Supplementary Material for: Comparison of Dolphins' Body and Brain Measurements with Four Other Groups of Cetaceans Reveals Great Diversity

We compared mature dolphins with 4 other groupings of mature
cetaceans. With a large data set, we found great brain diversity among 5
different taxonomic groupings. The dolphins in our data set ranged in
body mass from about 40 to 6,750 kg and in brain mass from 0.4 to 9.3
kg. Dolphin body length ranged from 1.3 to 7.6 m. In our combined data
set from the 4 other groups of cetaceans, body mass ranged from about 20
to 120,000 kg and brain mass from about 0.2 to 9.2 kg, while body
length varied from 1.21 to 26.8 m. Not all cetaceans have large brains
relative to their body size. A few dolphins near human body size have
human-sized brains. On the other hand, the absolute brain mass of some
other cetaceans is only one-sixth as large. We found that brain volume
relative to body mass decreases from Delphinidae to a group of
Phocoenidae and Monodontidae, to a group of other odontocetes, to
Balaenopteroidea, and finally to Balaenidae. We also found the same
general trend when we compared brain volume relative to body length,
except that the Delphinidae and Phocoenidae-Monodontidae groups do not
differ significantly. The Balaenidae have the smallest relative brain
mass and the lowest cerebral cortex surface area. Brain parts also vary.
Relative to body mass and to body length, dolphins also have the
largest cerebellums. Cortex surface area is isometric with brain size
when we exclude the Balaenidae. Our data show that the brains of
Balaenidae are less convoluted than those of the other cetaceans
measured. Large vascular networks inside the cranial vault may help to
maintain brain temperature, and these nonbrain tissues increase in
volume with body mass and with body length ranging from 8 to 65% of the
endocranial volume. Because endocranial vascular networks and other
adnexa, such as the tentorium cerebelli, vary so much in different
species, brain size measures from endocasts of some extinct cetaceans
may be overestimates. Our regression of body length on endocranial
adnexa might be used for better estimates of brain volume from endocasts
or from endocranial volume of living species or extinct cetaceans.