Orangutans employ unique strategies to control branch flexibility

Abstract

Orangutans are the largest habitually arboreal mammal. For them, as for all arboreal mammals, access to the abundant fruits and narrowest gaps found among the thin peripheral branches of tree crowns poses considerable safety risks and energetic demands. Most arboreal primates use flexed-limb postures to minimize problems caused by branch compliance and instability. Here, we show that Sumatran orangutans employ unique locomotor strategies to control compliance and allow access to the terminal branch niche for feeding and gap crossing. We calculated a ''stiffness score,'' which is a measure of the flexibility of the supports on which orangutans moved. We found that certain locomotor behaviors clearly are associated with the most compliant supports; these behaviors appear to lack regular limb sequences, which serves to avoid the risk of resonance in branch sway caused by high-frequency, patterned gait. Balance and increased stability are achieved through long contact times between multiple limbs and supports and a combination of pronograde (horizontal) and orthograde (vertical) body postures, used both above branches and in suspension underneath them. Overall, adult females seem to be the most conservative in their travel, selecting more solid and secure supports than males and adolescents. These results have implications for understanding locomotor diversity in fossil and extant apes and for orangutan conservation and reintroduction programs.biomechanics ͉ hominoid evolution ͉ locomotor ecology ͉ positional behavior ͉ compliance

Functional Anatomy and Adaptation of Male Gorillas (Gorilla gorilla gorilla) With Comparison to Male Orangutans (Pongo pygmaeus)

“…As they move across flexible, unstable supports, their considerable mass deforms branches downward and opens gaps between adjacent trees (Grand, 1984). Orangutans solve this problem by distributing their weight over multiple supports, preferably lianas less than 10 cm in diameter (Povinelli and Cant, 1995;van Schaik, 2004;Thorpe et al, 2009). Using their bulk, they sway branches and form bridges to adjacent trees, and use their long and powerful arms and hands to pull in flexible branches to cross gaps.…”

Perch size and structure have species-dependent effects on the arboreal locomotion of rat snakes and boa constrictors

“…Such structural variation in the environment often requires animals to balance or use some sort of adhering or gripping mechanism (Cartmill, 1974). Arboreal animals also must cope with secondary branches that emerge from the primary surfaces on which they move, and depending on size and body plan of an animal and the mode of locomotion, such structures may either impede (Jones and Jayne, 2010) or enhance the ability to move (Astley and Jayne, 2007;Thorpe et al, 2009).…”

Canopy structure drives orangutan habitat selection in disturbed Bornean forests

“…There were no significant differences in the way age and sex classes used canopies (Kruskal-Wallis test, P > 0.05 for all structural metrics) (Fig. 2), suggesting that structural drivers influence orangutans in similar ways despite pronounced sexual dimorphism and adult male bimaturism, as well as observed age and sex differences in terrestrial activity and branch use (14,24; but see refs. 22, 23, where no differences in locomotion between age and sex classes were detected).…”

“…Orangutans are the world's heaviest arboreal mammal and need sufficiently strong branches to cross gaps, with jumping being rare, biomechanically difficult, and energetically expensive (25). They must either descend to the ground (or lower levels of the canopy) or select gapfree pathways (i.e., continuous canopy cover) and tall trees that contain sufficiently strong branches and/or woody lianas to support their weight (24). Tall trees (including emergent crowns) are also important nesting locations (26, 27) and concentrated fruiting sites (11,28), especially in disturbed forests (29), likely contributing to their selection.…”

“…We further expected selection to differ among orangutan age and sex classes, with females being more conservative in their selection of movement pathways, opting to travel in directions of increased canopy height and cover that would provide stronger supports relative to males, which would take more risks (following refs. 23,24). Alternatively, heavier males could be expected to select closed canopy that would aid lateral movement across the forest because they would require stronger supports and be less able to cross areas of sparse canopy, whereas females could be less selective of closed canopy and focus selection instead on structurally complex, tall canopies because their smaller, lighter bodies would enable them to cross sparser canopy.…”