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

Abstract

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.

Neuroanatomy and inner ear labyrinths of the narwhal, Monodon monoceros, and beluga, Delphinapterus leucas (Cetacea: Monodontidae)

“…; Ridgway et al. ), and descriptions of monodontid arterial retia and related vasculature (Vogl & Fisher, ,b, ). Previous cranial endocast descriptions are of particular interest because the most detailed relevant data comprise the sister clade to monodontids, the true porpoises or phocoenids (Racicot & Colbert, ; Racicot & Rowe, ), and a successive sister taxon, a delphinid Tursiops truncatus (Colbert et al.…”

“…; Ridgway et al. ), in the case of fossils it is difficult to separate brain mass entirely from other aspects of the endocranial space. Endocranial volumes and other measurements from extant specimens, with comparisons with actual brain volumes, can inform estimates for fossil specimens, making broader comparisons more tenable.…”

Is Cetacean Intelligence Special? New Perspectives on the Debate

“…Table 1. The above image highlights the current available statistics for the cerebral cortex of cognitive sophisticated terrestrial mammal species as well as several species of cetaceans (see [2,3,12,13,17,29,30,48,53,67,70,[77][78][79][80][81][82][83][84][85]). The table on top of the figure provides estimations of different physiological parameter for the brains of great apes, humans, and African elephants.…”

“…Furthermore, one of the most fascinating characteristics of their brains is the high degree of cortical gyrification and the resulting neocortical surface area. Therefore, in spite of the fact that recent studies have revealed great diversity amongst the brains found across this order (e.g., differences in brain parts, as well as body and brain measurements) [6,29], based on available data it is plausible to argue that, generally speaking, their brains appear to have followed the second pathway (Route 2), presented by the mathematical model being used to support the hypothesis exploring how brains evolved naturally and the causes of increased cognitive capacities. More specifically, if the assumption being made is that the depth of the information-processing hierarchy in the cerebral cortex of cetaceans is lower when compared to most of the intelligent terrestrial mammals species, then those defending complex cognition in cetaceans would need to demonstrate the existence of a higher neural connectivity (and thus higher metabolic costs), but, at the same time, a comparable (or higher) number of neurons in their cortices when compared to terrestrial mammal species for which there is enough documented evidence (besides humans) for their overall cognitive superiority, for example the great apes.…”

“…A mathematical model that is suited for making quantitative predictions about the cognitive capacities of species for comparative purposes is also provided. As the order Cetacea is a very diverse group [29], and due to the aforementioned reasons, little is currently known about the intelligence of most cetaceans [30]; hence, it is important to emphasize that the focus of the present study is primarily on large-brained cetacean species and/or those species with brains of comparable size to terrestrial species on which there are sufficient neurophysiological statistics and also documented behavioural evidence of their overall cognitive sophistication. Furthermore, the principal conclusions derived from this study must be interpreted according to the premises of the scientific method, that is, as predictions resulting from the extension of the mathematical model and a hypothesis that must subsequently be verified or refuted by future studies.…”