Scaling of sensorimotor control in terrestrial mammals

Terrestrial mammals span a wide range of sizes, with the largest elephant being several million times more massive than the smallest shrew. This huge size range results in small and large animals experiencing very different physical challenges, yet all animals must effectively interact with their environment to survive. In order to sense and respond to stimuli with similar speed and precision, small and large animals may need to control their movement in different ways. To begin to understand whether and how small and large animals coordinate their movement with similar effectiveness despite their different physical challenges, I investigated how size influences the physiological mechanisms underlying sensorimotor control. My general hypothesis was that the sensorimotor systems of larger animals have longer delays and lower precision than those of smaller animals. To investigate the scaling of delays, I combined my own electrophysiology measurements with data from the literature to determine how total response time and its component delays changed with animal mass. To investigate the scaling of precision, I combined my own histology measurements with data from the literature to determine how nerve fiber number and size distribution changed with animal mass. As part of this, I developed a supervised image analysis method to measure nerve fiber characteristics in scanning electron microscope images. I found that larger animals have longer absolute delays and more nerve fibers than smaller animals. However, changes to movement times with animal size almost entirely compensate for increases in absolute delays, resulting in similar relative delays for all sizes of animals. Nerve fiber number increases more slowly than animal mass, area, and muscle force, suggesting that larger animals have relatively fewer sensors and motor units than smaller animals. Nerve fiber size distribution becomes more bimodal as animal size increases, ameliorating the potential tradeoff between speed and precision in peripheral nerves. While small and large animals seem to be able to sense and respond to stimuli within the same relative time, large animals may face challenges in situations requiring short absolute delays and high precision, and may need to rely more heavily on predictive methods of control.