Fasting durations of Steller sea lion pups vary among subpopulations—evidence from two plasma metabolites

Abstract

Geographic differences in population growth trends are well-documented in Steller sea lions (Eumetopias jubatus), a species of North Pacific pinniped listed under the U.S. Endangered Species Act in 1990 following a marked decline in population abundance that began during the 1970s. As population growth is intrinsically linked to pup production and survival, examining factors related to pup physiological condition provides useful information to management authorities regarding potential drivers of regional differences. During dam foraging trips, pups predictably transition among three fasting phases, distinguished by the changes in the predominant metabolic byproduct. We used standardized ranges of two plasma metabolites (blood urea nitrogen and β–hydroxybutyrate) to assign pups to fasting categories (n = 1528, 1990–2016, 12 subpopulations): Recently Fed–Phase I (digestion/assimilation–expected hepatic/muscle glycogen usage), Phase II (expected lipid utilization), transitioning between Phases II–III (expected lipid utilization with increased protein reliance), or Phase III (expected protein catabolism). As anticipated, the majority of pups were classified as Recently Fed–Phase I (overall mean proportion = 0.72) and few pups as Phase III (overall mean proportion = 0.04). By further comparing pups in Short (Recently Fed–Phase II) and Long (all other pups) duration fasts, we identified three subpopulations with significantly (P < 0.03) greater proportions of pups dependent upon endogenous sources of energy for extended periods, during a life stage of somatic growth and development: the 1) central (0.27 ± 0.09) and 2) western (0.36 ± 0.13) Aleutian Island (declining population trend) and 3) southern Southeast Alaska (0.32 ± 0.06; increasing population trend) subpopulations had greater Long fast proportions than the eastern Aleutian Islands (0.10 ± 0.05; stabilized population). Due to contrasting population growth trends among these highlighted subpopulations over the past 50+ years, both density-independent and density-dependent factors likely influence the dam foraging trip duration, contributing to longer fasting durations for pups at some rookeries.

Preliminary comparisons between a point-of-care ketometer and reference method using Steller sea lion pup whole blood and plasma

“…The ketometer underestimated [β-HBA] when Reference Assay measurements were <0.25 mmol/l, and conversely, the ketometer overestimated Reference Assay measurements were ≥0.25 mmol/l. The dashed blue line represents the metabolite threshold applied to categorize the fasting status of Steller sea lion pups ( Crawford et al ., 2023 ). The figure is sorted in ascending order by Reference Assay mean values.…”

“…Similarly, the difference in measurements for comparison C are all ≤0, resulting in an uneven distribution around the zero line. For the purposes of our larger study ( Crawford et al , 2023 ), our classification of the SSL pup fasting phase would not have differed as a result of any of the changes observed in [β-HBA] following Thaw 2. Comparisons D and E between ketometer and Reference Assay [β-HBA] (plasma, Thaw 1 ) were both found to be linearly correlated: whole blood (H = 5.082, P = 1.000) and plasma (H = 5.082, P = 0.938), respectively ( Table 4 , Fig.…”

“…Approximately nine months following the field collections, the [β-HBA] of thawed ( Thaw 1 ) plasma aliquoted from NaHep-treated blood collection tubes was measured using both the Precision Xtra™ ketometer and the Reference Assay , the established method utilized in our research laboratory ( Castellini and Costa, 1990 ; Castellini et al, 1993 ), via a commercially-available assay kit (Sigma Aldrich, now Millipore Sigma, St. Louis, MO, Kit #MAK041). The [β-HBA] plasma ( Thaw 1 ) data obtained via Reference Assa y for these 14 SSL pups were also utilized in Crawford et al ’s (2023) study that evaluated the fasting status of 1528 SSL pups.…”

“…As fasting-adapted species do not accumulate circulating ketone bodies in concentrations resulting in ketoacidosis, we defined a TP as a SSL pup sample with Reference Assay and ketometer [β–HBA] ≥0.3 mmol/l (i.e., SSL pups that are ‘positive’ for ketone production, documented by Rea et al (2000) when fasting durations exceeded 16 hours). While the condition of fasting is not a ‘disease’, one of our objectives was to evaluate this ketometer’s performance in the identification of plasma samples above and below a threshold of 0.3 mmol/l β-HBA, essential to categorizing the fasting status of free-ranging SSL pups as part of our larger investigation ( Crawford et al , 2023 ).…”

“…This ketometer’s utility has also been demonstrated in field research of wild birds ( Lindholm et al, 2018 ; Morales et al, 2020 ), but we are unaware of a validated point-of-care ketometer for any mammalian wildlife species. Our primary objectives were to assess the Precision Xtra TM ketometer’s (1) accuracy when testing thawed plasma samples, and (2) specifically to reliably allow us to identify plasma samples above and below a threshold of 0.3 mmol/l β-HBA, necessary for classifying the fasting category of free-ranging SSL pups ( Crawford et al, 2023 ).…”