Headfirst descent behaviors in a comparative sample of strepsirrhine primates

The ability to safely ascend and descend is critical to the success of arboreal animals. Nonprimate mammals typically descend supports headfirst aided by their claws, but primates must rely on grasping, and use a variety of behaviors to move down within an arboreal environment, including headfirst and tailfirst descending. This study assesses hypothesized body mass limits on vertical headfirst descent and identifies approximate support orientations and diameters at which headfirst descent is ceased in a sample of nine strepsirrhines species ranging in mass from 0.06 to 4.5 kg. Species under 1 kg, arboreal quadrupeds Cheirogaleus medius and Microcebus murinus, and slow climber Nycticebus pygmaeus, always descended supports headfirst regardless of orientation and diameter as long as a grasp could be established. Arboreal quadrupedal species above 1 kg, Daubetonia madagascariensis, Eulemur coronatus, Eulemur mongoz, Lemur catta, and Varecia variegata differed in the orientation at which they ceased using headfirst descent and the types of alternative descending behaviors they employed. Lemur catta, a highly terrestrial species, started to employ tailfirst descents at 45° and completely stopped using headfirst descent on steeper and thicker supports. Other arboreal quadrupeds, D. madagascariensis, E. coronatus, E. mongoz, and V. variegata, began using tailfirst descent at 60°. The vertical clinging and leaping species Propithecus coquereli rarely engaged in above branch quadrupedalism, and individuals were observed using tailfirst descents on supports as shallow as 15°. This study shows the ways in which mass and anatomy may constrain use of headfirst descent through arboreal environments, and the alternate strategies strepsirrhine primates employ to descend.