Influence of maternal characteristics and oceanographic conditions on survival and recruitment probabilities of Weddell seals

Seasonal habitat preference and foraging behaviour of post-moult Weddell seals in the western Ross Sea

Weddell seals (Leptonychotes weddellii) are important predators in the Southern Ocean and are among the best-studied pinnipeds on Earth, yet much still needs to be learned about their year-round movements and foraging behaviour. Using biologgers, we tagged 62 post-moult Weddell seals in McMurdo Sound and vicinity between 2010 and 2012. Generalized additive mixed models were used to (i) explain and predict the probability of seal presence and foraging behaviour from eight environmental variables, and (ii) examine foraging behaviour in relation to dive metrics. Foraging probability was highest in winter and lowest in summer, and foraging occurred mostly in the water column or just above the bottom; across all seasons, seals preferentially exploited the shallow banks and deeper troughs of the Ross Sea, the latter providing a pathway for Circumpolar Deep Water to flow onto the shelf. In addition, the probability of Weddell seal occurrence and foraging increased with increasing bathymetric slope and where water depth was typically less than 600 m. Although the probability of occurrence was higher closer to the shelf break, foraging was higher in areas closer to shore and over banks. This study highlights the importance of overwinter foraging for recouping body mass lost during the previous summer.

Reproductive senescence in polar bears in a variable environment

Reproductive senescence is ubiquitous in mammals. However, patterns of senescence vary across reproductive traits, even within populations, perhaps because of differences in selection pressures, physiological constraints, and responses to environmental conditions. We investigated reproductive senescence in wild female polar bears (Ursus maritimus), using 31 years of capture-recapture data from the Svalbard area. We studied the influence of environmental conditions on age-specific litter production and litter size using generalized linear mixed models. Further, using a capture-recapture model that handles the dependency between vital rates of individuals belonging to the same family unit, we assessed maternal-age-related changes in first year cub and litter survival. We provide clear evidence for reproductive senescence in female polar bears. Litter production and litter size peaked in middle-aged females and declined sharply afterward. By contrast cub and litter survival did not decline after prime age. We found no evidence of terminal investment. The reproductive output of all females was affected by sea-ice conditions during the previous year and the Arctic Oscillation, with some effects differing greatly between age groups. Old females were affected the most by environmental conditions. Our results suggest that the decline in reproductive output is a combination of fertility and body-condition senescence, with a weak contribution of maternal-effect senescence, possibly due to benefits of experience. Further, as predicted by evolutionary theory, senescence appears to be a consequence of failures in early stages of the reproductive cycle rather than in late stages, and environmental variation affected old females more than prime-aged females. Our study emphasizes the need to study several reproductive traits and account for environmental variation when investigating reproductive senescence. Differences in senescence patterns across reproductive traits should be interpreted in light of evolutionary theory and while considering underlying physiological drivers.

Iron mobilization during lactation reduces oxygen stores in a diving mammal

The profound impacts that maternal provisioning of finite energy resources has on offspring survival have been extensively studied across mammals. This study shows that in addition to calories, high hemoprotein concentrations in diving mammals necessitates exceptional female-to-pup iron transfer. Numerous indices of iron mobilization (ferritin, serum iron, total-iron-binding-capacity, transferrin saturation) were significantly elevated during lactation in adult female Weddell seals (Leptonychotes weddellii), but not in skip-breeders. Iron was mobilized from endogenous stores for incorporation into the Weddell seal’s milk at concentrations up to 100× higher than terrestrial mammals. Such high rates of iron offload to offspring drew from the female’s own heme stores and led to compromised physiologic dive capacities (hemoglobin, myoglobin, and total body oxygen stores) after weaning their pups, which was further reflected in shorter dive durations. We demonstrate that lactational iron transfer shapes physiologic dive thresholds, identifying a cost of reproduction to a marine mammal.

Here, the authors show that Weddell seal mothers mobilize endogenous iron stores during lactation to provide to pups, resulting in iron concentrations in milk 100x higher than terrestrial mammals. This was associated with reduced dive durations in the mother, a cost of reproduction.

Biomarkers of Animal Nutrition: From Seasonal to Lifetime Indicators of Environmental Conditions

Nutrition underpins survival and reproduction in animal populations; reliable nutritional biomarkers are therefore requisites to understanding environmental drivers of population dynamics. Biomarkers vary in scope of inference and sensitivity, making it important to know what and when to measure to properly quantify biological responses. We evaluated the repeatability of three nutritional biomarkers in a large, iteroparous mammal to evaluate the level of intrinsic and extrinsic contributions to those traits. During a long-term, individual-based study in a highly variable environment, we measured body fat, body mass, and lean mass of mule deer (Odocoileus hemionus) each autumn and spring. Lean mass was the most repeatable biomarker (0.72 autumn; 0.61 spring), followed by body mass (0.64 autumn; 0.53 spring), and then body fat (0.22 autumn; 0.01 spring). High repeatability in body and lean mass likely reflects primary structural composition, which is conserved across seasons. Low repeatability of body fat supports that it is the primary labile source of energy that is largely a product of environmental contributions of the previous season. Based on the disparate levels in repeatability among nutritional biomarkers, we contend that body and lean mass are better indicators of nutritional legacies (e.g., maternal effects), whereas body fat is a direct and sensitive reflection of recent nutritional gains and losses.

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.

Early diving behavior in Weddell seal (Leptonychotes weddellii) pups

During the dependency period in mammals with parental care, offspring must develop the behavioral skills that allow them to forage independently and thus survive into early adulthood. Deep-diving Weddell seals (Leptonychotes weddellii) are a model species for research on diving physiology, yet previous studies lack a thorough investigation into the diving behavior of dependent pups when they first begin to enter the water. To capture fine-scale dive behavior during the dependency period, we deployed time-depth recorders (TDRs) on Weddell seal pups (n = 18) from the age of 1 week through 7 weeks, during the 2017 and 2019 breeding seasons in McMurdo Sound, Antarctica. Dive parameters were correlated with time of day, age, sex, and weaning status, to characterize diving behavior, and we used raw wet/dry data to determine which times of the day pups were most likely to be in the water. Pups made their deepest and longest dives and had the longest post-dive durations in the morning hours. Pups were in the water more during the late night and morning hours than in the afternoon. Whereas dive depth significantly increased with age, dive duration significantly increased with age and after weaning. Post-dive duration significantly decreased with age and after weaning. We discuss how dependent pups may prioritize the development of swimming and navigational abilities as opposed to building and practicing foraging skills.

Temporal changes in Weddell seal dive behavior over winter: Are females increasing foraging effort to support gestation?

In capital-breeding marine mammals, prey acquisition during the foraging trip coinciding with gestation must provide energy to meet the immediate needs of the growing fetus and also a store to meet the subsequent demands of lactation. Weddell seals (Leptonychotes weddellii) that give birth following the gestational (winter) foraging period gain similar proportions of mass and lipid as compared to females that fail to give birth. Therefore, any changes in foraging behavior can be attributed to gestational costs. To investigate differences in foraging effort associated with successful reproduction, twenty-three satellite tags were deployed on post-molt female Weddell seals in the Ross Sea. Of the 20 females that returned to the area the following year, 12 females gave birth and eight did not. Females that gave birth the following year began the winter foraging period with significantly longer and deeper dives, as compared to non-reproductive seals. Mid- to late winter, reproductive females spent a significantly greater proportion of the day diving, and either depressed their diving metabolic rates (DMR), or exceeded their calculated aerobic dive limit (cADL) more frequently than females that returned without a pup. Moreover, non-reproductive females organized their dives into 2-3 short bouts per day on average (BOUTshort; 7.06 ± 1.29 hr; mean ± 95% CI), whereas reproductive females made 1-2 BOUTshort per day (10.9 ± 2.84 hr), comprising one long daily foraging bout without rest. The magnitude of the increase in dive activity budgets and depression in calculated DMR closely matched the estimated energetic requirements of supporting a fetus. This study is one of the first to identify increases in foraging effort that are associated with successful reproduction in a top predator and indicates that reproductive females must operate closer to their physiological limits to support gestational costs.

Forest structure provides the income for reproductive success in a southern population of Canada lynx

Understanding intrinsic and extrinsic drivers of reproductive success is central to advancing animal ecology and characterizing critical habitat. Unfortunately, much of the work examining drivers of reproductive success is biased toward particular groups of organisms (e.g., colonial birds, large herbivores, capital breeders). Long-lived mammalian carnivores that are of conservation concern, solitary, and territorial present an excellent situation to examine intrinsic and extrinsic drivers of reproductive success, yet they have received little attention. Here, we used a Canada lynx (Lynx canadensis) data set, from the southern periphery of their range, to determine if reproductive success in a solitary carnivore was consistent with capital or income breeding. We radio-marked and monitored 36 female Canada lynx for 98 lynx years. We evaluated how maternal characteristics and indices of food supply (via forest structure) in core areas influenced variation in body condition and reproductive success. We characterized body condition as mass/length and reproductive success as whether a female produced a litter of kittens for a given breeding season. Consistent with life-history theory, we documented a positive effect of maternal age on body condition and reproductive success. In contrast to predictions of capital breeding, we observed no effect of pre-pregnancy body condition on reproductive success in Canada lynx. However, we demonstrated statistical effects of forest structure on reproductive success in Canada lynx, consistent with predictions of income breeding. The forest characteristics that defined high success included (1) abundant and connected mature forest and (2) intermediate amounts of small-diameter regenerating forest. These attributes are consistent with providing abundant, temporally stable, and accessible prey resources (i.e., snowshoe hares; Lepus americanus) for lynx and reinforce the bottom-up mechanisms influencing Canada lynx populations. Collectively, our results suggest that lynx on the southern range periphery exhibit an income breeding strategy and that forest structure supplies the income important for successful reproduction. More broadly, our insights advance the understanding of carnivore ecology and serve as an important example on integrating long-term field studies with ecological theory to improve landscape management.

Coping with the loss of large, energy-dense prey: a potential bottleneck for Weddell Seals in the Ross Sea

Extraction of Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea began in 1997, following a management plan that targets the largest fish with a goal of reducing the spawning biomass by 50% over 35 yr. We investigate the potential long-term consequences of the reduced availability of this prey for Weddell seals (Leptonychotes weddellii). Energy demands in seals are acute, especially immediately following lactation, when females must recover substantial mass and cope with molting costs. We tested the hypothesis that toothfish are critically important for adult female seals during this period. Toothfish body mass is three orders of magnitude greater, and its energy density nearly double that of the most common seal prey, Antarctic silverfish (Pleuragramma antarcticum). Reduction or elimination of toothfish consumption could impair a female's ability to sufficiently recover and successfully produce a pup in the following pupping season. Our goals are to (1) illustrate mechanisms and conditions whereby toothfish depletion might plausibly affect seal population trends; (2) identify measurable parameters of the seals' ecology that may help better understand the potential negative impact of toothfish depletion on seal populations; and (3) promote a precautionary management approach for the fishery that includes monitoring of seal populations We constructed a set of inter-linked models of seal diving behavior, physiological condition, and demography based on existing information. We evaluate the effect of the following factors on seal mass recovery and intrinsic population growth rates: fishery depletion rate, daily diving limits, probability of a successful dive, and body mass recovery target. We show that loss of toothfish has the greatest potential impact on seal populations' growth rate. Under some scenarios, populations may decrease at >10% per year. Critical parameters to better understand fishery impacts include prevalence and size of toothfish in the seals' diet; the relationship between diet and the rate of mass recovery; and female breeding propensity in relation to body condition at the end of the molting period. Our results lend support to concerns about the potential negative impact of toothfish extraction in the Ross Sea; and to advocate for a precautionary management approach by the fishery.

Population Biology of Aging in the Wild

Empirical studies reveal aging occurs in wild populations. Consideration of the ecological and evolutionary consequences of these findings is critical for many areas of research, including life-history evolution, sexual selection, behavior, and applied ecology. Variation in the patterns of age-dependent declines of phenotypic traits has been found both within and among individuals, and this raises future questions aimed at understanding what determines these trajectories across traits and across the tree of life. The presence of older, aging, individuals in populations can have transgenerational effects on offspring and can influence how individuals interact. In some species older individuals in populations can have positive impacts, influencing knowledge and leadership, postreproductive care, and population cycle stabilization. Aging and long life span also need to be recognized in an applied ecology context including management plans, vector-borne disease transmission, and ecotoxicology.

The effects of birth weight and maternal care on survival of juvenile Steller sea lions (Eumetopias jubatus)

Steller sea lions were listed as endangered following a collapse of the western distinct population beginning in the late 1970s. Low juvenile survival has been implicated as a factor in the decline. I conducted a multistate mark-recapture analysis to estimate juvenile survival in an area of the western population where sea lions are showing signs of recovery. Survival for males and females was 80% between 3 weeks and 1 year of age. Approximately 20% of juveniles continued to be nursed by their mothers between ages 1 and 2 and 10% between ages 2 and 3. Survival for juveniles that suckled beyond 1 year was 88.2% and 89.9% to ages 2 and 3, respectively. In contrast, survival for individuals weaned by age 1 was 40.6% for males and 64.2% for females between ages 1 and 2. Birth mass positively influenced survival for juveniles weaned at age 1 but had little effect on individuals continuing to suckle. Cumulative survival to age 4 was double that estimated during the population decline in this region. Evidence suggests that western Steller sea lions utilize a somewhat different maternal strategy than those in the eastern distinct population. Western adult females generally invest more in their pups during the first year but wean offspring by age 1 more often. This results in better survival to age 1, but greater mortality between ages 1 and 3 compared to the eastern population. Different maternal strategies may reflect density dependent pressures of populations at opposite levels of abundance.

Individual heterogeneity in reproductive rates and cost of reproduction in a long-lived vertebrate

Individual variation in reproductive success is a key feature of evolution, but also has important implications for predicting population responses to variable environments. Although such individual variation in reproductive outcomes has been reported in numerous studies, most analyses to date have not considered whether these realized differences were due to latent individual heterogeneity in reproduction or merely random chance causing different outcomes among like individuals. Furthermore, latent heterogeneity in fitness components might be expressed differently in contrasted environmental conditions, an issue that has only rarely been investigated. Here, we assessed (i) the potential existence of latent individual heterogeneity and (ii) the nature of its expression (fixed vs. variable) in a population of female Weddell seals (Leptonychotes weddellii), using a hierarchical modeling approach on a 30-year mark–recapture data set consisting of 954 individual encounter histories. We found strong support for the existence of latent individual heterogeneity in the population, with “robust” individuals expected to produce twice as many pups as “frail” individuals. Moreover, the expression of individual heterogeneity appeared consistent, with only mild evidence that it might be amplified when environmental conditions are severe. Finally, the explicit modeling of individual heterogeneity allowed us to detect a substantial cost of reproduction that was not evidenced when the heterogeneity was ignored.

Reproductive improvement and senescence in a long-lived bird

Heterogeneity within a population is a pervasive challenge for studies of individual life-histories. Population-level patterns in age-specific reproductive success can be broken down into relative contributions from selective disappearance, selective appearance of individuals into the study population, and average change in performance for survivors (average ontogenetic development). In this article, we provide an exact decomposition. We apply our formula to data on the reproductive performance of a well characterized population of common terns (Sterna hirundo). We show that improvements with age over most of adult life and senescence at old ages are primarily due to a genuine change in the mean among surviving individuals rather than selective disappearance or selective appearance of individuals. Average ontogenetic development accounts for approximately 87% of the overall age-specific population change.

Parent age, lifespan and offspring survival: structured variation in life history in a wild population

1. Understanding the degree to which reproductive success varies with an individual's age and lifespan, and the degree to which population-level variation mirrors individual-level variation, is central to understanding life-history evolution and the dynamics of age-structured populations. We quantified variation in the survival probability of offspring, one key component of reproductive success and fitness, in relation to parent age and lifespan in a wild population of red-billed choughs (Pyrrhocorax pyrrhocorax). 2. On average across the study population, the first-year survival probability of offspring decreased with increasing parent age and lifespan; offspring of old parents were less likely to survive than offspring of young parents, and offspring of long-lived parents were less likely to survive than offspring of short-lived parents. 3. However, survival did not vary with parent age across offspring produced by groups of parents that ultimately had similar lifespans. 4. Rather, across offspring produced by young parents, offspring survival decreased with increasing parent lifespan; parents that ultimately had long lifespans produced offspring that survived poorly, even when these parents were breeding at young ages. 5. The average decrease in offspring survival with increasing parent age observed across the population therefore reflected the gradual disappearance of short-lived parents that produced offspring that survived well, not age-specific variation in offspring survival within individual parents. 6. The negative correlation between offspring survival and maternal lifespan was strongest when environmental conditions meant that offspring survival was low across the population. 7. These data suggest an environment-dependent trade-off between parent and offspring survival, show consistent individual variation in the resolution of this trade-off that is set early in a parent's life, and demonstrate that such structured life-history variation can generate spurious evidence of senescence in key fitness components when measured across a population.

An evaluation of density-dependent and density-independent influences on population growth rates in Weddell seals

Much of the existing literature that evaluates the roles of density-dependent and density-independent factors on population dynamics has been called into question in recent years because measurement errors were not properly dealt with in analyses. Using state-space models to account for measurement errors, we evaluated a set of competing models for a 22-year time series of mark-resight estimates of abundance for a breeding population of female Weddell seals (Leptonychotes weddellii) studied in Erebus Bay, Antarctica. We tested for evidence of direct density dependence in growth rates and evaluated whether equilibrium population size was related to seasonal sea-ice extent and the Southern Oscillation Index (SOI). We found strong evidence of negative density dependence in annual growth rates for a population whose estimated size ranged from 438 to 623 females during the study. Based on Bayes factors, a density-dependence-only model was favored over models that also included environmental covariates. According to the favored model, the population had a stationary distribution with a mean of 497 females (SD = 60.5), an expected growth rate of 1.10 (95% credible interval = 1.08-1.15) when population size was 441 females, and a rate of 0.90 (95% credible interval = 0.87-.93) for a population of 553 females. A model including effects of SOI did receive some support and indicated a positive relationship between SOI and population size. However, effects of SOI were not large, and including the effect did not greatly reduce our estimate of process variation. We speculate that direct density dependence occurred because rates of adult survival, breeding, and temporary emigration were affected by limitations on per capita food resources and space for parturition and pup-rearing. To improve understanding of the relative roles of various demographic components and their associated vital rates to population growth rate, mark-recapture methods can be applied that incorporate both environmental covariates and the seal abundance estimates that were developed here. An improved understanding of why vital rates change with changing population abundance will only come as we develop a better understanding of the processes affecting marine food resources in the Southern Ocean.

Nonlinear effects of winter sea ice on the survival probabilities of Adélie penguins

The population dynamics of Antarctic seabirds are influenced by variations in winter sea ice extent and persistence; however, the type of relationship differs according to the region and the demographic parameter considered. We used annual presence/absence data obtained from 1,138 individually marked birds to study the influence of environmental and individual characteristics on the survival of Adélie penguins Pygoscelis adeliae at Edmonson Point (Ross Sea, Antarctica) between 1994 and 2005. About 25% of 600 birds marked as chicks were reobserved at the natal colony. The capture and survival rates of Adélie penguins at this colony increased with the age of individuals, and five age classes were identified for both parameters. Mean adult survival was 0.85 (SE = 0.01), and no effect of sex on survival was evident. Breeding propensity, as measured by adult capture rates, was close to one, indicating a constant breeding effort through time. Temporal variations in survival were best explained by a quadratic relationship with winter sea ice extent anomalies in the Ross Sea, suggesting that for this region optimal conditions are intermediate between too much and too little winter sea ice. This is likely the result of a balance between suitable wintering habitat and food availability. Survival rates were not correlated with the Southern Oscillation Index. Low adult survival after a season characterized by severe environmental conditions at breeding but favorable conditions during winter suggested an additional mortality mediated by the reproductive effort. Adélie penguins are sensitive indicators of environmental changes in the Antarctic, and the results from this study provide insights into regional responses of this species to variability in winter sea ice habitat.