Boulinguez‐Ambroise, G. , Zablocki‐Thomas, P. , Aujard, F. , Herrel, A. and Pouydebat, E. (2019), Ontogeny of food grasping in mouse lemurs: behavior, morphology and performance. Journal of Zoology, vol. 308, pp. 1-8. https://doi.org/10.1111/jzo.12652
The stars of the study by Boulinguez-Ambroise and colleagues are tiny, furry and large-eyed critters: the grey mouse lemurs (Microcebus murinus). These strictly arboreal primates are excellent climbers and use both their hands (bimanual) in food grasping. Furthermore, they grow super fast which makes them an ideal model organism to study ontogenetic changes of grasping performance. In contrast to their adult counterparts, the juvenile mouse lemurs are physiologically immature which places them at greater risk of injury and mortality. In particular, it is assumed that the hindlimb musculature is not yet sufficiently developed to ensure secure contact with the arboreal substrate. This lead to the hypothesis that the young animals may additionally use one hand to support body balance when the other hand grasps the mobile food item.
To tackle this question, the authors of the present work first analysed the external morphology of the fore- and hindlimbs. “We measured the length of the segments to compare the limb proportions throughout development.”, explains G. Boulinguez-Ambroise, the first author of the study. “Next, we investigated possible differences in grasping performance across ontogeny using a force platform to quantify the maximal pull strength of the hands and feet.” In a third step, video analyses were performed to record the prehension strategy displayed to grasp the moving food item. Surprisingly, grasping behavior did not vary during development. “Juvenile mouse lemurs used bimanual grasps in the large majority of the sessions, the mouth alone being never used”, says Dr. E. Pouydebat, the leader of the project. In accordance with this, young animals showed relatively high grasping performance. Relative to their body mass, 1-month-old individuals displayed a grasping performance that was equivalent to that of 6-month-old individuals which corresponds to the adult state. Interestingly, the results indicated that juvenile mouse lemurs have disproportionally longer hindlimb segments when compared to older individuals. Furthermore, the authors found a positive relationship between size of the hindlimbs and feet performance (i.e., grasping strength). This suggests that the long legs may permit young primates to compensate for their immature musculature.
The study recently published in the Journal of Zoology shows that grey mouse lemurs are under selection for increased grasping ability early in life. The ability to grasp plays a vital role in many behaviors, most notably in arboreal locomotion and foraging and is closely linked to the origin of primates because it facilitated the exploitation of the terminal branch milieu.