Surface area of the cerebral cortex is directly related to brain weight among ... cerebral hemisphere was found to be larger in surface area for Tursiops (P < 0.05) ...
115 Acta Zool. Fennica 172:149-152. 1984
Relative brain sizes and cortical surface areas in odontocetes S.H. Ridgway & R.H. Brownson
Ridgway , S.H . & Brownson , R.H. 1984: Relative brain sizes and cortical surface areas in odontocetes. - Acta Zool. Fennica 172:149-152. Surface area of the cerebral cortex is directly related to brain weight among odontocetes studied including Delphinus, G/obicepha/a, Grampus, Orcin us, Physeler, Stenella, Tursiops and Ziphius (linear regression of brain weight (W) in g vs. surface area (A) of cortex in em revealed A = 330.4 2.17 W, with correlation coefficient r = 0.98). The genera differed greatly when cortical surface area and brain weight were related to body length, body weight, and encephalization quotient. Neonatal Tursiops and Orcinus had brain weights almost one-half as large as adult brains. As a percentage of adult brain weight Tursiops and Orcin us neonates had brains more developed than those of sixmonth-old humans. Two neonatal Orcinus had brains very similar in size to that of the Physeter neonate measured . Asymmetries were studied in Tursiops, Slenella and Delphinus. The right cerebral hemisphere was fo und to be larger in surface area for Tursiops (P < 0.05) and Delphinus (P < 0.07) but not in Slenella. There was no difference in total cortical thickness between various regions of right and left hemispheres. Total cortical thickness varied from 1.30 mm (visual) to 1.76 mm (motor). The auditory cortex of Tursiops brain was thicker than that of Delphinus (P < 0.01). S.H. Ridgway, Nayal Ocean Syslems Center, San Diego, California 92152, USA. R.H. Brownson, Department of Anatomy, Easlern Virginia Medical School. Norfolk, Virginia 23501, USA.
1. Introduction Very large brains are found in some of the s mall toothed whales (odontocetes) that are commonly called porpoises or dolphins. Although it is generally assumed that the human brain is more convoluted (cf. Teyler 1977) than that of all other mammals, Elias & Swartz (1969) have shown that the brains of some odontocetes are more convoluted - they have more surface area per unit volume than does the human brain . The "index of folding " was found to be 2.86 for a series of human brains and 4.47 for a bottlenosed dolphin brain of the same stze. The large brains of dolphins have prompted Lilly (1964) to speculate that dolphins possess language . Although one of Lilly's assumptions, that no deep sea adult cetacean possesses a brain less than 900-1000 g, has been disproved (Ridgway et al. 1966), it has been widely ass umed that all cetaceans have large brains. There are relatively few data on brain sizes or cortical surface areas in odontocetes. Elias & Swartz (1969) published a simple grid plate technique for measuring cortical surface area. We decided to apply this technique to brains we obtained for analysis at postmortem, and gradually accumulated a series that includes eight odontocetes - a significant addition to the available
data on brain size for the genera considered (Lilly 1964, Ridgway et al. 1966, Pilleri & Gihr 1969,197 1, Elias & Swartz 1969, von Bonin 1973). First we wanted to determine if surface area of the cerebral cortex increased in direct relation to brain size as it does for mammals (Jerison 1982). Secondly, we were interested in the concept of encephalization quotient (EQ) as developed by Jerison (1973) and wanted to compare the different species of odontocetes using Jerison's formula as had been done previously on a smaller series of specimens (Wood & Evans 1980). Thirdly, we were able to obtain several specimens of neonates from species such as Orcinus that, to our knowledge, had not been previously measured. Fourth, for those species for which we had more than just several specimens, we were interested to study the symmetry of the two hemispheres of the cortex by measuring their surface areas and comparing their weights. Finally, we were interested in measuring the thickness of the cortex in different areas of each hemisphere and comparing these measures between the two halves of the brain.
2. Material and methods Brain s we re re mo ved at postmortem from animals lhat died of
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Fig. 1. Surf~h.:c urea of the cen:bral conex plotted against brain weigh1. Linear regress ion of brain weight (W) in g vs. surface area of corto, in com (A) revealed A = 330A + 2.17 W, Wllh correlation coefficient r = 0.98.
natural ca uses or we re killed accidentally
In commercial fi shin g. Animal lengths (standard length; Norris 1961) and we ights were recorded. The Slenella, laken in (he eastern tropical Pacific, we re provided from frozen strorage b y (he Na lional Marine Fisheries Service; Marine Mammal Program . Oceanarium speci mens were pruvided fres h , frozen or in formalin . Length an d weight data we re taken from oceanarium records . Each brain was biseeted in the midline by cutting through the interhemispheric cleft, septum and raphe of the brains tern . The divided hemisp heres were we ighed, then placed in 10 % formalin and allowed to harden before we sliced them into sections of 510 10 mm depending upon the size of each brain. The hemispheres were sliced with a long knife guided through a guillotine type device, then analyzed by grid plate stere ology (Elias & Swartz 1969). Tissue sectio ns were removed, for measures of cortical thickness by light microscopy, from several areas of the cerebral cortex of Tursiops and Delphinus specimens. These sections were taken from functional areas such as auditory and visual cortex identified by A. Y. Supin and his colleagues in the Soviet Union (Bullock & Gurevich 1979).
The smallest specimen was a 7-kg Delphinus neonate with a brain of 442 g. The largest was an adult Orcin us of 2409 kg with a brain that weighed 6215 g. Thus the largest brain was over 14 times as heavy as the smallest and the largest animal weighed about 344 times as much as the smallest. The range of brain size could probably be increased to perhaps as much as 100 times by measuring the sma llest neonate brain of the smallest brained genus, Platanisla (Kamiya & Pirlot 1980) and the largest brain of Physeler (Pilleri & Gihr 1969). The range of body sizes in our data is, however, small when compared to the range possible from the smallest cetacean neonate to the largest adult - almost 20 000 times. This great size range offers interesting possibilities for comparison of brain size in relation to body size both within and between genera. Fig. I shows the results of our regression analysis of brain weight and cortical surface area. The surface area of the cerebral cortex is directly related to brain weight among these odontocetes. In Fig. 2 we have plotted Jerison's encephalization quotient against body length for Tursiops and Delphinus. The regression for the Tursiops data clearly shows that EQ decreases with increasing body length. Therefore it is ner:essary, when usi ng this measure for comparison bet ween genera , to use data from animals of the same relative maturity. Table I gives average EQ values for those genera for which we had at least one mature animal's body weight and brain weight represented . Table 2 gives data on brain weight in neonatal Tursiops, Orcin us, and Delphinus. These can be compared with average measures for adults in Table I. Table 3 shows a comparison of the two halves of the brain with respect to weights of right and left halves and to surface area of the cerebral cortex for each cerebral hemisphere. Complete tables of this data have been published elsewhere (Ridgway & Brownson 1979). In Tursiops the right hemisphere has a larger surface are (I test: P < 0.05). Delphinus gave a similar result though with less certainty (I test: P 0.07). Although the mean values for the Slenella showed a small advantage to the right, the difference was not significant (I test: P> 0.1). There were no significant differences in thickness of the
