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Keywords:

  • artiodactyls;
  • cetaceans;
  • cytoarchitecture;
  • delphinids;
  • neocortex;
  • odontocete

Abstract

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED

Cetaceans (dolphins, whales, and porpoises) have a long, dramatically divergent evolutionary history compared with terrestrial mammals. Throughout their 55–60 million years of evolution, cetaceans acquired a compelling set of characteristics that include echolocation ability (in odontocetes), complex auditory and communicative capacities, and complex social organization. Moreover, although cetaceans have not shared a common ancestor with primates for over 90 million years, they possess a set of cognitive attributes that are strikingly convergent with those of many primates, including great apes and humans. In contrast, cetaceans have evolved a highly unusual combination of neurobiological features different from that of primates. As such, cetacean brains offer a critical opportunity to address questions about how complex behavior can be based on very different neuroanatomical and neurobiological evolutionary products. Cetacean brains and primate brains are arguably most meaningfully conceived as alternative evolutionary routes to neurobiological and cognitive complexity. In this article, we summarize data on brain size and hemisphere surface configuration in several cetacean species and present an overview of the cytoarchitectural complexity of the cerebral cortex of the bottlenose dolphin. © 2005 Wiley-Liss, Inc.

Morphological and molecular evidence shows that cetacean ancestry is closely tied to that of Ungulata (the order of hooved mammals) and specifically Artiodactyla (the suborder of even-toed ungulates) (Gingerich et al., 2001; Thewissen et al., 2001; Geisler and Uhen, 2003). Molecular evidence shows a sister-group relationship between extant cetaceans and the artiodactyl family Hippopotamidae (Nikaido et al., 1996; Shimamura et al., 1997; Gatesy, 1998; Milinkovitch et al., 1998), though an early divergence with hippopotamids at least 52 million years ago (Gingerich and Uhen, 1998). The first cetacean suborder, Archaeoceti, derived from near-shore Indo-Pakistani locales (Thewissen et al., 1996) and survived until the late Eocene around 37 million years ago (Barnes et al., 1985) when the modern suborders, Mysticeti (comprising 13 species of baleen and rorqual whales) and Odontoceti (comprising 67 species of toothed whales, dolphins, and porpoises) appeared in the early Oligocene (Barnes et al., 1985).

Modern cetacean brains are among the largest in both absolute size and in relation to body size of all mammals [expressed here as encephalization level or encephalization quotient, EQ (Jerison, 1973)]. This is particularly striking in light of the fact that early cetaceans possessed small brains and, even more importantly, low EQs averaging about 0.5 (Marino et al., 2004). The largest brain on earth today belongs to the sperm whale (Physeter macrocephalus), with an average adult brain size of 8,000 g (Marino, 2002a). Furthermore, almost all odontocetes (toothed whales, dolphins, and porpoises) possess above-average encephalization levels (some EQs just under 5) compared with other mammals. Numerous odontocete species possess encephalization levels second only to modern humans (EQ ≃ 7) and significantly higher than any of the nonhuman anthropoid primates (highest EQ ≃ 3.3) (Marino, 1998; Marino et al., 2004).

EQs of mysticetes are all substantially below 1 (Marino, 2002a) because of nonlinearities in EQ for very large animals. However, the large absolute sizes, high degrees of cortical convolutedness, and highly derived morphology establish that mysticete brains have, in addition to odontocete brains, undergone substantial enlargement and elaboration during the course of their evolution (Oelschlager and Oelschlager, 2002). Table 1 displays average brain weight, average body weight, and EQ based on the formula derived by Jerison (1973) for 26 species of modern odontocetes and 3 mysticetes with sexes combined.

Table 1. Average brain weight, body weight, and EQ for 29 cetaean species
Species Common Name (Taxonomic Name)Brain wt (g)Body wt (g)EQ
  • Sources of data are Marino (1998) and Marino et al. (2004).

  • *

    Those species whose EQ (as well as brain-body residual) is statistically significantly higher than that of anthropoid primates and the other cetacean species.

Bottlenose Dolphin (Tursiops truncatus)*18242095304.14
Common Dolphin (Delphinus delphis)*815601704.26
Risso's Dolphin (Grampus griseus)23873280004.01
Pacific White-sided Dolphin (Lagenorynchus obliquidens)*1148910504.55
Atlantic White-sided Dolphin (Lagenorynchus acutus)11032446672.25
Long-finned Pilot Whale (Globicephala melas)28939432002.39
Killer Whale (Orcinus orca)505919554502.57
False Killer Whale (Pseudorca crassidens)25343500984.03
Spinner Dolphin (Stenella longirostris)660662003.24
Striped dolphin (Stenella coeruleoalba)9402610992.94
Routh-toothed Dolphin (Steno bredanensis)*15421248574.95
Tucuxi Dolphin (Sotalia fluviatilis)*688422404.56
Harbor Porpoise (Phocoena phocoena)540511932.95
Dall's Porpoise (Phocoenoides dalli)866868303.54
Chinese River Dolphin (Lipotes vexillifer)510820002.17
Ganges River Dolphin (Platanista gangetica)295596301.55
Amazon River Dolphin (Inia geoffrensis)634920042.51
Franciscana (Pontoporia blainvillei)221348591.67
Pygmy Sperm Whale (Kogia breviceps)10123050001.78
Dwarf Sperm Whale (Kogia simus)6221685001.63
Sperm Whale (Physeter macrocephalus)8028358333300.58
Cuvier's Beaked Whale (Ziphius cavirostris)200422730000.92
Gervais' Beaked Whale (Mesoplodon europaeus)214914650002.11
Blainville's beaked whale (Mesoplodon densirostris)14257705001.39
Beluga Whale (Delphinapterus leucas)20836360002.24
Narwhal (Monodon monoceros)299715783301.76
Fin Whale (Balaenoptera physalus)7085384215000.49
Blue Whale (Balaenoptera musculus)3636509040000.21
Humpback Whale (Megaptera novaeangliae)6411392950000.44

The cetacean telencephalon is arranged into three concentric tiers of limbic, paralimbic, and supralimbic tissue. The high degree of cortical gyrification and resulting expansive surface area of approximately 3,745 cm2 is unsurpassed among mammals, including humans (Ridgway and Brownson, 1984). Cerebral enlargement in cetaceans occurs most exuberantly in the parietal and temporal regions. Whereas primate brains feature large frontal lobes, no homologous frontal lobe region in the cetacean brain has been identified, leading many investigators to substitute the term “orbital lobe” for “frontal lobe” when referring to these modest, ventrally oriented hemispheric regions (Morgane et al., 1980). From a functional standpoint, a few electrophysiological mapping studies of cetacean cortex places primary visual cortex on the vertex of the hemisphere in the lateral gyrus and the primary auditory cortex lateral and directly adjacent to it in the suprasylvian gyrus. Secondary auditory cortex lies lateral to the primary auditory field in the medial ectosylvian gyrus (Supin et al., 1978) and somatosensory and motor cortices lie immediately adjacent and rostral to the visual and auditory regions (Lende and Akdikmen, 1968; Lende and Welker, 1972; Ladygina et al., 1978). Thus, the large mass and unusual surface configuration of cetacean brains sets the context for an interesting array of characteristics at the cytoarchitectural level. In this article, we explore several unusual features of cetacean neocortical architecture.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED

The brains of three adult male bottlenose dolphins (Tursiops truncatus) were analyzed in detail in this study. These specimens were perfused by gravity with 40 l of Windle's fluid in situ using a cannula inserted into the descending aorta in animals that had been euthanized for medical reasons. The brains were then extracted and postfixed in 8% formalin for 3 months (Jacobs et al., 1971, 1979). The brains were then dehydrated in graded alcohol solutions, embedded in celloidin, and cut serially at 35 μm on a modified large specimen microtome (Mico Instruments, Cambridge, MA). Each brain was cut in one of three planes (coronal, sagittal, horizontal) relative to the beak-fluke axis of the animal. Two 1:5 series of adjacent sections throughout these brains were stained for myelin with the Loyez-Weigert method or for Nissl substance with the Bielchowsky-Plien cresyl violet method (Bertrand, 1930). The sections were mounted on large glass slides and coverslipped in clarite for examination.

Additional adult specimens from Tursiops, a beluga whale (Delphinapterus leucas), a long-finned pilot whale (Globicephala melas), and a Cuvier's beaked whale (Ziphius cavirostris) were used for comparison across a few species. These specimens were obtained from stranded animals within a few hours of death and were fixed by immersion in neutral formalin for several months. Local samples of the regions corresponding to the primary visual and primary auditory cortex [i.e., from the mid-posterior portion of the lateral gyrus and the mid-posterior region of the suprasylvian gyrus, respectively (Sokolov et al., 1972; Supin et al., 1978; Morgane et al., 1988)] were obtained from these cases, cryoprotected in graded sucrose solution, and cut on a cryostat (Reichert Jung, Vienna, Austria) at 60 μm and series of sections were then stained with cresyl violet.

All histological preparations were examined on a Zeiss Axiophot 2 photomicroscope with 5×, 10×, and 20× Fluar and Apochromat objectives (Zeiss, Oberkochen, Germany). Photomicrographs were acquired using a 10× PlanApochromat lens and an Optronics Microfire digital camera (Optronics, Goleta, CA). Photomontages were digitally assembled with Virtual Slice software (MicroBrightField, Williston, VT) and processed with Adobe Photoshop CS 8.0. The nomenclature of gyri and sulci follows that proposed by Morgane et al. (1980).

RESULTS

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED

The histological organization of the cetacean cerebral cortex presents several unique features and appears complicated owing to the inordinate number of gyri and sulci that characterize the brains in these species (Fig. 1). In spite of a pervasive notion that neocortical structure is rather uniform throughout the cortical mantle with minor local variations, it is in fact quite complex, with a degree of regional parcellation at least comparable to that of large-brained terrestrial mammals such as anthropoid primates, carnivores, and ungulates. Short of a few studies, generally describing only restricted regions (Kojima, 1951; Kesarev, 1969; Kesarev and Malofeeva, 1969; Jacobs et al., 1971, 1979; Kesarev et al., 1977; Morgane et al., 1982, 1988; Garey et al., 1985; Manger et al., 1998), the regional organization of the cerebral cortex in cetaceans remains poorly understood. Overall, the dolphin neocortex is thin and characterized by a general absence of granularity, a very prominent, thick layer I, which is far more cellular than in most terrestrial species, the presence of large, atypical neurons in the dense layer II, and very large pyramidal neurons frequently forming clusters at the border between layers III and V (layer IIIc/V). Layers III and VI vary considerably in thickness and cellular density across regions. Here we summarize some preliminary observations on the major neocortical domains in the bottlenose dolphin and a few comparisons with other species of the primary visual and auditory cortices. The archicortex, paleocortex, and cingulate cortex of dolphins have been described in detail by Jacobs et al. (1971, 1979) and by Morgane et al. (1982).

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Figure 1. Views of brains from different cetacean species. Clockwise from lower left, lateral view of the left hemisphere of the fin whale brain (Balaenoptera physalus), posterior superior view of the killer whale brain (Orcinus orca), lateral view of the left hemisphere of the beluga whale brain (Delphinapterus leucas), mid-sagittal view of the brain of an Amazon river dolphin (Inia geoffrensis), frontal superior view of the brain of a tucuxi (Sotalia fluviatilis), and lateral view of the right hemisphere of an Atlantic white-sided dolphin (Lagenorhynchus obliquidens). The brains are not represented to scale. See Table 1 for details on size.

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Frontal Region

The anterior aspects of the lateral and medial orbital gyri are characterized by a well-defined laminar pattern with rather small pyramidal neurons in layers III and V (Fig. 2A). Layer II is very dense and thick. Posteriorly toward the olfactory lobe, the cortex becomes less differentiated with an increase in small neurons in the deep layers and considerable spacing of neurons in layer III, especially toward the junction with the ventral anterior insular cortex. At this level, layer II thins out and fragments into small islands. Medially, the cortex is thin on the gyrus proreus and adjacent gyri toward the pole of the frontal region, with fairly regularly distributed medium-sized pyramidal neurons in layer V. At more posterior levels, the cortex loses its lamination progressively and contains neurons of a smaller size (Fig. 2B), becoming a paraolfactory cortex in the subgenual region with no discernible laminar patterns. The frontopolar region, including the medial orbital gyrus and the cortex within most of the coronal gyrus, displays a cytoarchitecture comparable to the anterior orbital cortex. The dorsal and lateral aspects of the frontal region include the anterior reaches of the paralimbic cleft (entolateral sulcus), cruciate sulcus, and the superior lateral sulcus. These regions exhibit considerable diversity of cytoarchitecture. However, a dominant theme is the presence of large to gigantic pyramidal cells in many of these fields, comparable to the description of a putative motor cortex in the sperm whale (Kojima, 1951). In the cortex of the cruciate sulcus, there is a gigantocellular region with very large layer V pyramidal cells occurring alone or in small groups of 2–3 cells, along with cell-dense layers II and III (Fig. 2C). The cortex along the paralimbic cleft shows very large layer V pyramidal cells that tend to form larger clusters than in the cruciate sulcus. It is possible that these gigantopyramidal fields represent, on the basis of their layer V cell size and their topographic localization, the homologues of the primary, secondary, and supplementary motor cortices of other mammals. The anterior portion of the superior lateral sulcus is quite heterogeneous with regions showing similar characteristics as the large-celled field of the paralimbic and cruciate cortex, with a high degree of clustering, whereas other regions have slender and denser, elongated layer V pyramidal neurons.

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Figure 2. Examples of cytoarchitecture in the frontal cortex of the bottlenose dolphin. A: Lateral orbital gyrus. B: Posterior level of the gyrus proreus. C: Cortex on the lateral bank of the cruciate sulcus, possibly corresponding to a motor field. Layers are indicated by Roman numerals. wm, white matter. Scale bar (on C) = 100 μm.

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Insular Cortex

The insular lobe of the dolphin contains a variable number of transverse and vertical gyri (up to 16 in our specimens) that exhibit at least three major patterns of cytoarchitectural organization. The posterior and superior third of the insula and retroinsular region is characterized by a well-laminated type of cortex that is comparable to the immediately adjacent cortices in the lateral aspect of the sylvian cleft (Fig. 3A), with a thin, dense layer II, large pyramidal cells in layers III and V, but relatively small and nonclustering layer IIIc/V pyramids. The middle third of the insular cortex shows a less differentiated cortical lamination than its posterior region, with a large field of vertically organized modules that span all cortical layers from the very thin layer II to the white matter (Fig. 3B). The neuronal density between these modules is very sparse. Anteriorly and ventrally, the insular cortex is characterized by the appearance in layer II of large clusters of small neurons (Manger et al., 1998) resembling the layer II islands traditionally seen in the mammalian entorhinal cortex (Fig. 3C). This pattern is especially distinguishable where the basal ganglia encroaches the lateral surface of the brain. The other layers are not as identifiable and contain moderately large pyramidal cells and layers V and VI mix with claustral neurons. The insular cortex extends rostrally along the basal ganglia and merges with the orbital cortex, at which level the layer II islands become smaller and less conspicuous as the cortex thins out ventrally.

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Figure 3. Cytoarchitecture of the bottlenose dolphin insula. A: Posterior superior region. B: Middle third of insula. C: Anterior inferior part of insula with typical layer II islands. Layers are indicated by Roman numerals. Cl, claustrum. Scale bar (on C) = 100 μm.

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Temporoparietal Region

The temporoparietal region of the cetacean brain constitutes the largest extension of neocortical surface. It is organized in several roughly parallel deep sulci around the somewhat verticalized sylvian cleft. Although there are many species-specific variations in the gyrification patterns, generally three major sulci can be recognized around the sylvian cleft, namely, the inferior lateral fissure (ectosylvian sulcus), the intermediate lateral fissure (suprasylvian sulcus), and the superior lateral fissure (ectolateral sulcus). These sulci define major gyri: the lateral gyrus superiorly, and moving laterally on the hemisphere, the ectolateral, suprasylvian, and ectosylvian gyri. The cortex in each of these gyri includes a rich diversity of cytoarchitectonic fields in either their superior (above and anterior to the sylvian cleft) or their inferior extent (below and posterior to the sylvian cleft). Such a pattern is reminiscent of the organization of the visual and auditory primary fields and their associated regions in carnivores, such as cats, in which the functional organization of these fields is well known. The localization of such fields or even their homology to the situation in other species is, however, uncertain in cetaceans, as only the primary auditory cortex has been located with any certainty in the suprasylvian gyrus and the primary visual cortex in the lateral gyrus. It is possible that as many as 10 different cytoarchitectural fields exist in the rostrocaudal extent of both the suprasylvian and the ectosylvian cortex.

A large expanse of cortex in the mid-posterior segment of the lateral gyrus corresponds to the primary and probably secondary visual cortex (Sokolov et al., 1972; Supin et al., 1978; Garey et al., 1985; Morgane et al., 1988). This region is characterized by the presence of midsized pyramidal neurons in layer V compared to adjacent fields in the paralimbic cleft and to a certain degree of granularity owing to the presence of small cells in layer III. Anterior and lateral to it lies the possible somatosensory cortex that contains small cell in all layers and small clusters of layer V pyramids of about 2–3 cells each (Fig. 4A). This comparatively small-celled region abuts the gigantocellular motor regions further rostrally in the frontal cortex. The cortex of the suprasylvian gyrus contains the primary auditory cortex. This region displays clear clusters of large pyramidal cells in layer V distributed with regularity. Of note, the clusters and the pyramidal cells of the auditory cortex are larger than those in the primary visual cortex and putative somatosensory cortex. Laterally in the superior portion of the ectosylvian gyrus, a thinner cortex is present with increased modularity identifiable by large aggregates of neurons below layer II and smaller cells (Fig. 4B). The modules are separated by wide gaps of white matter possibly corresponding to the projection of subcortical afferents to a region that may be related to auditory function. A similar pattern of modular organization is seen in the superior aspect of the cortex within the sylvian cleft. Ventrally toward the pole of the temporal lobe, the cortex exhibits thinner columns of cells with intercalated bundles and increased density of neurons in all layers. The temporopolar region shows medially a fragmented layer II with islands similar to those seen in the insula, and a less well-defined lamination overall, with relatively small layer V cells (Fig. 4C). Posteriorly, the inferior temporal cortex contains a progressively less differentiated cortex medially toward the entorhinal cortex, with layers III through VI having neurons of about the same size with no evidence of grouping in the deep layers. In contrast, the lateral aspect of the inferior temporal lobe (i.e., the inferior ectosylvian cortex) shows distinct clustering in layer V, although these pyramidal cells remain comparatively small in this region (Fig. 4D). Neurons become larger in the cortex of the inferior aspect of the sylvian cleft but the cortex is rather thin in these regions compared to frontal and parietal regions. These general patterns are seen along much of the rostrocaudal extent of the gyri, with local variations in cortical thickness and density of neurons that may identify a large number of individual cortical fields. This issue will, however, require a detailed and systematic study.

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Figure 4. Cytoarchitecture of the bottlenose dolphin temporoparietal operculum. A: Anterior suprasylvian cortex, possibly corresponding to the somatosensory region. B: Superior ectosylvian cortex. C: Lateral temporopolar region. D: Inferior ectosylvian cortex. Layers III and V are identified. Scale bar (on D) = 100 μm.

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Posterior Polar Region

The posterior (occipital) cortex is carved by three major sulci, the paralimbic cleft that defines the posterior aspect dorsally of the lateral gyrus, the superior lateral fissure (ectolateral sulcus), and laterally the intermediate lateral fissure (suprasylvian sulcus). Medially it contains the lingual lobule, bounded anteriorly by the calcarine sulcus. The cortex of the lateral gyrus is slightly thicker than on the adjacent gyri and contains very well-defined clusters of midsized pyramidal neurons in layer V and dense layers II and III. It is reminiscent of the organization seen in the primary visual cortex more rostrally in the lateral gyrus and may represent an accessory visual cortex. This pattern changes laterally along the suprasylvian gyrus, where the neurons in layer V appear smaller and less clustered and the granularity of layers II and III increases (Fig. 5A). Further laterally in the posterior ectosylvian cortex, a strong columnar pattern becomes apparent with vertical aggregates of neurons, 5–10 cells wide, spanning layers III through VI and separated by thin gaps that may represent ascending axonal bundles (Fig. 5B). This pattern is seen with relative regularity where the cortex is not cut tangentially by the plane of section, downward to the inferior tip of the posterior temporoparietal cortex. The medial and ventral aspect of the posterior polar cortex is relatively thin and shows a more regular arrangement of neurons in layer V, with no or low degree of clustering and slender neurons in the more ventral aspects of the lingual lobule. Dorsally toward the paralimbic cleft, larger neurons are present in layer V with an increased cellularity of layers II and III.

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Figure 5. Cytoarchitecture of the posterior polar region and oval lobule. A: Posterior suprasylvian cortex. B: Posterior ectosylvian cortex. C: Cortex at mid-level of the oval lobule. Layers III and V are identified. Scale bar (on C) = 100 μm.

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Oval Lobule

The oval lobule is bounded ventrally by the limbic cleft, anteriorly by the superior marginal sulcus, dorsally by the paralimbic cleft, and posteriorly by the possible homologue of the calcarine sulcus. Its cortex is characterized by a well-demarcated layer II and a dense layer III containing small pyramidal cells. Layer V contains regularly spaced clusters of medium-sized pyramidal cells throughout most of the lobule (Fig. 5C). These neurons become smaller and less grouped posteriorly toward the junction with the lingual lobule. Dorsally, the pattern merges with the cortex within the paralimbic cleft, where the layer V pyramidal cells appear progressively larger and darker, and where the cellular density and thickness of layer II increase, rendering the layer II/III border difficult to assess.

Cingulate and Retrosplenial Cortex

It is convenient to subdivide the limbic lobe into five distinct domains or lobules (Morgane et al., 1982): the parolfactory (subgenual) lobule; the supracallosal lobule that contains the cingulate cortex proper with its pregenual, anterior, and posterior divisions; the retrosplenial lobule with its anterior and posterior divisions; and the temporal region (parahippocampal lobule and hippocampal formation). The posterior aspect of the subgenual cortex lacks distinct layers. At the more anterior level of the root of the internal intercalate limbic sulcus, the cortex widens and the layers are clearly apparent with a patchy thin layer II and a small-celled layer V (Fig. 6A). The pregenual region is marked by the appearance of two distinct sulci (the internal and external intercalate limbic sulci) that course throughout the rostrocaudal extent of the supracallosal lobule. A limbic cleft limits the limbic lobe dorsally. As in other mammals, the supracallosal cortex of the limbic lobe abuts the callosal sulcus with the induseum griseum and the subicular remnant. The cortex then becomes clearly laminated, with thin layers and densely packed small cells. The cortex remains parvocellular around the full extent of the internal intercalate gyrus. In the external intercalate gyrus, layer V becomes progressively thicker with larger cells that tend to cluster and form modular elements spanning layers V and VI (Fig. 6B). This pattern is best appreciated in the anterior and posterior domains of the cingulate cortex. Dorsal to the external intercalate cortex, a transition (marginal) zone exists that shows increased cellularity of the deep layers anteriorly, with larger pyramidal cells than in the intercalate cortex, and posteriorly a clear modular pattern in layers V and VI until it abuts the retrosplenial cortex at the inferior marginal sulcus. Dorsally, within the limbic cleft, the marginal cortex borders the cortex of the paralimbic lobe, which is at these levels characterized by prominent, large pyramidal cells in layer V, forming clusters and what resembles the patterns observed in the frontal lobe, particularly in the cruciate sulcus.

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Figure 6. Cytoarchitecture of the cingulate cortex and retrosplenial cortex. A: Anterior level of the anterior segment of the cingulate cortex through the internal intercalate sulcus. B: Mid-level of the anterior segment of the cingulate cortex through the external intercalate sulcus. C: Anterior level of the anterior segment of the retrosplenial cortex through the internal intercalate sulcus. D: Mid-level of the anterior segment of the retrosplenial cortex through the external intercalate sulcus. Layers III and V are identified. Scale bar (on D) = 100 μm.

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The retrosplenial cortex occupies the region between the callosal sulcus and the limbic cleft. The cortical plate next to the subiculum shows a progressive differentiation of layers with a cytoarchitecture radically different from that seen in the supracallosal lobule. The cortex that faces the corpus callosum is characterized by a thin layer II and a relatively cell-poor layer III, but the neurons in that layer aggregate in somewhat regular groups of neurons defining a tiling pattern, which disappears medially toward the tip of the gyrus (Morgane et al., 1988). The cortex lining the variable retrosplenial intercalate sulcus presents a more usual clustering of rather small layer V pyramidal cells (Fig. 6C). Dorsally toward the limbic cleft, the marginal retrosplenial zone shows a thicker cortex, with a better-defined layer II and larger pyramidal cells in layer V (Fig. 6D). These features remain fairly constant throughout the anteroposterior extent of the retrosplenial cortex. Dorsally the marginal zone abuts the cortex of the oval and lingual lobules.

Hippocampal Formation and Parahippocampal Cortex

Cetaceans have an extensive parahippocampal cortex, very large subicular complex, and a comparatively diminutive hippocampus proper and dentate gyrus (Fig. 7A). All fields traditionally described in other mammals exist with few variations in their architecture, except for the very small size of the dentate gyrus, whose granule cell layer is frequently reduced to a small smooth layer. The hippocampal fields are also somewhat small compared to the size of the subiculum. The entorhinal cortex can be divided into at least four lateral subregions in the dorsal lip of the parahippocampal sulcus, a medial field between the lateral field and the periamygdaloid cortex anteriorly and the pre-/parasubiculum posteriorly, and a posterior field. It has very clear layer II islands in the medial field and a well-defined lamina dissecans throughout (Fig. 7B) that makes its border with the inferior temporal neocortex unequivocal.

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Figure 7. Organization of the hippocampus and medial entorhinal cortex. A: The hippocampus proper (CA1 and CA3 fields) is relatively small and the dentate gyrus (DG) appears unfolded compared to the situation in other mammals. In contrast, there is a large subicular complex that extends over the adjacent gyrus at right. ProS, prosubiculum; SUB, subiculum; hf, hippocampal fissure. The medial-most region of the entorhinal shown here displays the typically large lamina dissecans (ld) and layer II islands (wm, white matter). Roman numerals on B indicate entorhinal cortex layers. The junction with the subicular complex (parasubiculum) occurs at the left side of the photomicrograph. Scale bar (on B) = 100 μm.

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Comparison of Areas V1 and A1 in Four Odontocete Species

In addition to a considerable variety of cytoarchitectural patterning in the cetacean brain, there exist differences in cortical organization among species for specific cortical regions. Here we provide a few examples based on the observation of the primary visual (area V1) and primary auditory (area A1) cortex in odontocete species, three delphinids, the bottlenose dolphin, the beluga whale, the long-finned pilot whale, and a ziphiid, Cuvier's beaked whale (Fig. 8). The interspecies differences concern mainly the size of layer V cells and the pattern of aggregation of neurons in layers V–VI. In both areas, the bottlenose dolphin and the beluga whale (Fig. 8A, B, E, and F) are characterized by fairly large neurons in layer V, while these neurons are fewer or smaller in the pilot whale (Fig. 8C and G) and the beaked whale (Fig. 8D and H). Also, these large pyramidal cells appear to be more clustered in the bottlenose dolphin than they are in the other species. The pilot whale exhibits very clear grouping of large numbers of cells in layer V and VI with relatively large cell-poor gaps in both areas, whereas such pattern is poorly defined or absent in the other species. The ziphiid species also shows a less prominent layer II (Fig. 8D and H). Whether this is a generalized phenomenon across the whole neocortex in the beaked whale cannot be established on our materials. That such differences exist in primary sensory regions across cetacean species suggests that larger variations in cortical organization are likely to occur in the association cortex and reflect taxon-specific functional specializations.

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Figure 8. Comparative cytoarchitecture of the primary auditory cortex (AD) and primary visual cortex (EH) in Tursiops truncatus (bottlenose dolphin; A and E), Delphinapterus leucas (beluga whale; B and F), Globicephala melas (long-finned pilot whale; C and G), and Ziphius cavirostris (Cuvier's beaked whale; D and H). Note the differences in cellular composition of the two primary regions across these species. Layers are indicated by Roman numerals. wm, white matter. Scale bar (on H) = 100 μm.

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DISCUSSION

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED

For several decades, the common view has been that cetacean cortex is homogeneous and fairly nondifferentiated in character. This view, exemplified by Kesarev (1971, 1975), has engendered much hypothesizing about where the structural (and functional) complexity of cetacean brains lies if not in highly modified and differentiated cortical units as is the case in primates (Glezer et al., 1988). This view of cetacean neocortex as a rather indistinct and also sparse structure had implications for views of cetacean cognitive complexity and intelligence. On the one hand, the apparent uniformity of the neocortex implied a low level of behavioral complexity in cetaceans (Gaskin, 1982; Aronson and Tobach, 1988). On the other hand, the homogeneous nature posed a perplexing inconsistency to those that accepted the accruing evidence for considerable cognitive and behavioral complexity in cetaceans (Glezer et al., 1988; Marino, 2002b).

The present results, however, show that the cytoarchitectural patterns in cetaceans, at least based on the bottlenose dolphin, are far more varied and complex than generally thought. Although regions of less distinct lamination do exist, we identified numerous features of various neocortical regions that exemplify cytoarchitectural differentiation and diversity. Despite the modest gross morphological appearance of the frontal lobes, this region is distinctly laminated and comprises several cortical fields, as in the other lobes. The cytoarchictecture in the frontal lobe of cetaceans is indeed different from that of primates but it displays its own unique pattern of differentiation. Also, highly distinct, vertically oriented modules are apparent in the insular cortex and posterior polar region, as well as in other areas, and such columnar or patchy modules vary considerably in length and width depending on the region considered. The patchiness or columnarity of these patterns may reflect the region-specific distribution of corticocortical or thalamocortical afferents and may be exaggerated in respect to other mammalian species due to the peculiar laminar organization of the neocortical mantle in cetaceans that lacks layer IV and exhibits a very thick layer I. As such, there appears to be a rich diversity of cytoarchitectural fields in most of the lobes, and the peculiar cortical cytoarchitecture in cetaceans may underlie different functional strategies for cortical processing compared to other mammals. Indeed, cetaceans, and especially odontocetes, have a highly developed auditory system and echolocation capabilities whose distribution and representation at the neocortical level are still poorly understood (Ridgway, 2000). Also, many aspects of cortical and subcortical connectivity are likely to differ in cetaceans from terrestrial species as exemplified by their unique patterns of sleep/wakefulness and hemispheric sleep regulation (Manger et al., 2003).

It is worth noting that the neocortex of cetaceans shares several cyto- and chemoarchitectural features with that of large terrestrial artiodactyls, consistent with the phylogenetic propinquity of these species (Nikaido et al., 1996; Shimamura et al., 1997; Gatesy, 1998; Gingerich and Uhen, 1998; Milinkovitch et al., 1998; Hof et al., 1999, 2000; Gingerich et al., 2001; Thewissen et al., 2001; Geisler and Uhen, 2003). From an evolutionary and developmental standpoint, in comparison to other mammals, both cetaceans and artiodactyls are born with early physical maturity, a key survival factor in the aquatic milieu and, for artiodactyls, to escape predation. The many specializations of the cetacean brain may also be related to the retention in adult forms of juvenile ancestral features, a phenomenon known as pedomorphosis. In cetaceans, possible pedomorphic features are the retention in adult stages of the pontine, mesencephalic and cephalic flexures found only in embryos in other mammals, and the very large brain size at birth (Glezer et al., 1988). Also consistent with pedomorphosis is the fact that the neocortex of cetaceans (and of large artiodactyls) is dominated by interneurons containing calbindin and calretinin, the calcium-binding proteins that appear first during development in rodents, carnivores, and primates (Hof et al., 1999), which may reflect the persistence in adults of an apparently less differentiated cortex compared to other mammals. Our findings reveal nonetheless that there are potentially as many neocortical regions that can be identified by cytoarchitectural criteria in cetaceans as in other mammals such as primates and carnivores. Part of the explanation for why cetacean neocortex has historically been considered so much less interesting and well organized is because of limitations on the number of species studied, the number of regions sampled, and methodological constraints owing to the large size of most specimens.

The present evidence for considerable complexity in cetacean neocortex, particularly when coupled with the large size of these brains, is clearly consistent with the large body of evidence for behavioral and social complexity in cetaceans (for a review, see Marino, 2002b). Decades of work on learning. memory, and artificial language comprehension have shown that bottlenose dolphins are at least as capable as chimpanzees in these domains (for a review, see Herman, 2002). Bottlenose dolphins have also demonstrated rare abilities related to self-awareness, such as mirror self-recognition (Reiss and Marino, 2001) and self-monitoring (Smith et al., 2003). Furthermore, many cetaceans, such as bottlenose dolphins, sperm whales, and killer whales exhibit complex social patterns that include coalition formation, cooperation, cultural transmission (Connor et al., 1992; Rendell and Whitehead, 2001), and tool use (Krutzen et al., 2005). Therefore, it is expected that these abilities would be underwritten by a similar level of complexity in brain organization in cetaceans as they are in primates. What remains a compelling question for future study, however, is how such dissimilar neocortical cytoarchitectural motifs, such as that found in cetaceans and primates, result in convergent cognitive and behavioral characteristics, and why.

Acknowledgements

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED

The authors thank Drs. P.J. Morgane and I.I. Glezer for generous donation of most of the brain specimens and histological slide collection, Dr. G. Meyer for providing the brain samples from Z. cavirostris, and Dr. E. van der Gucht for helpful discussion.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. MATERIALS AND METHODS
  4. RESULTS
  5. DISCUSSION
  6. Acknowledgements
  7. LITERATURE CITED
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