Huddling Soulairac has more recently given a formal demonstration of the familiar fact that laboratory rats do associate and huddle together in groups. He used a relatively large cage and allowed a group of four males and four females to live in it undisturbed. In these conditions, although there was opportunity to separate off into isolated individuals or pairs, in fact the rats slept together in a group. Only parturient females isolated themselves. Similar observations have been made on wild rats in large cages. If adult rats, strangers to each other and unfamiliar with the cage, are placed in it, they explore, but they also tend, even at this early stage to group together, especially in the corners of the main cage. Later, they can be studied during periods when they are inactive; these occur mainly during the day. They then sleep or rest in groups, either in the nest boxes attached to the main cage or in the corners of the main cage as before. This behavior is not merely a product of the shelter of the tactile stimuli offered by these places; if rats do rest on the floor of the cage they usually do so all in one corner; the corner chosen may vary, but the grouping remains rather constant. Similarly, when several nest boxes are available, only one or two may be occupied; when more than one, each is used by a group. Again, the choice of nest box may vary over a period, even though conditions in and around the cage remain unchanged. Rats often sleep, not merely lying alongside each other but piled in a heap. From time to time one at the top wakes or rolls off and then burrows its way into the bottom. This has often been observed in young wild rats. Huddling is one of a number of ways in which rats derive cutaneous stimulation from each other. What is its biological significance? It is commonly thought that huddling has a heat-conserving function. If this is its function, the stimuli to which a rat responds may be the temperature of the skin of other rats, the pressure of other rats' bodies or something else less obvious. It is certain that, in some circumstances, the heat provided by other animals is important for small mammals. This has been shown for rats by Gelineo & Gelineo. Nestling rats especially are dependent on heat from their mother, since they are unable to regulate their own temperature until they are at least eighteen days old. The colder the environment, the more important an external source of heat becomes. Nevertheless, Benedict has shown that in ordinary conditions in the laboratory huddling has no energy-conserving effect. Yet rats continue to huddle in these conditions. Huddling gives the impression of being an example of "innate" behavior but nothing is known of how it develops. The earliest contacts a rat has with other rats are with its mother and litter-mates in the nest immediately after birth; the first environment of a rat is in fact one in which cutaneous stimulation from other rats is almost continuous. Whether this influences later huddling has not been investigated. Source: the book "The Rat: A Study in Behaviour" by S.A. Barnett