Life is full of surprises. For an animal in the wild though, surprises could cost dearly; lurking predators might attack or rough terrain could turn every step into a precarious balancing act. As such, an animal's ability to quickly sense trouble and respond accordingly is crucial. Given that neural information must travel longer distances in larger animals, how do larger animals cope with these apparently longer delays? Heather More and Max Donelan from Simon Fraser University, Canada, decided to find out by studying how the size of an animal influences the delay between sensing and generating movements.
To test the delay between sensing and movement, the team decided to study the stretch reflex – the animals’ equivalent of the human knee-jerk reflex – that maintains favored muscle lengths by sensing length changes to the muscle and correcting the length accordingly. The reflex can be broken down into a series of events: sensors in the muscle generate an electrical signal when a tap of the tendon stretches the muscle; a nerve fiber transmits the signal to the spinal cord; another nerve fiber transmits the signal back and stimulates the muscle fibers so that they produce force and generate movement. Previously, the team had studied the reflex in a range of terrestrial animals, but to test their new hypothesis they searched the scientific literature to include stretch reflex delays from more animals, spanning a 5 g shrew to a 5000 kg elephant, to bolster their analysis.
Plotting animal size against total reflex delay, More and Donelan found that the delay strongly increased with animal size: a shrew had a 10 ms delay compared with an elephant's 180 ms delay – an 18-fold increase in delay across the size range. To understand why the reflex delay got longer with size, the team took a closer look at the time course of the signal's path from the stretch sensors and back to the muscle and found that large animals’ longer nerve fibers, spanning their longer limbs, mainly explained the longer delays. In contrast, other portions of the reflex's path, such as the time it took the stretch sensors to generate the electrical signal and the time it took the electrical signal to cross between the nerve fibers and from the nerve fiber to the muscle, remained about 1 ms and, therefore, made a negligible contribution to the total delay in large animals. This showed that the total delay increases strongly with animal size, despite partial delays not scaling uniformly with size.
Next, the team calculated the reflex delay relative to the contact time of the animals’ legs with the ground to account for the larger animals’ slower movement. As larger animals move more slowly, their reflex delay expressed as a percentage of the animals’ contact time did not increase 18-fold across the entire 5 g to 5000 kg size range, but only doubled. This means that larger animals benefit from their slower movements because the long movement times accommodate their longer delays – if they moved faster they wouldn't be able to use the reflex information to correct their movement.
Together, More and Donelan have shown that delays between sensing and movement increase strongly with animal size. Even though the larger animals’ slower movements reduce the severity of the delays, delay periods likely remain challenging for all animal sizes and for large animals in particular. Therefore, large animals might especially benefit from moving slowly to prolong the time in which their delayed information can usefully be implemented. Alternatively, large animals could avoid long delays by predicting the consequences of their movements – thereby relying less on slow sensory information – so that they better handle life's inevitable unwelcome surprises.