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Grade Changes in Brain-Body Allometry: Morphological and Behavioural Correlates of Brain Size in Miniature Spiders, Insects and Other Invertebrates

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  • Abstract We use a recent wave of data to confirm that Haller's rule of brainâ€"body allometry, for smaller species to have relatively larger brains, holds for invertebrates as well as vertebrates. But different invertebrate taxa fall on several different allometric lines (grades). In the smallest animals in several grades, the brain occupies a large fraction (up to approximately 16%) of the total body mass. The brain and the structures enclosing it show morphological alterations suggesting a lack of housing capacity in the head for the brain (e.g. the brain extends into other parts of the body such as the legs or thorax), and other structures normally enclosed in the same area are displaced. Miniature animals may thus sacrifice some morphological aspects of body design to accommodate their disproportionately large CNS. The smallest animals of one such group, orb web spiders, do not show signs of behavioural limitation in web construction compared with larger relatives. We speculate that, because of selection resulting from the high metabolic costs of nervous tissue, grade changes may involve substantial modifications of how brains function, and help explain differences between neuron-profligate vertebrates and invertebrates having far fewer neurons (as few as approximately 200â€"500 neurons in two groups). Scaling problems associated with small size are of general importance, because many moderate-sized animals have very small free-living immature stages.

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  • 2011

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