Endocrine response to realimentation in young northern elephant seals (Mirounga angustirostris): Indications for development of fasting adaptation.

Most organisms undergo changes in their environment, both predictably and unpredictably, which require them to alter priorities in nutrient allocation with regards to food availability. Species that more predictably encounter extended periods of limited food resources or intake while mitigating the negative effects of starvation are considered to be fasting adapted. Northern elephant seals (NES) are one such species and routinely undergo extended periods of fasting for breeding, molting, as well as a post-weaning fast at 6-8weeks of age. However, during unusual times of nutritional deprivation, animals may enter stage III fasting. While fasting and foraging in this species has been extensively studied, realimentation following fasting beyond normal life history parameters has not been investigated. In this study, changes in ghrelin, growth hormone (GH), and insulin-like growth factor (IGF)-I were compared across 8weeks of realimentation following emaciation in three age classes: neonates, post-molt pups, and yearlings. Longitudinal changes in hormone profiles indicate that neonate and post-molt pups are slow to recover mass and positive energy balance despite an energy dense diet fed at 10% body mass. In addition, ghrelin and GH concentrations remained elevated in post-molt pups compared to other age classes. Changes in hormone concentrations early in realimentation indicate that yearling animals recover more rapidly from periods of nutritional deprivation than do younger animals. Overall, this suggests that the ability to regulate metabolic homeostasis with regards to nutrient allocation may develop over time, even in a species that is considered to be fasting adapted.

Total mercury concentrations in Steller sea lion bone: Variability among locations and elements.

Mercury is a global contaminant that bioaccumulates in a tissue-specific manner in long-lived predators such as Steller sea lions (SSL). Bone is a well-preserved material amenable for studying millennial scale trends; however, little is known about the distribution and variability of total mercury concentrations ([THg]) within individual bones and among bone elements in SSL. We assessed SSL bone [THg] variability with respect to physiologic age, bone type, longitudinally within a bone, and among bone elements. Pup bones (mean ± SD; 31.4 ± 13.58 ppb) had greater [THg] than adults (7.9 ± 1.91 ppb). There were greater and more variable [THg] within individual long bones near epiphyses compared to mid-diaphysis. Pup spongy bone in ribs (62.7 ± 44.79 ppb) had greater [THg] than long bones (23.5 ± 8.83 ppb) and phalanges (19.6 ± 10.78 ppb). These differences are likely due to variability in bone composition, growth, and turnover rate. This study informs standardized sampling procedures for [THg] in bone to improve interpretations of mercury variability over time and space.

Total Mercury, Total Selenium, and Monomethylmercury Relationships in Multiple Age Cohorts and Tissues of Steller Sea Lions (Eumetopias jubatus).

Steller sea lion (Eumetopias jubatus) tissue mercury concentrations increasingly above thresholds of concern occur in regions of Alaska where lack of population recovery is noted. Selenium-monomethylmercury interactions may mitigate toxicosis but may also result in functional selenium deficiency, impacting essential selenium-dependent processes. Physiologically driven differences in tissue concentrations (organotropism) of total mercury ([THg]), total selenium ([TSe]), and monomethylmercury ([MeHg+ ]) confound interpretation for various age cohorts. Archived tissues from Alaska Steller sea lions (2002-2016) were used to compare [THg], [MeHg+ ], and [TSe] across age cohorts and tissue types. Liver [THg] ranged from 0.05 to 63.7 µg/g. Fetal and pup livers had significantly lower [THg] and [TSe], higher percentage MeHg+ , and greater range of molar TSe:THg than subadult and adult livers. Molar Se:MeHg+ ratios, including Se in excess of nonmethylmercury, were dependent on [MeHg+ ] in fetuses and pups. While [THg] varied significantly by muscle type (heart vs. skeletal) and anatomical location, concentrations were strongly correlated. Biomagnification and/or bioaccumulation of THg in liver of older animals confounded comparison with other tissues; however, in fetal and pup liver [THg] correlated with other tissues. In contrast, liver [MeHg+ ] correlated with other tissues across all age classes. Fetal and pup tissues, which reflect in utero exposure and are subject to limited bioaccumulation, are ideal for assessing mercury exposure related to dam diet, including intertissue comparison, and represent key cohorts of concern. Evaluating [MeHg+ ] and [TSe] in tissues from multiple age cohorts allows better intertissue comparison, providing insight into time courses, routes of exposure, and potential for adverse effects. Environ Toxicol Chem 2022;41:1477-1489. © 2022 SETAC.

Response of metabolic hormones and blood metabolites to realimentation in rehabilitated harbor seal (Phoca vitulina) pups.

Mammals with increased requirements for adipose tissue stores, such as marine mammals, have altered nutrient allocation priorities compared to many terrestrial mammals and thus the physiological response to undernutrition (low nutritional status) and realimentation (refeeding) may differ. Key regulators of nutrient allocation and tissue specific growth include metabolic hormones of the somatotropic axis, growth hormone (GH) and insulin-like growth factor (IGF)-I, as well as satiety and adipose promoting ghrelin and the stress hormone cortisol. Longitudinal measurements of metabolic hormones, blood metabolites, and morphometrics were collected over a 10-week period in twelve (male n = 3, female n = 9) harbor seal pups (< 6 weeks of age). Blood metabolites were used to indicate metabolic response during realimentation while morphometrics estimated tissue specific growth priorities. Harbor seal pups undergoing refeeding after nutritional deprivation show a preference for protein sparing despite severe malnutrition. Both BUN and total protein were negatively associated with GH and positively associated with IGF-I and ghrelin highlighting the importance of these metabolic hormones in the regulation of protein metabolism. While the response of the somatotropic axis to realimentation was typical of the mammalian pattern, the surprising increase of ghrelin across the study period suggests the priority of adipose accretion in addition to a possible mechanism regulating compensatory growth of vital adipose stores in a species, which prioritizes adipose accretion for survival.