Structure of the cerebral cortex of the humpback whale,Megaptera novaeangliae (Cetacea, Mysticeti, Balaenopteridae)

Abstract

Cetaceans diverged from terrestrial mammals between 50 and 60 million years ago and acquired, during their adaptation to a fully aquatic milieu, many derived features, including echolocation (in odontocetes), remarkable auditory and communicative abilities, as well as a complex social organization. Whereas brain structure has been documented in detail in some odontocetes, few reports exist on its organization in mysticetes. We studied the cerebral cortex of the humpback whale (Megaptera novaeangliae) in comparison to another balaenopterid, the fin whale, and representative odontocetes. We observed several differences between Megaptera and odontocetes, such as a highly clustered organization of layer II over the occipital and inferotemporal neocortex, whereas such pattern is restricted to the ventral insula in odontocetes. A striking observation in Megaptera was the presence in layer V of the anterior cingulate, anterior insular, and frontopolar cortices of large spindle cells, similar in morphology and distribution to those described in hominids, suggesting a case of parallel evolution. They were also observed in the fin whale and the largest odontocetes, but not in species with smaller brains or body size. The hippocampal formation, unremarkable in odontocetes, is further diminutive in Megaptera, contrasting with terrestrial mammals. As in odontocetes, clear cytoarchitectural patterns exist in the neocortex of Megaptera, making it possible to define many cortical domains. These observations demonstrate that Megaptera differs from Odontoceti in certain aspects of cortical cytoarchitecture and may provide a neuromorphologic basis for functional and behavioral differences between the suborders as well as a reflection of their divergent evolution. Anat Rec, 290:1-31, 2007. 2006 Wiley-Liss, Inc.Key words: balaenopterids; cetaceans; cytoarchitecture; dolphins; mysticete; neocortex; odontoceteThe order Cetacea comprises highly diversified species distributed into two suborders, the Mysticeti (baleen whales) and the Odontoceti (toothed whales). The suborder Mysticeti includes 13 species (the so-called right whales, the pygmy right whale, the gray whale, and the rorquals), while Odontoceti contains 72 species of toothed whales, dolphins, and porpoises, distributed into four superfamilies (Mann et al., 2000). Morphological features and molecular phylogenetics suggest that cetaceans are directly related to artiodactyls (even-toed ungulates) (Gingerich et al., 2001;

The von Economo neurons in frontoinsular and anterior cingulate cortex in great apes and humans

“…Instead it appears to be related to absolute brain size. The VENs are present in primates with adult brain sizes greater than about 300 g. They are also present in the apparent homologs of FI and LA in other mammals with very large brains, such as cetaceans and elephants (Hof and Van der Gucht 2007;Butti et al 2009;Hakeem et al 2009). Nearly all of these mammals are also highly social.…”

Neuronal factors determining high intelligence

“…In 'insectivores', the unrelated groups of eulipotyphlans and afrosoricidans, we find the dominance of the olfactory system, while an olfactory cortex is absent in cetaceans, which instead have a large auditory cortex. These animals have a relatively small hippocampus [44], which was the site of olfactory memory in the primitive state of mammals. In addition, the cetacean cortex lacks a prominent layer IV, which is called 'granular layer', because of the presence of many small-sized neurons.…”

“…However, such 'von Economo cells' have recently been found in some cetaceans and in elephants as well, but not consistently in all largebrained mammals [44,51]. Whether this mosaic existence of 'von Economo cells' is owing to independent evolution or, when absent, to secondary loss, is unclear, as is their specific significance for cognition [52].…”

A Neurological Comparative Study of the Harp Seal (Pagophilus groenlandicus) and Harbor Porpoise (Phocoena phocoena) Brain

“…However, it was not possible to identify the subdivisions of neocortex using distinct cytoarchitectural features as described by Hof and van der Gucht (2007) in the humpback whale (Megaptera novaeangliae) or the presence of well-visible cellular modules as seen in layer II in the occipital lobe of the humpback whale in either the harbor porpoise or harp seal (Fig. 1).…”

“…Much research has gone into the cetacean brain (Marino, 2004;Manger, 2006;Eriksen and Pakkenberg, 2007;Hof and Van Der Gucht, 2007) and most have focused on comparisons between cetacean species (e.g., Furutani, 2008). However, less attention has been given to comparison between cetaceans and other marine mammals such as seals.…”