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1. Reproductive success can be influenced by maternal physiological condition at the time of embryo implantation and by foraging success during gestation. Polar marine mammals experience drastic fluctuations in body composition (lipid stores) as a result of life-history events and large-scale changes in seasonal productivity and environmental conditions. These species provide the opportunity to explore physiological parameters important to reproductive success. 2. There are conflicting physiological demands on Weddell seal (Leptonychotes weddellii) females during the moult period, when animals are at their leanest but still must generate an energetically costly new pelage and begin active gestation. 3. To investigate the impact of post-moult condition and hormonal mediators on the reproductive success of the southernmost breeding mammal, body composition was determined for post-moult (fall; 53 non-reproductive) and pre-breeding (spring; 31 non-reproductive, 17 reproductive) adult female Weddell seals. Animals were significantly larger and had greater lipid stores in spring, after the winter foraging period. There were no differences in the proportion of mass or condition gained overwinter between females that gave birth (n = 12) and those that did not (n = 8) the following year. 4. Changes in body condition were correlated with endocrine factors that influence energy allocation, such as cortisol, growth hormone (GH), insulin-like growth factor (IGF)-1 and thyroid hormones (T 3 and T 4 ). Of these, GH and T 4 were significantly higher during the post-moult period, likely to promote protein sparing and hair regeneration. In addition, females that had higher T 4 concentrations in fall were significantly more likely to have a pup the following year, possibly due to the role of thyroid hormones in embryo attachment. This suggests that hormones influencing fuel use during the moult may also impact subsequent reproductive success. 5. Unlike some other large pinnipeds, Weddell seals are not capital breeders. This work indicates that gestating Weddell seals do not gain as much mass or energy overwinter in preparation for lactation the following year as lower-latitude phocid species, which might explain why female Weddell seals rely on foraging to meet energetic demands during lactation.

Mass and body composition are indices of overall animal health and energetic balance and are often used as indicators of resource availability in the environment. This study used morphometric models and isotopic dilution techniques, two commonly used methods in the marine mammal field, to assess body composition of Weddell seals (Leptonychotes weddellii, N = 111). Findings indicated that traditional morphometric models that use a series of circular, truncated cones to calculate marine mammal blubber volume and mass overestimated the animal’s measured body mass by 26.9±1.5% SE. However, we developed a new morphometric model that uses elliptical truncated cones, and estimates mass with only −2.8±1.7% error (N = 10). Because this elliptical truncated cone model can estimate body mass without the need for additional correction factors, it has the potential to be a broadly applicable method in marine mammal species. While using elliptical truncated cones yielded significantly smaller blubber mass estimates than circular cones (10.2±0.8% difference; or 3.5±0.3% total body mass), both truncated cone models significantly underestimated total body lipid content as compared to isotopic dilution results, suggesting that animals have substantial internal lipid stores (N = 76). Multiple linear regressions were used to determine the minimum number of morphometric measurements needed to reliably estimate animal mass and body composition so that future animal handling times could be reduced. Reduced models estimated body mass and lipid mass with reasonable accuracy using fewer than five morphometric measurements (root-mean-square-error: 4.91% for body mass, 10.90% for lipid mass, and 10.43% for % lipid). This indicates that when test datasets are available to create calibration coefficients, regression models also offer a way to improve body mass and condition estimates in situations where animal handling times must be short and efficient.