Sumatran orangutan mothers differ in the extent and trajectory of their expression of maternal behaviour.

Mothers play a crucial role in the early development and survival of mammalian offspring, and differences in maternal care may affect offspring's development. Whereas previous research has primarily focused on biological and socioecological factors to understand population-level variation in maternal behaviour, the individual as a source of variation remains understudied. We investigated between-individual variation in the average expression of, and plasticity in, six maternal behaviours in Sumatran orangutans (Pongo abelii), using 15 years of behavioural data. We found that mothers differed substantially in the average expression of four maternal behaviours, even after controlling for socioecological conditions, biological state characteristics and the offspring's influence on these behaviours. Furthermore, not controlling for these confounding effects exaggerated or masked between-individual variation. Mothers also substantially differed in how they adjusted three of the maternal behaviours during offspring development, meaning that mothers differed in behavioural plasticity. Our results suggest that Sumatran orangutan mothers are constrained in the average expression of maternal behaviours and their plastic responses, potentially resulting in consistent differences among mothers, otherwise called maternal personality. Our findings highlight that there is biologically meaningful variation around the population mean in maternal behaviour and present novel opportunities to study evolutionary processes that shape maternal behaviour.