We observed the spontaneous behavior of a laboratory marsupial - the gray short-tailed opossum (Monodelphis domestica) - in the elevated plus-maze (EPM) during six consecutive sessions and compared it with the behavior of Long-Evans rats. During the first exposure to the maze both species spent most of the time in the enclosed arms but opossums showed much higher frequency of entries into the open arms and stayed there longer. On the third and subsequent days opossums reduced their entries into the open arms and spent more time on the central square, where unlike rats they frequently groomed their lower belly and hind legs. During the last sessions they started spending more time in the enclosed arms. It is concluded that probably opossums, like rats show a stable anxiety evoked by open space. However, in the rat anxiety prevails over motivation to explore a new environment, while in the opossum it is initially at equilibrium with curiosity which habituates slower than in the rat. Results are discussed in the context of different ecology of the gray opossum that actively searches and hunts quickly moving insects. Thigmotaxic behavior, while strong in both species, dominates spontaneous behavior of the rat, but not opossum.
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