Variation in body size growth trajectories between ecologically distinct mountain gorilla populations

Body size is a fundamental feature of an organism's biology, and studies of body size growth can yield insights into factors that shape evolutionary change within and between species. However, apart from humans, variability in body size growth within primate species across environments remains poorly known. We used non-invasive parallel-laser photogrammetry to compare body size growth of two ecologically distinct populations of mountain gorillas, from Bwindi Impenetrable National Park, Uganda and Volcanoes National Park, Rwanda, in the Virunga Massif (n = 197, aged 0.3-42.7 years). Bwindi females attained a smaller adult size than Virunga females, despite continuing their growth over a longer duration. Population differences in male adult size were minimal, although Bwindi males continued growth in back breadth over a longer duration. For both sexes, Bwindi gorillas were smaller than Virunga gorillas as young infants, and these size differences decreased over ontogeny. This study provides the first evidence of within-species variation in body size growth among living wild apes between populations that exhibit ecological and life-history differences. This variation could be due to genetic variation, phenotypic plasticity, reproductive trade-offs and/or ecological constraints and have implications for understanding body size growth variation in humans and other species.