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Brain mass and cranial nerve size in shrews and moles.

Leitch DB, Sarko DK, Catania KC - Sci Rep (2014)

Bottom Line: We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size.Trigeminal nerve number was by far the largest and also increased with increasing brain and body size.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA.

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.

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Scaling of brain size, body size, and numbers of myelinated axons within select cranial nerves in seven insectivore species.(A) Scatterplot of brain mass and body mass relationships. Species are indicated. (B) Log-transformed brain mass as a function of log body mass. y = 0.796x − 3.352; R2 = 0.984, p < 0.0001. (C) Olfactory bulb mass (g) as a function of brain mass. y = 2.493x − 2.606, R2 = 0.980, p < 0.0001. (D) Cochlear nerve axons as a function of brain mass. y = 0.300x + 3.990, R2 = 0.906, p = 0.0127. (E) Leverage plot of the residuals of the number of trigeminal axons as a function of brain mass. The average number of trigeminal axons for the star-nosed mole is indicated with the black arrow. (F) Trigeminal nerve axons as a function of brain mass excluding the outlier value from the star-nosed mole. y = 0.179x + 4.486, R2 = 0.845, p = 0.0096. The species symbols in (A) also apply to (B) through (D).
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f3: Scaling of brain size, body size, and numbers of myelinated axons within select cranial nerves in seven insectivore species.(A) Scatterplot of brain mass and body mass relationships. Species are indicated. (B) Log-transformed brain mass as a function of log body mass. y = 0.796x − 3.352; R2 = 0.984, p < 0.0001. (C) Olfactory bulb mass (g) as a function of brain mass. y = 2.493x − 2.606, R2 = 0.980, p < 0.0001. (D) Cochlear nerve axons as a function of brain mass. y = 0.300x + 3.990, R2 = 0.906, p = 0.0127. (E) Leverage plot of the residuals of the number of trigeminal axons as a function of brain mass. The average number of trigeminal axons for the star-nosed mole is indicated with the black arrow. (F) Trigeminal nerve axons as a function of brain mass excluding the outlier value from the star-nosed mole. y = 0.179x + 4.486, R2 = 0.845, p = 0.0096. The species symbols in (A) also apply to (B) through (D).

Mentions: Mammalian brain and body weight data are typically reported on logarithmic scales. We followed this convention to make our data readily comparable to that in similar studies (however, a simple linear model of the untransformed data yielded an R2 of 0.984 for the correlation between body weight and brain weight) (Fig. 3A). We examined regression statistics from both the shrews and moles, and these appeared to fit the conventional allometric equation (log(y) = log (b) + m[log(x)] or y = bxm, where y is average brain mass and x is average body weight. The slope m for the brain-body weight regression on log/log scales was 0.796, with R2 of 0.981, p < 0.0001 (Fig. 3B). Furthermore, variance in the logarithm of the mass of olfactory bulbs appeared to be largely explained by changes in either log brain (R2 = 0.980, p < 0.0001) (Fig. 3C) or log body mass (R2 = 0.957, p < 0.0001 plot not shown).


Brain mass and cranial nerve size in shrews and moles.

Leitch DB, Sarko DK, Catania KC - Sci Rep (2014)

Scaling of brain size, body size, and numbers of myelinated axons within select cranial nerves in seven insectivore species.(A) Scatterplot of brain mass and body mass relationships. Species are indicated. (B) Log-transformed brain mass as a function of log body mass. y = 0.796x − 3.352; R2 = 0.984, p < 0.0001. (C) Olfactory bulb mass (g) as a function of brain mass. y = 2.493x − 2.606, R2 = 0.980, p < 0.0001. (D) Cochlear nerve axons as a function of brain mass. y = 0.300x + 3.990, R2 = 0.906, p = 0.0127. (E) Leverage plot of the residuals of the number of trigeminal axons as a function of brain mass. The average number of trigeminal axons for the star-nosed mole is indicated with the black arrow. (F) Trigeminal nerve axons as a function of brain mass excluding the outlier value from the star-nosed mole. y = 0.179x + 4.486, R2 = 0.845, p = 0.0096. The species symbols in (A) also apply to (B) through (D).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4150104&req=5

f3: Scaling of brain size, body size, and numbers of myelinated axons within select cranial nerves in seven insectivore species.(A) Scatterplot of brain mass and body mass relationships. Species are indicated. (B) Log-transformed brain mass as a function of log body mass. y = 0.796x − 3.352; R2 = 0.984, p < 0.0001. (C) Olfactory bulb mass (g) as a function of brain mass. y = 2.493x − 2.606, R2 = 0.980, p < 0.0001. (D) Cochlear nerve axons as a function of brain mass. y = 0.300x + 3.990, R2 = 0.906, p = 0.0127. (E) Leverage plot of the residuals of the number of trigeminal axons as a function of brain mass. The average number of trigeminal axons for the star-nosed mole is indicated with the black arrow. (F) Trigeminal nerve axons as a function of brain mass excluding the outlier value from the star-nosed mole. y = 0.179x + 4.486, R2 = 0.845, p = 0.0096. The species symbols in (A) also apply to (B) through (D).
Mentions: Mammalian brain and body weight data are typically reported on logarithmic scales. We followed this convention to make our data readily comparable to that in similar studies (however, a simple linear model of the untransformed data yielded an R2 of 0.984 for the correlation between body weight and brain weight) (Fig. 3A). We examined regression statistics from both the shrews and moles, and these appeared to fit the conventional allometric equation (log(y) = log (b) + m[log(x)] or y = bxm, where y is average brain mass and x is average body weight. The slope m for the brain-body weight regression on log/log scales was 0.796, with R2 of 0.981, p < 0.0001 (Fig. 3B). Furthermore, variance in the logarithm of the mass of olfactory bulbs appeared to be largely explained by changes in either log brain (R2 = 0.980, p < 0.0001) (Fig. 3C) or log body mass (R2 = 0.957, p < 0.0001 plot not shown).

Bottom Line: We found that moles and shrews have tiny optic nerves with fiber counts not correlated with body or brain size.Trigeminal nerve number was by far the largest and also increased with increasing brain and body size.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, Vanderbilt University, Nashville, TN, USA.

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.

Show MeSH
Related in: MedlinePlus