Cetacean Brain Evolution: Dwarf Sperm Whale <i>(Kogia sima)</i> and Common Dolphin <i>(Delphinus delphis)</i> – An Investigation with High-Resolution 3D MRI

Abstract

This study compares a whole brain of the dwarf sperm whale (Kogia sima) with that of a common dolphin (Delphinus delphis) using high-resolution magnetic resonance imaging (MRI). The Kogia brain was scanned with a Siemens Trio Magnetic Resonance scanner in the three main planes. As in the common dolphin and other marine odontocetes, the brain of the dwarf sperm whale is large, with the telencephalic hemispheres remarkably dominating the brain stem. The neocortex is voluminous and the cortical grey matter thin but expansive and densely convoluted. The corpus callosum is thin and the anterior commissure hard to detect whereas the posterior commissure is well-developed. There is consistency as to the lack of telencephalic structures (olfactory bulb and peduncle, olfactory ventricular recess) and neither an occipital lobe of the telencephalic hemisphere nor the posterior horn of the lateral ventricle are present. A pineal organ could not be detected in Kogia. Both species show a tiny hippocampus and thin fornix and the mammillary body is very small whereas other structures of the limbic system are well-developed. The brain stem is thick and underlies a large cerebellum, both of which, however, are smaller in Kogia. The vestibular system is markedly reduced with the exception of the lateral (Deiters’) nucleus. The visual system, although well-developed in both species, is exceeded by the impressive absolute and relative size of the auditory system. The brainstem and cerebellum comprise a series of structures (elliptic nucleus, medial accessory inferior olive, paraflocculus and posterior interpositus nucleus) showing characteristic odontocete dimensions and size correlations. All these structures seem to serve the auditory system with respect to echolocation, communication, and navigation.

Stereology of the Neocortex in Odontocetes: Qualitative, Quantitative, and Functional Implications

“…Interestingly, this phenomenon is in obvious contrast to the well-known allometric effects of (a) decreasing neuron numbers per cortical unit, i.e. below a standardized area of neocortical surface [Poth et al, 2005], and (b) decreasing cortical neuron density, respectively, in mammals with increasing brain mass [Manger, 2006;Oelschläger andOelschläger, 2002, 2009;Oelschläger et al, 2010]. Our data show generally higher neuron numbers per layer III volume unit in T. truncatus than in P. phocoena (see Results), and maximal densities were seen in areas A1 and V1.…”

“…Although a number of papers have dealt with the quantitative morphology of the cetacean neocortex [Tower, 1954;Hawkins and Olszewski, 1957;Kraus and Pilleri, 1969a;Rockel et al, 1980;Morgane et al, 1982;Jacobs et al, 1984, Garey et al, 1985Garey and Leuba, 1986;Haug, 1987;Poth et al, 2005;Eriksen and Pakkenberg, 2007;Butti et al, 2009;Oelschläger et al, 2010;Walloe et al, 2010], our knowledge of it is still rather limited. Brains differ in size and cortical thickness; larger animals with- Num bers in parentheses indicate the layer III/layer V ratio in 1 area of 1 species.…”

“…87 in a systematic group tend to have larger brains with a more convoluted cortex and a slightly thicker cortical grey but a lower number of subpial neurons [Haug, 1987;Poth et al, 2005] and also a lower neuron density [Tower, 1954;Rockel et al, 1980;Eriksen and Pakkenberg, 2007;Oelschläger andOelschläger, 2002, 2009]. Poth et al [2005], counted neurons below a standard (pial) surface in order to eliminate differences in cortical thickness in the comparison of various species within an 'ascending toothed whale series' with increasing body mass and brain mass as a parallel to the 'ascending primate series' [Stephan, 1975;Schwerdtfeger et al, 1984;Matano et al, 1985;Stephan et al, 1988;West, 1990;Oelschläger et al, 2010].…”

Evolution of brain elaboration

“…Interestingly, these structures do not appear to be capable of adapting to a return to an aquatic lifestyle, as olfactory bulbs and antennal lobes are lost or greatly reduced in secondarily aquatic animals such as cetaceans or whirligig beetles [26,27].…”

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

“…the supraorbital process of the maxilla formed the origin of a large volume of facial muscles) (Fordyce & de Muizon, 2001;Geisler et al, 2014) as modern dolphins. In the case of the sperm whale, these muscles reflect the profound phylogenetic modifications of the epicranial complex in this species which led to a unique situation with respect to that in terrestrial mammals (Oelschläger, 2008;Oelschläger et al, 2010). In parallel to the secondary rostral shift of the blowhole to the dorsal tip of the sperm whale snout, the course of the muscle fibre bundles from the margins of the skull roof to the blowhole has changed from a more concentric (radial) orientation of up to six muscle layers in dolphins and porpoises (Lawrence & Schevill, 1956;Mead, 1975;Heyning, 1989;Huggenberger et al, 2009) to a more longitudinal orientation of a single powerful muscle (maxillonasolabialis muscle) in sperm whales.…”