18 March 2013

The costs of being tall: lessons from giraffes

True facts about giraffes!

They’re tall. And I use the word precisely. They’re not just big; their legs are about half again as long as you’d predict based on their mass and bodies of other mammals.

Being tall has distinct consequences for the nervous system. The distances that signals have to travel might mean there is lots of lag between something happening out in the world, the signal getting to the brain, and the appropriate response going all the way back down to the muscles the animals use to move about.

There are ways around this distance problem. You can make axons bigger, which speeds up how fast they can send a signal, but that means you probably have fewer axons, which could mean lower sensitivity on the sensory side, or precision on the motor side.

A new paper by Heather More and colleagues try to figure out how the giraffe’s nervous system deals with all these great distances. They recorded the speed of reactions and the size of neurons in the sciatic nerve of giraffes. The average speed of signals in this giraffe nerve was about 50 meters per second, which is about the same as rats. Rats, it should be noted, are not tall. They’re not even big.

More and colleagues calculated that for a giraffe to be as quick and as responsive as a rat, the speed of signals would have to be around 200 m/s, which is the top speed in the entire animal kingdom (a record held by some shrimps). And to get to that point, their neurons would have to be two and a half times the diameter they actually found in the giraffes.

Looking at the sheer number of axons, More and colleagues also suggest that the giraffe is comparatively at a disadvantage compared to smaller mammals. If the giraffe’s sciatic nerve had the same number of axons for its size as the rat does, it would have about 50 times more axons than what giraffes actually have.

From this, the authors predict that giraffes are working with a bit of a neural and behavioural handicap. They should be less sensitive to the world around them, and slower to respond to it, than smaller animals. But this is still a prediction that needs testing. Getting some giraffes in the lab for the experiments might be a bit tricky.

P.S.—When I blogged about a previous paper with some of the same authors, one criticism in the comments was that the team used conduction velocity of action potentials to measure “responsiveness.” This paper does a much better job of laying out all the different elements that go into determining “responsiveness” in general. That said, they don’t make much progress on measuring all those other elements in this paper, but at least they recognize they exist.

Related posts

The elephant and the shrew, an axonal story


More HL, O'Connor SM, Brondum E, Wang T, Bertelsen MF, Grondahl C, Kastberg K, Horlyck A, Funder J,  Donelan JM. 2013. Sensorimotor responsiveness and resolution in the giraffe. The Journal of Experimental Biology 216 (6): 1003-1011. DOI:

Top photo by ucumari on Flickr; used under a Creative Commons license.

1 comment:

Neuroskeptic said...

Interesting. But giraffes are not too badly off, because most of their sense-organs are close to their brain. It's only somatosensation that's delayed... so they might face a somewhat harder job of coordinating their movements based on feedback than other animals.

Hmmm. I wonder, do they "know" that their touch sensations are delayed?

If you trained a giraffe to discriminate, say, two simultaneous touches from two touches one after the other, and then you touched it simultaneously on the nose and on the tail, what would it think?