Size dependence in prosimian locomotion and its implications for the distribution of body mass.

The mechanical requirements for arboreal life are reviewed and the constraints which these requirements impose on the body of a prosimian are defined. The mechanical necessities can be fulfilled only by animals which possess the appropriate morphological characters. It is incorrect to refer to these morphological traits directly as 'adaptations'. Instead their a priori existence must be considered as the precondition for the acquisition of a certain life-style. Once such a life-style has been acquired, a strong selective pressure acts towards a further refinement of such 'adaptations' or 'pre-adaptations'. Postcranial morphology must be seen in a context of following natural laws and is strictly related to the mechanics of posture and locomotion. The traits emphasised and explained here are body proportions--specifically the relative lengths of body segments and the distribution of (muscle) mass on these segments.