The influence of substrate size upon pulling and gripping forces in parrots (Psittaciformes: Agapornis roseicollis)

Quantitative assessment of grasping strength in platyrrhine monkeys

OBJECTIVES

Despite the longstanding importance of grasping adaptations in theories of primate evolution, quantitative data on primate grasping strength remain rare. We present the results of two studies testing the prediction that callitrichines-given their comparative retreat from a small-branch environment and specialization for movement and foraging on tree trunks and large boughs-should be characterized by weaker grasping forces and underdeveloped digital flexor muscles relative to other platyrrhines.

METHODS

First, we directly measured manual grasping strength in marmosets (Callithrix jacchus) and squirrel monkeys (Saimiri boliviensis), using a custom-constructed force transducer. Second, we reanalyzed existing datasets on the fiber architecture of forearm and leg muscles in 12 platyrrhine species, quantifying digital flexor muscle physiological cross-sectional area (i.e., PCSA, a morphometric proxy of muscle strength) relative to the summed PCSA across all forearm or leg muscles.

RESULTS

Callithrix was characterized by lower mean and maximum grasping forces than Saimiri, and callitrichines as a clade were found to have relatively underdeveloped manual digital flexor muscle PCSA. However, relative pedal digital flexor PCSA did not significantly differ between callitrichines and other platyrrhines.

CONCLUSIONS

We found partial support for the hypothesis that variation in predominant substrate usage explains variation in empirical measurements of and morphological correlates of grasping strength in platyrrhines. Future research should extend the work presented here by (1) collecting morphological and empirical metrics of grasping strength in additional primate taxa and (2) extending performance testing to include empirical measures of primate pedal grasping forces as well.

A functional framework for interpreting phalangeal form

Across tetrapods, the proportional lengths of the manual and pedal phalanges are highly constrained, following a generalized blueprint of shortening in a proximodistal gradient. Despite this, several lineages of both mammals (e.g. sloths, bats and colugos) and birds (e.g. raptors, parrots and woodpeckers) have broken this pattern, shortening the proximal phalanx while elongating more distal elements. As yet, no unifying explanation for this convergence has been empirically evaluated. This study combines a comparative phylogenetic assessment of phalangeal morphology across mammals and birds with a novel bioinspired robotics approach to explicitly test functional hypotheses relating to these morphotypes. We demonstrate that shortening the proximal phalanx allows taxa to maximize forces produced at the proximal interphalangeal joint, while elongation of subsequent elements maintains total ray length-ensuring arboreal species can still enclose large-diameter supports. Within suspensory and vertically clinging mammals, we additionally observe a secondary adaptation towards maximizing grip strength: namely increasing the height of the trochleae to increase the moment arm of digital flexor muscles that cross the joint. Together, our analyses highlight that numerous tetrapod lineages independently converged upon this morphotype to maximize proximal gripping strength, an adaptation to support specialized hunting and locomotor behaviours.