Nuevo State Reserve Research Articles

How northern elephant seals mobilise blubber for fuel when fasting

When food is scarce, most animals go into survival mode, reducing their activity to conserve energy. But not northern elephant seals (Mirounga angustirostris). Leaving their ocean feeding grounds and hauling themselves onto Californian and Mexican beaches for weeks, the females give birth, nursing their pups, while the males posture as they protect their harems; all of which they do while fasting. Jane Khudyakov from the University of the Pacific, USA, explains that during this time the animals depend on their extensive fat reserves for survival. However, it wasn't clear how the animals coordinate their tissue energy use while going without. ‘The cellular mechanisms that regulate these adaptations are still not fully understood’, she says. Teaming up with collaborators Dan Crocker (Sonoma State University, USA) and Dan Costa [University of California, Santa Cruz (UCSC), USA], Khudyakov and Rachel Holser (UCSC) headed down to the elephant seals’ breeding grounds at Año Nuevo State Reserve, USA, when the animals returned to moult, to find out.‘They shed old skin and hair within several weeks’, says Khudyakov, adding, ‘it literally peels off in sheets, which makes the beach very smelly’. However, the moulting females are easier to approach than when fasting in the breeding season while protecting their pups. Surrounded by the colony, Holser with colleagues from Costa's lab gently anaesthetised five animals that had just returned from 3–4 month foraging trips, ready for Khudyakov to take small blood, muscle and fat samples at the beginning and end of the moult. In addition, the team weighed the females, recording that the animals lost ∼3 kg per day – totalling ∼28% of their body mass over the ∼33 day period – in the form of skin, hair and fat.Back in the lab, Khudyakov and University of the Pacific undergraduates Serena Ly and Theron Niel extracted proteins from the muscle and blubber before Craig Vierra and Basma Hasan (University of the Pacific) joined them to analyse the protein content, alongside that of the blood sample. ‘We identified 1053 proteins in blubber, 711 in muscle and 260 in blood, most of which have not been studied before in seals’, says Khudyakov. Yet, when the team compared the composition of the muscle and blubber over the month-long fast, they found they had changed little. ‘This suggests that protein turnover in these tissues may be tightly controlled’, says Khudyakov. However, the quantities of three key proteins involved in fat accumulation reduced in all three tissues as the animals switched from storing to consuming fat.In contrast, 53 blood proteins involved in controlling and transporting lipids around the body increased dramatically, including hormones that regulate the release and transport of lipids and those that control how the body uses lipids as a fuel. ‘We think that changes in these proteins may help seals quickly switch their metabolism between feeding and fasting without any of the adverse effects that such rapid fat gain and loss would cause in humans’, says Khudyakov. And when the team checked on the quantities of fat circulating in the animals’ blood, they found that triglycerides – a form of fat – and high-density lipoproteins – ‘good’ cholesterol – did not change significantly, ‘suggesting that seals very tightly regulate their fat reserves, despite using them for energy, during fasting’, says Khudyakov.So, fasting elephant seals are able to sustain an active lifestyle by mobilising fats from their blubber in a controlled way. And Khudyakov is keen to find out how the proteins that regulate fat use change when the females produce full-fat (60%) milk for their pups, although that will require running the gauntlet of the fiercely protective mothers’ teeth.

Changes in stable isotope compositions during fasting in phocid seals.

RationaleThe grey seal, Halichoerus grypus (GS), and the northern elephant seal, Mirounga angustirostris (NES), come ashore for reproduction. This period involves intense physiological processes such as lactation in females and a developmental post‐weaning fast in juveniles. Previous studies have shown that δ13C and δ15N values are affected by starvation, but the precise effects of fasting associated with lactation and post‐weaning fast in seals remain poorly understood.MethodsTo examine the effect of lactation and post‐weaning fast on stable isotope ratios in GS and NES, blood and hair were sampled from 21 GS mother‐pup pairs on the Isle of May and on 22 weaned NES pups at Año Nuevo State Reserve during their respective breeding seasons. Milk samples were also collected from GS mothers. Stable isotope measurements were performed with an isotope ratio mass spectrometer coupled to an N‐C elemental analyser.ResultsChanges in stable isotope ratios in blood components during fasting were similar and weak between GS and NES mothers especially in blood cells (GS: Δ15N = 0.05‰, Δ13C = 0.02‰; NES: Δ15N = 0.1‰, Δ13C = 0.1‰). GS showed a 15N discrimination factor between maternal and pup blood cells and milk, but not for 13C. The strongest relationship between the isotopic compositions of the mother and the pup was observed in the blood cells.ConclusionsIsotopic consequences of lactation, fasting, and growth seem limited in NES and GS, especially in medium‐term integrator tissues of feeding activity such as blood cells. Stable isotope ratios in the blood of pups and mothers are correlated. We observed a subtle mother‐to‐pup fractionation factor. Our results suggest that pup blood cells are mostly relevant for exploring the ecology of female seals.

Terrestrial locomotion of the northern elephant seal (Mirounga angustirostris): limitation of large aquatically adapted seals on land?

The aquatic specializations of phocid seals have restricted their ability to locomote on land. The amphibious northern elephant seal, Mirounga angustirostris, is the second largest phocid seal in the world, with males reaching 2700 kg. Although elephant seals are proficient swimmers and deep divers, their extreme size and aquatic specializations limit terrestrial movement. The kinematics of terrestrial locomotion in northern elephant seals were analyzed from video recordings of animals observed on the beach of Año Nuevo State Reserve, CA, USA. The seals moved using a series of rhythmic undulations produced by dorsoventral spinal flexion. The traveling spinal wave moved anteriorly along the dorsal margin of the body with the chest, pelvic region and foreflippers serving as the main points of contact with the ground. The hindflippers were not used. The spinal wave and foreflippers were used to lift the chest off the ground as the body was pushed forward from the pelvis as the foreflippers were retracted to pull the body forward. Seals moved over land at 0.41-2.56 m s-1 (0.12-0.71 body lengthss-1). The frequency and amplitude of spinal flexions both displayed a direct increase with increasing speed. The duty factor for the pelvic region decreased with increasing velocity while the duty factor of the foreflipper remained constant. Kinematic data for elephant seals and other phocids were used in a biomechanical model to calculate the mechanical energy expended during terrestrial locomotion. The elephant seals were found to expend more energy when traveling over land for their size than smaller phocids. The unique method of terrestrial movement also exhibited greater energy expenditure on land than values for large quadrupeds. The trade-off for the northern elephant seal is that its massive size and morphology have well adapted it to an aquatic existence but limited its locomotor performance (i.e. speed, endurance) on land.

Serum POP concentrations are highly predictive of inner blubber concentrations at two extremes of body condition in northern elephant seals

Long-lived, upper trophic level marine mammals are vulnerable to bioaccumulation of persistent organic pollutants (POPs). Internal tissues may accumulate and mobilize POP compounds at different rates related to the body condition of the animal and the chemical characteristics of individual POP compounds; however, collection of samples from multiple tissues is a major challenge to ecotoxicology studies of free-ranging marine mammals and the ability to predict POP concentrations in one tissue from another tissue remains rare. Northern elephant seals (Mirounga angustirostris) forage on mesopelagic fish and squid for months at a time in the northeastern Pacific Ocean, interspersed with two periods of fasting on land, which results in dramatic seasonal fluctuations in body condition. Using northern elephant seals, we examined commonly studied tissues in mammalian toxicology to describe relationships and determine predictive equations among tissues for a suite of POP compounds, including ΣDDTs, ΣPCBs, Σchlordanes, and ΣPBDEs. We collected paired blubber (inner and outer) and blood serum samples from adult female and male seals in 2012 and 2013 at Año Nuevo State Reserve (California, USA). For females (N = 24), we sampled the same seals before (late in molting fast) and after (early in breeding fast) their approximately seven month foraging trip. For males, we sampled different seals before (N = 14) and after (N = 15) their approximately four month foraging trip. We observed strong relationships among tissues for many, but not all compounds. Serum POP concentrations were strong predictors of inner blubber POP concentrations for both females and males, while serum was a more consistent predictor of outer blubber for males than females. The ability to estimate POP blubber concentrations from serum, or vice versa, has the potential to enhance toxicological assessment and physiological modeling. Furthermore, predictive equations may illuminate commonalities or distinctions in bioaccumulation across marine mammal species.

Parasitological examination of northern elephant seal (Mirounga angustirostris) pups for presence of hookworms (Uncinaria spp.) on San Miguel Island, California

Summary Necropsy and extensive parasitological examination of dead northern elephant seal (NES) pups was done on San Miguel Island, California, in February, 2015. The main interest in the current study was to determine if hookworms were present in NESs on San Miguel Island where two hookworm species of the genus Uncinaria are known to be present - Uncinaria lyonsi in California sea lions and Uncinaria lucasi in northern fur seals. Hookworms were not detected in any of the NESs examined: stomachs or intestines of 16 pups, blubber of 13 pups and blubber of one bull. The results obtained in the present study of NESs on San Miguel Island plus similar finding on Año Nuevo State Reserve and The Marine Mammal Center provide strong indication that NES are not appropriate hosts for Uncinaria spp. Hookworm free-living third stage larvae, developed from eggs of California sea lions and northern fur seals, were recovered from sand. It seems that at this time, further search for hookworms in NESs would be nonproductive.

Development of an animal-borne “sonar tag” for quantifying prey availability: test deployments on northern elephant seals

Developments in electronic tagging technologies have provided unprecedented insight into the movements and behavior of marine predators. Concurrent information on the prey of these tracked animals, however, is mostly lacking. We developed and tested a prototype autonomous echosounder (aka the sonar tag) for deployment on large marine animals intended to provide quantification of their prey fields. The resulting fully autonomous, internally recording prototype sonar tag operated at a power of 1 W and a frequency of 200 kHz. A series of test deployments were successfully conducted on four juvenile female elephant seals (Mirounga angustirostris) captured at the Ano Nuevo State Reserve, California, and released short distances away. Translocated seals were instrumented with a sonar tag and a Fastloc GPS tag with an integrated time-depth recorder (TDR). All four animals returned to land after 3–18 days, making dives to depths of up to 778 m. Strong backscattering from the bottom was observed during many dives, indicating an often close association with the seafloor. Numerous observations of strongly scattering targets, potentially indicative of prey, were also made in the water column, often associated with particular dive and movement behaviors. During dives identified as foraging-type and also travel-type, one animal with the acoustic transducer on its head showed successive targets getting increasingly closer to the animal, possibly consistent with prey pursuit. These results demonstrate the value of active acoustic backscattering measurements made from free-ranging animals, complementing the ecological insight afforded by traditional depth- and position-logging tags. Future refinements will include further miniaturization, performance optimization, and extensions in the deployment duration.

Mobilisation of blubber fatty acids of northern elephant seal pups (Mirounga angustirostris) during the post-weaning fast

Northern elephant seal pups were longitudinally sampled at Año Nuevo State Reserve during the post-weaning fast, in order to evaluate the changes of fatty acid (FA) profiles in serum as well as in the inner and outer layers of blubber. The major FAs of inner and outer blubber layers were broadly similar to those found in NES maternal milk previously measured, suggesting a direct deposit of dietary FAs in the blubber during the suckling period. The outer blubber layer contained more medium-chain monounsaturated FAs that contribute in keeping the fluidity of this tissue at cold temperatures. It was compensated by higher proportions of saturated FAs in the inner blubber layer. The FA signature of inner blubber, the layer that is mainly mobilised during energy deprivation, slightly differed from the signature of serum. There were greater proportions of medium-chain saturated FAs and ω-6 polyunsaturated FAs, and lower proportions of long-chain saturated FAs, medium-chain monounsaturated FAs and long-chain monounsaturated FAs in serum as compared to inner blubber. We also demonstrated that lipophilicity is the main factor governing the mobilisation of FAs from blubber. The least lipophilic FAs were preferentially hydrolysed from blubber, leading to an enrichment of the more lipophilic FAs in this tissue with the progression of the fast. The expression levels of HSL and ATGL, which are two enzymes involved in the lipolytic process, remained stable during the post-weaning fast. This suggests that the pups have developed the enzymatic mechanisms for an efficient lipolysis as soon as the first week of fast.

Bioaccumulation of hydroxylated polychlorinated biphenyls and pentachlorophenol in the serum of northern elephant seal pups (Mirounga angustirostris)

Northern elephant seals (NES) (Mirounga angustirostris) from the Año Nuevo State Reserve (CA, USA) were sampled at 1-, 4-, 7- and 10-week post-weaning. Concentrations of hydroxylated polychlorinated biphenyls (HO-PCBs) and their parent PCBs were measured in the serum of each individual. The ΣHO-PCB concentrations in the serum increased significantly between early and late fast (from 282±20 to 529±31pg/mL). This increase might result from a mobilisation of HO-PCBs transferred from the mother during gestation and/or lactation and stored in the pup's liver. Food deprivation has been shown to exacerbate biotransformation capacities in mammals, birds and fish. The HO-penta-CBs was the predominant homologue group, followed by HO-hexa-CBs and HO-hepta-CBs. No preferential pathway for the metabolism of HO-PCBs (HO-direct insertion or NIH-shift of a chlorine atom) could be evidenced. The concentrations of pentachlorophenol (PCP) in the serum of weaned NES increased from 103±7pg/mL at early fast to 246±41pg/mL at late fast, which is within the range of PCP concentrations usually encountered in marine mammals.

Mobilisation of lipophilic pollutants from blubber in northern elephant seal pups (Mirounga angustirostris) during the post-weaning fast

Northern elephant seals (NES) (Mirounga angustirostris) from the Año Nuevo State Reserve (CA, USA) were longitudinally sampled during the post-weaning fast in order to study the mobilisation and redistribution of various classes of persistent organic pollutants (POPs), such as polybrominated diphenyl ethers (PBDEs), polychlorinated biphenyls (PCBs), dichlorodiphenyldichloroethylene (p,p′-DDE) and hexachlorobenzene (HCB) between blubber and blood. Inner and outer blubber layers were analysed separately. Organohalogenated compounds were detected in all blubber samples in the decreasing order of their concentrations: p,p′-DDE>PCBs⪢HCB>PBDEs. The concentrations of all studied compounds were homogeneously distributed in the blubber layer at early fast, since the concentrations of POPs were statistically not different in the inner and outer layers. With the progression of the fast, the concentrations of PBDEs, PCBs and p,p′-DDE increased more sharply in inner blubber than in outer blubber. As a result, their levels became significantly higher in inner blubber as compared to outer blubber at late fast. The rise of pollutant concentrations in blubber might result from a less efficient mobilisation than triglycerides and/or a reuptake by adipocytes of some of the pollutants released into the circulation. The mobilisation of pollutants from blubber was higher at late fast. An increase of pollutant concentrations was observed in serum between early and late fast. Lower halogenated congeners (i.e. tetra-CBs) were present in higher proportions in serum, whereas the higher halogenated congeners (i.e. hepta-CBs) were mainly found in the inner and outer blubber layers. The transfer ratios of both PBDEs and PCBs from inner blubber to serum decreased with the number of chlorine and bromine atoms. In addition, the distribution of both types of compounds between serum and blubber was strongly influenced by their lipophilic character (logKow values), with more lipophilic compounds being less efficiently released from blubber to serum.

Causes of death in preweaned northern elephant seal pups (Mirounga angustirostris, Gill, 1866), Año Nuevo State Reserve, California, 2012

During an ongoing physiological ecology study on pups and adult female northern elephant seals (Mirounga angustirostris, Gill, 1866) on the mainland rookery at Año Nuevo State Reserve (California), an opportunity was afforded to collect fresh dead pups for parasitology and necropsy. The investigation was undertaken to delineate the causes of death of northern elephant seals recovered from Año Nuevo State Reserve. Prior to this study, there was no evidence of increased mortality or health problems on this rookery. Necropsies, histology, and ancillary diagnostic studies were conducted on 21 fresh dead preweaned pups. Ages ranged from 1 stillbirth to pups approximately 2 weeks of age. Gross lesions included varying degrees of bruising, hemorrhage, lacerations, and fractures attributed to blunt force trauma to the head, chest, and/or abdomen in 16 pups; starvation in 6 pups; bite wounds in 2 pups; generalized icterus in 2 pups; presumptive drowning in 2 pups; and 1 stillbirth. Most pups had multiple gross lesions. Following light microscopic examination, pups could be assigned into 4 general diagnostic categories: 1) trauma, 2) nutritional status, 3) infectious conditions, and 4) congenital anomalies. This investigation of preweaned pup mortality of northern elephant seals in California further refines diagnostic categories for perinatal pup mortality.

Parasitological examination for presence of hookworms (Uncinaria spp.) in northern elephant seals (Mirounga angustirostris) at Año Nuevo State Reserve, California (2012)

Northern elephant seals (Mirounga angustirostris Gill, 1866), inhabiting rookeries on the mainland of Año Nuevo State Reserve in central California, were investigated in 2012 for presence of hookworms (Uncinaria spp.). Material collected and examined for hookworms included: blubber (n = 15), stomach and intestines (n = 21) from dead pups; feces from the rectum of weaned pups (n = 23); sand containing apparent feces in areas of weaned pups (n = 28) and sand without apparent feces in areas of weaned pups (n = 54); milk from females (n =23) at 5 days and about 23 to 26 days postpartum; and placenta from one female. Evidence of hookworm presence was not detected in any of the samples examined. Possible reasons why hookworms were not found in northern elephant seals on the mainland of Año Nuevo State Reserve are discussed.

Do male northern elephant seals recognize individuals or merely relative dominance rank?

Vocal recognition was tested in a socially dynamic context where many individuals interact: the female defense polygyny practiced by male northern elephant seals. The goal was to tease apart whether animals recognize other individuals or instead use a simple rule-based category (i.e., relative dominance rank). A total of 67 playback experiments conducted with 18 males at Año Nuevo State Reserve, California, tested three aspects of recognition: (1) recognition of relative rank; (2) whether such recognition was continuous or categorical; and (3) recognition of familiarity. Results indicate that males recognize familiar individuals although responses are primarily based on relative dominance rank.

Vocal recognition of individuals versus relative dominance rank among breeding male northern elephant seals (Mirounga angustirostris).

Whether an animal truly recognizes an individual or a simple rule-based category (e.g. neighbor or offspring) has important behavioral and evolutionary implications such as the accuracy of social reciprocity. Many tests of individual recognition have focused on neighboring territorial males (dear enemy or neighbour-stranger recognition). Unfortunately the static territorial context of these tests, mostly with male songbirds, opens them to the criticism of being merely associative habituation. More dynamic mating assemblages, such as leks where vocally advertising animals encounter numerous others, are a potentially rich and largely untested alternative. The female defense polygyny practiced by male northern elephant seals during terrestrial breeding is such a dynamic system. To examine whether elephant seals were recognizing individuals or dominance categories we conducted a total of 53 playback experiments to 18 males at Ano Nuevo State Reserve. Each playback was a series of threat calls assigned to four dominance conditions relative to the subject. Dominance was based on the outcomes of interactions among contesting male dyads. Responses were measured using three assays in situ and from video records of each experiment. Results indicate that males do recognize familiar individuals although responses are primarily based on relative dominance rank.

Click and tone-pip auditory evoked potentials in a large marine mammal, the northern elephant seal

The use of auditory-evoked potentials (AEPs) to study the hearing of mysticete whales is challenged by access to animals, their large size, and proportionately smaller brain relative to odontocetes. One means by which AEP techniques can be adapted to these larger animals is by application to more readily available proxy species. The northern elephant seal (Mirounga angustirostris) is a large pinniped, potentially in excess of 2000 kg, with a thick dermis, large skull, relatively small auditory nerve, and a low-frequency vocal communication system. AEP collection in elephant seals provides similar challenges to those of the mysticetes but at a scale that provides a greater opportunity for success. AEP tests were conducted on northern elephant seals at Año Nuevo State Reserve, the natural haul-out site of the elephant seal. Subjects were chemically immobilized with tiletamine/zolazepam and chemical restraint was maintained with bolus injections of ketamine. Click-evoked potentials were collected from four weanling and two adult male elephant seals and tone-pip-evoked potentials were collected from a 2-year-old female. Results demonstrate that AEPs can be recorded from large pinniped species, providing a step towards the application of similar techniques to larger cetacean species.

Source levels of northern elephant seal vocalizations in-air

Accurate measurements of vocalization sound-pressure levels are necessary to determine the acoustical active space of animals in natural and human-altered ambient noise conditions. Despite this basic need, such data are limited or nonexistent for most species. Our study characterized aerial ambient noise and vocalization source levels for northern elephant seals during the breeding season. Subjects were adult males, lactating females, and dependent offspring (pups) at Año Nuevo State Reserve. Source level measurements were made using a Type 1 sound level meter and calibrated microphones on-axis: (1) at 1 m; (2) at several known distances (laser measured); and (3) simultaneously at 1 m and a second known distance. Concurrent ambient noise conditions were measured in situ (non-weighted 5 min Leq integrated averages) and recorded for later spectral analysis. Measurements were made at two sites, one relatively noisy and the other relatively quiet, to determine whether animals compensate for higher noise conditions by increasing source levels (Lombard effect). Results indicate a wide range in signal strength, particularly for adult males whose vocalization source levels appear to be correlated with dominance rank and related to ambient noise conditions. The Lombard effect was not observed for adult females or elephant seal pups.

Maternal Traits and Reproductive Effort in Northern Elephant Seals

The aim of this study was to determine the effects of maternal traits on reproductive expenditure and energy delivery to the offspring in a capital breeder, the northern elephant seal (Mirounga angustirostris). Changes in maternal and offspring energy reserves and milk-energy delivery were examined in relation to maternal parturition mass, body composition, and age in females and pups breeding at Año Nuevo State Reserve, California. Maternal body mass and composition had significant effects on maternal energy expenditure over lactation. Path analysis suggested no significant effects of maternal age on reproductive effort of parous females. The efficiency of milk production increased significantly with maternal age. Offspring metabolism was a relatively small component of maternal energy expenditure, with pups storing 84% of the energy obtained from milk. These effects are an important consequence of the phocid strategy for enabling terrestrial parturition despite marine feeding. This strategy has resulted in an abbreviated and highly efficient lactation system that is strongly impacted by body reserves, linking foraging success at sea with reproductive success on land. Maternal size, body composition, and age were important features of reproduction in northern elephant seals. These characteristics are rarely considered concurrently in life history studies.

MATERNAL TRAITS AND REPRODUCTIVE EFFORT IN NORTHERN ELEPHANT SEALS

The aim of this study was to determine the effects of maternal traits on reproductive expenditure and energy delivery to the offspring in a capital breeder, the northern elephant seal (Mirounga angustirostris). Changes in maternal and offspring energy reserves and milk-energy delivery were examined in relation to maternal parturition mass, body composition, and age in females and pups breeding at Ano Nuevo State Reserve, California. Maternal body mass and composition had significant effects on maternal energy expenditure over lactation. Path analysis suggested no significant effects of maternal age on reproductive effort of parous females. The efficiency of milk production increased significantly with maternal age. Offspring metabolism was a relatively small component of maternal energy expenditure, with pups storing 84% of the energy obtained from milk. These effects are an important consequence of the phocid strategy for enabling terrestrial parturition despite marine feeding. This strategy has resulted in an abbreviated and highly efficient lactation system that is strongly impacted by body reserves, linking foraging success at sea with reproductive success on land. Maternal size, body composition, and age were important features of reproduction in northern elephant seals. These characteristics are rarely considered concurrently in life history studies.

Effects of buoyancy on the diving behavior of northern elephant seals.

Marine mammals experience radical seasonal changes in body composition, which would be expected to affect their buoyancy in the water. The aim of this study was to examine the relationship between such changes in buoyancy and diving behavior in northern elephant seals Mirounga angustirostris. This was achieved by modifying the buoyancy of 13 juvenile elephant seals translocated from Año Nuevo State Reserve, CA, USA, and released at various sites in Monterey Bay, CA, USA. The buoyancy of each seal was calculated and was increased or decreased using syntactic foam or lead weights, and their diving behavior was recorded as they returned to Año Nuevo. The seals were divided into three groups: increased buoyancy (B+), reduced buoyancy (B-) and control seals (Bc). Mean descent rates were 0.77+/-0.3 ms-1 for the B+ seals, 0.82+/-0.2 ms-1 for the control seals and 0.87+/-0.3 ms-1 for the B- seals, and were significantly different. Mean ascent rates for the three treatments were 0.82+/-0.3 ms-1 for the B+ seals, 0.86+/-0.3 ms-1 for the control seals and 0.82+/-0.3 ms-1 for the B- seals. All the B+ seals ascended faster than they descended, while four of the five B- seals descended faster than they ascended. There was a significant negative correlation between buoyancy and descent rate, with less buoyant seals descending faster than more buoyant seals. There was, however, no correlation between ascent rate and buoyancy. This suggests that seals may use negative buoyancy to drift passively during descent, but that all seals may swim continuously during ascent. There was a significant correlation between buoyancy and the drift descent rate of C-type drift dives, including upwards drift in the most buoyant seal. Buoyancy was not correlated with diving depth, trip duration, dive duration or surface-interval duration. This study demonstrates that buoyancy plays a significant role in shaping diving behavior in northern elephant seals and that elephant seals may adjust their behavior to suit their buoyancy, rather than adjusting their buoyancy to suit a dive. This study also validated the truncated cones method of calculating body composition in this species by comparing it with body composition determined using tritium dilution.

HOMING BEHAVIOR OF JUVENILE NORTHERN ELEPHANT SEALS

AbstractThe aim of this study was to determine if juvenile northern elephant seals, Mirounga angustirostris, translocated from their rookery would return to it quickly and reliably. During the spring and fall of 1990 and 1991 we captured 75 seals at Año Nuevo State Reserve, CA, U. S. A. and translocated them to release sites up to 100 km away. Eighty‐eight percent of the seals returned to the capture site within 4.7 ± 4.3 d. Homing rate increased with age, but even the youngest seals (8–10 mo) homed at a 73% rate. Homing rate did not vary significantly with sex, season, or year. Data from diving instruments suggested that the seals often followed direct routes home, arrived on the rookery significantly more often at night than during the day, and when released together, returned separately. Mean homing speed of 18 seals with complete diving records was 39 km/d (range 3–70 km/d). Instruments on seals had no detectable effect on homing rate or homing speed. The translocation paradigm provides a powerful tool for conducting intensive shortterm studies on free‐ranging seals.

A METHOD FOR ESTIMATING MASS OF LARGE PINNIPEDS

AbstractFifty‐two male elephant seals were weighed and photographed at Año Nuevo State Reserve, California, to establish a predictive relationship between photographically measured morphological variables (length, side area, and girth area) and body mass. Regression of mass on these variables revealed that side area, roughly equivalent to a longitudinal cross‐section, was the most useful single variable for predicting mass, and that adding the other two variables to side area slightly improved the accuracy of the photogrammetric technique. Curvilinear regressions based on a power model provided the best predictive relationships. This technique may prove useful for estimating body mass of other pinnipeds.