Brain Structure and Correlation Patterns in Insectivora, Chiroptera, and Primates

Abstract

Multiple discriminant analysis and bivariate correlations of logarithms of progres- sion indices are applied to volumetric data on 11 major brain regions of 139 species of Insecti, vora, Chiroptera, and Primates. The pattern of projections of the coordinate axes of original variates on the first two discriminant functions appears to correspond to MacLean's (1970) division of the brain into the neomammalian, paleomammalian and protoreptilian regions. However, the peculiar position of the neocortex may be somewhat exaggerated by collinearity. The brain components which undergo the greatest relative volumetric increase from basal In- sectivora to higher Primates are, by order of decreasing importance, the neocortex, cerebellum, striatum, and diencephalon. The bulbus olfactorius and paleocortex are strongly correlated in Insectivora and prosimians, but this correlation drops abruptly in Chiroptera and in simians, where olfaction has regressed markedly while the paleocortex remained relatively stable, pre- sumably by developing its non-olfactory functions. The neocortex, bulbus olfactorius, septum, schizocortex, and hippocampus are positively correlated in Chiroptera, apparently because they are large relative to body weight in Megachiroptera and frugivorous Microchiroptera but small in insectivorous Microchiroptera. The neocortex may be more useful in frugivorous and nectarivo- rous bats-for environment recognition based on visual and olfactory stimuli-and relatively less useful in insectivorous bats, which often must react rapidly to echolocative signals. The negative correlations of the medulla oblongata with the bulbus olfactorius and paleocortex, in progressive Insectivora, appear to reflect the evolutionary regression of olfaction and the com- pensatory progression of the tactile trigeminal system (vibrissae) in aquatic Insectivora. (Brain structure; Chiroptera; correlation patterns; evolutionary compensations; evolutionary regres- sions; Insectivora; multiple discriminant analysis; Primates; progression indices.)