Hand and body position during locomotor behavior in the aye‐aye (Daubentonia madagascariensis)

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Abstract
Aye‐ayes (Daubentonia madagascariensis) have unique hands among primates, with extraordinarily long fingers in relation to body size. These long digits may be vulnerable to damage from forces during locomotion, particularly during head‐first descent—a locomotor mode that the aye‐aye utilizes frequently. Previous behavioral studies of aye‐aye locomotion reported that Daubentonia must curl its fingers during horizontal quadrupedalism and/or descent to reduce potential stresses on its long fingers. To test this hypothesis, we examined hand and body position in three captive adult aye‐ayes while they walked quadrupedally on horizontal and oblique branches. Substantial variation in hand position was observed among individuals for each substrate orientation. While hand postures with curled fingers were preferred by one individual during descent, they were not preferred by the other two individuals, contrary to our expectations. Differences in body position were more consistent among all three individuals. The angle of the body relative to the substrate was significantly reduced during descent (8.4°) compared to horizontal locomotion (16.9°). These results suggest that changes in body position, rather than hand position, may help reduce stresses on the digits. A biomechanical model is proposed that demonstrates how a reduction in the body angle in relation to substrate may act to move the center of mass more caudally. This mechanism of moderating loads by altering body position, rather than hand position, may represent an important functional aspect of arboreal locomotion in aye‐ayes and other primates.