Molting strategies of Arctic seals drive annual patterns in metabolism

Linking ringed seal foraging behaviour to environmental variability

BACKGROUND

Foraging rates directly influence animals' energetic intake and expenditure and are thus linked to body condition and the ability to survive and reproduce. Further, understanding the underlying processes driving a species' behaviour and habitat use is important as changes in behaviour could result from changes in environmental conditions.

METHODS

In this study, the dives of Saimaa ringed seals (Pusa hispida saimensis) were classified for the first time using hidden Markov models and telemetry data collected on individual dives, and the behavioural states of the diving seals were estimated. In addition, we used generalized additive mixed models on the foraging probability of the seals to identify environmental and temporal drivers of foraging behaviour.

RESULTS

We inferred three (in winter) or four (in summer) different dive types: sleeping/resting dives, shallow inactive dives, transiting dives and foraging dives, based on differences in dive metrics logged by or derived from data from telemetry tags. Long and relatively deep sleeping/resting dives were missing entirely in the winter, compensated by an increased proportion of time used for haul-out. We found profound differences in the behaviour of Saimaa ringed seals during the open water season compared to the ice-covered winter, with the greatest proportion of time allocated to foraging during the summer months (36%) and the lowest proportion in the winter (21%). The seals' foraging probability peaked in summer (July) and was highest during the daytime during both summer and winter months. Moreover, foraging probability was highest at lake depths of 7-30 m in the winter and at depths > 15 m in the summer. We also found some evidence of sex-specific foraging strategies that are adapted seasonally, with females preferring more sheltered water areas during winter.

CONCLUSIONS

We suggest that the foraging behaviour of Saimaa ringed seals is largely influenced by diel vertical movements and availability of fish, and that the seals optimize their energy acquisition while conserving energy, especially during the cold winter months. Further, the seals display some flexibility in foraging strategies, a feature that may help this endangered subspecies to cope with the ongoing climate change.

An annual cycle perspective on energetics and locomotion of migratory animals

Animal migrations, or long-distance movements, on land, through water or in the air, are considered energetically costly because of the investment in persistent locomotion typical for migration. Diverse strategies exist to manage these energetic costs. Yet migration is only one stage in an annual cycle and may not be the most energetically costly. To better understand how free-ranging animals adaptively organize energy expenditure and locomotion, an annual cycle perspective is needed. Bio-logging data are collected for a range of animal species and could facilitate a life cycle approach to study energy expenditure. We provide examples from several studies across different taxa, as well as a more in-depth exploration from our own recent research on time activity budgets based on field observations and bio-logging data to estimate daily energy expenditure in a migratory seabird throughout a year. Our research has shown that daily energy expenditure is highest (1.7× average daily energy expenditure) during the spring migration of long-distance migratory gulls, whereas short-distance migrants expend the most energy (1.4× average daily energy expenditure) during the breeding season. Based on the examples provided, we show how bio-energetic models create exciting opportunities to study daily energetics and behaviour of migratory animals, although limitations also still exist. Such studies can reveal when, where and why peaks and lulls in energy expenditure arise over the annual cycle of a migrant, if long-distance movements are indeed energetically expensive and how animals can adapt to fluctuating demands in their natural environment throughout the year.

The nasal cavity of the bearded seal: An effective and robust organ for retaining body heat and water

We report the effects of varying physiological and other properties on the heat and water exchange in the maxilloturbinate structure (MT) of the bearded seal (Erignathus barbatus or Eb) in realistic environments, using a computational fluid dynamics (CFD) model. We find that the water retention in percent is very high (about 90 %) and relatively unaffected by either cold (-30 °C) or warm (10 °C) conditions. The retention of heat is also high, around 80 % . Based on a consideration of entropy production by the maxilloturbinate system, we show that anatomical and physiological properties of the seal provide good conditions for heat and water exchange at the mucus lining in the seal's nasal cavity. At normal values of tidal volume and maxilloturbinate (MT) length, the air temperature in the MT reaches the body temperature before the air has left the MT channels. This confers a safety factor which is expected to be helpful in exercise, when ventilation increases.

SEASONAL VARIATION OF BLOOD ANALYTES ASSOCIATED WITH SKIN HEALTH IN ALASKAN ICE SEALS

Recent unusual mortality events involving skin pathology in bearded (Erignathus barbatus), ringed (Pusa hispida), and spotted seals (Phoca largha) in Alaska highlight the potential sensitivity of ice-associated species to the complex effects of climate change. The regulation of thyroid hormones, cortisol, and vitamin A have been shown to play essential roles in skin health and seasonal molt in some pinnipeds. Unfortunately, the lack of available reference data for healthy Alaskan ice seals has prevented the adequate evaluation of these factors in cases associated with mortality events. To improve understanding of blood analytes that may serve as useful markers of skin health, we compiled diagnostic information for individuals in long-term managed care following short-term rehabilitation. Thyroid hormones (TT4, TT3, and FT4d), cortisol, and vitamin A levels are reported for four ringed seals, four spotted seals, and three bearded seals at the Alaska SeaLife Center in Seward, Alaska, with serial samples obtained when possible and referenced to time of year. For ringed and spotted seals, the thyroid hormones and vitamin A showed a strong seasonal pattern with peak values obtained during the annual molt in spring, whereas cortisol values did not vary predictably between molting and non-molting periods. Trends were similar for individuals across both sex and species. Bearded seals had fewer available data for younger individuals only, but observed analyte values are provided for this understudied species. Collectively, these measurements can be used to support veterinary management of ice seals under human care and to provide initial baselines for skin health monitoring in wild populations and in stranded individuals with known skin lesions or pathology.

Spring haul-out behavior of seals in the Bering and Chukchi Seas: implications for abundance estimation

Ice-associated seals rely on sea ice for a variety of activities, including pupping, breeding, molting, and resting. In the Arctic, many of these activities occur in spring (April through June) as sea ice begins to melt and retreat northward. Rapid acceleration of climate change in Arctic ecosystems is therefore of concern as the quantity and quality of suitable habitat is forecast to decrease. Robust estimates of seal population abundance are needed to properly monitor the impacts of these changes over time. Aerial surveys of seals on ice are an efficient method for counting seals but must be paired with estimates of the proportion of seals out of the water to derive population abundance. In this paper, we use hourly percent-dry data from satellite-linked bio-loggers deployed between 2005 and 2021 to quantify the proportion of seals hauled out on ice. This information is needed to accurately estimate abundance from aerial survey counts of ice-associated seals (i.e., to correct for the proportion of animals that are in the water, and so are not counted, while surveys are conducted). In addition to providing essential data for survey 'availability' calculations, our analysis also provides insights into the seasonal timing and environmental factors affecting haul-out behavior by ice-associated seals. We specifically focused on bearded (Erignathus barbatus), ribbon (Histriophoca fasciata), and spotted seals (Phoca largha) in the Bering and Chukchi seas. Because ringed seals (Phoca (pusa) hispida) can be out of the water but hidden from view in snow lairs analysis of their 'availability' to surveys requires special consideration; therefore, they were not included in this analysis. Using generalized linear mixed pseudo-models to properly account for temporal autocorrelation, we fit models with covariates of interest (e.g., day-of-year, solar hour, age and sex class, wind speed, barometric pressure, temperature, precipitation) to examine their ability to explain variation in haul-out probability. We found evidence for strong diel and within-season patterns in haul-out behavior, as well as strong weather effects (particularly wind and temperature). In general, seals were more likely to haul out on ice in the middle of the day and when wind speed was low and temperatures were higher. Haul-out probability increased through March and April, peaking in May and early June before declining again. The timing and frequency of haul-out events also varied based on species and age-sex class. For ribbon and spotted seals, models with year effects were highly supported, indicating that the timing and magnitude of haul-out behavior varied among years. However, we did not find broad evidence that haul-out timing was linked to annual sea-ice extent. Our analysis emphasizes the importance of accounting for seasonal and temporal variation in haul-out behavior, as well as associated environmental covariates, when interpreting the number of seals counted in aerial surveys.

Isochrony in barks of Cape fur seal (Arctocephalus pusillus pusillus) pups and adults

Animal vocal communication often relies on call sequences. The temporal patterns of such sequences can be adjusted to other callers, follow complex rhythmic structures or exhibit a metronome-like pattern (i.e., isochronous). How regular are the temporal patterns in animal signals, and what influences their precision? If present, are rhythms already there early in ontogeny? Here, we describe an exploratory study of Cape fur seal (Arctocephalus pusillus pusillus) barks-a vocalisation type produced across many pinniped species in rhythmic, percussive bouts. This study is the first quantitative description of barking in Cape fur seal pups. We analysed the rhythmic structures of spontaneous barking bouts of pups and adult females from the breeding colony in Cape Cross, Namibia. Barks of adult females exhibited isochrony, that is they were produced at fairly regular points in time. Instead, intervals between pup barks were more variable, that is skipping a bark in the isochronous series occasionally. In both age classes, beat precision, that is how well the barks followed a perfect template, was worse when barking at higher rates. Differences could be explained by physiological factors, such as respiration or arousal. Whether, and how, isochrony develops in this species remains an open question. This study provides evidence towards a rhythmic production of barks in Cape fur seal pups and lays the groundwork for future studies to investigate the development of rhythm using multidimensional metrics.

What are the Metabolic Rates of Marine Mammals and What Factors Impact this Value: A review

Over the past several decades, scientists have constructed bioenergetic models for marine mammals to assess potential population-level consequences following exposure to a disturbance, stressor, or environmental change, such as under the Population Consequences of Disturbance (pCOD) framework. The animal's metabolic rate (rate of energy expenditure) is a cornerstone for these models, yet the cryptic lifestyles of marine mammals, particularly cetaceans, have limited our ability to quantify basal (BMR) and field (FMR) metabolic rates using accepted 'gold standard' approaches (indirect calorimeter via oxygen consumption and doubly labeled water, respectively). Thus, alternate methods have been used to quantify marine mammal metabolic rates, such as extrapolating from known allometric relationships (e.g. Kleiber's mouse to elephant curve) and developing predictive relationships between energy expenditure and physiological or behavioral variables. To understand our current knowledge of marine mammal metabolic rates, we conducted a literature review (1900-2023) to quantify the magnitude and variation of metabolic rates across marine mammal groups. A compilation of data from studies using 'gold standard' methods revealed that BMR and FMR of different marine mammal species ranges from 0.2 to 3.6 and 1.1 to 6.1 x Kleiber, respectively. Mean BMR and FMR varied across taxa; for both measures odontocete levels were intermediate to higher values for otariids and lower values of phocids. Moreover, multiple intrinsic (e.g. age, sex, reproduction, molt, individual) and extrinsic (e.g. food availability, water temperature, season) factors, as well as individual behaviors (e.g. animal at water's surface or submerged, activity level, dive effort and at-sea behaviors) impact the magnitude of these rates. This review provides scientists and managers with a range of reliable metabolic rates for several marine mammal groups as well as an understanding of the factors that influence metabolism to improve the discernment for inputs into future bioenergetic models.

A Review of Circumpolar Arctic Marine Mammal Health-A Call to Action in a Time of Rapid Environmental Change

The impacts of climate change on the health of marine mammals are increasingly being recognised. Given the rapid rate of environmental change in the Arctic, the potential ramifications on the health of marine mammals in this region are a particular concern. There are eleven endemic Arctic marine mammal species (AMMs) comprising three cetaceans, seven pinnipeds, and the polar bear (Ursus maritimus). All of these species are dependent on sea ice for survival, particularly those requiring ice for breeding. As air and water temperatures increase, additional species previously non-resident in Arctic waters are extending their ranges northward, leading to greater species overlaps and a concomitant increased risk of disease transmission. In this study, we review the literature documenting disease presence in Arctic marine mammals to understand the current causes of morbidity and mortality in these species and forecast future disease issues. Our review highlights potential pathogen occurrence in a changing Arctic environment, discussing surveillance methods for 35 specific pathogens, identifying risk factors associated with these diseases, as well as making recommendations for future monitoring for emerging pathogens. Several of the pathogens discussed have the potential to cause unusual mortality events in AMMs. Brucella, morbillivirus, influenza A virus, and Toxoplasma gondii are all of concern, particularly with the relative naivety of the immune systems of endemic Arctic species. There is a clear need for increased surveillance to understand baseline disease levels and address the gravity of the predicted impacts of climate change on marine mammal species.

Seasonal changes in diel haul-out patterns of a lacustrine ringed seal (Pusa hispida saimensis)

Seasonal changes in diel haul-out patterns of the lacustrine Saimaa ringed seal (Pusa hispida saimensis) were studied using a combination of satellite telemetry and camera traps during 2007-2015. We found the haul-out activity patterns to vary seasonally. Our results show that during the ice-covered winter period before the seals start their annual molt, the peak in haul-out generally occurs at midnight. Similarly, during the postmolt season of summer and autumn when the lake is free of ice, the haul-out is concentrated in the early hours of the morning. In contrast, over the spring molting period, Saimaa ringed seals tend to haul out around the clock. The spring molt is also the only period when a slight difference in haul-out behavior between the sexes is observed, with females having a haul-out peak at nighttime while the males have a less visible diel pattern. According to our results, the diel haul-out patterns of Saimaa ringed seals are similar to the ones of marine ringed seals. Gathering information on haul-out activity is important in order to safeguard the natural patterns of Saimaa ringed seals in areas that are prone to disturbance from human activities.

Growth in marine mammals: a review of growth patterns, composition and energy investment

Growth of structural mass and energy reserves influences individual survival, reproductive success, population and species life history. Metrics of structural growth and energy storage of individuals are often used to assess population health and reproductive potential, which can inform conservation. However, the energetic costs of tissue deposition for structural growth and energy stores and their prioritization within bioenergetic budgets are poorly documented. This is particularly true across marine mammal species as resources are accumulated at sea, limiting the ability to measure energy allocation and prioritization. We reviewed the literature on marine mammal growth to summarize growth patterns, explore their tissue compositions, assess the energetic costs of depositing these tissues and explore the tradeoffs associated with growth. Generally, marine mammals exhibit logarithmic growth. This means that the energetic costs related to growth and tissue deposition are high for early postnatal animals, but small compared to the total energy budget as animals get older. Growth patterns can also change in response to resource availability, habitat and other energy demands, such that they can serve as an indicator of individual and population health. Composition of tissues remained consistent with respect to protein and water content across species; however, there was a high degree of variability in the lipid content of both muscle (0.1-74.3%) and blubber (0.4-97.9%) due to the use of lipids as energy storage. We found that relatively few well-studied species dominate the literature, leaving data gaps for entire taxa, such as beaked whales. The purpose of this review was to identify such gaps, to inform future research priorities and to improve our understanding of how marine mammals grow and the associated energetic costs.

Maintaining control: metabolism of molting Arctic seals in water and when hauled out

Seals haul out of water for extended periods during the annual molt, when they shed and regrow their pelage. This behavior is believed to limit heat loss to the environment given increased peripheral blood flow to support tissue regeneration. The degree to which time in water, particularly during the molt, may affect thermoregulatory costs is poorly understood. We measured the resting metabolism of three spotted seals (Phoca largha), one ringed seal (Pusa hispida) and one bearded seal (Erignathus barbatus) during and outside the molting period, while resting in water and when hauled out. Metabolic rates were elevated in spotted and ringed seals during molt, but comparable in water and air for individuals of all species, regardless of molt status. Our data indicate that elevated metabolism during molt primarily reflects the cost of tissue regeneration, while increased haul out behavior is driven by the need to maintain elevated skin temperatures to support tissue regeneration.

Metabolic cost of thermoregulation decreases after the molt in developing Weddell seal pups

Allocation of energy to thermoregulation greatly contributes to the metabolic cost of endothermy, especially in extreme ambient conditions. Weddell seal (Leptonychotes weddellii) pups born in Antarctica must survive both on ice and in water, two environments with very different thermal conductivities. This disparity likely requires pups to allocate additional energy toward thermoregulation rather than growth or development of swimming capabilities required for independent foraging. We measured longitudinal changes in resting metabolic rate (RMR) for Weddell seal pups (n=8) in air and water from one to seven weeks of age, using open-flow respirometry. Concurrently, we collected molt, morphometric, and dive behavior data. Absolute-MR in air followed the expected allometric relationship with mass. Absolute-MR in water was not allometric with mass, despite a 3-fold increase in mass between one and seven weeks of age. Developmental stage (or molting stage), rather than calendar age, determined when pups were thermally capable of being in the water. We consistently observed post-molt pups had lower RMR in air and water (6.67±1.4 and 7.90±2.38 ml O2 min-1kg-1, respectively) than pre-molt (air: 9.37±2.42 ml O2 min-1kg-1, water: 13.40±3.46 ml O2 min-1kg-1) and molting pups (air: 8.45±2.05 ml O2 min-1kg-1, water: 10.4±1.63 ml O2 min-1kg-1). RMR in air and water were equivalent only for post-molt pups. Despite the increased energy cost, molting pups spent 3x more time in the water than other pups. These results support the idea of an energetic trade-off during early development; pups expend more energy for thermoregulation in water, yet gain experience needed for independence.