| Abstract: | How does body size determine the locomotor performance and proportions of leapers? In an analysis of the mechanics of leaping we derived two principles that explain the kinematic and morphological differences between leaping prosimian primates of different body size. 1. In small animals, the distance through which the body can be accelerated during take-off, and the time available for acceleration, are short. In small-bodied leapers we therefore find adaptations that increase the distance or length of time for propulsion and maximize speed. These are: great angular excursions at the joints of the hindlimb, long load arms of body weight and short power arms for the muscles, elongated hindlimbs with a disproportionate lengthening of the distal segments, and additional joints in the tarsus. 2. With increasing body size, the time for accelerating the body is no longer a problem. Instead, the ratio of muscle force available for acceleration to mass to be accelerated is unfavorable. Accordingly, large-bodied leapers have adaptations that allow optimal use of available muscle force. These include: acceleration in energetically profitable joint positions, avoidance of acute joint angles especially at the distal joints (where the muscles work against the highest percentage of body mass), only moderate elongation of the hindlimbs with rather short distal segments, and long lever arms of those muscles that extend the hindlimb joints. In addition, take-offs of the larger-bodied leapers are characterized by a regularly occurring arm swing movement, thus making additional use of nonhindlimb muscles for acceleration. The mass-dependent differences in forces and velocities have consequences for the energy budget. As the muscles of the small species must contract very rapidly against high loads, they consume more energy per unit of mechanical work. It is not possible to optimize speed and force in the same animal. Body size in conjunction with the laws of mechanics determines how maximum leaping potential will be realized. |
