| Summary: | Reproduction is costly and involves a number of sequential physiological processes that require different levels of energetic investment. In mammalian species gestation and lactation require the most energy and the amount of energy invested in reproduction is reflected in litter size at birth and by offspring growth through weaning. The object of this study was to describe variation in Weddell seal (Leptonychotes weddellii) pup mass at birth and during several ages of the lactation/nursing period, and to evaluate the ability of several maternal traits to explain this variation. Mass measurements were collected from 887 pups at parturition and throughout lactation in Erebus Bay, Antarctica during the 2004 through 2010 field seasons and maternal traits were taken from a long term database. Analysis demonstrated high individual variation in pup mass within a season and modest variation among seasons which suggests that pup mass may be correlated with individual animal attributes rather than annual variation in environmental conditions. Maternal age, a female's reproductive status the previous season and their interaction were found to be the most influential maternal traits. Pup body mass at parturition and during lactation showed maternal age-related variation with evidence of senescence during pre-partum investment and terminal investment during post-partum investment. A female's reproductive status in one season affected her reproductive investment during lactation in consecutive seasons but that effect was correlated with age. Younger females who pupped in consecutive seasons weaned larger pups than those females who were of the same age and did not give birth in consecutive seasons. The opposite was found to be true for older females. The variation in the affect of maternal traits on maternal investment may be due to the increased energy requirement of lactation, age specific changes in reproductive costs, and a senescent decline in physiological function.
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