An examination of cetacean brain structure with a novel hypothesis correlating thermogenesis to the evolution of a big brain.

School of Anatomical Sciences, Faculty of Health Sciences, University of the Witwatersrand, 7 York Road, Parktown, 2193, Johannesburg, Republic of South Africa.
Biological Reviews (Impact Factor: 9.79). 06/2006; 81(2):293-338. DOI: 10.1017/S1464793106007019
Source: PubMed

ABSTRACT 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.

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Available from: Paul Manger, Jul 13, 2015
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    • "In comparison to a dolphin skull, such as Tursiops, Pontoporia has a long narrow bony rostrum, with a concave ventral profile in lateral view, and exhibits the most symmetrical skull of any extant odontocetes (Ness 1967, Barnes 1985, Cranford et al. 1996) (Fig. 1). The small braincase with one of the smallest brains among toothed whales (Schwerdtfeger et al. 1984, Manger 2006) may correlate with the elongation of Pontoporia's nasal complex. Huggenberger et al. (2010) have noted that this elongation is marked by topographical relations of the dorsal bursae with the cranial vault and melon, which is located further caudally in relation to the brain case. "
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    • "Testes and brain tissues are energetically expensive, so a reduction in the mass of one or the other would evolve in species that did not need or was unable to maintain large amounts of costly tissue (Pitnick et al. 2006). Thermogenesis was suggested as a possible explanation for the evolution of large brains in cetaceans (Manger 2006), but there is overwhelming evidence suggesting that cetacean brain size is linked with cognitive capacities (Herman 1980; Ridgeway 1986; Connor et al. 1992; Marino 1998; Connor 2007; Marino et al. 2007). There was no significant correlation between residual testes mass and residual brain mass after controlling for phylogeny, which weakens support for the Machavellian intelligence or sexual conflict hypothesis that predicts increased brain mass with increasing sperm competition. "
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    Canadian Journal of Zoology 06/2014; 92(8):721-726. DOI:10.1139/cjz-2014-0044 · 1.35 Impact Factor
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    • "His assumption, then, is that primate-like cognition depends on a primate-like cortical structure. As the dolphin cortex is so different from that of the primates, the similarity in their cognition is, according to Manger (2006), an illusion based on the over-interpretation of an inadequate experimental research base. As Kirsh, Güntürkün, and Rose (2008) have shown for avian species, however, very different kinds of brains can converge on similar kinds of cognitive abilities. "
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