Variation in the timing of developmental milestones in Gorilla does not support the ecological risk aversion hypothesis

Species comparisons of developmental milestones (behavioural and physical markers of neurodevelopment) allow empirical testing of hypotheses of how distinctive life-history characteristics may have evolved. Here, we analysed developmental milestone emergence in Gorilla using data from six mountain gorilla groups and one western gorilla group, collected from 1998 to 2023. The study included 36 mountain gorillas and 7 western gorillas, tracking 28 developmental milestones. Bayesian mixed-effects models (which allowed us to measure and account for the uncertainty from the small sample size in western gorillas) compared milestone timing between species. We then compared developmental milestone emergence in Gorilla to available data on western chimpanzees using visual inspection of species averages. In line with the expectations of the ‘life-history brain development’ and ‘delayed benefits’ hypotheses, we observed that developmental milestones emerged gradually across ontogeny. However, we found limited species differences among Gorilla, and the differences between chimpanzees and gorillas did not conform to the ‘ecological risk aversion’ hypothesis. Ecological risk aversion may predict growth rates, but it is not a consistent predictor of developmental milestone emergence. Instead, the interplay between ecology, social opportunity and social constraints may be more promising predictors of the development of behavioural and cognitive milestones.