Conditions for Conflict 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. Or a single adult male with one or more females may be used. In either case conflict is exceedingly unlikely; there are instances of males attacking females, but they are rare. 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. THis holds even if the resident male or males have been brought up from birth only with their mother and siblings, and consequently have never fought before, except in play. A series of experiments was carried out with small artificial colonies, some consisting only of adult males, some of males and females, some of rats which had been brought up together from the age of six weeks or younger. Of twenty males added to such colonies, eighteen died. There was no corresponding mortality among the residents. Similar observations, some unpublished, have also been made on the effect of introducing males into a large cage containing only one resident male. By contrast, the addition of females rarely resulted in injury or death. No attacks on your rats were seen. The account so far given does not, however, prove that fighting between males is purely, or even mainly, territorial. It might be that males sometimes fight each other "spontaneously", or for a female. It has already been stated that males brought up together from youth do not fight; but even adult males, strangers to each other, put together in an artificial colony may live tranquilly together. For this to happen, two conditions must be observed; first, they must all be strange to the cage or enclosure; second, they must all be introduced at the same time. Even an interval of ten minutes between one male and the next may alter the situation. Provided these conditions are satisfied, even all-male colonies may be maintained without conflict. Hence an encounter between males need not involve fighting; indeed, in a settled colony, a clash between males is unusual; the rats either ignore each other or behave in one of the "amicable" ways already described. These observations show that crowding is not by itself a cause of conflict among wild rats. Source: the book "The Rat: A Study in Behaviour" by S.A. Barnett