The humpback whale is known as the gregarious, singing “gentle giant” of the sea. But the herring it inventively preys upon—one whale in a gang blows “air bubble nets” around a school of fish while another screams until the poor things are scared to the surface—would probably disagree with this assessment.
In any case, the auditory and communicative behaviors within groups of humpbacks reveal remarkable intelligence. However, since whale specimens are rare—either harvested from beached whales or sick aquarium residents—scientists know only the basics of their brain surface anatomy and are virtually ignorant about what goes on underneath.
But last summer, neuroscientists from Mount Sinai School of Medicine got their hands on one of these rare brain samples and studied it. Now they’ve published a thorough morphological analysis of the humpback brain, and have compared it to a host of other species. Their study, published in the Nov. 27 early online edition of the journal The Anatomical Record, reveals that the humpback brain contains many anatomical curiosities, including one type of neuron involved in high-level cognitive functions previously thought to be unique to primates.
“Methodologically, it’s a beautifully done study, and the results are really stunning,” said neuroscientist Lori Marino of Emory University, who was not involved in the research. “[The presence of these neurons] tells us that these are animals with possibly a very sophisticated sense of social cognition.”
Scientists have only just begun to study the wide range of social behaviors exhibited by cetaceans—whales, dolphins, and porpoises. “These animals can clearly recognize other individuals quite specifically,” said neuroscientist Patrick Hof, lead author of the study. “They form coalitions, and hunting parties, and they definitely play for fun, too.”
When Hof first looked at his humpback specimen this July, he spotted something unexpected.
“I was looking at the section, not even under the microscope yet,” he recounted, “and I see a very strange streak in the occipital lobe and said to myself, ‘What is that?!’ It was like little balls of yarn covering a vast expanse of cortex. I was really surprised.”
It turns out that these balls of yarn are actually clumps of spherical cells—“islands”—only found in the largest of whales and a few other mammals. No one knows the islands’ function, though Hof speculates that they may have evolved to promote fast communication between neurons.
But Hof discovered the most surprising feature of the humpback brain only after looking under the microscope: elongated neurons called spindle cells. In humans, spindle cells are thought to be involved in high-level cognitive processes, such as self-awareness, social cognition, and possibly even communication, researchers said.
Hof found spindle cells in human brains about a decade ago and spotted them in great apes a few years later. But up until now, spindle cells seemed to have been limited to these subsets of primates.
“In the last 11 years, we never found them in lesser apes, or in any other species other than great apes and humans. So we thought OK, we have a marker of the most recent primates, and that spindle cells evolved in the last 13 million years,” Hof explained. “It was making sense, a very nice story—until July, when I found them in the humpback.”
Hof then started looking for spindle cells in other kinds of cetaceans, and found them in the dolphins and toothed whale species that had the largest brains of their class. This means, he said, that the cells must have evolved from cetaceans’ ancestors at least 33 million years ago.
The appearance of spindle cells in both cetacean and primate brains, the authors speculate, may be an example of parallel evolution—the development of similar characteristics in organisms that are not closely related. In this case, cetaceans and primates may have each evolved spindle neurons independently and at different times, because the cells somehow aid in the formation of the complex social patterns and communicative abilities that are so similar between the two groups.
Still, Hof said that when studying whale brains, we shouldn’t be too anthropocentric. “How much can we learn about ourselves by studying whales? It’s probably fairly limited, compared to studying chimpanzees or macaques. What is really important here is learning about the biological diversity of mammalian species.”
Originally published January 11, 2007