Ecology, postnatal development, morphometrics, and taxonomic status of the short-tailed opossum, Monodelphis dimidiata, an apparently semelparous annual marsupial

Abstract

Field data for M. dimidiata were collected from trapping sites near Balcarce, Buenos Aires Province, Argentina. Fallow pastures and native grasslands were live-trapped and snap-trapped as part of a rodent population of dynamics study, extending from Dec. 1968 to April 1970. The opposums represented only several percent of the total small mammal community capture each month. The greatest number were captured in the autumn (March-May), coinciding with the dispersal of the young. The animals were diurnal, with most of the activity concentrated in the late afternoon. Rarely were the opossums captured more than once. The species appears to be predominately insectivorous; it is not believed to be a major predator of rodents. Litters were documented for 2 summer months (Dec.-Jan.), with a high of 16 young recorded for 1 litter. Data on postnatal development were recorded for a limited number of young during the first 5 wk after birth, close to the time of eye opening. Morphometric data from lab and field animals, and skull measurements show that the young developed rather slowly through the cooler months; spring growth, especially in the males, increased drastically, with sexual maturation occurring in both sexes by summer. M. dimidiata is an apparently semelparous and annual species. Temporal distribution of age group classes and patterns of non-geographic variation provide the primary support for this conclusion. In keeping with its apparent reproductive strategy, M. dimidiata shows extreme sexual dimorphism. Non-geographic variation is documented. The differences between the sexies in M. dimidiata and certain other Monodelphis have sometimes resulted in taxonomic problems. Previously confounded with M. henseli (probably = M. sorex), M. dimidiata seems to be known with certainty only from Argentina and Uruguay. Although sometimes regarded as conspecific with M. fosteri conspecificity has not been demonstrated.