The correlation between the biomass of forest primates and a chemical index of the average nutritional quality of leaves in tropical forests has been repeatedly documented since 1990. We tested the role played by protein : fibre on lemur biomass in a gallery forest in southern Madagascar. Plant species abundance was determined based on transect censuses. We calculated an average ratio of protein-to-fibre in leaves and an abundance-weighted ratio, i.e. the mean weighted by the basal area of tree species, to be compared with the figures available for other forest ecosystems in Madagascar and a number of anthropoid habitats. Lemur densities were evaluated through compilation of previous studies made from prior to 1975 and up until 2011 based on strip censuses and/or identification of all groups supplemented with new censuses. A high mean ratio of protein to fibre (> 0.4) supports high folivore biomass at 390 kg km−2 (reaching 630 kg km−2 in the closed-canopy forest area) compared with primate communities in other Malagasy forests (protein : fibre: < 0.5; folivore biomass: < 440 kg km−2), as predicted. However, the data corroborate the finding that the total biomass of lemur communities as well as the biomass of folivorous lemur species are low compared with those of African and Asian primate communities for a given protein : fibre ratio. Tree diversity and leaf production do not consistently explain this pattern. In contrast, the extinction of large folivorous lemurs during the past two millennia presumably allowed too little time for smaller-sized species to evolve equally effective morphological and physiological specializations for processing a large range of fibrous foods.