Abstract
We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles. Species include tiny masked shrews (S. cinereus) weighing only a few grams and much larger mole species weighing up to 90 grams. It also includes closely related species with very different sensory specializations – such as the star-nosed mole and the common, eastern mole. We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size. Auditory nerves were similarly small but increased in fiber number with increasing brain and body size. Trigeminal nerve number was by far the largest and also increased with increasing brain and body size. The star-nosed mole was an outlier, with more than twice the number of trigeminal nerve fibers than any other species. Despite this hypertrophied cranial nerve, star-nosed mole brains were not larger than predicted from body size, suggesting that magnification of their somatosensory systems does not result in greater overall CNS size.
Highlights
We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles
The shrews used in this analysis ranged in total body mass from approximately 3.8 grams (S. cinereus) to 16 g (S. palustris) (Table S1 and Fig. 2A, B) whereas the mole body sizes ranged from roughly 50 grams for the smaller P. breweri and C. cristata to 90 grams for S. aquaticus (Table S1 and Fig. 2C, D)
The relationship between brain size and body size - and the significance of this ratio for behavior or information processing capabilities - has long been a topic of interest[2,23,24]
Summary
We investigated the relationship between body size, brain size, and fibers in selected cranial nerves in shrews and moles. In light of previous investigations of CNS adaptation found in the star-nosed mole (Condylura cristata) which have focused on the magnification within primary and secondary somatosensory cortex[13], principal trigeminal nucleus[14], and spinal trigeminal subnuclei[15] this tactile specialist offers the chance to compare potential changes of overall brain size as a possible consequence of increased sensory input. For example it is possible that natural selection has produced a larger than expected brain corresponding to the large amount of CNS devoted to the nasal representation This would be in keeping with investigations of a wide range of mammals of different niches and brain sizes across developmental time, indicating that enlargement of the entire brain (minus the olfactory system) is associated with selection for particular behavioral abilities[16]. These findings suggest a ‘‘simple rule’’ concerning the volume of particular areas of the brain increasing in a regular, predictable manner over development, with elaborations to particular areas occurring as consequence of alterations to the timing and duration of isocortical neurogenesis, resulting in a larger brain[4,17]
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