Higher neuron densities in the cerebral cortex and larger cerebellums may limit dive times of delphinids compared to deep-diving toothed whales

Abstract

Since the work of Tower in the 1950s, we have come to expect lower neuron density in the cerebral cortex of larger brains. We studied dolphin brains varying from 783 to 6215g. As expected, average neuron density in four areas of cortex decreased from the smallest to the largest brain. Despite having a lower neuron density than smaller dolphins, the killer whale has more gray matter and more cortical neurons than any mammal, including humans. To begin a study of non-dolphin toothed whales, we measured a 596g brain of a pygmy sperm whale and a 2004g brain of a Cuvier’s beaked whale. We compared neuron density of Nissl stained cortex of these two brains with those of the dolphins. Non-dolphin brains had lower neuron densities compared to all of the dolphins, even the 6215g brain. The beaked whale and pygmy sperm whale we studied dive deeper and for much longer periods than the dolphins. For example, the beaked whale may dive for more than an hour, and the pygmy sperm whale more than a half hour. In contrast, the dolphins we studied limit dives to five or 10 minutes. Brain metabolism may be one feature limiting dolphin dives. The brain consumes an oversized share of oxygen available to the body. The most oxygen is used by the cortex and cerebellar gray matter. The dolphins have larger brains, larger cerebellums, and greater numbers of cortex neurons than would be expected given their body size. Smaller brains, smaller cerebellums and fewer cortical neurons potentially allow the beaked whale and pygmy sperm whale to dive longer and deeper than the dolphins. Although more gray matter, more neurons, and a larger cerebellum may limit dolphins to shorter, shallower dives, these features must give them some advantage. For example, they may be able to catch more elusive individual high-calorie prey in the upper ocean.

Comparative morphology of the spinal cord and associated vasculature in shallow versus deep diving cetaceans

“…Thus, the spinal cord represents a relatively smaller proportion of TBM in the larger bodied, deep diving beaked whales than the small bodied, shallow diving delphinids. These results are consistent with the “inexpensive body hypothesis” and are similar to the pattern observed for the metabolically expensive brain in shallow versus deep divers (reviewed in Pabst et al, 2016; Ridgway et al, 2019). However, the slope and intercept for this allometric relationship is similar between the shallow and deep diving species, which suggests a shared growth pattern for all species investigated herein.…”

“…The spinal cord mass for each vertebral element was estimated by multiplying the spinal cord volume by a standard neural density of 1.036 g/cm 3 , the specific gravity of gray matter in humans (Gompertz, 1902; Ridgway et al, 2019; Stephan, 1960; Stephan et al, 1981). A standard spinal cord density was not found within the literature, so the density of brain, a CNS component, was used as a substitute.…”

Systematic validation and assessment of immunohistochemical markers for central nervous system pathology in cetaceans, with emphasis on auditory pathways

“…1:200), intranuclear neuronal IR was evident in at least one of the two auditory nuclei in all investigated specimens. The reasons could include species-specific functional specializations [ 57 ], and the higher intensity of the heat-induced antigen retrieval method used by the semi-automatic immunostainer used in our study. A higher initial demasking may have exposed more of the antigen, making it more available for the antibody, even at a higher dilution of the latter.…”

The Relevance of Ecological Transitions to Intelligence in Marine Mammals

“…Obtaining neuron counts used to be prohibitively time consuming, but newer methods allow much more rapid counting (von Bartheld et al, 2016). Notably, the current record holder for total number of cortical neurons is the killer whale, with over 40 billion cortical neurons (Ridgway et al, 2019). The pilot whale is a close second, with 32 billion (Mortensen et al, 2014), twice what humans average at 16 billion (Herculano-Houzel et al, 2015).…”