Reproductive Strategies of Adult Female Weddell Seals (Leptonychotes weddellii) and their Implications for Pup Survival
Life history strategies reflect variation in the allocation of an individual's resources (i.e. time, effort and energy expenditure) to competing life functions such as growth, survival and reproduction. For mammals, producing milk is one of the most energetically expensive activities for females, so factors determining its delivery to offspring essentially define the reproductive strategy a species evolves. The efficiency with which energy is transferred via milk also determines the reproductive investment trade off that exists between survival and future reproduction. The key objective of this study was to examine physiological aspects of lactation in Weddell seals (Leptonychotes weddellii) to gain a better understanding of reproductive strategies of an upper trophic level predator that must cope with unpredictable food availability in an extreme and highly variable environment. Female body mass (and absolute body fat) at parturition differed between the two years of study and this difference appeared to drive the length of the lactation period, maternal energy expenditure, pup mass gain and weaning mass. Effects were more marked in smaller individuals that did not increase energy expenditure to reconcile this disparity. Milk composition was independent of maternal post partum mass (MPPM) and condition, but did change over lactation. Protein tripled from post partum (PP) to end lactation (EL) while lipid and energy increased to mid lactation (ML) then slightly decreased. This pattern of changes may be related to the relatively long lactation period demonstrated by this species and the energetic and physiological needs of the mother and pup. There was evidence through both milk energy output and fatty acid transfer that feeding occurred in some individuals later in lactation. A major source of energy during lactation is provided through the mobilisation of blubber fatty acids (FA) I investigated the extent to which FA were mobilised to support both maternal metabolic requirements and milk production, and how this was reflected in the FA composition of the pups at EL. Fatty acid composition at PP was similar in females from both years indicating similar diets. However, selective mobilisation and transfer did occur during lactation which not only affected the vertical stratification of FA within the blubber layer but also the composition of the pups at EL. This was related to total body lipid stores of females at PP. It appears that selective mobilisation was most likely related to the physiological requirements of the developing pup. Highly mobilised fatty acids are underestimated in the blubber and affect diet predictions. Failing to account for mobilisation during periods of high turn over may seriously bias FASA diet estimates. Results suggest that dietary predictions will be improved when samples are taken at parturition. Differences in MPPM between years reflects environmental variability during the period of prey acquisition, and this manifests as differences in expenditure during lactation. These differences translate to changes in pup mass and condition at weaning with consequences for future survival and recruitment. My results confirm that differences in life history strategies exist within lactating Weddell seals and the trade off between long term survival in breeding females and the success of their offspring is contingent on individual size, which is further complicated by feeding to offset nutritional constraints imposed during poor resource years.