Ecometric modelling assesses how the functional morphology of ecogeographic communities relates to environmental variables. This improves understanding of how the interaction between organism and environment can result in morphological adaptation. This technique has mainly been used to model paleoenvironments, but has the capacity to aid conservation by quantifying how communities are structured through space and time. Here we test the relationship between limb proportions and the habitat ecology of South American non-human primates. There is a significant but weak fit between limb proportions and habitat, consistent with the environment exerting weak selective pressure on limb proportions. In contrast, body size and phylogeny are strongly correlated with IMI. Together, these findings suggest that habitat was a selection pressure that shaped how New World monkeys' limb proportions evolved but this selection pressure was secondary to that of body size. Research into these functional relationships is important not only to improve scientific understanding of their evolutionary pathways but also in order to aid their protection by informing conservation practices. Ensuring these species have the capacity to move with their niche is an immediate concern, as they face mounting pressure due to deforestation of the Amazon basin.