Birth-year and current-year influences on survival and recruitment rates of female Weddell seals

In long-lived species, juvenile survival typically is lower and more variable than adult survival, and modeling such variation is important for understanding population dynamics. Variability in juvenile survival can be related to birth- or current-year influences, and the birth-year influences can be transient, persistent, or intermediate in duration. We used multi-state models and data collected from 5,459 known-aged prebreeder female Weddell seals (Leptonychotes weddellii Lesson) tagged in Erebus Bay, Antarctica from 1980–2007 to evaluate the duration of potential birth-year influences on survival rates and the importance of birth- and current-year influences on survival and recruitment rates. Survival rates differed for each birth cohort and were positively related to current-year winter sea-ice conditions. The estimated duration of birth-cohort effects on survival was intermediate (6 years) rather than transient (2 years) or permanent. Estimated survivorship from birth to 6 years of age varied among cohorts from 0.13 (SE = 0.04) to 0.42 (SE = 0.06), and averaged 0.25 (SE = 0.02). Recruitment rates (probability of transitioning from prebreeder to breeder state) varied annually but apparently were not related to birth-year conditions. Our results provide evidence that birth- and current-year conditions act in combination to influence survival. Although for many long-lived species the influences of either birth- or current-year conditions on survival are well-studied, we suggest that modeling survival rates as a function of birth- and current-year influences simultaneously could lead to better understanding of survival and improved stochastic models to project population dynamics.     

Influence of maternal mass and condition on energy transfer in Weddell seals

1. Environmental variation influences food abundance and availability, which is reflected in the reproductive success of top predators. We examined maternal expenditure, offspring mass and condition for Weddell seals in 2 years when individuals exhibited marked differences in these traits. 2. For females weighing > or = 355 kg there was a positive relationship between maternal post-partum mass (MPPM) and lactation length, but below this there was no relationship, suggesting that heavier females were able to increase lactation length but lighter females were restricted to a minimum lactation period of 33 days. 3. Overall, females were heavier in 2002, but in 2003 shorter females were lighter than similar-sized females in 2002 suggesting that the effects of environmental variability on foraging success and condition are more pronounced in smaller individuals. 4. There was no relationship between MPPM and pup birth mass, indicating pre-partum investment did not differ between years. However, there was a positive relationship between MPPM and pup mass gain. Mass and energy transfer efficiency were 10.2 and 5.4% higher in 2002 than 2003, which suggests costs associated with a putatively poor-resource year were delayed until lactation. 5. Heavier females lost a higher proportion of mass during lactation in both years, so smaller females may not have been able to provide more to their offspring to wean a pup of similar size to larger females. 6. MPPM had only a small influence on total body lipid; therefore, regardless of mass, females had the same relative body composition. Females with male pups lost a higher percentage of lipid than those with female pups, but by the end of lactation female pups had 4.5% higher lipid content than males. 7. It appears that for Weddell seals the consequences of environmentally induced variation in food availability are manifested in differences in maternal mass and expenditure during lactation. These differences translate to changes in pup mass and condition at weaning with potential consequences for future survival and recruitment.     

Spatial variation in age-specific probabilities of first reproduction for Weddell seals

Spatial variation in vital rates can affect the dynamics and persistence of a population. We evaluated the prediction that age-specific probabilities of survival and first reproduction for Weddell seals would vary as a function of birth location in Erebus Bay, Antarctica. We used multi-state mark–resight models and 25 years of data to estimate demographic rates for female seals. We predicted that probabilities of survival and first reproduction would be higher for seals born at near-shore colonies or more southerly-located colonies with consistent ice conditions. Contrary to predictions, results revealed higher age-specific probabilities of first reproduction at offshore colonies relative to near-shore colonies and no spatial variation in survival rates. For 7-year old females (average age at 1st reproduction=7.6 years old) born at offshore colonies to mothers aged 10.8 years (average maternal age), probability of first reproduction was 0.43 (SE=0.07), whereas probability of first reproduction for females born at near-shore colonies was 0.30 (SE=0.05) based on estimates from our top-ranked model. Breeding probabilities following first reproduction were also higher at offshore colonies. Thus, our results (1) provide evidence of spatial variation in breeding probabilities, (2) reveal the importance of birth location on a female's vital rates, and (3) suggest that the effect persisted for many years. Birth-colony effects may be attributed to spatial variation in prey availability, or to heterogeneity in female quality in this population. If females who are superior competitors consistently chose offshore colonies for pupping, pups born at these locations may have inherited those superior qualities and displayed higher probabilities of first reproduction, relative to seals born at other colonies. Further research into physical or food-related differences among colonies may offer insight into spatial variation in breeding probabilities documented in this paper.     

Effects of pup age,maternal age,and birth date on pre‐weaning survival rates of Weddell seals in Erebus Bay,Antarctica

Identifying factors affecting juvenile survival is important to understanding the dynamics of populations and may also provide insights into life history theory and the selective forces shaping evolution. Quantifying the relative influence of the various potential selective forces for the post‐birth, maternal dependency, and independent periods is difficult and often limits investigators to estimating a single juvenile survival rate for the first year of life, or from birth to recruitment. We examined survival of individually marked Weddell seal Leptonychotes weddellii pups during the maternal dependency period in Erebus Bay, Antarctica from 2005–2007. We used mark‐recapture models to evaluate competing a priori hypotheses regarding variation in daily pre‐weaning survival rates (φ) during the first 42 days of age. The a priori model receiving the most support from the data supported several of our predictions: increased with pup age and was higher for pups born later in the season and to older mothers. Increases in with increasing pup age may have been due to improved resilience to the environment, phenotypic selection against the frailest pups, or both. Maternal age was more important to than was maternal experience or age of primiparity, potentially indicating that age‐related increases in body mass allow greater offspring provisioning, or age‐related improvements in competitive abilities benefit offspring during the period of maternal care. Depending on the timing of birth and the age of the mother, ^{42 days} ranged from 0.79 (SE = 0.05) to 0.98 (SE = 0.01). These exceptionally high pre‐weaning survival rates contrast with estimates from other large terrestrial and marine mammal species where neonate survival is considerably lower and suggest that in species with similar life histories, pre‐weaning survival probability may be buffered from both predators and environmental fluctuations during the period of maternal nutritional dependency. Climatic changes affecting stability of ice used for pupping substrate or extent of fast‐ice buffering pupping colonies from predators have the potential to reduce pre‐weaning survival and may have important implications for population growth rates.     

Blood rheology of Weddell seals and bowhead whales

Red blood cell (RBC) aggregation and blood viscosity are important determinants of in vivo blood flow dynamics and, in marine mammals, these parameters may impact diving physiology by altering blood oxygen delivery during the diving response. Weddell seals are superb divers and exhibit age-related patterns in blood oxygen chemistry and diving ability. By contrast, bowhead whales are not long duration divers, and little is known of their blood properties relative to diving. The present study was designed to compare rheological characteristics of blood from Weddell seal pups, Weddell seal adults, and from adult bowhead whales: blood viscosity and RBC aggregation in plasma and in polymer solutions (i.e., RBC "aggregability") were measured. Salient findings included: (1) significant 4- to 8-fold greater aggregation in blood from adult seals compared with pups and human subjects; (2) 2-to 8-fold greater aggregation in bowhead whale blood compared with human blood; (3) compared to human red cells, enhanced RBC aggregability of RBC from adult seals and whales as determined by their greater aggregation in polymer solutions; (4) increasing RBC aggregation and aggregability of seal pup blood over a seven day period following birth; (5) significantly greater blood viscosity for adult seals compared with pups at both native and standardized hematocrits. These results indicate that, for both species, hemorheological parameters differ markedly from those of humans, and suggest progressive changes with seal age; the physiological implications of these differences have yet to be fully defined.     

Three-dimensional dive profiles of free-ranging Weddell seals

We used an acoustic tracking system to record under-ice movements of two free-ranging adult male Weddell seals. The two males were unconstrained and interacting with other Weddell seals at a breeding colony in McMurdo Sound, Antarctica. We reconstructed three-dimensional paths of 279 dives by these seals. All dives were less than 20-min duration and none were deeper than 220 m. These three-dimensional dive profiles were compared with conventional time-depth dive profiles recorded using microprocessor loggers. We assigned each of the 279 dives to 1 of 6 classes using an existing classification scheme on the basis of the time-depth trace. Within these, two-dimensionally derived, classes the actual three-dimensional dive profiles at times varied profoundly. Additional parameters obtained with the acoustic system, such as bearing and distance travelled between diving and surfacing points, demonstrate that significant, additional, biologically important information can be derived from the three-dimensional data. Accepted: 19 December 1999     

Serum chemistry and antibodies against pathogens in antarctic fur seals, Weddell seals, crabeater seals, and Ross seals

Information on health parameters, such as antibody prevalences and serum chemistry that can reveal exposure to pathogens, disease, and abnormal physiologic conditions, is scarce for Antarctic seal species. Serum samples from Antarctic fur seals (Arctocephalus gazella, n=88) from Bouvet?ya (2000-2001 and 2001-2002), and from Weddell seals (Leptonychotes weddellii, n=20), Ross seals (Ommatophoca rossii, n=20), and crabeater seals (Lobodon carcinophagus, n=9) from the pack-ice off Queen Maud Land, Antarctica (2001) were analyzed for enzyme activity, and concentrations of protein, metabolites, minerals, and cortisol. Adult Antarctic fur seal males had elevated levels of total protein (range 64-99 g/l) compared to adult females and pups (range 52-79 g/l). Antarctic fur seals had higher enzyme activities of creatine kinase, lactate dehydrogenase, and amylase, compared to Weddell, Ross, and crabeater seals. Antibodies against Brucella spp. were detected in Weddell seals (37%), Ross seals (5%), and crabeater seals (11%), but not in Antarctic fur seals. Antibodies against phocine herpesvirus 1 were detected in all species examined (Antarctic fur seals, 58%; Weddell seals, 100%; Ross seals, 15%; and crabeater seals, 44%). No antibodies against Trichinella spp., Toxoplasma, or phocine distemper virus (PDV) were detected (Antarctic fur seals were not tested for PDV antibodies). Antarctic seals are challenged by reduced sea ice and increasing temperatures due to climate change, and increased anthropogenic activity can introduce new pathogens to these vulnerable ecosystems and represent a threat for these animals. Our data provide a baseline for future monitoring of health parameters of these Antarctic seal species, for tracking the impact of environmental, climatic, and anthropogenic changes in Antarctica over time.     

A safe and practical inhalation anaesthesia for Weddell seals

Nine adult female Weddell seals were anaesthetized during the breeding season using the inhalation anaesthetic sevoflurane. The seals were captured with a head bag. Sevoflurane was administered and vaporized into the bag. A dose of 5 ml sevoflurane induced narcosis 1 min after application. Anaesthesia was maintained by multiple follow-up doses of 2 ml at an interval of 2 min. The average duration for complete recovery was 11Dž min after the terminal application. Sevoflurane is a reliable volatile anaesthetic that requires a minimum of equipment even under the low temperature regime of Antarctica.     

Microcomputer monitor and blood sampler for free-diving Weddell seals

The equipment used for the first sampling of arterial blood at depth on free-diving Weddell seals Leptonychotes weddelli is described. Blood was withdrawn through an aortic catheter by a submersible, peristaltic roller pump and stored in a single- or multiple-sample collection device. The multiple sampler allowed up to eight individual blood samples to be collected during a single dive. The blood pump was controlled by a dedicated microcomputer that allowed initiation of blood sampling at flexible combinations of depth and/or time during either the descending or ascending phase of the dive. The dedicated microcomputer also recorded swimming depth, velocity, heart rate, and body temperature at selectable time intervals. These data were transmitted to a laboratory computer, and blood samples were retrieved, when the seal surfaced to breathe.     

Successful foraging zones of southern elephant seals from the Kerguelen Islands in relation to oceanographic conditions

Southern elephant seals, Mirounga leonina, undertake large-scale oceanic movements to access favourable foraging areas. Successful foraging areas of elephant seals from the Kerguelen Islands are investigated here in relation to oceanographic parameters. Movements and diving activity of the seals as well as oceanographic data were collected through a new generation of satellite relayed devices measuring and transmitting locations, pressure, temperature and salinity. For the first time, we have associated foraging behaviour, determined by high increased sinuosity in tracks, and dive density (i.e. number of dives performed per kilometre covered), and changes in body condition, determined by variations in drift rate obtained from drift dives, to identify the oceanographic conditions of successful foraging zones for this species. Two main sectors, one close to the Antarctic continent and the other along the Polar Front (PF), where both foraging activity and body condition increase, seem to be of particular interest for the seals. Within these regions, some seals tended to focus their foraging activity on zones with particular temperature signatures. Along the Antarctic continent, some seals targeted colder waters on the sea bottom during benthic dives, while at the PF the favourable zones tended to be warmer. The possible negative effect of colder waters in Antarctic on the swimming performances of potential fish or squid prey could explain the behaviour of elephant seals in these zones, while warmer waters within the PF could correspond to the optimal conditions for potential myctophid prey of elephant seals.     

Hemoglobin concentrations and blood gas tensions of free-diving Weddell seals

Arterial blood gas tensions, pH, and hemoglobin concentrations were measured in four free-diving Weddell seals Leptonychotes weddelli. A microprocessor-controlled sampling system enabled us to obtain 24 single and 31 serial aortic blood samples. The arterial O2 tension (PaO2) at rest [78 +/- 13 (SD) Torr] increased with diving compression to a maximum measured value of 232 Torr and then rapidly decreased to 25-35 Torr. The lowest diving PaO2 we measured was 18 Torr just before the seal surfaced from a 27-min dive. A consistent increase of arterial hemoglobin concentrations from 15.1 +/- 1.10 to 22.4 +/- 1.41 g/100 ml (dives less than 17 min) and to 25.4 +/- 0.79 g/100 ml (dives greater than 17 min) occurred during each dive. We suggest that an extension of the sympathetic outflow of the diving reflex possibly caused profound contraction of the Weddell seal's very large spleen (0.89% of body wt at autopsy), although we have no direct evidence. This contraction may have injected large quantities of red blood cells (2/3 of the total) into the seal's central circulation during diving and allowed arterial O2 content to remain constant for the first 15-18 min of long dives. The increase of arterial CO2 tensions during the dive and the compression increase of arterial N2 tensions were also moderated by injecting red blood cells sequestered at ambient pressure. After each dive circulating red blood cells are oxygenated and rapidly sequestered, possibly in the spleen during the first 15 min of recovery.     

Variation in probability of first reproduction of Weddell seals

1. For many species, when to begin reproduction is an important life-history decision that varies by individual and can have substantial implications for lifetime reproductive success and fitness. 2. We estimated age-specific probabilities of first-time breeding and modelled variation in these rates to determine age at first reproduction and understand why it varies in a population of Weddell seals in Erebus Bay, Antarctica. We used multistate mark-recapture modelling methods and encounter histories of 4965 known-age female seals to test predictions about age-related variation in probability of first reproduction and the effects of annual variation, cohort and population density. 3. Mean age at first reproduction in this southerly located study population (7.62 years of age, SD=1.71) was greater than age at first reproduction for a Weddell seal population at a more northerly and typical latitude for breeding Weddell seals (mean=4-5 years of age). This difference suggests that age at first reproduction may be influenced by whether a population inhabits the core or periphery of its range. 4. Age at first reproduction varied from 4 to 14 years, but there was no age by which all seals recruited to the breeding population, suggesting that individual heterogeneity exists among females in this population. 5. In the best model, the probability of breeding for the first time varied by age and year, and the amount of annual variation varied with age (average variance ratio for age-specific rates=4.3%). 6. Our results affirmed the predictions of life-history theory that age at first reproduction in long-lived mammals will be sensitive to environmental variation. In terms of life-history evolution, this variability suggests that Weddell seals display flexibility in age at first reproduction in order to maximize reproductive output under varying environmental conditions. Future analyses will attempt to test predictions regarding relationships between environmental covariates and annual variation in age at first reproduction and evaluate the relationship between age at first reproduction and lifetime reproductive success.     

Long-term evaluation of body mass at weaning and postweaning survival rates of Weddell seals in Erebus Bay, Antarctica

Variability in juvenile survival rate is expected to be an important component of the dynamics of long-lived animal populations. Across a range of species, individual variation in juvenile body mass has been shown to be an important cause of variation in fates of juveniles. Our goal in this paper was to estimate age-specific apparent survival rates for Weddell seals ( Leptonychotes weddellii ) in Erebus Bay, Antarctica, and to investigate hypotheses about relationships between body mass at weaning and apparent survival rate for juveniles. Mark–resighting models found average apparent juvenile survival rate (survival from weaning to age 3) was similar between males and females, and revealed positive relationships between body mass at weaning and apparent juvenile survival rate. The effects of body mass on juvenile survival rate differed between the sexes and the relationship between body mass and survival rate was stronger in males. These results indicate that the magnitude of energy transferred from mother to pup during lactation likely has important consequences on offspring survival rate and maternal fitness.     

Offspring size at weaning affects survival to recruitment and reproductive performance of primiparous gray seals

Offspring size affects survival and subsequent reproduction in many organisms. However, studies of offspring size in large mammals are often limited to effects on juveniles because of the difficulty of following individuals to maturity. We used data from a long‐term study of individually marked gray seals (Halichoerus grypus; Fabricius, 1791) to test the hypothesis that larger offspring have higher survival to recruitment and are larger and more successful primiparous mothers than smaller offspring. Between 1998 and 2002, 1182 newly weaned female pups were branded with unique permanent marks on Sable Island, Canada. Each year through 2012, all branded females returning to the breeding colony were identified in weekly censuses and a subset were captured and measured. Females that survived were significantly longer offspring than those not sighted, indicating size‐selective mortality between weaning and recruitment. The probability of female survival to recruitment varied among cohorts and increased nonlinearly with body mass at weaning. Beyond 51.5 kg (mean population weaning mass) weaning mass did not influence the probability of survival. The probability of female survival to recruitment increased monotonically with body length at weaning. Body length at primiparity was positively related to her body length and mass at weaning. Three‐day postpartum mass (proxy for birth mass) of firstborn pups was also positively related to body length of females when they were weaned. However, females that were longer or heavier when they were weaned did not wean heavier firstborn offspring.     

Immobilisation of free-living Weddell seals Leptonychotes weddellii using midazolam and isoflurane

Eleven lactating female Weddell seals were immobilised using inhaled isoflurane and oxygen, having initially been sedated using an intramuscular injection of midazolam. The seals were selected from colonies in Long Fjord, East Antarctica. Isoflurane was delivered using a precision, out-of-circle vaporiser in a portable, heated, semi-closed circle system anaesthetic machine. Induction time (time from injection of midazolam to detected maximal effect of midazolam) ranged from 12 min to 29 min. The maximal effect of midazolam was assessed as being either moderate sedation (n=9) or heavy sedation (n=2), and the maximal effect of inhaled isoflurane and oxygen was assessed as being light anaesthesia (n=11). The level of chemical restraint achieved using this combination allowed attachment of heart rate monitoring units and collection of biological samples. Recovery time ranged from 1 min to 11 min. The anaesthetic regime proved a practical, safe and reliable method for the immobilisation of lactating Weddell seals under conditions of low environmental temperature.     

Annual and diurnal variations in the underwater vocalizations of Weddell seals

Summary Weddell seal vocalizations were recorded once a fortnight throughout 1984 at a major pupping site near Davis, Antarctica. Few vocalizations were heard between January and June. The number of vocalizations recorded increased from July to a peak in November and early December during the breeding period and then fell off rapidly through December. The increase in the number of seal vocalizations was correlated with a build up in seal numbers but some of the increase may be explained by an increased rate of vocalization by males.     

Individual variation in pup vocalizations and absence of behavioral signs of maternal vocal recognition in Weddell seals (Leptonychotes weddellii)

Individually stereotyped vocalizations often play an important role in relocation of offspring in gregarious breeders. In phocids, mothers often alternate between foraging at sea and attending their pup. Pup calls are individually distinctive in various phocid species. However, experimental evidence for maternal recognition is rare. In this study, we recorded Weddell seal (Leptonychotes weddellii) pup vocalizations at two whelping patches in Atka Bay, Antarctica, and explored individual vocal variation based on eight vocal parameters. Overall, 58% of calls were correctly classified according to individual. For males (n= 12) and females (n= 9), respectively, nine and seven individuals were correctly identified based on vocal parameters. To investigate whether mothers respond differently to calls of familiar vs. unfamiliar pups, we conducted playback experiments with 21 mothers. Maternal responses did not differ between playbacks of own, familiar, and unfamiliar pup calls. We suggest that Weddell seal pup calls may need to contain only a critical amount of individually distinct information because mothers and pups use a combination of sensory modalities for identification. However, it cannot be excluded that pup developmental factors and differing environmental factors between colonies affect pup acoustic behavior and the role of acoustic cues in the relocation process.