The social structure in rodents is highly variable. Some species, like pocket gophers, pocket mice, and kangaroo rats, are solitary. In the case of many fossorial species like pocket gophers (family Geomyidae) and Mediterranean blind molerats (Nannospalax), individuals tend to occupy burrow systems that do not overlap spatially, except for males and females during the breeding season. Multiple individuals in a burrow system are usually restricted to mother and offspring. In most cases, individuals tend to demonstrate high levels of aggression when confined in the same cage. Some fossorial species are considerably more social and maintain overlapping burrow systems. Many of these forms are highly colonial. For instance, African mole-rats of the family Bathyergidae have some solitary forms, yet several species including the naked mole-rat (Heterocephalus glaber), the Damaraland mole-rat (Cryptomys damarensis), and the common mole-rat (C. hotten-totus) are highly colonial. In fact, the naked mole-rat and the Damaraland mole-rat have a social system analogous to eu-social insects (e.g., ants and termites). Multiple females of social tuco-tucos (Ctenomys sociabilis) share the same nest and burrow system along with their young. Group size in these social fossorial rodents varies greatly, to over 100 in some species. The degree of group living and the length of times that individuals remain in the group are influenced by multiple factors related to resource availability and cost of dispersal. In fact, individual naked mole-rats that disperse from their natal group appear to have a different phenotype characterized by large fat stores.

Other species of rodents tend to be gregarious. Prairie dogs, ground squirrels, and marmots live in colonies that have a well-defined structure consisting of related females that tend to stay within their natal group and males that disperse from neighboring groups. All of these species demonstrate sophisticated alarm calls in response to predators and other disturbances to alert other members of the colony, especially related individuals. In the case of Belding's ground squirrel (Spermophilus beldingi), the frequency of alarm calls, a behavior that can prove detrimental to the caller, tends to increase among related individuals. Under certain circumstances, both ground squirrels and prairie dogs display a form of infanticide known as marauding behavior, whereby adult individuals kill the young of a lactating female. In the case of ground squirrels this behavior appears to be directed towards non-kin.

Beavers live in colonies consisting of offspring and an adult male and female, and each colony occupies a defined territory. Individuals in the colony assist with construction of dams and lodges. Other rodents are also found to tend towards monogamy, e.g., South American acouchis and pacas, although they are less social than beavers. Additionally, some species of voles and deermice live in family groups like beavers. These species show strong incest avoidance.

Many species of South American caviomorph rodents are colonial, living in colonies with established male linear hierarchies maintained through dominance and aggression. Nearly all colonial species of caviomorph rodents are highly vocal and use a series of sounds to communicate warning, courtship, play, and aggression. Even some species of mice and rats are colo-

nial and have dominance-based social systems similar to other unrelated social species of rodents.

Species of the order Rodentia communicate using visual, aural, olfactory, and vocal signaling. Vocalizations such as squeaks, grunts, and barks are used as alarm calls, for sexual and aggressive functions, and for seeking misplaced young. Tree squirrels of the family Sciuridae have been known to use visual communication during mating by waving and shivering their tails. Several Rodentia species employ footdrumming, a characteristic thumping or stamping pattern made with the feet, to differentiate between neighbors and outsiders—though in African mole-rats (Bathyergidae), footdrumming is also used during the breeding season to announce an individual's presence to potential mates. Olfactory messages are also vital in marking territorial boundaries and recognizing colony members. Scent-producing facial and anal glands, as well as urine and fecal matter, are used to produce these signals.

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