Calvo-Torrent, A., Brain, P. F., Martinez, M. 1999. Effect of predatory stress on sucrose intake and behavior on the plus-maze in male mice. Physiology and Behavior 67, 189-196.

The authors accomodated mice in the same room as rats, and noticed that these mice left a higher number of fecal boli in the elevated plus-maze test and that they showed a higher level of anxiety than control rats. It was concluded that the presence of rats was a source of stress for the mice and, therefore, that these species should not be routinely housed together.