Information from Lay-Language Summaries is Embargoed Until the Conclusion of the Scientific Presentation
173—Evolution and Comparative Anatomy I
Sunday, November 10, 2013, 8:00 am - 12:00 noon
173.10: Strength in numbers: Behavioral specialization, neuron number, and neuron density in three carnivore species
Location: Halls B-H
*B. M. ARSZNOV1, S. T. SAKAI2; 1Psychology, 2Dept. of Psychology and Neurosci. Program, Michigan State Univ., East Lansing, MI
Abstract Body: A principle of comparative neurology is that behavioral specialization corresponds to enlargement of neural regions subserving that behavior. Such neural expansions are hypothesized to also correspond to an increase in neuron number in these regions. Primates possess relatively large brains largely due to an expansion of neocortex. This increase in neocortical size is largely attributed to an increase in neuron number. But both neuron densities and regional brain size vary across cortical regions making it difficult to determine if larger brains and expanded regional brain volumes are due to increased neuron number in other species. Our previous work in Hyaenidae and Procyonidae found that species exhibiting behavioral specializations related to social and physical environments possess larger functionally related brain regions. However, it remains unclear if the regional brain expansions found in these species is due to an increase in neuron number since these results were based on virtual endocasts created from computed tomography scans of the skulls. Here we sought to determine the relationship between total brain size, regional brain volume (frontal and somatosensory cortices) and neuron number in 3 carnivore species: spotted hyena (Crocuta crocuta), raccoon (Procyon lotor), and dog (Canis familiaris). Nissl stained sections through the orbital frontal cortex, proreal cortex, and somatosensory cortex were stereologically analyzed. Endocranial volume was obtained from three-dimensional virtual endocasts created from computed tomography scans for the spotted hyena and the raccoon, and a volumetric reconstruction from a magnetic resonance image scan for the dog. Here, we found an increase in brain size is associated with a decrease in neuron density, as indicated by a strong negative correlation between endocranial volume and neuron density. Second, variations in neuron density patterns were noted. Somatosensory cortex contained the highest density of neurons compared to more anterior regions of cortex, including proreal and orbital frontal cortex in all three species. The raccoon noted for its dexterous forepaw use also possessed the greatest neuron density, neuron number, and cortical volume in somatosensory cortex in comparison to either the dog or spotted hyena. The spotted hyena noted for social complexity possessed the greatest cortical volume and neuron number in the orbital frontal cortex compared to the dog and raccoon. Overall, these preliminary findings in selected carnivore species suggest that behavioral differences correspond to regional brain variations including neuron number and density differences.
Lay Language Summary: Are we smarter because we have big brains? Although scientists have long pondered this question, we still do not have a clear answer. We often think that big brains have more neurons than small brains and that more neurons should mean better cognition. These assumptions were addressed in this preliminary analysis of brain volume and neuron number in three carnivore species: the spotted hyena, raccoon, and dog. These species all differ in overall brain size and behaviors. We found that the spotted hyena has the largest overall brain size and lowest neuron density, while the raccoon possesses the smallest brain and greatest neuron density with the dog brain size and neuron density falling between the two species. Our most interesting findings suggest that despite these overall differences, regional variations including differences in volume, neuron number, and density correspond to behavioral specializations. The raccoon, a species noted for its forepaw use and sensitivity, has the greatest relative volume, number of neurons, and density in its somatosensory cortex compared with other species. Notably, the spotted hyena, a social species that lives in complex hierarchically organized groups, possesses the largest cortical volume and neuron number in the orbital frontal cortex, a brain area thought to be involved in mediating a variety of social behaviors. These initial findings suggest that greater brain region size and neuron number relate to increased information processing demands, such as the raccoon’s specialized sensory processing required by highly discriminative forepaw use and the gregarious spotted hyena’s social information processing. Although it may seem intuitive that large brains should contain more neurons than smaller brains, a recent study in selected primate, rodent, and insectivore species found that neuron density decreases as brain size increases (Herculano-Houzel, 2011). The scaling relationship between neuron density and brain size differs across these mammalian orders. Here, we provide new data in three carnivore species that support the inverse relationship between brain size and neuron density, but we lack sufficient comparison species to determine the scaling relationship for these variables. Additionally, the enlarged somatosensory cortex in the raccoon supports the principle that the relative amount of brain tissue devoted to a particular behavior increases with a greater demand for processing that information. This concept that both cortical volume and neuron number increase with the demands for information processing has been well known for sensory systems. However, we demonstrate for the first time in the spotted hyena that the frontal cortex, a brain area associated with executive and cognitive functions, is larger in volume and neuron number in this social carnivore relative to the other species. This finding suggests that the demand for processing social cognitive information is greater in the hyena. Also, based on previous work, the spotted hyena has the largest relative frontal cortex volume compared to less social hyena species. To examine the relationship between total brain size, regional brain variation, and neuron number in the three species, two neuroimaging techniques (computed tomography scans of skulls and magnetic resonance imaging) were integrated with stereological analysis of Nissl stained brain sections through the orbital frontal cortex, proreal cortex, and somatosensory cortex. Our future studies will examine additional carnivore species varying in overall brain size and specialized behaviors in order to determine the relationship between brain size, regional brain areas and neuron number with behavior.
Neuroscience 2013 (43rd annual meeting of the Society for Neuroscience)Exit