Abstract
Natural selection should favour strategies that maximise reproductive success. Females may use different resources during progressive stages of reproduction according to energetic demands, behavioural constraints and prey availability. We used South American fur seal, Arctocephalus australis australis, pup whisker isotope values as proxies for maternal diet and habitat use to determine how resource use (1) changes throughout pup development from in utero growth to mid-end of lactation and (2) how it differs among individuals. The longest whisker was cut from 5 male and 5 female fur seal pups (of approximately 8 months of age) at Bird Island, Falkland Islands, in 2018, and δ15N values and δ13C values were analysed every 5 mm along the length of each whisker. Patterns in δ13C values indicated that mothers used different habitats during the annual cycle, likely coinciding with seasonal shifts in prey availability or distribution. The individual specialisation index based on δ13C values was 0.34, indicating that adult females used different habitats, which could reduce intra-specific competition and ultimately enhance pup growth and survival. An increase in δ15N values occurred along every pup whisker from pup birth to mid-end of lactation, which likely reflected trophic enrichment related to suckling and fasting by pups, overriding the maternal isotopic signature. Pup whisker stable isotopes are useful proxies of maternal foraging ecology. However, physiological processes complicate interpretations by altering δ15N values. Interpreting these values therefore requires additional knowledge of the species’ ecology and physiology.
Highlights
In mammals, gestation and lactation have different energetic requirements that influence maternal resource use (Gittleman and Thompson 1988)
Ratios were corrected for instrument drift and linearity using interspersed samples of reference materials with known stable isotope values: GEL, ALAGEL and GLYGEL, each dried for 2 h at 70 °C
We tested whether body morphology significantly differed between 4 male pups and five female pups by running a Principal Components Analysis (PCA) on body measurements and testing the output from Principal Component 1 (PC1) and Principal Component 2 (PC2) in Welch’s t-tests
Summary
Gestation and lactation have different energetic requirements that influence maternal resource use (Gittleman and Thompson 1988). Female mammals must grow and maintain foetal, uterine, placental and mammary tissues, which is demanding towards the end of gestation when the foetus is large (Gittleman and Thompson 1988; Hückstädt et al 2018). Maternal metabolism dramatically increases as nutrients are transferred to the mammary glands to produce milk for offspring (Crocker et al 1998). Some otariid (eared seal) species are concurrently pregnant while lactating, so have higher costs of breeding than otariids that do not lactate and gestate simultaneously (Lima and Páez 1995). The distance and duration of successful foraging trips are limited by the fasting ability of offspring (Villegas-Amtmann et al 2017). As a result of these changing demands, females may alter their
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