Physiological regulation of annual life history events in adult female Weddell seals

Abstract

Predicting the impacts of environmental changes on animal populations requires a comprehensive understanding of the life history and physiological ecology of organisms in the wild, including the precise timing and regulation of annual biological events. In this dissertation, I assess how intrinsic factors including serum hormone concentrations and nutritional status relate to the critical annual events of reproduction, seasonal foraging, and pelage molt in an Antarctic marine predator, the Weddell seal (Leptonychotes weddellii). A large component of these assessments are comparisons between female Weddell seals that pupped in the current season (postpartum females) and females that have pupped in the past but did not give birth in the most recent pupping period (skip females). First, I examine how reproductive hormone profiles (serum progesterone, estrogen, prolactin, luteinizing hormone and follicle-stimulating hormone) vary between postpartum females and skip females across the austral summer. I found that skip females ovulated earlier than postpartum females and had higher serum estrogen and progesterone concentrations during early pregnancy. I also report a distinct midsummer decline in serum prolactin concentrations in both skip and postpartum seals. This decline may be due to seals becoming photorefractory at the time that the breeding period ends. Second, I characterize patterns in body mass changes and associated metabolic hormones and serum binding proteins (cortisol, growth hormone, insulin-like growth factor-1 (IGF-1), total and free thyroxine, total triiodothyronine, and IGF binding proteins 2 and 3) in Weddell seals across summer. I found that endocrine profiles of lactating seals reflected their depleted nutritional states, though their serum cortisol remained low. Postpartum females rapidly gained lean mass after weaning, which was likely supported by high serum growth hormone concentrations, while skip females lost body mass, probably due in part to their low serum concentrations of growth hormone. Next, I detail the Weddell seal hair cycle and molt, using histology to determine the timing of hair follicle activation, hair growth, and shedding of old fur, and the relationship between molt and wound healing. Hair follicle activity began at a similar time (~October) in all seals but proceeded significantly faster in skip females; the full molt process took 131±23 days in postpartum seals and 86±11 days in skip females. Seals that spent more time hauled out molted more quickly, indicating behavior likely drove the variation in molt timing between postpartum and skip females by affecting their exposure to different thermal conditions. Finally, I examined differential gene expression in skin across the Weddell seal hair cycle using next generation RNA sequencing (RNA-Seq). I report large shifts in the skin transcriptome between hair cycle phases, including changes in expression of genes related to hormone signaling. Together, my findings provide an in-depth examination of annual life history events in adult female Weddell seals. Findings identify physiological drivers and reflections of critical seasonal processes, which may be applied to predict future life history outcomes for Weddell seals and similar species.