Human Brain Hubris

Human brains are walnut-shaped but dolphin brains are nearly spherical -- so in terms of volume, side views (or sagittal sections) are misleading. If you've seen a dolphin brain in a museum you may have noticed that the neocortex appears to be even more convoluted than a human brain. Dolphin researchers have found that one hemisphere can sleep while the other is awake. The brain of a bottle-nosed dolphin is heavier than a human brain, but neuroscientists say that the important factor is the ratio of brain weight to body weight. Dolphins have a larger volume of body fat for insulation than humans. There are some small rodent species which have an even higher ratio than humans. The tree shrew has the largest brain to body ratio of all mammals (see: Is Bigger Always Better?).

Both whales and dolphins can communicate with each other through intricate patterns of sound. So far, the nature of their communication system is not fully understood by PhD-grade hominids.

Whale and elephant brains are considerably larger than human brains. Eric Chudler, an associate professor at the University of Washington, has compiled a page of Brain Facts and Figures, which includes a comparison of brain weights for a variety of species. Reported average brain weights differ from one academic source to another, but Eric Chudler has listed the textbooks he used to gather the figures. The following table shows a selection from the data:

Species	Average Brain Weight (grams)
Sperm whale *	7,800
Elephant	6,000
Bottle-nosed dolphin	1,500-1,600
Adult human	1,300-1,400
Walrus	1,020-1,126
Gorilla	465-540
Chimpanzee	420

* Blue whales are the largest mammal, but sperm whales have the largest brain.

Neuroscientist Lori Marino (Emory University in Atlanta, Georgia, USA) and her colleagues used computed tomography to estimate the brain and body size of 36 fossil whale species and compared them to those of modern toothed whales. Dolphins are a type of toothed whale. She reports that there have been dolphins swimming in the oceans with huge brains for more than 15 million years. And although whales haven't shared a common ancestor with primates for 94 million years, she says:

"Nevertheless, during evolution, whales have converged upon very similar capacities and behaviours to those of primates, including a highly developed social structure, which tells us that there is more than one way to evolve a complex intelligence."

References:

• (1) Lori Marino's research is reported in the Public Library of Science Biology journal: Molecular Insights into Human Brain Evolution.
• (2) The top image is based on Michigan State University's Human Brain Atlas and Dolphin Brain Atlas.

• Related blog article:
  Mass Extinctions and Super-Predators