Post-lactation mass recovery and metabolic hormone dynamics in adult female Weddell seals.

Weddell seal (Leptonychotes weddellii) females lose substantial body mass across an intensive, nutritionally restricted lactation period and then must rapidly recover mass during the short Antarctic summer. In this study, we examined endocrine dynamics associated with mass loss across lactation and subsequent realimentation in Weddell seals, comparing patterns between seals that recently gave birth and demographically similar non-reproductive females (skip females) in McMurdo Sound, Antarctica. Postpartum seals near weaning (∼35 days postpartum, n = 64) and skip females (n = 32) were handled during early austral summer (November/December) and rehandled in late summer (January/February). Body mass, body composition (% lipid), and a suite of metabolic hormones (growth hormone (GH), insulin-like growth factor (IGF)-I, cortisol, total thyroxine (tT), free thyroxine (fT), and total triiodothyronine (tT) and IGF binding protein (IGFBP)-2 and -3) were measured. Postpartum seals gained mass after weaning (0.98 ± 0.56 kg·day (mean ± SD)), primarily as lean tissue rather than lipid, while their serum concentrations of tT and fT, IGF-I, and cortisol increased. Their circulating GH and IGFBP-2 concentrations decreased and correlated negatively with mass. Skip females had greater body masses and lipid stores than postpartum seals at the end of the lactation period in early summer, but they lost mass (-1.03 ± 0.35 kg·day) and lipid stores over summer while their serum cortisol concentrations increased. Overall, body mass and composition of postpartum and skip females converged across summer. This convergence, likely driven in large part by contrasting endocrine profiles between the groups, may allow female Weddell seals to reach an advantageous seasonal body mass "set point" by onset of winter.