Brain-Body Size Relations in Grasshopper Mice

Abstract

Cranial volumes were measured on museum specimens of two species of grasshopper mice, Onychomys leucogaster brevicaudus and Onychomys torridus pulcher. These were compared with body weights and body lengths, as recorded on the museum tags. On average, females were heavier and slightly longer than males, but they had smaller cranial volumes by about 3%. Statistically, the differences were marginally significant (with no difference in body length for one species), but the pattern of differences was the same in both species, and therefore probably real. This casts suspicion on the common notion that a larger body size requires a larger brain. It also questions the real meaning of encephalization quotients.

Cortical Cyto- and Chemoarchitecture in Three Small Australian Marsupial Carnivores: <i>Sminthopsis macroura</i>, <i>Antechinus stuartii</i> and <i>Phascogale calura</i>

“…The cytoarchitectural similarity of the region we have defined as M1 with motor cortex in both diprotodont Neylon, 1978, 1979;Ashwell et al, 2005], polyprotodont [Haight and Neylon, 1981] and small bodied eutherian species [Franklin and Paxinos, 2007] argues for the existence of a separate motor cortex in these small marsupial carnivores of comparable size to that seen in rodents of similar body size and neocortical specialization [Franklin and Paxinos, 2007]. Our cyto-and chemoarchitectural findings are also consistent with the existence of architectonically distinct multiple sensory cortical fields as seen in larger marsupials [Beck et al, 1996;Karlen and Krubitzer, 2007] and eutherians of similar brain size [Hutcheon et al, 2002;Mann and Towe, 2003;Franklin and Paxinos, 2007] and support the contention that marsupials have evolved an array of cortical specializations that are similar to those seen in eutherians occupying similar habitats or consuming similar diets [Hutcheon et al, 2002;Mann and Towe, 2003;Karlen and Krubitzer, 2007]. The absence of a corpus callosum in the marsupials does not appear to have any significant influence on cortical cytoarchitecture relative to placental mammals.…”

“…In the present study, brain mass compared to body mass was similar in these species compared to other small carnivorous eutherians, e.g., several species of bats [Hutcheon et al, 2002] and one of the few carnivorous rodent genera, the grasshopper mouse [ Onychomys leucogaster and O. torridu s; Mann and Towe, 2003]. All three small marsupial carnivores were found to have primary somatosensory and visual cortices occupying 8 to 9% of the total pallial area.…”

Correlated Changes in Occlusal Pattern and Diet in Stem Murinae During the Onset of the Radiation of Old World Rats and Mice

“…Moreover, a strict insect‐dominated carnivorous diet might have also led to cognition modifications due to development of the brain, because insectivorous small mammals display comparatively larger brain sizes when compared to generalist and herbivorous related species (Harvey et al. ; Mann and Towe ). Such a diet might also have increased the emergence of territoriality and nocturnal activity in this taxon.…”

“…Such a diet might also have increased the emergence of territoriality and nocturnal activity in this taxon. Extant carnivorous rodents occur at relatively low densities and adults are indeed solitary and territorial, being extremely aggressive and lethal to their congeners (Nowak ; Mann and Towe ) and are nocturnal species (Kingdon ; Nowak ). However, the supposition of solitary, low‐density populations for Potwarmus and Antemus may be contradicted by the fossil record: both genera are very common in their assemblages (e.g., Mein and Ginsburg ).…”

“…Such dispersal could be explained by the fact that the acquisition of an insect-dominated carnivorous diet may have reduced competition with the plant-dominated omnivorous muroid species. Moreover, a strict insect-dominated carnivorous diet might have also led to cognition modifications due to development of the brain, because insectivorous small mammals display comparatively larger brain sizes when compared to generalist and herbivorous related species (Harvey et al 1980;Mann and Towe 2003). Such a diet might also have increased the emergence of territoriality and nocturnal activity in this taxon.…”