Population-level shifts in reproductive phenology in response to environmental change are common, but whether individual-level responses are modified by demographic and genetic factors remains less well understood. We used mixed models to quantify how reproductive timing varied across 1,755 female southern elephant seals (Mirounga leonina) breeding at Marion Island in the Southern Ocean (1989–2019) and to identify the factors that correlate with phenological shifts within- and between individuals. We found strong support for covariation in the timing of breeding arrival dates and the timing of the preceding moult. Breeding arrival dates were more repeatable at the individual-level, as compared to the population-level, even after accounting for individual traits (wean date as a pup, age and breeding experience) associated with phenological variability. Mother-daughter similarities in breeding phenology were also evident, indicating that additive genetic effects may contribute to between-individual variation in breeding phenology. Over 30 years, elephant seal phenology did not change towards earlier or later dates, and we found no correlation between annual fluctuations in phenology and indices of environmental variation. Our results show how maternal genetic (or non-genetic) effects, individual traits and linkages between cyclical life-history events can drive within- and between-individual variation in reproductive phenology.

From 1983 to 2019, nearly all elephant seal pups born at Marion Island were uniquely marked with two hind-flipper tags (n = 8,985 female pups) (Bester et al. 2011, Pistorius et al. 2011). The pups were marked at the first encounter after weaning, after their mothers had abandoned them. However, since 2006, we also marked some pre-weaned pups with ancillary tags (de Bruyn et al. 2008), thereby collecting pedigree information on 2,115 mother-daughter pairs. Data were processed in R 4.1.3 and later versions (R Core Team 2022) (see R script).

• Bester MN, de Bruyn PJN, Oosthuizen WC, Tosh CA, McIntyre T, Reisinger, RR et al. 2011 The Marine Mammal Programme at the Prince Edward Islands: 38 years of research. Afr. J. Mar. Sci. 33, 511-521.
• De Bruyn PJN, Tosh CA, Oosthuizen WC, Phalanndwa MV, Bester MN. 2008 Temporary marking of unweaned southern elephant seal (Mirounga leonina L.) pups. S. Afr. J. Wildl. Res. 38, 133-137.
• Pistorius PA, de Bruyn PJN, Bester MN. 2011 Population dynamics of southern elephant seals: a synthesis of three decades of demographic research at Marion Island. Afr. J. Mar. Sci. 33, 523-534.
• R Core Team. 2022 R: A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna, Austria. URL https://www.R-project.org/.

Data were processed in R 4.1.3 and later versions (R Core Team 2022). Anyone interested in using this long-term data is encouraged to collaborate with the authors, as their understanding of the data collection methods and availability of other meta-data is likely to enhance future analysis and interpretation.