Age-specific cost of first reproduction in female southern elephant seals

When to commence breeding is a crucial life-history decision that may be the most important determinant of an individual''s lifetime reproductive output and can have major consequences on population dynamics. The age at which individuals first reproduce is an important factor influencing the intensity of potential costs (e.g. reduced survival) involved in the first breeding event. However, quantifying age-related variation in the cost of first reproduction in wild animals remains challenging because of the difficulty in reliably recording the first breeding event. Here, using a multi-event capture–recapture model that accounts for both imperfect detection and uncertainty in the breeding status on an 18-year dataset involving 6637 individuals, we estimated age and state-specific survival of female elephant seals (Mirounga leonina) in the declining Macquarie Island population. We detected a clear cost of first reproduction on survival. This cost was higher for both younger first-time breeders and older first-time breeders compared with females recruiting at age four, the overall mean age at first reproduction. Neither earlier primiparity nor delaying primiparity appear to confer any evolutionary advantage, rather the optimal strategy seems to be to start breeding at a single age, 4 years.     

Equal investment in male and female offspring in southern elephant seals

Sex ratio theory predictions concerning differential parental investment in offspring by sex were tested on southern elephant seals, Mirounga leonina , breeding at Península Valdés, Argentina. Females invested equally in sons and daughters, as reflected by the similar mass at birth (mean ± 1 S.D.) of 14 males (44.1 ± 6.5 kg) and 14 females (43.4 ± 3.8 kg), and similar mass at weaning of 52 males (131.5 ± 22.4 kg) and 38 females (131.4 ± 18.3 kg). There were also no sex differences in the rate of mass gain during nursing (males = 4.0 ± 0.9 kg/day; females = 3.9 ± 0.8 kg/day), rate of mass loss during the first month of post-weaning fast (males = 0.85 ± 0.19 kg/day; females = 0.92 ± 0.15 kg/day), mean age at weaning (males = 22.3 ± 1.6 days; females = 22.7 ± 1.7 days), and female nursing behaviour. Mother's size accounted for most of the variation in mass of pups at weaning. Mothers ranked as small, medium and large, weaned pups with a mean mass of 102, 130 and 145 kg, respectively. The sex ratio of weanlings did not differ from unity. These data are consistent with Fisher's (1930) sex ratio theory.     

Growth of female southern elephant seals Mirounga leonina at Macquarie Island

Body growth of 137 female southern elephant seals (Mirounga leonina) over 1 year of age was investigated at subantarctic Macquarie Island. An asymptotic straight line, snout–tail body length of 2.57±0.03 m was estimated to be attained at 9 years of age, using a three-parameter Gompertz equation. A significant increase of approximately 0.1 m (5%) in mean body length of females between 1 and 10 years of age was estimated to have occurred between the 1950–1960s and 1990s at Macquarie Island. This is consistent with a reduction in both the rate of population decline and the age of onset of sexual maturity. Age determination using dental cementum layers and the importance of standardised measurements in pinniped growth studies are discussed.     

Extreme polygyny among southern elephant seals on Sea Lion Island, Falkland Islands

Elephant seals are known from long-term behavioral studies tobe highly polygynous and to show high variance in reproductivesuccess among males. However, genetic studies have determinedthat the level of polygyny varies between the closely relatednorthern and southern elephant seals. In the present study,we investigate paternal success at the Sea Lion Island southernelephant seal colony in the Falkland Islands by using both behavioralmeasures and genetic markers. We find that the average successof harem holding males at Sea Lion Island is significantly higherthan both the northern species and the nearby southern elephantseal population at Punta Delgada. We compare genetic paternitywith various behavioral indices of male mating success, andwe find that the behavioral measures provide a good estimateof the variance in male reproductive success. Only 28.2% ofmales achieved paternities, and among these, harem holders accountedfor 89.6%. We discuss the implications of our results in thecontext of the demographic and physical environment. Specifically,a comparatively high variance in resource holding potentialamong males, differences in male social behavior, and a smalltidal cycle limiting peripheral male access during female departurefrom the harem at this colony may be important factors leadingto the comparatively high variance in male reproductive successat Sea Lion Island.     

Diving behaviour and foraging location of female southern elephant seals from Patagonia

Our aim was to describe the free-ranging diving pattern and to determine the location of foraging of pregnant female southern elephant seals, Mirounga leonina , from Peninsula Valdes, Argentina. This colony is unusual in two respects: it is removed from deep water by a broad shallow shelf (345–630 km wide), and colony numbers have been increasing in recent years in contrast to numbers from other southern hemisphere colonies that are stable or in decline. Microprocessor controlled, geolocation-time-depth recorders were deployed on four females, recording a total of 15,836 dives (270 dive days) during the period February to April, 1992. Departing seals crossed the continental shelf quickly (54–5–62–1 h) and did not show signs of foraging until reaching deep water, due east of the colony in the South Atlantic Ocean. Diving was virtually continuous (93% of the time underwater) with overall mean (±S.D.) rates of 2.5±0.2 dives/h, mean dive durations of 22.8 ± 7.1 min (maximum dive duration = 79 min) with 1.6±0.6min surface intervals between dives, and dive depths of 431±193m (maximum dive depth = 1,072 m). The diving pattern of females from Patagonia is similar to that of seals from colonies where numbers are decreasing (Macquarie stock) or are stable (South Georgia Island). Our subjects did not, however, feed in or south of the Antarctic Polar Front, or in cold waters along the Antarctic coast, where seals from declining or stable colonies forage.     

The role of eddies in the diving behaviour of female southern elephant seals

As the Antarctic Circumpolar Current crosses the South-West Indian Ocean Ridge, it creates an extensive eddy field characterised by high sea level anomaly variability. We investigated the diving behaviour of female southern elephant seals from Marion Island during their post-moult migrations in relation to this eddy field in order to determine its role in the animals’ at-sea dispersal. Most seals dived within the region significantly more often than predicted by chance, and these dives were generally shallower and shorter than dives outside the eddy field. Mixed effects models estimated reductions of 44.33 ± 3.00 m (maximum depth) and 6.37 ± 0.10 min (dive duration) as a result of diving within the region, along with low between-seal variability (maximum depth: 5.5 % and dive duration: 8.4 %). U-shaped dives increased in frequency inside the eddy field, whereas W-shaped dives with multiple vertical movements decreased. Results suggest that Marion Island’s adult female elephant seals’ dives are characterised by lowered cost-of-transport when they encounter the eddy field during the start and end of their post-moult migrations. This might result from changes in buoyancy associated with varying body condition upon leaving and returning to the island. Our results do not suggest that the eddy field is a vital foraging ground for Marion Island’s southern elephant seals. However, because seals preferentially travel through this area and likely forage opportunistically while minimising transport costs, we hypothesise that climate-mediated changes in the nature or position of this region may alter the seals’ at-sea dispersal patterns.     

First-year survival of southern elephant seals,Mirounga leonina,at sub-Antarctic Macquarie Island

Juvenile seals branded on the isthmus of Macquarie Island as pups displayed a high degree of philopatry. They returned more often and in greater densities to the northern third of the island within 10 km of their birth sites. Juvenile seals were observed to haul out more frequently and in greater numbers on the east coast as opposed to the west. Juvenile seals typically hauled out on two occasions, once during the winter, and once to moult. The probability of recapturing (resighting) branded and tagged seals was greater during the mid-year haulout. First-year survival estimates were obtained from searches of all Macquarie Island beaches for marked (branded and tagged) seals. From a branded population of 2000 seals, 897 were known to be alive at age 1 year, and minimum first-year survival was calculated at 44.85%. To this minimum estimate was added the number of seals overlooked during systematic and standardised searches of the island, and a revised estimate of 65.60% was calculated. Survival rates calculated using a custom model and a conventional mark-recapture model (MARK) were compared and no differences detected. Actual survival data and probability of sighting estimates were included in the revised estimate of first-year survival of southern elephant seals at Macquarie Island. There were no differences in the number of surviving males and females. Accepted: 25 October 1998     

Lactation costs in southern elephant seals at King George Island,South Shetland Islands

Labelled-water methodology was used to quantify energy costs and energy transfer efficiency in 18 mother-pup pairs of southern elephant seals (Mirounga leonina) during lactation. During the lactation period, mothers lost a mean mass of 227±47 kg. Mass loss included 22% of the protein, 60% of the fat, and 51% of the energy in the mothers body upon arrival. Total body-energy reserves at parturition explained 69% of the variation in the total lactation costs and 50% of the variation in the pups body energy at weaning. On average, pups retained 48% of the mass, 49% of protein, 53% of fat and 51% of energy lost by their mothers. Greater, fatter females showed a decrease in the efficiency of energy and fat transfer and, at the same time, an increase in the efficiency of protein transfer. This may be due to an increased use of protein as metabolic fuel, as fat demands for milk production increase. There was no evidence that greater total lactation costs influence the ability of mothers to produce a pup in the next breeding season.     

Population changes,movements of southern elephant seals on Crozet and Kerguelen Archipelagos in the last decades

Summary The elephant seal populations breeding on the Crozet and Kerguelen Archipelago were surveyed during the eighties. Elephant seals were observed moving between Kerguelen, Amsterdam, Heard Islands and Vestfold Hills and between Crozet and Prince-Edward Archipelagos. No exchanges were observed between Crozet and Kerguelen Archipelagos suggesting that the two populations are more isolated than previously stated. On the Crozet Archipelago, since 1966, the Possession Island population showed at 70% reduction in numbers of cows ashore and the population is still decreasing. On Kerguelen Island there has been a decline of 44% from 1956 to 1989 but the population appears to have stabilized since 1984. It is suggested that elephant seal populations in the Southern Indian Ocean may have been affected by a change at the trophic level over the last four decades. But the highest rate of decrease observed on the Crozet Archipelago and the fact that the population is still decreasing may be explained by additional factors, in particular by killer whale predation.     

A lifetime at depth: vertical distribution of southern elephant seals in the water column

Although numerous studies have addressed the migration and dive behaviour of southern elephant seals (Mirounga leonina), questions remain about their habitat use in the marine environment. We report on the vertical use of the water column in the species and the potential lifetime implications for southern elephant seals from Marion Island. Long-term mark-resight data were used to complement vertical habitat use for 35 known individuals tagged with satellite-relay data loggers, resulting in cumulative depth use extrapolated for each individual over its estimated lifespan. Seals spent on average 77.59% of their lives diving at sea, 7.06% at the sea surface, and 15.35% hauled out on land. Some segregation was observed in maximum dive depths and depth use between male and female animals—males evidently being physiologically more capable of exploiting increased depths. Females and males spent 86.98 and 80.89% of their lives at sea, respectively. While at sea, all animals spent more time between 300 and 400 m depth, than any other depth category. Males and females spent comparable percentages of their lifetimes below 100 m depth (males: 65.54%; females: 68.92%), though males spent 8.98% of their lives at depths in excess of 700 m, compared to females’ 1.84% at such depths. Adult males often performed benthic dives in excess of 2,000 m, including the deepest known recorded dive of any air-breathing vertebrate (>2,133 m). Our results provide a close approximation of vertical habitat use by southern elephant seals, extrapolated over their lifespans, and we discuss some physiological and developmental implications of their variable depth use.     

Aggressive and maternal activities of female southern elephant seals (Mirounga leonina)

The aggressive and maternal behaviours of 44 individually marked female southern elephant seals, Mirounga leonina, were studied at South Georgia. Evidence of dominance and subordination could be determined in only 29% of 1315 encounters, but dominance-subordination relationships were observed in 49% of known cow dyads. Status was not necessarily size- or age-related, but most young, small cows were of low status. High-status cows reared larger pups which were bitten less frequently than pups of low-status cows. Orphaned pups were bitten at least three times as frequently as pups with mothers. Cows with pups initiated more interactions than pregnant cows, and were dominant more frequently. Older cows responded to their newborn pup more rapidly and intensively than did younger cows and this difference can be important in crowded breeding assemblies. Cow and pup behaviour in M. leonina at South Georgia is compared with that of M. angustirostris in California. The main differences are thought to relate to differences in population density.     

Behavioural and morphometric measurements of parental investment in southern elephant seals at the Falkland Islands

Parental investment is a key variable in the study of breeding strategies and life-histories evolution. In Pinnipedia, parental investment is usually calculated from direct measurements of pup weight gain or energy transfer between the mother and the pup. These direct methods always involve handling and restraining procedures that pose practical, logistical and ethical problems. To evaluate if weighing can be substituted by indirect observational estimates of parental investment, we analysed the relationship among various behavioural measures of suckling and post-natal growth in the southern elephant seal population of Sea Lion Island (Falkland Islands). Behavioural measures were in all cases a poor predictor of true investment as estimated by weighing. We concluded that there are currently no effective alternatives to direct handling, and that the best way to reduce the potential adverse impact of investment studies is the improvement of the handling protocol, which should include an estimation of the long-term effects on the health of handled animals. Further research is needed to test the validity of non-behavioural indirect methods (e.g. 3D photogrammetry).     

Inter-population differences in diving behaviour of adult male southern elephant seals (Mirounga leonina)

Access to different environments may lead to inter-population behavioural changes within a species that allow populations to exploit their immediate environments. Elephant seals from Marion Island (MI) and King George Island (KGI) (Isla 25 de Mayo) forage in different oceanic environments and evidently employ different foraging strategies. This study elucidates some of the factors influencing the diving behaviour of male southern elephant seals from these populations tracked between 1999 and 2002. Mixed-effects models were used to determine the influence of bathymetry, population of origin, body length (as a proxy for size) and individual variation on the diving behaviour of adult male elephant seals from the two populations. Males from KGI and MI showed differences in all dive parameters. MI males dived deeper and longer (median: 652.0?m and 34.00?min) than KGI males (median: 359.1?m and 25.50?min). KGI males appeared to forage both benthically and pelagically while MI males in this study rarely reached depths close to the seafloor and appeared to forage pelagically. Model outputs indicate that males from the two populations showed substantial differences in their dive depths, even when foraging in areas of similar water depth. Whereas dive depths were not significantly influenced by the size of the animals, size played a significant role in dive durations, though this was also influenced by the population that elephant seals originated from. This study provides some support for inter-population differences in dive behaviour of male southern elephant seals.     

Dispersion during the moult haulout of southern elephant seals at the Courbet Peninsula, Iles Kerguelen

We studied the dispersion of 4-year-old southern elephant seals (Mirounga leonina) along a 75.5 km coastal area at the Courbet Peninsula, Iles Kerguelen, relative to their birth site when they were ashore to moult in early 1984. The seals were mostly faithful to their natal sites, but availability of suitable moulting habitat (e.g. wallows, vegetated areas) influenced seal dispersion. As moult progressed, the seals moved farther away from their initial moult sites and natal sites, but remained largely on the easterly beaches of the Courbet Peninsula. This behaviour would facilitate mark-recapture estimates of age and sex specific survival.     

Temporal changes in fecundity and age at sexual maturity of southern elephant seals at Marion Island

Our objective was to examine the effect of variation in reproductive parameters on the demography of southern elephant seals at Marion Island. We used age-specific capture probabilities of breeding females in a Cormack-Jolly-Seber context to derive reproductive rates. We found that age at maturity declined and fecundity rates increased as the population declined, indicating a compensatory response. Fecundity rates ranged from 0.03 to 0.29 among 3-year-olds (mean=0.16), 0.18 to 0.50 in 4-year-olds (mean=0.40), and 0.28 to 0.50 in 5-year-olds (mean=0.45). We think that a relative increase in food availability, concomitant with the population decline, promoted earlier sexual maturity correlated with more rapid growth of juveniles when population abundance was lower. It is suggested that the relative importance of fecundity in population regulation in elephant seals has been underestimated. Moreover, it appears that the onset of sexual maturity may be the first demographic variable to change in response to a change in population density.     

Mass changes during their annual cycle in females of southern elephant seals at King George Island

Mass changes in female southern elephant seals, sampled sequentially at different points through their annual cycle, were measured at King George Island, South Shetland Islands, during the 1995/1996 and 1996/1997 field seasons. Females weighed after they had given birth showed an increase of 37 ± 36 kg (mean ± SD), which represented 6.2 ± 6.4% in relation to their mass in the first breeding season. During the first aquatic phase, between the end of lactation and the beginning of moult, females gained a mean of 128 ± 35 kg, (n = 18) (2.19 ± 0.65 kg day⁻¹), which represented between 27 and 83% of the mass they had lost during lactation. Nine females followed during moulting showed a mass loss rate of 5.0 ± 0.4 kg day⁻¹, which was half the rate during lactation. Total mass loss during moulting (129 ± 22 kg) was not significantly different from mass gain for the same females between lactation and moult (135 ± 37 kg). Furthermore, at the end of moulting, female mass was not significantly different from the mass at the end of lactation. These masses represented 65 ± 5% and 64 ± 5%, respectively, of their initial mass after parturition. During the second period at sea, from the end of the moult until females hauled out to give birth in the following breeding season, the estimated mass gain was 1.45 ± 0.24 kg day⁻¹ (n = 5), which was not significantly different to the rate of mass gain shown by the same females during the first period at sea (2.26 ± 0.70 kg day⁻¹). Total mass gain during the second aquatic phase (364 ± 63 kg) was not correlated with the mass at the end of moulting, but it was positively related to the mass loss experienced by females from parturition until the end of the moulting period in the first breeding season. Accepted: 5 September 1998     

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.     

Body mass loss of moulting female southern elephant seals,Mirounga leonina,at Macquarie Island

Thirteen female southern elephant seals moulting at Macquarie Island lost an average of 4.46±0.80 kg/day (10.01±1.20g/kg/day). There was no significant difference between this rate of body mass loss and that reported for moulting female southern elephant seals from South Georgia. Moulting female southern elephant seals however exhibited larger mass specific mass loss than either female northern elephant seals or male southern elephant seals, indicating a higher metabolic cost of moult in these animals.