt hole. Radio extension of tunnels which emerge to the surface form these bolt holes. Departure from burrow systems is frequently with considerable caution. Trails from burrows may lead through holes and fences from which the rat can proceed to its source of food or it may join other rats before it again returns to its home burrow by way of the trailway system. Structure their environment in many ways including the denuding of vegetation. One of the important ways in which rats structure their environment is by plugging entrances into burrows. Here, a lactating female is sealing the entrance with dirt and rocks from the inside. The sealing of entrances is most frequently exhibited by lactating females. Low social rank also contributes to the increase in frequency of sealing burrows. After completing her job, the female comes out and inspects her handiwork. Other rats which might have entered and approached her young pass by. A single entrance may be used by several rats as the four rats shown here indicate. A female begins to modify this situation by plugging this entrance with grass. Her work is soon taken over by a male. Note how this rat pulls on the grass with both teeth and fore feet and that the fore feet are used alternately as in the overhand pulling of a rope. Several rats immediately inspect the recently plugged entrances. Sticks are also dragged over burrow entrances where they form a partially effective barrier. Rats are continually altering the appearance of their environment by gnawing on wooden objects. They are agile climbers. Frequently trees at night become festooned with rats. The tail assists in balancing. Manipulation of food forms a major category of manual abilities. Food objects held in the fore paws are rotated as eating progresses. When the objects cannot be reached with the teeth, they are raked toward the rat with the fore feet. Self manipulation as in this cleaning of the tail frequently occurs. Included in self manipulation is the grooming of the fur. The structuring of the environment by rats produces situations in which social interaction is increased. Trails are one such situation. Trails connect goals. They lead from the burrow to the source of food. They connect adjoining burrows. Increased contacts also occur where trails from burrows converge and pass through a barrier as at this fence. Structuring of the environment makes it easier for young to orient between goals. Note how this mother is followed by one of her young. Tails enable month old young to find water at some distance from their homes and there to meet strange adults. On the long course of maturation and integration into society, the rats' first associations are with its mother. She delivers them into the world. Naked, blind and helpless, they would not long survive without her maternal care. By two weeks of age, they are well furred but their eyes have just begun to open. They rarely venture out of the nest. It is also during this period of life of the young that the mother is particularly aggressive in driving away any other rats which approach that portion of the burrow where her litter is located. She even repels the dominant male of the entire colony of 150 adults as he approaches from the right. Or in this case, she nips a male who attempts to invade her burrow. Between two and three weeks of age, initial explorations within the burrow are begun. However, if the young venture forth, particularly if they get outside the burrow, the mother will retrieve them. Up to 21 days of age, their only contact with adults is usually with their mother. By the time they are 23 to 25 days of age, they are quite active, make independent excursions within the burrow and in its immediate vicinity. But even here, they frequently remain close association with their mother. The 4 to 5 weeks old rat is characterized by hesitancy in leaving its burrow and by dashing motions to and from burrows. These scenes illustrate the utilization of pre-existing trails by juveniles. This hypersensitivity of the 4 to 5 weeks old rat is also exhibited at a source of food it encountered for the first time. It finally obtains a piece of food and runs away with it. Here again is the dashing movement by a juvenile in food procurement. Note the 5 weeks old rat drinking under the drain spout. He dashes away upon the approach of an adult. The beginnings of social adjustment are visible in these scenes of a 5 weeks old rat upon its first approach to the source of water. Each time an adult approaches, it darts away. This is unlike the behavior of adults who much less frequently avoid the approach of another adult. Note how this juvenile rat reacts to adults whom he is first encountering at the source of food. He darts about the periphery of the group and runs away each time an adult approaches. The adults show much less avoidance of each other despite a rapidly dwindling food supply. By the time the rat is nearly a year old, it may actively compete over some restricted goal as these two males are doing over scent left by an estrus female. Most encounters between adults are relatively placid. The most common behavior is nose touching. Two individuals approach cautiously and nearly or actually touch noses. Following recognition, each individual goes its own way or the one of lower social rank nearly retreats. On guard or boxing behavior may be exhibited by one or both rats in an encounter. A rat which is stationary will frequently run away from a rat in motion. Throwing is a form of threat exhibited by dominant rats. Trife with their fellows accentuates the caution frequently exhibited in departing from the burrow and upon returning, they dart back in. Caution is also exhibited along trails. Caution is the prime characteristic of rats as in approaching the food source beyond these steps or in coming through a passage which necessitates their passing near the burrow of more dominant rats or in approaching a strange object as these traps. Strange noises as the rattling of paper by the rat entering the garbage can causes withdrawal as by the rats on the left. This particular burrow system and its radial extensions housed 13 females, most of whom had litters. They formed the harem of the most dominant male of the population. This dominant male may be recognized by his crumpled hairless left ear at the food source or by a burrow hole. He roams above his harem's burrow at will. He guards the passage through the fence near his burrow. Rats living on the opposite side of the fence must pass through it to obtain food. After one sneaked past him, there is elicited this dirt pushing which has erotic implications and may be exhibited by dominant males following aggressive acts. Finally, he blocks the passage with his body. As males approach this passage, he assiduously chases them away. Only after the dominant male departs can these rats whose home is farther from the food supply go back and forth undisturbed. Males who have contended unsuccessfully possess much scar tissue and often large open wounds. Note the two males to the left. Such males are frequently forced to live in exposed suboptimal harbors as under this ladder. Although the dominant male was aggressive about his home burrow, he rarely exhibited aggression at the source of food. Here, he is the central rat in the garbage can. Water consumes a large part of each rat's nightly activity. Water may be lapped up directly or it may be secured by sitting in the forepaw and licking off the water. Or do may be licked off of grass. When the water supply is ample, several rats may drink together with little antagonism. When the food supply is ample, little antagonism is exhibited at the source, even though rats from different local colonies associate there. At a source of food which is ample but restricted in availability, there is usually pushing and mild threats. The antagonism exists among rats and an ample and readily available food supply. Food is continually being removed and transported. Rats of 35 days of age and older transport food. A single individual may make many consecutive trips. Most of the food is taken to the rat's home burrow. As the food is depleted, more rats aggregate at the food source. Even so, fighting such as occurs in territorial defense rarely occurs. The characteristic behavior is an increased irritability in the form of more frequent threats. Fighting may occur as over this last piece of palatable food, a crust of rye bread. Prior deprivation of water for 48 hours fosters large aggregates. There is a marked increase in irritability and the frequency of threats. Social rank influences the ability of maintaining position at the water. Males are attracted to the scent of females in estrus. Males inspect and follow females. The female leaves scent at the burrow entrance over sticks and on the ground. Note the feces. The scent of the estrus female forms a signpost. The male drags his genitalia over the sticks, gnaws upon them and saturates his fur by rubbing under them. Males inspect the scent about the burrow entrance. A male drags his genitalia over a scent post. Rocks become scent posts. Saturated with estrus scent is eaten. Other females compete over the priority to inspect the scent. Though they are members of his harem, this male approaches with caution, as one of the females also rubs against the passage through the fence. The predominant pre-copulatory behavior elicited by the estrus scent is erotic rolling about burrow entrances. The estrus female may remain just inside the burrow entrance. The male rarely enters after her and then only momentarily. Sexual rolling apparently may elicit an orgasm. At least the penis is cleaned as after copulation. The receptive female spends considerable time above ground. The male cleans its penis following mounting. Note the raising of the pelvic region by the female. The male sometimes falls over. The female, after being frequently mounted, becomes antagonistic toward males. The stress experienced by the female is aggravated by many males joining the pursuit. As soon as one male descends, another mounts. She finally escapes. When she emerges, the pursuit again ensues. Such intensive pursuit only exists at colonies lacking a clearly dominant male. Tension also develops between males. Such chases last many hours. This is in contrast to those colonies where a highly dominant male protects his harem from such stress. As the night wears on, the antagonism of the female toward males increases, as also does the tension among males. When females do manage to reach the water, males prevent them from satisfying their needs. In such colonies having an unstable social structure, the females have a lowered rate of conception, despite much copulation, and even those which conceive rarely rear their young. In the past scenes, we have shown some of the behaviors exhibited by rats, as well as a few of the situations in which these behaviors take place. Even so, it is apparent that the life of this so-called lower mammal is quite complex.