The intelligence and cognitive capabilities of dolphins and their aquatic cousins have long fascinated the public and the scientific community, but the question of how and why they have such large brains has mostly gone unanswered.
In the first-ever comprehensive analysis of its kind, a new study maps how brain size changed in dolphins and their relatives the past 47 million years, and helps to provide some answers to how the species evolved in relation to humans.
The study, published October 20, 2004 in online edition of The Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology, was conducted by psychologist Lori Marino, a faculty member from Emory University's Neuroscience and Behavioral Biology Program, and her colleagues Daniel McShea from Duke University and Mark D.Uhen from the Cranbrook Institute of Science. This study will also appear in the December 2004 print issue of the journal.
The study, which was funded by the National Science Foundation and the SETI Institute, investigates the fossil record of the toothed whales (which includes dolphins, porpoises, belugas and narwhals) from the order Cetacea and suborder Odontoceti. Many modern toothed whale species (odontocetes) have extremely high encephalization levels – possessing brains that are significantly larger than expected for their body sizes 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 says.
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 gathering the data and tracking down fossils at The Smithsonian Institution and other museums. A total of 66 fossil crania were scanned and measured. 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 the pattern of change in brain size relative to body size in cetaceans over 47 million years. They found that encephalization level increased significantly in two critical phases in the evolution of odontocetes.
The first increase occurred with the origin of odontocetes from the ancestral group Archaeoceti nearly 39 million years ago, and was accompanied by both an increase in brain size and a decrease in body size. This change in encephalization occurred with the emergence of the first cetaceans to possess echolocation - the processing of high frequency acoustic information within a perceptual-communicative system used by modern dolphins and other odontocetes, Marino says. The second major change occurred in the origin of the superfamily Delphinoidea (oceanic dolphins, porpoises, belugas and narwhals) by about 15 million years ago. Both increases probably relate to changes in social ecology (the animals' social lifestyle) as well, Marino says.
In addition to their large brains, odontocetes have demonstrated behavioral faculties previously only ascribed to humans and, to some extent, other great apes. These abilities include mirror self-recognition, the comprehension of artificial, symbol-based communication systems and abstract concepts, and the learning and intergenerational transmission of behaviors that have been described as cultural.
Despite cognitive commonalities, the odontocete evolutionary pathway has proceeded under a very different set of independent circumstances from that of primates, Marino explains. The highly expanded brain size and behavioral abilities of odontocetes are, in a sense, convergently shared with humans, she says.
"Dolphin brains are four to five times larger for their body size when compared to another animal of similar size. 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" Marino says. "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, 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."
Marino's previous research has showed how dolphins have the capacity for mirror self-recognition, a feat of intelligence previously thought to be reserved only for Homo sapiens and their closest primate cousins. Marino also holds adjunct appointments in Emory's psychology department and Center for Behavioral Neuroscience, and is a research associate at the Smithsonian's National Museum of Natural History, and in the Living Links Center for the Advanced Study of Ape and Human Evolution at Yerkes National Primate Research Center.
Article: "Origin and Evolution of Large Brains in Toothed Whales," by Lori Marino, Daniel W. McShea, and Mark D. Uhen, Anatomical Record Part A: Discoveries in Molecular, Cellular, and Evolutionary Biology, Published Online: October 20, 2004 (DOI: 10.1002/ar.a.20128); Print Issue Date: December 2004.
This study is available online via Wiley InterScience at http://www.interscience.wiley.com/ar.