About 3.5 million years ago that the genus Rattus evolved from the family of Murid, and from the emergence of the rattus genus are now what we call the Norway rats or the common house rats which took place sometime around 2 million years ago. Long ago, Norway rats originally came from the present Northern China. Due to the constant influx of trade and commercialism during those times, Norway rats have found their way to ships and caravans that travel across the continents transporting themselves into many different places in the world. Initially, they were prevalently transported to Europe as the center of ancient trade and commerce, most merchant travels towards there. Eventually, as the span of trade increased its coverage all over the world, it wasn't then long until Norway rats inhabited everywhere. It was around 18th century that Norway rats have arrived in the new world, or what is now US. At present, rats are found everywhere in the world, inhabiting in human and non-human territories, but mostly dependent on human beings for resources in a relationship called commensalism.
Most wild rats would live in their own colonies. In female rats, which are more than half the time family related, would live altogether in a system of burrows of about 6-8 groups at a time. All of them would have their own nest chambers where they could breed their young. However there are instances wherein two or more female rats coming from the same familial origins would live together in one burrow raising their young. This occurrence is a common thing in female rats. It's called communal nesting. Right after the weaning period, the males of the younglings, will immediately disperse and live life independently.
There is actually an organization in social structure among rats. Would you actually believe that the mating system for male rats would contingently depend on the population density of a particular colony? If the population of a specific colony has low densities, one male would most likely monopolize a burrow of females. By saying monopolize, it simply means that the male rat would mark that particular burrow as his territory, preventing all other male rats in the colony from ever getting into that burrow and would only mate with the females on that group inside the burrow. Basically, male rats preserve a polygynous mating system wherein they could have more than one female mate at a time. Therefore, in a low density population colony, rats are most likely to become polygynous and territorial.
However, there are differences when a colony's population is of high density. If a colony is heavily populated, there is already a slim to no chance at all that a male rat would be able to defend its territory, let alone own one at all. The reason is simply because there are just too many possible intruders in a high density colony population. In this type of colony, male rats are mostly like to follow a despotic system, wherein other male rats would become subordinate to socially dominant ones. In this system, males will no longer feel the need of ever having to defend their burrows. Whenever a female comes into heat, males would immediately rush towards to mate with the female. Male rats could practically mate with multiple females and vice versa. They would feel no or less competition among themselves when mating with a female. Therefore, in a high density population colony, we can say that male rats are despotic and polygynandrous. They also work in individual level in order to search and find food resources.
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