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FIG U R E 2. A reconstructed map of somatosensory cortex in possum 6. Filled circles represent cutaneous neuronal responses to peripheral stimulation, open circles represent responses to stim ulation of deep receptors and dashes highlight regions of no clear response. Thick lines delineate the primary somatosensory area (SI) forelimb and face representations, thin lines delineate cortex immediately rostral to, caudal to, and interdigitating between the SI discontinuities that was responsive to stim ulation of deep receptors: boundaries were determ ined by electrophysiological mapping. Thin grey lines within the SI forelim b representation highlight receptive field progressions from digit 1 (d1) to digit 5 (d5). Insert shows the orientation of these somatosensory fields in the intact brain. See Appendix for naming conventions. Scale bar = 2 m m.
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Microelectrode mapping techniques were used to determine the organization of somatosensory cortex in the Australian brush-tailed possum (Trichosurus vulpecula). The results of electrophysiological mapping were combined with data on the cyto- and myeloarchitecture, and patterns of corticocortical connections, using sections cut tangential to the pia...
Citations
... Both the location and relative size of area PV in the paca follows the pattern described for other mammalian species [42,50,52,54,57,58]; however, this is the first report of the existence of area PV in a hystricomorph rodent, as it has not been identified in either the capybara [59] or the agouti [17]. The absence of PV in the capybara may be attributed to technical limitations in the available study, which used macroelectrodes for electrophysiological mappings [59]. ...
Introduction: The study of non-laboratory species has been part of a broader effort to establish the basic organization of the mammalian neocortex, as these species may provide unique insights relevant to cortical organization, function, and evolution. Methods: In the present study, the organization of three somatosensory cortical areas of the medium-sized (5-11 kg body mass) Amazonian rodent, the paca (Cuniculus paca), was determined using a combination of electrophysiological microelectrode mapping and histochemical techniques (Cytochrome oxidase and NADPH diaphorase) in tangential sections. Results: Electrophysiological mapping revealed a somatotopically organized primary somatosensory cortical area (S1) located in the rostral parietal cortex with a characteristic foot-medial/head-lateral contralateral body surface representation similar to found in other species. S1 was bordered laterally by two regions housing neurons responsive to tactile stimuli, presumably the secondary somatosensory (S2) and parietal ventral (PV) cortical areas, that evinced a mirror-reversal representation (relative to S1) of the contralateral body surface. The limits of the putative primary visual (V1) and primary auditory (A1) cortical areas, as well as the complete representation of the contralateral body surface in S1, were determined indirectly by the histochemical stains. Like the barrel field described in small rodents, we identified a modular arrangement located in the face representation of S1. Conclusions: The relative location, somatotopic organization, and pattern of neuropil histochemical reactivity in the three paca somatosensory cortical areas investigated are similar to those described in other mammalian species, providing additional evidence of a common plan of organization for the somatosensory cortex in the rostral parietal cortex of mammals.
... Evidence from other mammalian species support the notion that S2 is very well interconnected ipsilaterally with other somatosensory cortical fields [primates (Burton and Carlson, 1986;Weller and Kaas, 1987;Krubitzer and Kaas, 1990;Disbrow et al., 2003), carnivores (Hartenstein et al., 1980;Herron and Johnson, 1987), insectivores (Catania and Kaas, 2001), rodents (Krubitzer et al., 1986;Carvell and Simons, 1987;Koralek et al., 1990), marsupials (Beck et al., 1996;Elston and Manger, 1999)]. In the present study, we found a small focus of projection to deep layers of a lateral region of the parietal cortex, next to the rhinal sulcus, not reported in a previous work with the agouti (Pimentel-Souza et al., 1980). ...
In order to understand how the mammalian sensory cortex has been structured during evolution, it is necessary to compare data from different species across distinct mammalian lineages. Here, we investigated the organization of the secondary somatosensory area (S2) in the agouti (Dasyprocta aguti), a medium-sized Amazonian rodent, using microelectrode mapping techniques and neurotracer injections. The topographic map obtained from multiunit electrophysiological recordings were correlated with both cytochrome oxidase (CO) histochemistry and with patterns of corticocortical connections in tangential sections. The electrophysiological mapping of the lateral strip of parietal cortex adjacent to the primary somatosensory area (S1) revealed that S2 displays a mirror-reversed topographical representation of S1, but with a smaller cortical magnification factor. The caudal border of S2 is surrounded by sensory fields which also respond to auditory stimulation. BDA injections into the forelimb representation of S2 revealed a dense homotopic ipsilateral projection to S1, supplemented by a less dense projection to the caudolateral cortex located near the rhinal sulcus (parietal rhinal area) and to a frontal region probably associated with the motor cortex. Our findings were similar to those described in other mammalian species, reinforcing the existence of a common plan of organization for S2 in the mammalian parietal cortex.
... However, different research groups seem to agree that most mammals share five somatosensory areas in the parietal lobe, including a primary somatosensory field (S1), a secondary area (S2), a parietal ventral area (PV) located immediately rostral to S2, and two narrow bands of somatosensory cortex along the rostral and caudal borders of S1 (Krubitzer et al., 1986;Beck et al., 1996;Slutsky et al., 2000;Santiago et al., 2007). Each one of these somatosensory fields contains a complete representation of the animal body and their boundaries can be visualized with different histological methods, especially those revealing myelin and cytochromeoxidase (CO) activity (Land and Simons, 1985;Krubitzer et al., 1986;Elston and Manger, 1999;Slutsky et al., 2000;Elston et al., 2006;Campi et al., 2007;Rocha et al., 2007;Anomal et al., 2011). In some of these areas, especially in S1, an internal modular organization of peripheral isomorphs can also be revealed using these techniques. ...
We analyzed the organization of the somatosensory and visual cortices of the agouti, a diurnal rodent with a relatively big brain, using a combination of multiunit microelectrode recordings and histological techniques including myelin and cytochrome oxidase staining. We found multiple representations of the sensory periphery in the parietal, temporal and occipital lobes. While the agouti's primary (V1) and secondary visual areas seemed to lack any obvious modular arrangement, such as blobs or stripes, which are found in some primates and carnivores, the primary somatosensory area (S1) was internally subdivided in discrete regions, isomorphically associated to peripheral structures. Our results confirm and extend previous reports on this species, and provide additional data to understand how variations in dimensions of lifestyle can influence brain organization in rodents. J. Comp. Neurol., 2014. © 2014 Wiley Periodicals, Inc.
... In previous studies, marsupials (including the short-tailed opossum) have been shown to have at least two complete somatotopic representations within the neocortex: S1 and the second somatosensory area, S2 (Beck et al., 1996;Huffman et al., 1999b;Catania et al., 2000;Frost et al., 2000; for review see Karlen and Krubitzer, 2007). Further analysis of electrophysiological recordings, architecture, and connectional patterns by our own and other laboratories has supported the presence of additional fields associated with somatosensory processing, including a rostral field (SR) as well as a caudal field (SC; Beck et al., 1996;Elston and Manger, 1999;Huffman et al., 1999b;Wong and Kaas, 2009;Anomal et al., 2011;Dooley et al., 2013). A third complete somatotopic representation (the parietal ventral area, PV) has been identified in numerous marsupials investigated (Beck et al., 1996;Elston and Manger, 1999;Huffman et al., 1999b), however despite careful exploration, a separate, third somatotopic representation has not been identified in the short-tailed opossum (Catania et al., 2000;Frost et al., 2000). ...
... Further analysis of electrophysiological recordings, architecture, and connectional patterns by our own and other laboratories has supported the presence of additional fields associated with somatosensory processing, including a rostral field (SR) as well as a caudal field (SC; Beck et al., 1996;Elston and Manger, 1999;Huffman et al., 1999b;Wong and Kaas, 2009;Anomal et al., 2011;Dooley et al., 2013). A third complete somatotopic representation (the parietal ventral area, PV) has been identified in numerous marsupials investigated (Beck et al., 1996;Elston and Manger, 1999;Huffman et al., 1999b), however despite careful exploration, a separate, third somatotopic representation has not been identified in the short-tailed opossum (Catania et al., 2000;Frost et al., 2000). However, the region which has been identified shows characteristics of both S2 and PV, thus we have elected to refer to this area as S2/PV throughout the text. ...
... Among marsupials, SR and SC have been identified based on receptive field characteristics and stimulus preference in the Virginia opossum (Beck et al., 1996), the brush-tailed opossum (Elston and Manger, 1999), the northern quoll, the striped possum (Huffman et al., 1999b), and the big-eared opossum (Anomal et al., 2011). Across the species investigated, neurons in SR and SC respond predominantly to stimulation of deep receptors in muscles and joints, show a coarse topographical organization, and have larger receptive fields compared to neurons in S1. ...
The current experiments build upon previous studies designed to reveal the network of parietal cortical areas present in the common mammalian ancestor. Understanding this ancestral network is essential for highlighting the basic somatosensory circuitry present in all mammals, and how this basic plan was modified to generate species specific behaviors. Our animal model, the short-tailed opossum (Monodelphis domestica), is a South American marsupial that has been proposed to have a similar ecological niche and morphology to the earliest common mammalian ancestor. In this investigation, we injected retrograde neuroanatomical tracers into the face and body representations of primary somatosensory cortex (S1), the rostral and caudal somatosensory fields (SR and SC), as well as a multimodal region (MM). Projections from different architectonically defined thalamic nuclei were then quantified. Our results provide further evidence to support the hypothesized basic mammalian plan of thalamic projections to S1, with the lateral and medial ventral posterior thalamic nuclei (VPl and VPm) projecting to S1 body and S1 face, respectively. Additional strong projections are from the medial division of posterior nucleus (Pom). SR receives projections from several midline nuclei, including the medial dorsal, ventral medial nucleus, and Pom. SC and MM show similar patterns of connectivity, with projections from the ventral anterior and ventral lateral nuclei, VPm and VPl, and the entire posterior nucleus (medial and lateral). Notably, MM is distinguished from SC by relatively dense projections from the dorsal division of the lateral geniculate nucleus and pulvinar. We discuss the finding that S1 of the short-tailed opossum has a similar pattern of projections as other marsupials and mammals, but also some distinct projections not present in other mammals. Further we provide additional support for a primitive posterior parietal cortex which receives input from multiple modalities.
... Marsupials including M. domestica have been shown to have two complete somatotopic representations within the cortex, S1, and the second somatosensory area, S2 (Huffman et al., 1999;Catania et al., 2000;Frost et al., 2000;Karlen and Krubitzer, 2007). Based on electrophysiological recordings, architectonic analysis, and patterns of connections, some studies indicate that marsupials have two to three additional fields associated with somatosensory processing; a rostral field termed R or SR, a caudal field termed C or SC, and a parietal ventral area, PV (Beck et al., 1996;Elston and Manger, 1999;Huffman et al., 1999;Wong and Kaas, 2009;Anomal et al., 2011). Despite the important phylogenetic position of marsupials, and the implications that cortical processing networks of early mammals may be more complex than previously thought, little is known about the cortical connectivity of somatosensory areas in this group of mammals (see Discussion). ...
... To date, the cortical connectivity of primary somatosensory cortex has only been investigated in three marsupials (Fig. 10). These include the North American Virginia opossum, Didelphis virginiana (Beck et al., 1996), the Australian brush-tailed possum, Trichosurus vulpecula (Elston and Manger, 1999), and the South American big-eared opossum, Didelphis aurita (Anomal et al., 2011). All of these studies report dense ipsilateral connections with other somatosensory areas, including S2, PV, SR, SC, and a region of cortex immediately lateral to S1, in and around the rhinal sulcus (we term this the entorhinal cortex; EC). ...
... Additionally, injections placed both medially and laterally in the Virginia opossum show similar topographic restriction between body and face representations (see figs. 16, 17 of Beck et al., 1996). The more distantly related brush-tailed possum's S1 also has topographically restricted intrinsic projections (Elston and Manger, 1999). ...
The current experiment is one of a series of comparative studies in our laboratory designed to determine the network of somatosensory areas that was present in the neocortex of the mammalian common ancestor. Such knowledge is critical for appreciating the basic functional circuitry that all mammals possess and how this circuitry was modified to generate species specific, sensory mediated behavior. Our animal model, the gray short-tailed opossum (Monodelphis domestica) is a marsupial that is proposed to represent this ancestral state more closely than most other marsupials and to some extent, even monotremes. We injected neuroanatomical tracers into the primary somatosensory area (S1), rostral and caudal somatosensory fields (SR and SC, respectively), and multimodal cortex (MM) and determined their connections with other architectonically defined cortical fields. Our results show that S1 has dense intrinsic connections, dense projections from the frontal myelinated area (FM), and moderate projections from S2 and SC. SR has strong projections from several areas, including S1, SR, FM and piriform cortex. SC has dense projections from S1, moderate to strong projections from other somatosensory areas, FM, along with connectivity from the primary (V1) and second visual areas. Finally, MM had dense intrinsic connections, dense projections from SC and V1, and moderate projections from S1. These data support the proposition that ancestral mammals likely had at least four specifically interconnected somatosensory areas, along with at least one multimodal area. We discuss the possibility that these additional somatosensory areas (SC and SR) are homologous to somatosensory areas in eutherian mammals. J. Comp. Neurol., 2013. © 2013 Wiley Periodicals, Inc.
... In the first study, performed in Didelphis virginiana,Beck et al. (1996)made injections located in S1 (briefly described above), but connections of other somatosensory fields, like the somatosensory rostral and caudal fields, were not investigated. The second anatomical tracer study was performed in the Australian marsupial Trichosurus vulpecula, also known as the brush-tailed possum (Elston and Manger, 1999). Similar to Didelphis virginiana, the brush-tailed possum presented at least three somatosensory fields that were topographically organized: S1, S2, and PV. ...
... A field caudal to S1 was named PP cortex, in which neurons were responsive to deep somatosensory stimulation and received ipsilateral projections from S1, S2, and PV. This cortical region was additionally connected to other sensory and motor areas, such as the primary and secondary visual areas, and motor cortex (Elston and Manger, 1999), and might be homologous to area SC described in Didelphis virginiana. Although precious for comparative studies, from a phylogenetic point of view data obtained in the brush-tailed possum may not be ideal for reconstructing the organization of the primitive brains of ancestral mammals. ...
... Typically, dysgranular cortex is less responsive to cutaneous stimulation than the granular cortex (Krubitzer et al., 1986). Similar to what has been described in rodents (Zilles and Wree, 1985;Krubitzer et al., 1986;Santiago et al., 2007), and also in primates (Fang et al., 2002), a region of dysgranular cortex interposing between the representation of the face and forepaw was detected in other marsupials as well (Elston and Manger, 1999;Huffman et al., 1999), but not in representatives of the genus Didelphis. Nevertheless, it is conceivable that these opossum species present a circuitry in the hand–face border that is differentiated from the rest of S1, thus resulting in neurons displaying attenuation of cutaneous responsiveness and larger receptive fields in anesthetized preparations. ...
In small-brained mammals, such as opossums, the cortex is organized in fewer sensory and motor areas than in mammals endowed with larger cortical sheets. The presence of multimodal fields, involved in the integration of sensory inputs has not been clearly characterized in those mammals. In the present study, the corticocortical connections of the forepaw representation in the somatosensory caudal (SC) area of the Didelphis aurita opossum was studied with injections of fluorescent anatomical tracers in SC. Electrophysiological mapping of S1 was used to delimit its respective rostral and caudal borders, and to guide SC injections. The areal borders of S1 and the location of area SC were further confirmed by myeloarchitecture. In S1, we found a well-delimited forepaw representation, although it presented a crude internal topographic organization. Cortical projections to S1 originate in somatosensory areas of the parietal cortex, and appeared to be mostly homotopic. Physiological and connectional evidence were provided for a topographic organization in opossum area SC as well. Most notably, corticocortical projections to the forepaw representation of SC originated from somatosensory cortical areas and from cortex representing other sensory modalities, especially the visual peristriate cortex. This suggests that SC might be involved in multimodal processing similar to the posterior parietal cortex of species with larger brains.
... In other marsupials, such as the brush-tailed possum [Gates and Aitkin, 1982] and the Northern quoll [Aitkin et al., 1986], primary auditory area (A1) is tono-topically organized and receives projections from the medial geniculate nucleus [Goldby, 1943;Neylon and Haight, 1983;Aitkin and Gates, 1983;Kudo et al., 1989] as in placental mammals [Ryugo and Killackey, 1974;Merzenich et al., 1976;Luethke et al., 1988;Wong et al., 2008]. Architectonically, the auditory cortex of short-tailed opossums has characteristics that are similar to those in other marsupials, including the Virginia opossum, brush-tailed possum and Northern quoll [Beck et al., 1996;Huffman et al., 1999;Elston and Manger, 1999;Catania et al., 2000b], such as dense myelination, dark CO staining, and a thick, densely packed layer 4 with granule cells. Here, we have further characterized the architectonic properties of the auditory cortex in short-tailed opossums. ...
... In addition, there is denser staining of VGluT2 and PV immunopositive terminations compared to the surrounding cortical areas, suggesting the presence of a higher proportion of thalamocortical terminations in area 3b, possibly originating from the ventroposterior nucleus, as has been shown in the Virginia opossum [Diamond and Utley, 1963;Pubols, 1968]. Unlike other marsupials, such as the brush-tailed possum, Virginia opossum, striped possum or Tammar wallaby, the short-tailed opossum has no architectonically definable barrel field or barrel-like cortex in area 3b [Woolsey et al., 1975;Waite et al., 1991Waite et al., , 1998Weller, 1993;Beck et al., 1996;Elston and Manger, 1999]. ...
Short-tailed opossums (Monodelphis domestica) belong to the branch of marsupial mammals that diverged from eutherian mammals approximately 180 million years ago. They are small in size, lack a marsupial pouch, and may have retained more morphological characteristics of early marsupial neocortex than most other marsupials. In the present study, we used several different histochemical and immunochemical procedures to reveal the architectonic characteristics of cortical areas in short-tailed opossums. Subdivisions of cortex were identified in brain sections cut in the coronal, sagittal, horizontal or tangential planes and processed for a calcium-binding protein, parvalbumin (PV), neurofilament protein epitopes recognized by SMI-32, the vesicle glutamate transporter 2 (VGluT2), myelin, cytochrome oxidase (CO), and Nissl substance. These different procedures revealed similar boundaries among areas, suggesting that functionally relevant borders were detected. The results allowed a fuller description and more precise demarcation of previously identified sensory areas, and the delineation of additional subdivisions of cortex. Area 17 (V1) was especially prominent, with a densely populated layer 4, high myelination levels, and dark staining of PV and VGluT2 immunopositive terminations. These architectonic features were present, albeit less pronounced, in somatosensory and auditory cortex. The major findings support the conclusion that short-tailed opossums have fewer cortical areas and their neocortex is less distinctly laminated than most other mammals.
... Both the cortical and subcortical connections of area 3b (S1) have been described in a number of mammals such as rodents (e.g., Wise & Jones, 1976;Akers & Killackey, 1978;Krubitzer et al., 1986;Chapin et al., 1987;Krubitzer & Kaas, 1987;Koralek et al., 1990;Fabri & Burton, 1991;Paperna & Malach, 1991) carnivores (Alloway & Burton, 1985;Barbaresi et al., 1987;Herron & Johnson, 1987), and marsupials (Beck et al., 1996;Elston & Manger, 1999; see Johnson, 1990 for review). As in primates, S1 is densely connected with area 3a (R, UZ, dysgranular cortex, see below), M1, and S2/PV. ...
... All species examined have a primary somatosensory area (S1) and a second somatosensory area (S2) located caudolateral to S1 (for review see Johnson, 1990;Rowe, 1990). The functional organization of S1 has been described using electrophysiological mapping techniques in several species ( Figure 5), including the brush-tailed possum (Trichosurus vulpecula; Adey and Kerr, 1954;Elston and Manger, 1999;Haight and Weller, 1973;Weller, 1993), the Virginia opossum Bodemer and Towe, 1963;Lende, 1963c;Pubols et al., 1976), the wallaby (Thylogale eugenii; Lende, 1963a), the white-eared opossum (Didelphis albiventris, previously Didelphis azarae azarae; Magalhaes-Castro and Saraiva, 1971), the Tasmanian wombat (Johnson et al., 1973), the short-tailed opossum (Monodelphis domestica; Catania et al., 2000;Frost et al., 2000;Huffman et al., 1999), the striped possum , and the Northern quoll (Dasyurus hallucatus, also called the native cat; Huffman et al., 1999). In all species examined, S1 contains a complete and inverted representation of the contralateral body surface with the tail represented most medially, followed by the representation of the hindlimb, trunk, forelimb, forepaw, face, and oral structures in a medial to lateral progression ( Figure 5). ...
... Using a variety of staining procedures, the cortical architecture of S1 has been described in a number of different marsupials. First, S1 is readily identified by its well-defined granular layer (layer IV), which contains densely packed cells that stain darkly for Nissl (Adey and Kerr, 1954;Ashwell et al., 2005;Elston and Manger, 1999;Foster et al., 1981). In fact, many of the same cell types, as defined by Golgi-Cox and Nissl staining, are found in the parietal cortex (which contains S1) of both marsupials and placental mammals (Walsh and Ebner, 1970). ...
... In fact, many of the same cell types, as defined by Golgi-Cox and Nissl staining, are found in the parietal cortex (which contains S1) of both marsupials and placental mammals (Walsh and Ebner, 1970). In several marsupials, a distinct barrel field (in the brush-tailed possum) or barrel-like subdivisions (in the Virginia opossum, Tammar wallaby (Macropus eugenii), and striped possum) have been found within S1 in both coronally sectioned and tangentially sectioned tissue processed for Nissl, succinic dehydrogenase (SDH), or myelin Elston and Manger, 1999;Waite et al., 1998;Waite et al., 1991;Weller, 1993;Woolsey et al., 1975). Unlike the barrel fields in mice and rats that are characterized by densely packed cells surrounding a loosely packed center, the barrel field in marsupials contains loosely packed cells surrounding a densely packed center (Possibly add figure of barrel cortex; Weller, 1993). ...
Marsupials are a diverse group of mammals that occupy a large range of habitats and have evolved a wide array of unique adaptations. Although they are as diverse as placental mammals, our understanding of marsupial brain organization is more limited. Like placental mammals, marsupials have striking similarities in neocortical organization, such as a constellation of cortical fields including S1, S2, V1, V2, and A1, that are functionally, architectonically, and connectionally distinct. In this review, we describe the general lifestyle and morphological characteristics of all marsupials and the organization of somatosensory, motor, visual, and auditory cortex. For each sensory system, we compare the functional organization and the corticocortical and thalamocortical connections of the neocortex across species. Differences between placental and marsupial species are discussed and the theories on neocortical evolution that have been derived from studying marsupials, particularly the idea of a sensorimotor amalgam, are evaluated. Overall, marsupials inhabit a variety of niches and assume many different lifestyles. For example, marsupials occupy terrestrial, arboreal, burrowing, and aquatic environments; some animals are highly social while others are solitary; different species are carnivorous, herbivorous, or omnivorous. For each of these adaptations, marsupials have evolved an array of morphological, behavioral, and cortical specializations that are strikingly similar to those observed in placental mammals occupying similar habitats, which indicate that there are constraints imposed on evolving nervous systems that result in recurrent solutions to similar environmental challenges.
... Such anatomically identifiable modules are found in the primary sensory cortices of other mammals with specialised sensory systems, e.g. platypus , insectivores (Catania, 2000), marsupials (Elston & Manger, 1999;Huffman et al., 1999), rodents (Woolsey, Welker & Schwartz, 1975), and primates (Mountcastle, 1997) among many others. Second, mammals with specialised sensory systems do not have the encephalisation quotient (EQ) grossly altered due to brain-body mass scaling differences that is seen in cetaceans. ...
This review examines aspects of cetacean brain structure related to behaviour and evolution. Major considerations include cetacean brain-body allometry, structure of the cerebral cortex, the hippocampal formation, specialisations of the cetacean brain related to vocalisations and sleep phenomenology, paleoneurology, and brain-body allometry during cetacean evolution. These data are assimilated to demonstrate that there is no neural basis for the often-asserted high intellectual abilities of cetaceans. Despite this, the cetaceans do have volumetrically large brains. A novel hypothesis regarding the evolution of large brain size in cetaceans is put forward. It is shown that a combination of an unusually high number of glial cells and unihemispheric sleep phenomenology make the cetacean brain an efficient thermogenetic organ, which is needed to counteract heat loss to the water. It is demonstrated that water temperature is the major selection pressure driving an altered scaling of brain and body size and an increased actual brain size in cetaceans. A point in the evolutionary history of cetaceans is identified as the moment in which water temperature became a significant selection pressure in cetacean brain evolution. This occurred at the Archaeoceti - modern cetacean faunal transition. The size, structure and scaling of the cetacean brain continues to be shaped by water temperature in extant cetaceans. The alterations in cetacean brain structure, function and scaling, combined with the imperative of producing offspring that can withstand the rate of heat loss experienced in water, within the genetic confines of eutherian mammal reproductive constraints, provides an explanation for the evolution of the large size of the cetacean brain. These observations provide an alternative to the widely held belief of a correlation between brain size and intelligence in cetaceans.
