Modern primates are special. They have no difficulty bounding through trees and over uneven terrain. Manuela Schmidt from Friedrich Schiller Universität Jena, Germany, explains that modern primates' agility is due to their unusual footfall pattern, which allows them to carry their weight on their rear feet before moving the front feet forward to test a new foothold. But how did they become so agile in a niche where few other mammals have succeeded? According to Schmidt ancient primates were probably more similar to the tiniest modern primates than to baboons and chimpanzees; which made her wonder how similar the movements of the smallest modern primates are to the crouched movements of other small animals. Schmidt decided to investigate how four of the smallest primates move to understand more about their ancestors(p. 3775).
Designing a specialised rope treadmill, Schmidt used X-rays to film grey mouse lemurs, brown lemurs, cotton-top tamarins and squirrel monkeys as they walked comfortably along the moving rope. `If they are not happy, they just jump off,' explains Schmidt and adds that working with the animals was a`great adventure'. After months of patiently filming the animals, ranging in size from 100 g to 3.5 kg, Schmidt analysed 13,000 frames of film covering more than 100 steps to find out how the animal's forelimbs move.
Knowing that the primates' hindlimb movements are similar to the crouched hindlimb movements of other small mammals, Schmidt was very surprised to see that their forelimb movements were very different. Instead of placing their forefeet on the ground beneath their eyes (like tree shrews, squirrels and other small mammals), the primates were reaching far in front of their heads. But why do their forelimbs perform so differently from the forelimbs of other small tree-dwelling mammals?
Schmidt decided to take a look at the forelimb proportions of over 100 small mammals, ranging from primates and marsupials to rodents and carnivores,and found that the primates' arms were much longer than small mammals' front legs, and the relative proportions of each segment of the primates' limbs were completely different from those of the other small animals. The scapula, top of the arm and forearm of small animals tend to be the same length, but the primates' scapulas were much shorter than their upper and lower arm sections. Schmidt explains that this probably increases the arm's mobility allowing the animal to reach well beyond the level of its head while walking.
But why have small primates, and possibly their ancestors, opted for such a different walking style? Schmidt suspects that it is due to the animals' long limbs and unusual footfall pattern. She explains that most small animals start by moving their front feet first so that they are clear of their rear feet when they follow: but not primates. Schmidt explains that by moving their rear feet first (to support their weight), primates run the risk of the rear foot colliding with the inline front foot when gripping onto a narrow branch. Primates overcome this problem by overreaching during the preceding step so that the front foot does not swing as far back during the next step, avoiding the collision. Small mammals don't encounter this problem. They use a different footfall pattern and their legs are often much shorter than their body length, avoiding foot collisions regardless of the surface they scamper on.
