Effects of Increased Distal Limb Mass on Stability During Arboreal Locomotion in the Laboratory Rat (Rattus norvegicus).

Traveling on arboreal substrates requires behavioral and morphological adaptations to reduce the likelihood of falls. Many arboreal specialists have greater distal limb mass compared with closely related terrestrial taxa. In this study, we test the hypothesis that augmenting the distal limb mass will increase stability during locomotion, allowing a quadrupedal mammal to rely less on static methods of stability when moving on a cylindrical support. Thus, we predict that weighted bracelets will result in less crouching, greater vertical oscillation, lower duty factor, and higher limb phase. We trained four laboratory rats to walk on a "rope-mill," the arboreal equivalent of a treadmill. We marked the fur over the glenohumeral joint and the greater trochanter, and then encouraged the animals to run either unmodified, wearing sham bracelets, or weighted bracelets (1.8 g each) on the wrists and ankles. From two video cameras, we extracted sequences of 15 strides for each experimental setup and digitized the right shoulder, hand, hip, and foot. When the rats walked with weighted distal limbs, the limb phase was lower, but duty factor was unchanged. The shoulder height was somewhat greater, whereas vertical accelerations were lower. The hip height and vertical accelerations were unchanged. Each individual adjusted to increased distal limb mass in a variety of ways. We conclude that adding mass to the distal parts of limbs did not enhance stability. We suggest that neuromuscular and developmental differences among individuals contribute to the variability among animals in responding to wearing weighted bracelets.