There is limited funding to study wildlife, so typically the species that are selected for study are those that are showing population declines or are of interest to the researcher for a specific reason. In the case of opposums, they do very well in the midwest, so they are not often studied. There is some published literature; this is just a sample: 1. Holmes, A. C. V., and G. C. Sanderson. 1965. Populations and Movements of Opossums in East-Central Illinois. Journal of Wildlife Management 29:287–295. 2. Hoffmeister, D. F. 1989. Mammals of Illinois. University of Illinois Press, Urbana and Chicago, Illinois, USA. 3. Hofmann, J. E. 2008. Field Manual of Illinois Mammals. Illinois Natural History Survey, Champaign, Illinois, USA.