Whiskers as hydrodynamic prey sensors in foraging seals.

The darkness of the deep ocean limits the vision of diving predators, except when prey emit bioluminescence. It is hypothesized that deep-diving seals rely on highly developed whiskers to locate their prey. However, if and how seals use their whiskers while foraging in natural conditions remains unknown. We used animal-borne tags to show that free-ranging elephant seals use their whiskers for hydrodynamic prey sensing. Small, cheek-mounted video loggers documented seals actively protracting their whiskers in front of their mouths with rhythmic whisker movement, like terrestrial mammals exploring their environment. Seals focused their sensing effort at deep foraging depths, performing prolonged whisker protraction to detect, pursue, and capture prey. Feeding-event recorders with light sensors demonstrated that bioluminescence contributed to only about 20% of overall foraging success, confirming that whiskers play the primary role in sensing prey. Accordingly, visual prey detection complemented and enhanced prey capture. The whiskers' role highlights an evolutionary alternative to echolocation for adapting to the extreme dark of the deep ocean environment, revealing how sensory abilities shape foraging niche segregation in deep-diving mammals. Mammals typically have mobile facial whiskers, and our study reveals the significant function of whiskers in the natural foraging behavior of a marine predator. We demonstrate the importance of field-based sensory studies incorporating multimodality to better understand how multiple sensory systems are complementary in shaping the foraging success of predators.

Individual variation in life-history timing: synchronous presence, asynchronous events and phenological compensation in a wild mammal.

Many animals and plants have species-typical annual cycles, but individuals vary in their timing of life-history events. Individual variation in fur replacement (moult) timing is poorly understood in mammals due to the challenge of repeated observations and longitudinal sampling. We examined factors that influence variation in moult duration and timing among elephant seals (Mirounga angustirostris). We quantified the onset and progression of fur loss in 1178 individuals. We found that an exceptionally rapid visible moult (7 days, the shortest of any mammals or birds), and a wide range of moult start dates (spanning 6-10× the event duration) facilitated high asynchrony across individuals (only 20% of individuals in the population moulting at the same time). Some of the variation was due to reproductive state, as reproductively mature females that skipped a breeding season moulted a week earlier than reproductive females. Moreover, individual variation in timing and duration within age-sex categories far outweighed (76-80%) variation among age-sex categories. Individuals arriving at the end of the moult season spent 50% less time on the beach, which allowed them to catch up in their annual cycles and reduce population-level variance during breeding. These findings underscore the importance of individual variation in annual cycles.

Adrenal response to ACTH challenge alters thyroid and immune function and varies with body reserves in molting adult female northern elephant seals.

Intrinsic stressors associated with life-history stages may alter the responsiveness of the hypothalamic-pituitary-adrenal axis and responses to extrinsic stressors. We administered adrenocorticotropic hormone (ACTH) to 24 free-ranging adult female northern elephant seals (NESs) at two life-history stages: early and late in their molting period and measured a suite of endocrine, immune, and metabolite responses. Our objective was to evaluate the impact of extended, high-energy fasting on adrenal responsiveness. Animals were blood sampled every 30 min for 120 min post-ACTH injection, then blood was sampled 24 h later. In response to ACTH injection, cortisol levels increased 8- to 10-fold and remained highly elevated compared with baseline at 24 h. Aldosterone levels increased 6- to 9-fold before returning to baseline at 24 h. The magnitude of cortisol and aldosterone release were strongly associated, and both were greater after extended fasting. We observed an inverse relationship between fat mass and the magnitude of cortisol and aldosterone responses, suggesting that body reserves influenced adrenal responsiveness. Sustained elevation in cortisol was associated with alterations in thyroid hormones; both tT3 and tT4 concentrations were suppressed at 24 h, while rT3 increased. Immune cytokine IL-1ß was also suppressed after 24 h of cortisol elevation, and numerous acute and sustained impacts on substrate metabolism were evident. Our data suggest that female NESs are more sensitive to stress after the molt fast and that acute stress events can have important impacts on metabolism and immune function. These findings highlight the importance of considering life-history context when assessing the impacts of anthropogenic stressors on wildlife.

Mercury Bioaccumulation and Cortisol Interact to Influence Endocrine and Immune Biomarkers in a Free-Ranging Marine Mammal.

Mercury bioaccumulation from deep-ocean prey and the extreme life history strategies of adult female northern elephant seals (Mirounga angustirostris) provide a unique system to assess the interactive effects of mercury and stress on animal health by quantifying blood biomarkers in relation to mercury (skeletal muscle and blood mercury) and cortisol concentrations. The thyroid hormone thyroxine (tT4) and the antibody immunoglobulin E (IgE) were associated with mercury and cortisol concentrations interactively, where the magnitude and direction of the association of each biomarker with mercury or cortisol changed depending on the concentration of the other factor. For example, when cortisol concentrations were lowest, tT4 was positively related to muscle mercury, whereas tT4 had a negative relationship with muscle mercury in seals that had the highest cortisol concentrations. Additionally, we observed that two thyroid hormones, triiodothyronine (tT3) and reverse triiodothyronine (rT3), were negatively (tT3) and positively (rT3) associated with mercury concentrations and cortisol in an additive manner. As an example, tT3 concentrations in late breeding seals at the median cortisol concentration decreased by 14% across the range of observed muscle mercury concentrations. We also observed that immunoglobulin M (IgM), the pro-inflammatory cytokine IL-6 (IL-6), and a reproductive hormone, estradiol, were negatively related to muscle mercury concentrations but were not related to cortisol. Specifically, estradiol concentrations in late molting seals decreased by 50% across the range of muscle mercury concentrations. These results indicate important physiological effects of mercury on free-ranging apex marine predators and interactions between mercury bioaccumulation and extrinsic stressors. Deleterious effects on animals' abilities to maintain homeostasis (thyroid hormones), fight off pathogens and disease (innate and adaptive immune system), and successfully reproduce (endocrine system) can have significant individual- and population-level consequences.

Changes in apolipoprotein abundance dominate proteome responses to prolonged fasting in elephant seals.

Unlike many animals that reduce activity during fasting, northern elephant seals (NES) undergo prolonged fasting during energy-intensive life-history stages such as reproduction and molting, fueling fasting energy needs by mobilizing fat stores accrued during foraging. NES display several unique metabolic features such as high fasting metabolic rates, elevated blood lipid and high-density lipoprotein (HDL) cholesterol levels, efficient protein sparing and resistance to oxidative stress during fasting. However, the cellular mechanisms that regulate these adaptations are still not fully understood. To examine how metabolic coordination is achieved during prolonged fasting, we profiled changes in blubber, skeletal muscle and plasma proteomes of adult female NES over a 5 week fast associated with molting. We found that while blubber and muscle proteomes were remarkably stable over fasting, over 50 proteins changed in abundance in plasma, including those associated with lipid storage, mobilization, oxidation and transport. Apolipoproteins dominated the blubber, plasma and muscle proteome responses to fasting. APOA4, APOE and APOC3, which are associated with lipogenesis and triglyceride accumulation, decreased, while APOA1, APOA2 and APOM, which are associated with lipid mobilization and HDL function, increased over fasting. Our findings suggest that changes in apolipoprotein composition may underlie the maintenance of high HDL levels and, together with adipokines and hepatokines that facilitate lipid catabolism, may mediate the metabolic transitions between feeding and fasting in NES. Many of these proteins have not been previously studied in this species and provide intriguing hypotheses about metabolic regulation during prolonged fasting in mammals.

Density-dependent effects on reproductive output in a capital breeding carnivore, the northern elephant seal (Mirounga angustirostris).

All organisms face resource limitations that will ultimately restrict population growth, but the controlling mechanisms vary across ecosystems, taxa, and reproductive strategies. Using four decades of data, we examine how variation in the environment and population density affect reproductive outcomes in a capital-breeding carnivore, the northern elephant seal (Mirounga angustirostris). This species provides a unique opportunity to examine the relative importance of resource acquisition and density-dependence on breeding success. Capital breeders accrue resources over large temporal and spatial scales for use during an abbreviated reproductive period. This strategy may have evolved, in part, to confer resilience to short-term environmental variability. We observed density-dependent effects on weaning mass, and maternal age (experience) was more important than oceanographic conditions or maternal mass in determining offspring weaning mass. Together these findings show that the mechanisms controlling reproductive output are conserved across terrestrial and marine systems and vary with population dynamics, an important consideration when assessing the effect of extrinsic changes, such as climate change, on a population.

Changes in serum adipokines during natural extended fasts in female northern elephant seals.

Adipose tissue is essential to endotherms for thermoregulation and energy storage as well as functioning as an endocrine organ. Adipose derived hormones, or adipokines, regulate metabolism, energy expenditure, reproduction, and immune function in model systems but are less well studied in wildlife. Female northern elephant seals (NES) achieve high adiposity during foraging and then undergo natural fasts up to five weeks long during haul-outs associated with reproduction and molting, resulting in large changes in adipose reserves. We measured circulating levels of four adipokines: leptin, resistin, adiponectin, and kisspeptin-54, in 196 serum samples from female NES at the beginning and end of their breeding and molting fasts. We examined the relationships between these adipokines and life-history stage, adiposity, mass, cortisol, and an immune cytokine involved in the innate immune response interleukin 6 (IL-6). All four adipokines varied with life-history stage. Leptin concentrations were highest at the beginning of the breeding haul-out. Resistin concentrations were higher throughout the breeding haul-out compared to the molt haul-out. Adiponectin concentrations were highest at the beginning of both haul-outs. Kisspeptin-54 concentrations were highest at the end of the breeding haul-out. Leptin, resistin, and adiponectin were associated with measures of body condition, either adiposity, mass, or both. Resistin, adiponectin, and kisspeptin-54 were associated with circulating cortisol concentrations. Resistin was strongly associated with circulating IL-6, a multifunctional cytokine. Adiponectin was associated with glucose concentrations, suggesting a potential role in tissue-specific insulin sensitivity during life-history stages categorized by high adiposity. Increased cortisol concentrations late in lactation were associated with increased kisspeptin-54, suggesting a link to ovulation initiation in NES. This study suggests dramatic changes in circulating adipokines with life-history and body condition that may exert important regulatory roles in NES. The positive relationship between adiponectin and adiposity as well as the lack of a relationship between leptin and kisspeptin-54 differed from model systems. These differences from biomedical model systems suggest the potential for modifications of expression and function of adipose-derived hormones in species that undergo natural changes in adiposity as part of their life-history.

Acceleration-triggered animal-borne videos show a dominance of fish in the diet of female northern elephant seals.

Knowledge of the diet of marine mammals is fundamental to understanding their role in marine ecosystems and response to environmental change. Recently, animal-borne video cameras have revealed the diet of marine mammals that make short foraging trips. However, novel approaches that allocate video time to target prey capture events is required to obtain diet information for species that make long foraging trips over great distances. We combined satellite telemetry and depth recorders with newly developed date-/time-, depth- and acceleration-triggered animal-borne video cameras to examine the diet of female northern elephant seals during their foraging migrations across the eastern North Pacific. We obtained 48.2 h of underwater video, from cameras mounted on the head (n=12) and jaw (n=3) of seals. Fish dominated the diet (78% of 697 prey items recorded) across all foraging locations (range: 37-55°N, 122-152°W), diving depths (range: 238-1167 m) and water temperatures (range: 3.2-7.4°C), while squid comprised only 7% of the diet. Identified prey included fish such as myctophids, Merluccius sp. and Icosteus aenigmaticus, and squid such as Histioteuthis sp., Octopoteuthis sp. and Taningia danae Our results corroborate fatty acid analysis, which also found that fish are more important in the diet, and are in contrast to stomach content analyses that found cephalopods to be the most important component of the diet. Our work shows that in situ video observation is a useful method for studying the at-sea diet of long-ranging marine predators.

What's in a whisker? Disentangling ecological and physiological isotopic signals.

RATIONALE: Stable isotope analysis of keratinized tissues is an informative tool for quantifying foraging ecology that can address questions related to niche specialization and temporal variation in behavior. Application of this approach relies on an understanding of tissue growth and how isotope ratios relate to physiological and ecological processes, data that are lacking for many species. METHODS: We collected paired whisker length measurements from northern elephant seals to estimate growth and shedding patterns (n = 16). A subset of seals (n = 5) carried a satellite tag and time-depth recorder across the 7+ month foraging trip following the annual pelage molt. Stable isotopes of carbon and nitrogen were measured in whisker segments grown across the 6+ week fasting on land and the subsequent foraging trip; profiles were combined with growth parameters to timestamp each segment and investigate relationships with foraging behavior. RESULTS: Whisker loss and initial regrowth primarily occurred during the annual pelage molt, but newly grown whiskers exhibited active, nonlinear growth across the foraging trip. The δ13 C and δ15 N values were higher in segments grown on land than at sea and exhibited a characteristic decline upon departure from the rookery. There was a relationship between latitude and longitude and δ15 N values, and individual whisker segments grown at sea could be classified to the correct ecoregion with 81% accuracy. CONCLUSIONS: Fasting affected both δ13 C and δ15 N values and the ability to exclude these values from ecological investigations is crucial given the temporal overlap with tissue growth. The rapid decline in isotope ratios upon departure can be used to isolate portions of the whisker with a strong physiological signal, even for whiskers with unknown growth histories. The active growth across the foraging trip combined with the ability to identify differences in foraging behavior validates the utility of this approach for addressing ecological questions.

The extra burden of motherhood: reduced dive duration associated with pregnancy status in a deep-diving mammal, the northern elephant seal.

The cost of pregnancy is hard to study in marine mammals, particularly in species that undergo pregnancy while diving continuously at sea such as elephant seals (genus Mirounga). We analysed the diving behaviour of confirmed pregnant and non-pregnant northern elephant seals (M. angustirostris, n = 172) and showed that after an initial continuous increase in dive duration, dives of pregnant females become shorter after week 17. The reasons for this reduction in dive duration remain unknown, but we hypothesize that increased fetal demand for oxygen could be the cause. Our findings reveal an opportunity to explore the use of biologging data to investigate pregnancy status of free-ranging marine mammals and factors that could affect pregnancy success.

Foraging behavior and age affect maternal transfer of mercury to northern elephant seal pups.

Deep ocean foraging northern elephant seals (Mirounga angustirostris) consume fish and squid in remote depths of the North Pacific Ocean. Contaminants bioaccumulated from prey are subsequently transferred by adult females to pups during gestation and lactation, linking pups to mercury contamination in mesopelagic food webs (200-1000 m depths). Maternal transfer of mercury to developing seal pups was related to maternal mercury contamination and was strongly correlated with maternal foraging behavior (biotelemetry and isotopes). Mercury concentrations in lanugo (hair grown in utero) were among the highest observed worldwide for young pinnipeds (geometric mean 23.01 µg/g dw, range 8.03-63.09 µg/g dw; n = 373); thus, some pups may be at an elevated risk of sub-lethal adverse health effects. Fetal mercury exposure was affected by maternal foraging geographic location and depth; mercury concentrations were highest in pups of the deepest diving, pelagic females. Moreover, pup lanugo mercury concentrations were strongly repeatable among successive pups of individual females, demonstrating relative consistency in pup mercury exposure based on maternal foraging strategies. Northern elephant seals are biosentinels of a remote deep-sea ecosystem. Our results suggest that mercury within North Pacific mesopelagic food webs may also pose an elevated risk to other mesopelagic-foraging predators and their offspring.

Effects of disease on foraging behaviour and success in an individual free-ranging northern elephant seal.

Evaluating consequences of stressors on vital rates in marine mammals is of considerable interest to scientific and regulatory bodies. Many of these species face numerous anthropogenic and environmental disturbances. Despite its importance as a critical form of mortality, little is known about disease progression in air-breathing marine megafauna at sea. We examined the movement, diving, foraging behaviour and physiological state of an adult female northern elephant seal (Mirounga angustirostris) who suffered from an infection while at sea. Comparing her to healthy individuals, we identified abnormal behavioural patterns from high-resolution biologging instruments that are likely indicators of diseased and deteriorating condition. We observed continuous extended (3-30 minutes) surface intervals coinciding with almost no foraging attempts (jaw motion) during 2 weeks of acute illness early in her post-breeding foraging trip. Elephant seals typically spend ~ 2 minutes at the surface. There were less frequent but highly extended (30-200 minutes) surface periods across the remainder of the trip. Dive duration declined throughout the trip rather than increasing. This seal returned in the poorest body condition recorded for an adult female elephant seal (18.3% adipose tissue; post-breeding trip average is 30.4%). She was immunocompromised at the end of her foraging trip and has not been seen since that moulting season. The timing and severity of the illness, which began during the end of the energy-intensive lactation fast, forced this animal over a tipping point from which she could not recover. Additional physiological constraints to foraging, including thermoregulation and oxygen consumption, likely exacerbated her already poor condition. These findings improve our understanding of illness in free-ranging air-breathing marine megafauna, demonstrate the vulnerability of individuals at critical points in their life history, highlight the importance of considering individual health when interpreting biologging data and could help differentiate between malnutrition and other causes of at-sea mortality from transmitted data.

Brain activity of diving seals reveals short sleep cycles at depth.

Sleep is a crucial part of the daily activity patterns of mammals. However, in marine species that spend months or entire lifetimes at sea, the location, timing, and duration of sleep may be constrained. To understand how marine mammals satisfy their daily sleep requirements while at sea, we monitored electroencephalographic activity in wild northern elephant seals (Mirounga angustirostris) diving in Monterey Bay, California. Brain-wave patterns showed that seals took short (less than 20 minutes) naps while diving (maximum depth 377 meters; 104 sleeping dives). Linking these patterns to accelerometry and the time-depth profiles of 334 free-ranging seals (514,406 sleeping dives) revealed a North Pacific sleepscape in which seals averaged only 2 hours of sleep per day for 7 months, rivaling the record for the least sleep among all mammals, which is currently held by the African elephant (about 2 hours per day).

Impacts of marine heatwaves on top predator distributions are variable but predictable.

Marine heatwaves cause widespread environmental, biological, and socio-economic impacts, placing them at the forefront of 21st-century management challenges. However, heatwaves vary in intensity and evolution, and a paucity of information on how this variability impacts marine species limits our ability to proactively manage for these extreme events. Here, we model the effects of four recent heatwaves (2014, 2015, 2019, 2020) in the Northeastern Pacific on the distributions of 14 top predator species of ecological, cultural, and commercial importance. Predicted responses were highly variable across species and heatwaves, ranging from near total loss of habitat to a two-fold increase. Heatwaves rapidly altered political bio-geographies, with up to 10% of predicted habitat across all species shifting jurisdictions during individual heatwaves. The variability in predicted responses across species and heatwaves portends the need for novel management solutions that can rapidly respond to extreme climate events. As proof-of-concept, we developed an operational dynamic ocean management tool that predicts predator distributions and responses to extreme conditions in near real-time.

Animal personality: Worn whiskers reveal resilience.

Consistent individual differences in behavior, or personality, may buffer populations against environmental changes. A long-term study of Galápagos sea lions reveals foraging polymorphisms with different levels of reproductive resilience as ocean temperatures increase.

Forced into an ecological corner: Round-the-clock deep foraging on small prey by elephant seals.

Small mesopelagic fishes dominate the world's total fish biomass, yet their ecological importance as prey for large marine animals is poorly understood. To reveal the little-known ecosystem dynamics, we identified prey, measured feeding events, and quantified the daily energy balance of 48 deep-diving elephant seals throughout their oceanic migrations by leveraging innovative technologies: animal-borne smart accelerometers and video cameras. Seals only attained positive energy balance after feeding 1000 to 2000 times per day on small fishes, which required continuous deep diving (80 to 100% of each day). Interspecies allometry suggests that female elephant seals have exceptional diving abilities relative to their body size, enabling them to exploit a unique foraging niche on small but abundant mesopelagic fish. This unique foraging niche requires extreme round-the-clock deep diving, limiting the behavioral plasticity of elephant seals to a changing mesopelagic ecosystem.

Changes in Northern Elephant Seal Skeletal Muscle Following Thirty Days of Fasting and Reduced Activity.

Northern elephant seals (NES, Mirounga angustirostris) undergo an annual molt during which they spend ∼40 days fasting on land with reduced activity and lose approximately one-quarter of their body mass. Reduced activity and muscle load in stereotypic terrestrial mammalian models results in decreased muscle mass and capacity for force production and aerobic metabolism. However, the majority of lost mass in fasting female NES is from fat while muscle mass is largely preserved. Although muscle mass is preserved, potential changes to the metabolic and contractile capacity are unknown. To assess potential changes in NES skeletal muscle during molt, we collected muscle biopsies from 6 adult female NES before the molt and after ∼30 days at the end of the molt. Skeletal muscle was assessed for respiratory capacity using high resolution respirometry, and RNA was extracted to assess changes in gene expression. Despite a month of reduced activity, fasting, and weight loss, skeletal muscle respiratory capacity was preserved with no change in OXPHOS respiratory capacity. Molt was associated with 162 upregulated genes including those favoring lipid metabolism. We identified 172 downregulated genes including those coding for ribosomal proteins and genes associated with skeletal muscle force transduction and glucose metabolism. Following ∼30 days of molt, NES skeletal muscle metabolic capacity is preserved although mechanotransduction may be compromised. In the absence of exercise stimulus, fasting-induced shifts in muscle metabolism may stimulate pathways associated with preserving the mass and metabolic capacity of slow oxidative muscle.