Functional Correlates of Brain and Brain Region Sizes in Nonmammalian Vertebrates

Abstract

The relative size of the brain and its constituent regions varies greatly among vertebrate clades. This variation has long been associated with differences in behavior in accordance with several highly influential theories in evolutionary neurobiology. Although most of these theories were originally formulated to explain variation in mammalian brains, they are equally applicable to nonmammalian vertebrates. Comparative studies have demonstrated that relative brain size is associated with some measurements of cognition. This was recently corroborated by an artificial selection experiment in guppies in which large-brained fish have cognitive and other advantages over small-brained fish. The size of telencephalic regions is often associated with cognition as well, but studies of the avian hippocampus and spatial memory suggest that size might not be the only factor determining cognitive abilities. In contrast, there is overwhelming evidence that the size of sensory regions is associated with sensory abilities and specific ecological niches. Species that rely heavily on vision have enlarged visual processing brain regions, whereas species that rely more upon olfactory or somatosensory cues have enlarged brain regions associated with those senses. Multivariate analyses in fishes further demonstrate that increases and decreases in the relative size of sensory brain regions are related to the occupation of specific habitats. Finally, parental investment and energetics also play significant roles in the diversification of relative brain size in nonmammalian vertebrates. Greater parental investment in offspring is associated with an increase in relative brain size. The relative size of the brain is also the product of energetic trade-offs with other organ systems, including fat deposition and digestive system size. Variation in relative brain and brain region sizes is therefore an important component of understanding the evolution of the vertebrate brain.