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July 04, 2013 | By:  Bradley Voytek
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The measure of a whale

Cetaceans are the marine animals that we more commonly know and love as whales and dolphins. These animals exhibit complex social behaviors usually only seen in higher-order mammals such as primates (which include us humans). They play, care for their young, communicate with one another.

They're smart critters. And their brians, as we will see, are incredibly advanced as well. (This fact plays into the ongoing controversies surrounding whaling practicies. See this post by Scitable co-blogger Kate Whittington regarding the ongoing debates.)

One simple metric for assessing relative "intelligence" is the encephalization quotient (EQ), which is (roughly) an animals brain mass to body mass ratio. The logic behind this is simple: as an animal's overall size is larger, the amount of their brain's total area devoted to dealing with the increased body size also increases. Think about how much bigger an elephant is than us, and how much more skin they have to feel with, and how much more muscle mass their brains need to control. So given a certan body size, we know that a certain amount of brain is needed just for basic movement and sensation. If the brain is bigger than what we'd expect given their body size, that may be an indication of greater intelligence.

By way of comparison, humans have an EQ around 7.5, other primates are around 2 to 3, and dogs and cats are around 1.0.

Whales and dolphins are around 3 to 6, and their brains are highly complex (check out this great write-up on Scientific American).

But one of the most interesting things to me about the brains of cetaceans is how big some of their neurons need to be in order to move information from their bodies to their brains and back.

A while back someone asked me over on Quora what the longest axon is. Now, for those of you who don't know, axons are the "cables" that connect neurons with one another and carry information between them (technically most neurons don't actually physically connect because they're separated by small gaps called synapses, but for the purposes of this, you can think of them as connected).

After thinking about it for a bit, my first guess that the blue whale-equivalent of the motor axon that carries information along the sciatic nerve would be the longest because, in humans, the sciatic nerve is the longest. But then I remembered it doesn't have the longest axons.

Check out the figure on the left there. See that second neuron, the "unipolar neuron"? That long cable moving vertically in the image is one long axon.

Those can get really long.

Mammals have a cell called the dorsal root ganglion (DRG). The DRG carries sensory information from the body to the brain. It's unipolar, so it's got a loooong axon, where one end has receptors in the skin and the other end enters the spinal cord, ascends in the fasciculus gracilis and synapses in the nucleus gracilis all the way up in the brainstem.

This means that for the sensations in the toes, the DRG axon goes all the way from the toe to the brainstem, which is at about the same height as the mouth. This can be more than 2 meters long in tall people!

Now think about what this means for a blue whale. Blue whales are almost certainly the largest mammale to have ever existed, which means they're the largest animal that we now to have a very long axon like a DRG (we don't know a whole lot of details about dinosaur nervous systems).

Therefore the longest axon in the blue whale, which is itself the largest mammal, is probably the DRG.

When trying to confirm my guess about the longst axon, however, I learned a lot of crazy stuff.

For example, the largest blue whales are around 30 m long. This would suggest a DRG axon of at least 25 m, or 75 feet, long.

Here's where it gets really nuts and things stop making sense to me...

Axons typically conduct signals between a wide range of speeds: 0.5 to 100 m/s. This means that if I were to flick a whale's tail (as one might do), it could take anywhere from a third of a second (a long time in brain time!) to more than SIX SECONDS to reach the whales' "conscious" perception (assuming they have consciousness).

Even more wild, according to this paper by Douglas H. Smith published in 2009 in Progress in Neurobiology: whale spinal axons growing at 3 cm/day represent an increase in volume that is likely more than double the volume of the entire neuron cell body—each day. This rapid volume increase for neurons is akin to the peak cellular growth rate observed for rapidly dividing cancerous cells.

(bold emphasis mine)

Basically, these axons are growing faster than cancerous cells and the speed at which they stretch should cause them to tear or rupture.


Man, whale brains are cool.

(This post is adapted from my personal blog, Oscillatory Thoughts.)

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