A quantitative comparison of the parts of the brains of two Australian marsupials and some eutherian mammals.

The size and composition of the brains of the quokka wallaby (Setonix brachyurus) and the brush-tailed possum (Trichosurus vulpecula) were compared with those of the rat, rabbit, cat, sheep, and human. Separate structures were taken from the brains by standardized dissection and their weights compared. Particular attention was paid to the relationships between the size of the amygdala and other brain structures. Marsupials were within the eutherian range in the ratio of whole brain weight to body weight. They were also within eutherian ranges in the proportion of the total brain weight formed by all structures, except the striatum, and in the proportion of the non-neocortical brain weight contributed by all structures but the olfactory bulbs. In all measures the marsupials were within the general range for eutherian mammals reported in the literature. The size of the amygdala was related significantly to that of the septum, hypothalamus, thalamus, lower brain stem, and olfactory bulbs.