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October 25 , 2004
Study details dolphin brain evolution for first time

BY Beverly Clark

The intelligence and cognitive capabilities of dolphins and their aquatic cousins have long fascinated the public and the scientific community, but the questions of how and why they have such large brains have mostly gone unanswered.

In the first comprehensive analysis of its kind, a new study maps how brain size has changed in dolphins and their relatives over the past 47 million years, and helps to provide some answers to how the species evolved in relation to humans. The study, which will appear in the December issue of Anatomical Record, was done by Emory psychologist Lori Marino, senior lecturer in neuroscience and behavioral biology, and colleagues Daniel McShea from Duke University and Mark Uhen from the Cranbrook Institute of Science.

The study investigates the fossil record of the toothed whales (dolphins, porpoises, belugas and narwhals) from the order Cetacea and suborder Odontoceti. Many modern toothed whale species (odontocetes) have extremely high encephalization levels, meaning they have brains significantly larger than expected for their body size and second only to those of modern humans.

“A description of the pattern of encephalization in toothed whales has enormous potential to yield new insights into odontocete evolution, whether there are shared features with hominoid brain evolution, and more generally how large brains evolve,” Marino said.

To investigate how the large brains of odontocetes changed over time, Marino and her colleagues quantified and averaged estimates of brain and body size for fossil cetacean species using computed tomography, and analyzed these data along with those for modern odontocetes.

The only data previously available were a small handful of fossils that provided a very limited record. Marino and her colleagues spent four years tracking down fossils at the Smithsonian Institution and other museums, scanning and measuring a total of 66 fossil crania. This subset was added to brain and body weight data from 144 modern cetacean specimens for a total sample of 210 specimens representing 37 families and 62 species.

Their work produced the first description and statistical tests of changes in brain size relative to body size in cetaceans over 47 million years. They found that encephalization level increased significantly in two critical phases.

The first increase occurred with the origin of odontocetes from the ancestral group Archaeoceti nearly 39 million years ago, and was accompanied both by an increase in brain size and a decrease in body size. This occurred, Marino said, with the emergence of the first cetaceans to possess echolocation, the high-frequency, perceptual-communicative system used by modern dolphins and other odontocetes. The second major change occurred in the origin of the superfamily Delphinoidea (oceanic dolphins, porpoises, belugas and narwhals) about 15 million years ago. Both increases probably relate to changes in social ecology (the animals’ social lifestyle) as well, Marino added.

In addition to their large brains, odontocetes have demonstrated behavioral faculties previously ascribed only to humans and, to some extent, great apes. These include mirror self-recognition, comprehension of symbol-based communication systems and abstract concepts, and the learning and intergenerational transmission of cultural behaviors.

Despite cognitive commonalities, the odontocete evolutionary pathway has proceeded under a very different set of independent circumstances from that of primates, Marino explained; the highly expanded brain size and behavioral abilities of odontocetes are, in a sense, convergently shared with humans.

“Dolphin brains are four to five times larger for their body size when compared to another animal of similar size,” she said. “In humans, the measure is seven times larger—not a huge difference. Essentially, the brains of primates and cetaceans arrived at the same cognitive space while evolving along quite different paths. What the data say to me is that we, as humans, are not that special. Although we are highly encephalized, it’s not by much or for that long compared with odontocetes.”

Marino and her colleagues add that the observation that there is a single remaining human lineage “pruned down from a bushier tree” has led to a popular view that several species of highly encephalized animals cannot co-exist at the same time.

“However,” they wrote, “our results show that not only do multiple highly encephalized delphinoids coexist in similar and overlapping environments today, but this situation arose as early as 20 million years ago and has persisted for at least 15 million years.”

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