MONOGAMY IN HYPOGEOMYS ANTIMENA, AN ENDEMIC RODENT OF THE DECIDUOUS DRY FOREST IN WESTERN MADAGASCAR

Hypogeomys antimena, the largest endemic rodent of Madagascar, is excl usively found in a restricted area of less than 1000 km(2) in the deci duous 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 stud y (Cook er nl., 1991) suggest monogamy, a rare social system occurring in only 3% of mammals. By using the same methods of trapping and radi otracking during different seasons, investigations in the proposed soc ial system were carried out and provided a medium-term view of the soc ial structure. The data on Hypogeomys antimena were then used to test hypotheses about the evolution of monogamy in rodents by comparing spa cing behaviour and the size of home ranges of males and Females before (dry season) and after (rainy season) the birth of offspring. Males a nd females from neighbouring burrows have similarly-sized but mutually exclusive home ranges. Home ranges of pairs trapped at the same burro ws, however, always overlapped extensively. The morphological correlat es (no sexual differences in the body measurements, small testes in re lation to body size) are those of monogamous species. Pairs remain ass ociated during both breeding and non-breeding periods and pair bonds s eem 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 Hypogeom ys antimena lives in a monogamous social system. Home ranges are large r during the dry season before birth than during the rainy season afte r 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 i nfanticidal conspecifics and/or predators could explain the male's ass istance in rearing the young and the evolution of monogamy in Hypogeom ys.