Cetacean Explained

Variation in absolute brain size, relative brain size (as expressed by EQ = encephalization quotient), and body size of cetaceans in an evolutionary context.  Representatives of seven cetacean families are shown and are scaled to body length.  EQ and absolute brain size are indicated to the right of each species name by brain drawings of brains. The width of each brain is proportioned to the value of EQ or brain weight for each species.  Evolutionary tree and ages for divergence events are from McGowen et al. (2009). The nature of cetacean evolution is currently under debate, specifically with respect to the development of a large, relative brain size. Some experts maintain that cetaceans acquired their large brain sizes in two large and punctuated increases beginning in the Eocene Epoch. Other scientists counter that the fossil record is incomplete and that what appears as an abrupt increase was actually gradual.

To explain the observed sudden increases, South African scientist Paul Manger, has attempted to link the increases in brain size to decreases in average global water temperatures (Manger, 2006). That is, according to Manger, cetaceans evolved larger brains to resist heat loss to cooler oceanic temperatures. This is supported by data that show that cetacean brains are very efficient when it comes to heat loss, particularly neuronal support cells (glial cells). To further support his idea that cooler water selects for larger brains, estimated water temperatures were compared to the evolution of brain size in cetaceans over the last 50 million years. He found that the two greatest periods of relative brain size increase coincided with the two most dramatic drops in global oceanic water temperatures. Manger also asserts that his hypothesis challenges the hypothesis that a larger brain correlates with higher cognitive function and intelligence.

In addition, Manger investigated the unique aspects of the brain activity of cetaceans while they sleep. Earlier work had shown that dolphins are capable of entering REM sleep with only one brain hemisphere at a time. The pontomesencephalon part of the brain contains several nuclei that are responsible for different types of sleep. Further study of this part of the brain could help to reveal the origin of the dolphin's remarkable ability and how it evolved from more typical sleep behaviors of other mammals. Manger proposed that the ability to have one hemisphere sleep at a time is another way to conserve heat because brain cells that are not in REM sleep would be more metabolically active, and thus produce more heat. If true, this would be further evidence that the cetacean brain is optimized for the production and conservation of heat.

A large group of scientists including biologists, psychologists, and geologists (Marino et al., 2008) have responded to Manger’s claims. These scientists have effectively shown that the purported large and punctuated increases in relative brain size were misinterpreted by Manger. Relative brain size is scaled to body mass, and they showed that some of the increases in relative brain size were actually the result of a decrease in overall body size, not an increase in brain size. Secondly, they showed that gaps in the fossil record make some of the changes seem more drastic than they actually were. The group also provided evidence that the large size of cetacean brains can be better explained by the development of echolocation and complex social hierarchies. With respect to brain anatomy, this team of researchers were able to show that cetacean brains had a normal ratio of glial cells and neurons, and as compared to other mammals, a typical proportion of the brain was dedicated to actual mental functions, not just generation of heat.  Finally, this group also provided evidence that REM sleep actually generates more heat in some regions of the brain, on average, than when the brain is fully awake. It seems that this team of researchers have effectively countered most of Manger’s arguments, although the discovery of new fossils could necessitate another look at his provocative hypothesis References Cited:

Manger, P. R. 2006. An examination of cetacean brain structure with a novel hypothesis correlating thermogenesis to the evolution of a big brain. Biological Review 81: 293-338.

Marino, L., Butti, C., Connor, R.C., Fordyce, R.E., Herman, L.M., Hof, P.R., Lefebvre, L., Lussaeu, D., McCowan, B., Nimchinsky, E.A., Pack, A.A., Reidenberg, J.S., Reiss, D., Rendell, L., Uhen, M.D., Van der Gucht, E., Whitehead, H. 2008. A claim in search of evidence: reply to Manger's thermogenesis hypothesis of cetacean brain structure. Biological Review: 1-23.

Mike is a freshman at Georgia Southern University majoring in Biology (update: Mike has since enrolled in the University of Georgia).

He is interested in neuroscience but is excited to learn more about other aspects of biology.

In his senior year at Milton High School (Milton, Georgia), he went to the Institute on Neuroscience (Yerkes National Primate Research Center at Emory University) for a summer internship that was hosted by the Center for Behavioral Neuroscience.

There Mike studied the correlations between the dominance of male anoles (a type of small lizard) and levels of neurotrophins in certain areas of the brain.