Hypogeomys antimena, the largest endemic rodent of Madagascar, is exclusively found in a restricted area of less than 1000 km2 in the deciduous dry forest near the western coast. The facts so far known about its biology, like pair bonds, 1–2 offspring, lack of sexual dimorphism, and the results of a nine-week trapping and radiotracking field study (Cook et al, 1991) suggest monogamy, a rare social system occurring in only 3% of mammals. By using the same methods of trapping and radiotracking during different seasons, investigations in the proposed social system were carried out and provided a medium-term view of the social structure. The data on Hypogeomys antimena were then used to test hypotheses about the evolution of monogamy in rodents by comparing spacing behaviour and the size of home ranges of males and females before (dry season) and after (rainy season) the birth of offspring.

Males and females from neighbouring burrows have similarly-sized but mutually exclusive home ranges. Home ranges of pairs trapped at the same burrows, however, always overlapped extensively. The morphological correlates (no sexual differences in the body measurements, small testes in relation to body size) are those of monogamous species. Pairs remain associated during both breeding and non-breeding periods and pair bonds seem to continue for more than one reproductive season. Offspring stay together with their parents for at least one more year, implying that sexual maturation is probably not reached before the age of two years. The results are consistent with the previous suggestion that Hypogeomys antimena lives in a monogamous social system.

Home ranges are larger during the dry season before birth than during the rainy season after the birth of offspring. Pairs with offspring had smaller home ranges than those without. Males tended to stay closer to the offspring than females. This suggests that the protection of the offspring against infanticidal conspecifics and/or predators could explain the male's assistance in rearing the young and the evolution of monogamy in Hypogeomys.