Ontogenesis of the sperm whale brain

Abstract

The development of the sperm whale brain (Physeter macrocephalus) was investigated in 12 embryos and early fetuses to obtain a better understanding of the morphological and physiological adaptations in this most exotic cetacean concerning locomotion, deep diving, and orientation. In male adult sperm whales, the average absolute brain mass and the relative size of the telencephalic hemisphere are the largest within the mammalia, whereas the ratio of the brain mass to the total body mass is one of the smallest. In the early sperm whale fetus, the rostral part of the olfactory system (olfactory nerves and bulbs) is lost, whereas the nervus terminalis seems to persist. Several components of the limbic system show signs of regression (hippocampus, fornix, mamillary body). In contrast, some components of the auditory system (trapezoid body, inferior colliculus) show marked enlargement in the early fetal period, thereby reflecting their dominant position in the adult. The cerebellum and pons grow slower than in most smaller toothed whales. The pyramidal tract develops poorly (reduction of the limbs), whereas marked growth of the striatum and inferior olive may be related to the animal's locomotion via trunk and tail. In the early fetal period, the trigeminal, vestibulocochlear, and facial nerves are the dominant cranial nerves (besides the vagus nerve). Whereas the number of axons in the vestibulocochlear nerve is high in adult, toothed whales and their diameters are considerable, the trigeminal nerve of the sperm whale may be the thickest of all cranial nerves and has the largest number of axons (innervation of the huge forehead region). A similar situation seems to exist for the facial nerve: It innervates the blowhole musculature that surrounds the very large spermaceti organ and melon (generation and emission of sonar clicks).

The nose of the sperm whale: overviews of functional design, structural homologies and evolution

“…In contrast to the situation in non-cetacean mammals, this cranial nerve runs without significant ramification from the tympanoperiotic complex to and below the orbita, turns upward around the lateral margin of the skull roof via the antorbital notch, and then ramifies strongly in order to innervate the blowhole musculature (Rauschmann, 1992;Rommel et al, 2002Rommel et al, , 2009). In the sperm whale, not only the diameter but also the number of axons of the facial nerve is much higher than in other toothed whales investigated so far, and also in comparison to baleen whales of the same body size (Oelschläger & Kemp, 1998;Oelschläger & Oelschläger, 2002). This correlates, on the one hand, with the extreme absolute and relative size of the head and the nasal musculature and, on the other hand, with the fact that baleen whales do not echolocate and possess a much less developed nasal area (no epicranial complex).…”

Anatomy and three‐dimensional reconstructions of the brain of a bottlenose dolphin (Tursiops truncatus) from magnetic resonance images

“…The differences between dolphin and other mammalian brains of similar size have been noted at the level of cortical cytoarchitecture and immunohistochemistry (Garey et al, 1985;Garey and Leuba, 1986;Glezer et al, , 1992aGlezer et al, ,b, 1993Glezer et al, , 1998Hof et al, 1992Hof et al, , 1995, cortical surface morphology (Haug, 1987;Jacobs et al, 1979;Morgane et al, 1980) and subcortical structures (Glezer et al, 1995a,b;Tarpley and Ridgway, 1994). These differences are also manifest during ontogenesis (Buhl and Oelschlager, 1988;Oelschlager and Buhl, 1985;Oelschlager and Kemp, 1998).…”

Neuroanatomy of the common dolphin (Delphinus delphis) as revealed by magnetic resonance imaging (MRI)

“…The differences between cetacean and other mammalian brains of similar size have been noted at the level of cortical cytoarchitecture and histochemistry (Garey et al, 1985;Garey and Leuba, 1986;Glezer et al, , 1992aGlezer et al, , b, 1993Glezer et al, , 1998Hof et al, 1992Hof et al, , 1995, cortical surface configuration (Jacobs et al, 1979;Morgane et al, 1980;Haug, 1987), and subcortical structural morphology (Tarpley and Ridgway, 1994;Glezer et al, 1995a, b). These differences are also manifest during ontogeny (Oelschlager and Buhl, 1985;Buhl and Oelschlager, 1988;Oelschlager and Kemp, 1998).…”