Fig 1.
PET scans show that gray matter of the dolphin cerebrum and cerebellum have the highest metabolism in the brain.
Frontal (A, B) and horizontal (C, D) views of a Tursiops truncatus brain after FDG PET (A, C) and MRI (B, D) scans. Images have been modified from previous publications with the authors’ permission [7,8]. The color map indicates the relative degree of glucose metabolism. The images demonstrate that high metabolic areas (i.e., areas of increased glucose consumption; red) are mainly concentrated in the gray matter of the cerebral cortex and cerebellum, with the exception of smaller sub-cortical nuclei (e.g., inferior colliculi; thalamic gray matter). The PET and MRI scans are from the same healthy dolphin that was trained to lie still in the scanner.
Fig 2.
Boxes indicate where the brain samples were taken for the study.
Anterior-superior view of the Tursiops truncatus (Tt) brain. The supralimbic (auditory; red) and anterior paralimbic (motor; blue) areas are marked on either side of the brain.
Table 1.
A trend for larger paralimbic neurons in larger brains.
Table 2.
Neuron density by cortex layer.
Table 3.
Glia density by cortex layer.
Fig 3.
Comparison of a smaller short diver and a smaller long diver.
An illustration of the differences in brain mass, cortical surface area, cerebellum mass, average neuron density, and maximum dive time between one female adult delphinid (Tursiops truncatus; A) and one female subadult individual from the family Kogiidae (Kogia breviceps; B) of similar body size.
Fig 4.
Comparison of a larger short diver and a larger long diver.
An illustration of the differences in brain mass, cortical surface area, cerebellum mass, average neuron density, and maximum dive time between one female adult delphinid (Oo; A) and one female adult ziphiid (Zc; B) of similar body size. Zc, which can dive deeper than a mile and longer than an hour, has a smaller brain with fewer neurons than that of Oo, an animal of similar body size that performs shallower, shorter dives.
Fig 5.
Comparison of neonates and adults for the only two cetacean species where such data is available.
Brain and body size, brain cell density, and cortical surface area comparisons between neonates and adults representing two dolphin species, Tt (A) and Oo (B). All data are from individual animals. Brain and body mass data as well as neuron and glial cell densities for the neonate Tt were published previously [40].
Fig 6.
Distribution of total brain mass for mature Oo, Tt, Dd, Zc, and H. sapiens.
Each bar represents the percentage of total brain mass in the cerebellum (pink), gray matter of the cerebral cortex (green), and the remaining areas of the brain (ROB: brainstem, white matter of the cerebral cortex) are shown in blue. Mass of the gray matter was calculated by multiplying the cortex surface area and cortex thickness (see S1 Table) by the specific gravity of the gray matter of the cerebral cortex [52]. For average cortex thickness and surface area we used our own measurements. For the human cortex thickness, we used a value of 2.5 mm. The calculated values here are similar to H. sapiens values averaged from previously published data [74]. Cerebellum masses are values from a previous publication [37]. Kb was not included because we only had a brain from an immature animal.
Fig 7.
Dive time by neurons/kg of body mass.
Maximum dive time by total number of neurons in the cerebral cortex per kilogram of body mass in three delphinids, one member of family Ziphiidae, one member of family Kogiidae, and humans. Dive time data come from previous publications [12, 50, 78–80].