A mechanistic approach to understanding birth timing, calf growth, and residency of southern right whale mother–calf pairs on their calving ground

Understanding residency patterns provides valuable insights into animal life history and is fundamental for establishing protected areas. Southern right whales (Eubalaena australis) are migratory capital breeders, with females accumulating energy reserves on summer feeding grounds to sustain late gestation and early lactation while fasting on winter calving grounds. On these oligotrophic calving grounds, mothers must grow their calves to a size suitable for migration without depleting their finite energy stores. To better understand this trade-off, we developed a mechanistic model to estimate the residency duration of mother–calf pairs at Australia's largest calving aggregation. By linking maternal body size and condition to key reproductive traits, including birth timing, size at birth, calf growth rate, and departure timing, we quantified the effect of maternal phenotype on residency duration. Maternal size and condition positively influenced both calf birth size and growth rate. Absolute calf size was the primary driver of departure, with pairs leaving once the calf reached approximately 8 m in length, which likely improves calf locomotor performance and reduces predation risk. Larger, better-conditioned mothers could grow their calves more rapidly, enabling earlier departure and a shorter residency period. This, in turn, allowed mothers to return to feeding grounds sooner and replenish depleted energy reserves. Our findings highlight how the early lactation phase in SRWs is shaped by the energetic constraints of capital breeding and the demands of migration and underscore the importance of maternal traits in determining residency duration on calving grounds.