When you're stretching for something that is just out of reach do you ever envy the world's tallest land mammal – the giraffe? Although its size might seem like the perfect solution for reaching the highest leaves, being tall does have drawbacks. As animals grow bigger, electric impulses have to travel farther along nerve fibres to deliver their commands. This could have important consequences for how quickly – and appropriately – these towering animals sense and respond to the world around them. Heather More, Shawn O'Connor (both from Simon Fraser University, Canada) and their colleagues travelled to South Africa to investigate just how much of a sensing and responding delay the giraffes' colossal dimensions caused (p. 1003).
The team captured eight giraffes and administered gentle electric jolts to the sciatic nerve and part of the muscle in the hind leg. Carefully placed electrodes showed that the impulses travelled at a nifty velocity of 50.4ms−1 and that it took 13.4ms for muscle contraction to begin and 45.9ms to reach full force. Although this may seem speedy, the team points out that in rats, muscle contraction begins after 4.5ms and nerve impulses travel at 59.4m s−1. These delays mean that responses to stimuli can take at least 100ms, and as this is about half the time that a galloping giraffe's foot is on the ground, it doesn't leave much room for error.
Although giraffes are not as quick at sensing and responding to stimuli as their rodent compatriots, perhaps they sense and respond more precisely? To investigate, the team measured how many nerve fibres made up the giraffe's sciatic nerve. As the number of fibres increases, so does the ability to fine-tune muscle movements and precisely detect stimuli. They found that giraffes have, on average, an impressive 106,697 fibres per sciatic nerve. However, to match the precision of the nimble rat they would need 5.6 million fibres per nerve. So, again it seems that rats may be more accurate at sensing and responding than their tall friends. However, as giraffes don't often stumble, they must have developed ways to overcome these disadvantages.