Morphology and Evolutionary Biology of the Dolphin (<i>Delphinus</i> sp.) Brain – MR Imaging and Conventional Histology

Abstract

Whole brains of the common dolphin (Delphinus delphis) were studied using magnetic resonance imaging (MRI) in parallel with conventional histology. One formalin-fixed brain was documented with a Siemens Trio Magnetic Resonance scanner and compared to three other brains which were embedded in celloidin, sectioned in the three main planes and stained for cells and fibers. The brain of the common dolphin is large, with the telencephalic hemispheres dominating the brain stem. The neocortex is voluminous and the cortical grey matter thin but extremely extended and densely convoluted. There is no olfactory ventricular recess due to the lack of an anterior olfactory system (olfactory bulb and peduncle). No occipital lobe of the telencephalic hemisphere and no posterior horn of the lateral ventricle are present. A pineal organ could not be detected. The brain stem is thick and underlies a very large cerebellum. The hippocampus and mammillary body are small and the fornix is thin; in contrast, the amygdaloid complex is large and the cortex of the limbic lobe is extended. The visual system is well developed but exceeded by the robust auditory system; for example, the inferior colliculus is several times larger than the superior colliculus. Other impressive structures in the brainstem are the peculiar elliptic nucleus, inferior olive, and in the cerebellum the huge paraflocculus and the very large posterior interpositus nucleus. There is good correspondence between MR scans and histological sections. Most of the brain characteristics can be interpreted as morphological correlates to the successful expansion of this species in the marine environment, which was characterized by the development of a powerful sonar system for localization, communication, and acousticomotor navigation.

Brain Size and Brain Organization of the Whale Shark, <i>Rhincodon typus</i>, Using Magnetic Resonance Imaging

“…Although histological techniques provide higher spatial resolution (up to the cellular level) than MRI data, these more traditional methods are highly manual and constrained to two dimensions. MRI is able to efficiently and non-invasively identify and delineate major brain areas to produce volumetric data [Yopak and Frank, 2007;Yopak et al, 2009] that is comparable to existing data on chondrichthyan brain morphology and is similar to traditional methods (such as histology) [e.g., Marino et al, 2003;Kaufman et al, 2005;Oelschlager et al, 2008], and thus has the po- Fig. 6.…”

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

“…The dolphin brains in our series (total mass: 685-830 g; table 1 ) are considerably to slightly smaller than the average in adult animals of this species (835.6 8 79.9 g) relating to an average body length of 193.1 8 5.8 cm for both sexes and an average body weight of 67.6 8 11.7 kg [Ridgway and Brownson, 1984;Oelschläger et al, 2008].…”

“…The highest encephalization is known from marine delphinid species [Schwerdtfeger et al, 1984;Ridgway and Tarpley, 1996;Manger, 2006;Oelschläger et al, 2008, Oelschläger and Oelschläger, 2002, whereas pygmy and dwarf sperm whales of about the same body dimensions show distinctly smaller brains per body mass. In adult male giant sperm whales (Physeter macrocephalus) , the average absolute brain mass, on the one hand, is the largest within the mammalia and only rivalled by those in the largest male killer whales [ Orcinus orca ;Osborne and Sundsten, 1981;Ridgway and Tarpley, 1996].…”

“…This sonar system allows targeted locomotion by active orientation and echolocation as well as extensive communication independent from the time of day and water depth or quality. Therefore, it is not surprising that the auditory system has stamped its influence on the brain at any of its levels by the hypertrophy of the relevant structures involved in the processing of auditory information and acousticomotor (audiomotor) navigation [Ridgway, 1983[Ridgway, , 1986[Ridgway, , 2000Ridgway and Au, 1999;Schulmeyer et al, 2000;Oelschläger et al, 2008;Oelschläger, 2008;Oelschläger, 2002, 2009].…”

The Central Vestibular Complex in Dolphins and Humans: Functional Implications of Deiters’ Nucleus

“…Apart from multiple diffuse projections into the reticular formation, the LVN sends efferents into the anterior motor horn of the spinal cord via the lateral vestibulospinal tract, thus providing a major source for spinal activation and control [Bütt ner-Ennever, 1992;Voogd 1998a, b;Kugler, 2004;Oelschläger et al, 2008]. Interestingly, the LVN is already large in Pontoporia , although this plesiomorphic estuarine/coastal species is relatively small in body size and brain size (encephalization index EI: 5.69; table 1 ) [Schwerdtfeger et al, 1984] ) might again be due to differences in the ecophysiological characteristics of their respective habitats (coastal vs. pelagic).…”