EVOLUTIONARY RADIATION OF VISUAL AND OLFACTORY BRAIN SYSTEMS IN PRIMATES, BATS AND INSECTIVORES

How brains have evolved in response to particular selection pressures is illuminated by ecological correlates of differences in brain struct ure among contemporary species. The focus of most comparative studies has been on the overall size of brains relative to body size, hence ig noring the ways in which selection operates on specific neural systems . Here we investigate evolutionary radiations in the size of visual an d olfactory brain structures within three orders of mammals: primates, bats and insectivores. The comparative relationships within these thr ee orders show both similarities and differences. After removal of the allometric effect of overall brain size, the sizes of different struc tures within each sensory modality are positively correlated in all th ree orders. Correlations between visual and olfactory structures, howe ver, are negative in primates, negative but non-significant in insecti vores, and positive in bats. In both primates and insectivores, noctur nal lineages tend to have larger olfactory structures than do diurnal or partly diurnal lineages, and among the primates diurnal lineages ha ve larger striate visual cortexes. Hence the apparent trade-off betwee n vision and olfaction in primates seems to be related to the divergen ce of nocturnal and diurnal forms. However, negative correlations betw een visual and olfactory structures were also found when nocturnal str epsirhines and diurnal haplorhines were analysed separately, suggestin g that ecological Variables in addition to activity timing may be sign ificant. Indeed, there were also associations with diet: frugivory was associated with enlargements of the geniculostriate visual system in diurnal primates, enlargements of olfactory structures in nocturnal pr imates, and possibly enlargements of both in bats. Further ecological associations were found within insectivores: aquatic lineages had smal ler olfactory structures than in their non-aquatic counterparts, and f ossorial lineages had smaller optic nerves than in non-fossorial forms . We conclude that activity timing, diet and habitat have each played a role in the evolutionary radiation of mammalian sensory systems, but with varying effects in the different taxa. Some of the associations between ecology and sensory systems suggest alternative explanations f or correlates of overall brain size, which have in the past commonly b een interpreted in terms of selection on intelligence.