Social structure in synanthropic populations of a murid rodent Mastomys natalensis in Senegal

Synanthropic populations of Mastomys natalensis (A. Smith, 1834) were studied in southeastern Sénégal in order to evaluate some elements of their social structure. Analysis of trapping data consisting in single and multiple captures indicates that adult males were less likely to be involved in multiple captures than any other cate­gory of individuals. Sex-ratio in adults was significantly female-biased, and multiple captures involving 4 individuals or more (up to 13) were characterized by the small number of adult males involved (usually one per capture). Dyadic encounters between males revealed a higher rate of aggressive behaviours in M. natalensis than in its sibling species M. huberti. These observations suggest that these synanthropic populations are subdivided in polygynous units in which: (1) a part of males are evicted from the social group before their maturity and (2) adult males of different social units display aggressive behaviour towards each other.