Early-life environmental conditions shape demographic responses to climate change in male Antarctic fur seals

Climate change intensifies selection, driving population declines and eroding genetic diversity, yet the mechanisms linking environmental perturbations to long-term demographic and evolutionary outcomes remain elusive. In particular, deteriorating early-life environments can have lasting impacts on fitness and population dynamics, but detecting these carry-over effects requires multidecadal, individual-based datasets. Long-term monitoring of Antarctic fur seals at Bird Island, South Georgia, has revealed a decline in breeding female numbers associated with persistent positive phases of the Southern Annular Mode (SAM), when warmer sea surface temperatures reduce local krill abundance. However, male responses remain understudied as their large size and aggressiveness preclude conventional tagging. To investigate male population dynamics, we combined 27 years of field observations of nearly 2,000 individually paint-marked territorial males with genetic recapture analysis of over 11,000 tissue samples, testing for both direct effects of the SAM on male abundance and lagged effects operating through recruitment. Territorial male numbers declined by 61%, yet male abundance was decoupled from the contemporary SAM, reflecting a delayed recruitment strategy that allows resource accumulation over multiple years and buffers short-term environmental variation. By contrast, recruitment probability fell by 59% annually and was strongly linked to natal conditions. Males born during increasingly positive SAM phases were significantly less likely to recruit, whereas heavier and more heterozygous individuals showed higher recruitment success, indicating that adverse natal environments act as selective filters. These findings identify a pathway linking environmental variability to population decline through recruitment dynamics, highlighting the importance of accounting for early-life carry-over effects when predicting the resilience of long-lived species to rapid environmental change.