This article is from the Dolphin
FAQ, by Jaap van der Toorn
Whales and dolphins have quite large brains. Most authors agree that the size should be viewed in relation to the body size. In some comparisons, brain weight to body weight ratio is used, but it is now more common to use the so-called Encephalization Quotient (EQ), which is calculated as: EQ = brain weight / (0.12 * (body weight ^ (2/3))) (brain weight, divided by 0.12 time the body weight to the power (2/3)). In this formula, brain and body weight should be expressed in grammes. In bottlenose dolphins, the EQ lies between 4 and 5, in the killer whale between 2.5 and 3, in humans in the 6.5-7.5 range.
However, the structure of the dolphin brain is quite different from most land mammals and shows a lot of similarities with so-called archetypal mammals like hedgehogs and bats (mammals in which the brain structure has changed little since the middle of the Tertiary period).
Dolphin and whale ancestors returned to the sea 50-70 million years ago. (Bats have presumably developed their aerial lifestyle in the same period). In their adaptation to the aquatic environment, they seem to have retained characteristic features of the brain of the primitive mammalian species of the time. In cetaceans, the neocortex has expanded greatly, but without the substantial reorganization in 6 layers seen in most land mammals. The main features in the cetacean brain which differ from land mammals are:
- a thin neocortex (about 1.5 mm, compared to 2.9 mm in humans)
- rather uniform structure of the cortex
- low degree of differentiation between cortical layers and cells
- massive development of the (phylogenetically older) layers I and VI
- poor development of layers II, III and IV (which are well developed in land mammals)
- the neurons have relatively few primary dendrites and these are weakly branched.
References and suggested reading:
A. Berta & J.L. Sumich (1999) Marine mammals - Evolutionary biology Academic Press, San Diego, London (ch. 7.3 focuses on senses and the nervous system)
I.I. Glezer, P.R. Hof, C. Leranth & P.J. Morgane (1992) Morphological and histochemical features of odontocete visual neocortex: immunocytochemical analysis of pyramidal and non-pyramidal populations of neurons. in: J.A. Thomas, R.A. Kastelein & A.Y. Supin (eds.): Marine Mammal Sensory Systems, pp. 1-38 Plenum Press, New York, London
L. Marino (1997) The relationship between gestation length, encephalization, and body weight in odontocetes. Marine Mammal Science 13(1):133-138
P.J. Morgane, M.S. Jacobs & A. Galaburda (1986a) Evolutionary morphology of the dolphin brain in: R.J. Schusterman, J.A. Thomas & F.G. Wood (eds.): Dolphin cognition and behavior: a comparative approach, pp. 5-29 Lawrence Erlbaum Associates, New Jersey
P.J. Morgane, M.S. Jacobs & A. Galaburda (1986b) Evolutionary aspects of cortical organization in the dolphin brain. in: M.M. Bryden & R. Harrison (eds.): Research on Dolphins, pp. 71-98 Oxford Science Publications, Clarendon Press, London
P.J. Morgane & I.I. Glezer (1990) Sensory neocortex in dolphin brain in: J.A. Thomas & R.A. Kastelein (eds.): Sensort Abilities of Cetaceas - Laboratory and Field Evidence, pp, 107-136. NATO ASI Series, Series A: Life Sciences vol. 196 Plenum Press, New York, London
S.H. Ridgway (1986a) Physological observations on dolphin brains in: R.J. Schusterman, J.A. Thomas & F.G. Wood (eds.): Dolphin cognition and behavior: a comparative approach, pp. 31-59 Lawrence Erlbaum Associates, New Jersey
S.H. Ridgway (1986b) Dolphin brain size in: M.M. Bryden & R. Harrison (eds.): Research on Dolphins, pp. 59-70 Oxford Science Publications, Clarendon Press, London