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Evolutionary Trends And GRasping form and function In Primates and other tetrapod lineages

Periodic Reporting for period 1 - GETAGRIP (Evolutionary Trends And GRasping form and function In Primates and other tetrapod lineages)

Reporting period: 2016-01-18 to 2018-01-17

Hand grasping is a behaviour that has been suggested to play a key role in the evolutionary success of tetrapods (frogs, lizards and mammals) because of its implication in vital behaviours such as feeding, mating, moving or social exchanges. However, even if the ability to grab food, to hold it with hand, and to manipulate it with fingers has been observed in a variety of species including frogs and lizards, it is thought to be essentially a primate trait. Moreover, fine prehensile abilities are thought to be uniquely associated with the evolution of human hands and with tool-making and tool-use in the earliest humans. However, given the single evolutionary origin of primates and the unique anatomical features they possess, quantitative tests of evolutionary scenarios pertaining to the origin of grasping behavior remain difficult. This is why this project aimed to explore the origin of this complex behavior among tetrapods more generally.
Two main hypotheses have been suggested to explain the origin of manual grasping: 1) grasping may be derived from arboreal locomotion on thin branches. As the importance of the forelimb in body weight support is largely diminished in arboreal animals, the forelimbs are functionally decoupled and can become specialized for novel behaviours. 2) grasping may have originated in the context of the capture of mobile prey which initiates forelimb grasping movements. Both hypotheses are not mutually exclusive and could confer the selective environment driving the evolution of grasping ability and fine manipulation skills.

This project aimed to understand the origin and evolution of manual grasping in tetrapods by studying the morphology of the forelimb in relation to its function and manual grasping behaviour. We tested these two-main hypotheses in three groups of tetrapods showing grasping abilities by studying the morphology, behavior and locomotion in species that are either arboreal or terrestrial and species with different manual grasping abilities. Our results show significant diversity in forelimb morphology related to and arboreal lifestyle in all clades examined. Moreover, behavioral trial suggests that moving objects elicited more manual grasping compared to stationary objects. Overall our results support both the 'arboreal origins' and 'predation' hypotheses and suggest that manual grasping may have originated in different ecological and functional contexts.
An integrative approach was taken examining the occurrence of grasping behavior, the anatomy and function of the forelimb, and the evolution of grasping across three major tetrapod clades that show manual grasping abilities: mammals, reptiles, and amphibians. To do so, arboreal and terrestrial species with different grasping ability were compared in each group as well as the use of the hand during manual grasping. Our project showed that arboreal species used much more their hand during manipulation of food items than terrestrial ones. They also display morphological adaptations that are different, with muscles and bones allowing greater mobility of the forelimb, greater flexion of the wrist and fingers and rotation of the hand. These morphological adaptations can help to better position the hand on substrates of different size, orientation and compliance during locomotion in arboreal milieu. Furthermore, the impact of mobile food items on grasping behaviour was tested and our results showed that species used their hands more when grasping mobile prey. To conclude, a common trend on morphological and behavioural adaptations emerged in species showing manual grasping behaviour irrespective of their phyogenetic affinity. This suggests that the constraints associated with manual grasping induce convergence in clades as disparate as mammal, frogs and lizards.These results have been presented in 5 papers and 4 congresses at international meetings, 2 videos of myself presenting my works on this project are also available on you tube.
This project quantitively tested the origin of grasping in tetrapods. By means of an integrative approach combining morphological, behavioral and locomotor analysis I was able to demonstrate that grasping is likely derived from both arboreal locomotion, the pressures imposed by the need to capture mobile food, and possibly other behaviours such as infant carrying. I here show for the first time that neither of the previously proposed hypotheses is mutually exclusive. This project highlighted trends in morphology, behaviour and locomotion of species with strongly developed manual grasping abilities. This result is novel and opens up new avenues of research focusing on the evolution of this key-behaviour. Specifically, analyses of diversification in clades showing developed manual grasping versus those that do not are needed to evaluate whether this behavior can indeed be considered as a key innovation.