Cellular scaling rules of insectivore brains
They are the smallest living mammals, smaller than what was supposedly the common ancestor that all current mammals share. Hence the question: are their miniaturized brains off the scales? Do they vary in size according to special scaling rules?
We determined the cellular composition of five insectivore species (smoky shrew, short-tailed shrew, star-nosed mole, hairy-tailed mole and eastern mole) by using the isotropic fractionator. Comparative analysis of the average cell numbers obtained for each species demonstrates that the insectivore cellular scaling rules overlap somewhat with those for rodents and primates.
The insectivore cortex shares scaling rules with rodents (increasing faster in size than in numbers of neurons), but the insectivore cerebellum shares scaling rules with primates (increasing isometrically). Brain structures pooled as “remaining areas” appear to scale similarly across all three mammalian orders with respect to numbers of neurons, and the numbers of non-neurons appear to scale similarly across all brain structures for all three orders.
Therefore, common scaling rules exist, to different extents, between insectivore, rodent and primate brain regions, and it is hypothesized that insectivores represent the common aspects of each order. The olfactory bulbs of insectivores, however, offer a noteworthy exception in that neuronal density increases linearly with increasing structure mass. This implies that the average neuronal cell size decreases with increasing olfactory bulb mass in order to accommodate greater neuronal density, and represents the first documentation of a brain structure gaining neurons at a greater rate than mass. This might allow insectivore brains to concentrate more neurons within the olfactory bulbs without a prohibitively large and metabolically costly increase in structure mass.