Lieu: Brunoy

Abstract: The environment of juvenile primates is very challenging, especially when the parents do not carry their babies. Juveniles have to forage and move on the same substrates as adults do and escape the same predators, despite their immature state. In this study, we explore the developmental strategies that may provide effective locomotor abilities early in life. This could provide new insights into the selective pressures acting on juvenile primates and into evolution of primate locomotion. We conducted an ontogenetic study of 36 arboreal grey mouse lemurs from birth to adulthood (6 months). The investigated parameters were for both limbs, 1) grasping behaviour during locomotion (i.e., grip postures), 2) grasping performance (i.e., pull strength) and 3) motor coordination (i.e., RotaRod® test). Our results show that as early as 8 days of age babies are able to climb substrates of various slopes and diameters outside of their nest. Although juveniles cannot successfully complete a motor coordination test before 30 days of age, young individuals display relative pull strengths that are very high or even on par with adults, guaranteeing stability on narrow substrates. Moreover, individuals of all ages use pedal secure grasps on all substrates; however, the use of manual secure grasps decreases during development. These behavioral transitions are particularly marked on vertical and narrow substrates. From an evolutionary point of view our results suggest that vertical and narrow branches may drive the evolution of strong grasping abilities in primates. Our results further suggest different functional roles of the hands and feet, with the hind limbs ensuring body balance on the substrates, freeing the upper limbs for manipulation.