When it comes to brains, size isn't everything. Good wiring counts for a lot too. In humans and apes, this wiring takes the form of special neural circuits that are thought to boost cognitive powers. Now, researchers have found the same circuits in whales--and it looks like whales evolved them first.
The link between neural connectivity and cognition was tightened back in 1999, when researchers discovered that the brains of humans and great apes such as chimpanzees and gorillas contain a special kind of elongated nerve cell called a spindle neuron, which is lacking in all other primates (ScienceNOW, 28 April 1999). The cells have recently been linked to making quick, intuitive decisions during social situations. They are also implicated in higher cognitive functions such as consciousness and emotions.
The discovery that these cells are not unique to primates came about when neuroscientist Patrick Hof of Mount Sinai School of Medicine in New York City, the leader of the group that made the 1999 discovery, teamed up with Mount Sinai neuroscientist Estel Van der Gucht to study the detailed anatomy of whale brains. To their surprise, they found spindle neurons in several large-brained species, including the humpback whale and the fin whale. These cells were concentrated the same regions of the brains as in primates. Yet the spindle neurons were absent from the brains of smaller-brained whales, as well as the brains of dolphins, the team reports online this week in The Anatomical Record.
By comparing the evolutionary histories of the spindle neuron haves and have-nots, Hof and Van der Gucht estimate that whales evolved spindle neurons between 22 million and 30 million years ago--at least 7 million years before great apes evolved them. The most likely scenario is that they arose independently, a case of so-called convergent or parallel evolution. The researchers speculate that the cognitive talents of some whales, including the formation of cohesive social groups and complex communication such as singing in humpback whales, might be linked to the cells.
Todd Preuss, a neuroscientist at Emory University in Atlanta, Georgia, says that he is "really impressed" with the paper, although he cautions against concluding that spindle neurons have the same functions in both primates and whales. We may soon learn whether other large-brained, intelligent animals contain this special wiring. John Allman, a neurobiologist at the California Institute of Technology in Pasadena, who worked with Hof on the original spindle-neuron discovery, is currently looking for spindle neurons in elephant brains.