Development of body oxygen stores in harbor seals: effects of age, mass, and body composition

Harbor seal pups are highly precocial and can swim and dive at birth. Such behavioral maturity suggests that they may be born with mature body oxygen stores or that stores develop quickly during the nursing period. To test this hypothesis, we compared the blood and muscle oxygen stores of harbor seal pups, yearlings, and adults. We found that pups had smaller oxygen stores than adults (neonates 57%, weaned pups 75%, and yearlings 90% those of adults), largely because neonatal myoglobin concentrations were low (1.6+/-0.2 g% vs. 3.8+/-0.3 g% for adults) and changed little during the nursing period. In contrast, blood oxygen stores were relatively mature, with nursing pups having hematocrit (55%+/-0.2%), hemoglobin (21.7+/-0.4 g%), and blood volume (12.3+/-0.5 mL/kg) only slightly lower than the corresponding values for adults (57%+/-0.2%, 23.8+/-0.3 g %, and 15.0+/-0.5 mL/kg). Because neonatal pups had relatively high metabolic rates (11.0 mL O2/kg min), their calculated aerobic dive limit was less than 50% that of adults. These results suggest that harbor seals' early aquatic activity is primarily supported by rapid development of blood, with immature muscle oxygen stores and elevated use rates limiting aerobic diving ability.     

Aerobic dive limit. What is it and is it always used appropriately?

The original definition of aerobic dive limit (ADL) was the dive duration after which there is an increase in post-dive concentration of lactate in the blood of Weddell seals freely diving in the field. The only other species in which such measurements have been made is the emperor penguin. For all other species, aerobic dive limit has been calculated (cADL) by dividing usable oxygen stores with an estimation of the rate of oxygen consumption during diving. Unfortunately, cADL is often referred to as the aerobic dive limit, implying that it is equivalent to that determined from the measurement of post-dive blood lactate concentration. However, this is not so, as at cADL all of the usable oxygen would have been consumed, whereas Weddell seals and emperor penguins can dive for at least 2-3 times longer than their ADL. Thus, at ADL, there is still some usable oxygen remaining in the stores. It is suggested that to avoid continued confusion between these two terms, the former is called diving lactate threshold (DLT), as it is somewhat analogous to the lactate threshold in exercising terrestrial vertebrates. Possible explanations of how some species routinely dive beyond their cADL are also discussed.     

Development of the blood and muscle oxygen stores in gray seals (Halichoerus grypus): implications for juvenile diving capacity and the necessity of a terrestrial postweaning fast

To successfully transition from nursing to foraging, phocid seal pups must develop adequate diving physiology within the limited time between birth and their first independent foraging trip to sea. We studied the postpartum development of oxygen stores in gray seals (Halichoerus grypus, n=40) to better understand the ontogeny of diving capacity in phocids. Hemoglobin (Hb), hematocrit (Hct), blood volume (BV), and myoglobin (Mb) levels in newborn (3 d postpartum [DPP]) and newly weaned (17+/-0.4 DPP) pups were among the lowest measured across age classes. During the pups' terrestrial postweaning fast (PWF), Hb, Hct, mass-specific BV, and Mb increased by 28%, 21%, 13%, and 29%, respectively, resulting in a 35% increase in total body mass-specific oxygen stores and a 23% increase in calculated aerobic dive limit (CADL). Although Hb and Hct levels at the end of the PWF were nearly identical to those of yearlings, total body mass-specific oxygen stores and CADL of weaned pups departing for sea were only 66%-67% and 32%-62%, respectively, of those for yearlings and adult females. The PWF represents an integral component of the physiological development of diving capacity in phocids; however, newly independent phocids still appear to have limited diving capabilities at the onset of foraging.     

Movements and diving behavior of weaned Weddell seal (Leptonychotes weddellii ) pups

Between 1993 and 1995, the diving behavior and movement patterns of 23 weaned Weddell seal pups (Leptonychotes weddellii) were tracked in the Ross Sea. Antarctica, using satellite-linked time-depth recorders. Regression analyses revealed that for seals of between 8 and 27 weeks old, age was poorly correlated with the dive depth, duration, or frequency. However, changes in dive parameters suggested that Weddell seal pups were attempting to maximize dive time, but the manner in which this was done depended on age and time of day. Movement patterns indicated that most Weddell seal pups left their natal area by the end of February, and traveled north along the Antarctic continent coastline. Several individuals returned to McMurdo Sound, but others were last located more than 400 km from McMurdo. Routes followed suggest that pups can use the pack ice habitat, but prefer to remain closer to the coastline than do adults. Accepted: 21 July 1998     

Ontogeny of total body oxygen stores and aerobic dive potential in Steller sea lions (Eumetopias jubatus)

Two key factors influence the diving and hence foraging ability of marine mammals: increased oxygen stores prolong aerobic metabolism and decreased metabolism slows rate of fuel consumption. In young animals, foraging ability may be physiologically limited due to low total body oxygen stores and high mass specific metabolic rates. To examine the development of dive physiology in Steller sea lions, total body oxygen stores were measured in animals from 1 to 29 months of age and used to estimate aerobic dive limit (ADL). Blood oxygen stores were determined by measuring hematocrit, hemoglobin, and plasma volume, while muscle oxygen stores were determined by measuring myoglobin concentration and total muscle mass. Around 2 years of age, juveniles attained mass specific total body oxygen stores that were similar to those of adult females; however, their estimated ADL remained less than that of adults, most likely due to their smaller size and higher mass specific metabolic rates. These findings indicate that juvenile Steller sea lion oxygen stores remain immature for more than a year, and therefore may constrain dive behavior during the transition to nutritional independence.     

Aerobic dive limit does not decline in an aging pinniped

Apneustic hunters such as diving mammals exploit body oxygen stores while submerged; therefore, any decline in oxygen handling at advanced life stages could critically impair foraging ability. We calculated the aerobic dive limit (cADL = 17.9 ± 4.4 min SD) from blood and muscle oxygen stores and published metabolic rates of Weddell seals within (9-16 years, n = 24) and beyond peak-reproductive age (17-27 years, n = 26), to investigate (1) senescent constraints in apneustic hunting, and (2) whether mass or age primarily determines oxygen stores and ADL in older seals. We compared cADL with behavioral ADL from 5,275 free-ranging dives (bADL = 24.0 ± 5.3 min, n = 18 females). We observed no changes in Weddell seal oxygen stores, its determinants, or in ADLs late in life. Oxygen stores were better predicted by mass than age, consistent with published findings for young adults. Hematological panels (n = 6) were consistent across mass and age, though hematocrit (females > males, 6% elevation) and mean corpuscular hemoglobin content (females < males, 8% reduction) varied by sex. Whole blood viscosity was decreased with increasing mass in females and was higher than in males overall (+18%). This was largely due to elevated hematocrit in females, although plasma viscosity also varied under some conditions. Females had higher blood volume and elevated blood oxygen stores (vol% body mass), which did not translate into significantly higher cADL (18.1 vs. 17.1 min for males). Neither cADL nor bADL were mass- or age-dependent.     

Balancing conflicting metabolic demands of exercise and diving

During enforced diving, aquatic animals activate a set of physiological reflexes (apnea, bradycardia, peripheral vasoconstriction), which are termed the diving response and are in effect the first line of defense against hypoxia. At least in the Weddell seal, this strategy is now known also to be used in voluntary diving at sea, but the response is necessarily modified to accommodate potentially conflicting demands of diving and swimming exercise. The main modification appears to involve skeletal muscles used in swimming, which, because of their high energy requirements, must be powered by aerobic metabolism. Thus they must remain perfused at rates porportional to swimming velocity (which is why heart rates are adjusted to swimming velocity). The required regulation of O2 delivery is achieved at least in part by a well-paced release of oxygenated red blood cells, stored at the beginning of the dive apparently in the spleen. The main metabolic difference between laboratory and voluntary diving is that, in the latter, working muscles serve as a sink for lactate and thus the entry rates of lactate into the plasma can be balanced by exit rates from the plasma; the maintenance of this balance means that no excess lactate remains for a lactate washout in postdiving exercise except under long, exploratory diving. Even in the latter long dives, however, the amount of lactate formed is far less than would be expected if the energetic shortfall caused by hypoperfusion and O2 lack were made up by anaerobic glycolysis (Pasteur effect). Consequently, during diving, hypoperfused tissues necessarily sustain a metabolic arrest of variable degrees as a mechanism of defense against hypoxia.     

Heart rate and plasma lactate responses during submerged swimming and trained diving in California sea lions, Zalophus californianus

California sea lions, Zalophus californianus, were trained to elicit maximum voluntary breath holds during stationary underwater targeting, submerged swimming, and trained diving. Lowest heart rate during rest periods was 57 bpm. The heart rate profiles in all three protocols were dominated by a bradycardia of 20–50 bpm, and demonstrated that otariid diving heart rates were at or below resting heart rate. Venous blood samples were collected after submerged swimming periods of 1–3 min. Plasma lactate began to increase only after 2.3-min submersions. This rise in lactate and our inability to train sea lions to dive or swim submerged for periods longer than 3 min lead us to conclude that an aerobic limit had been reached. Due to the similarity of heart rate responses and swimming velocities recorded during submerged swimming and trained diving, this 2.3-min limit should approximate the aerobic dive limit in these 40-kg sea lions. Total body O₂ stores, based on measurements of blood and muscle O₂ stores in these animals, and prior lung O₂ store analyses, were 37–43 ml O₂ kg⁻¹. The aerobic dive limit, calculated with these O₂ stores and prior measurements of at-sea metabolic rates of sea lions, is 1.8–2 min, similar to that measured by the change in post-submersion lactate concentration. Accepted: 7 July 1996     

Effects of age and body mass on development of diving capabilities of gray seal pups: costs and benefits of the postweaning fast

Development of adequate diving capabilities is crucial for survival of seal pups and may depend on age and body size. We tracked the diving behavior of 20 gray seal pups during their first 3 mo at sea using satellite relay data loggers. We employed quantile analysis to track upper limits of dive duration and percentage time spent diving, and lower limits of surface intervals. When pups first left the breeding colony, extreme (ninety-fifth percentile) dive duration and percentage time spent diving were positively correlated with age, but not mass, at departure. Extreme dive durations and percentage time spent diving peaked at [Formula: see text] d of age at values comparable with those of adults, but were not sustained. Greater peaks in extreme percentage time spent diving occurred in pups that had higher initial values, were older at their peak, and were heavier at departure. Pups that were smaller and less capable divers when they left the colony improved extreme dive durations and percentage time spent diving more rapidly, once they were at sea. Minimum survival time correlated positively with departure mass. Pups that were heavier at weaning thus benefitted from being both larger and older at departure, but smaller pups faced a trade-off. While age at departure had a positive effect on early dive performance, departure mass impacted on peak percentage time spent diving and longer-term survival. We speculate that once small pups have attained a minimum degree of physiological development to support diving, they would benefit by leaving the colony when younger but larger to maximize limited fuel reserves, rather than undergoing further maturation on land away from potential food resources, because poor divers may be able to "catch up" once at sea.     

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.     

Acquisition of gliding skills by Weddell seal (Leptonychotes weddellii) pups during lactation

Pinnipeds give birth to their pups ashore or on ice and forage in water. Therefore, neonates initially lack the adaptations to sustain prolonged underwater diving activity. Although the physiological development for breath-holding during diving has been investigated in seal pups, little is known about the concurrent development of behavioral adaptations during lactation. In this study, multisensor data loggers were used to record diving behavior and swimming gaits of pre-weaned Weddell seal (Leptonychotes weddellii) pups. Experiments were conducted in 16 pups at the Syowa Station, Antarctica, from November to December 1999 and 2004. Swimming speeds, dive depths and flipper stroking rates were recorded for each individual for about 24 h. We found that the glide index during ascending was increased with body length, whereas the dominant stroke cycle frequency were not affected by body length, dive depth and descent or ascent phase. All pups had significantly higher stroke rates in descent than in ascent, but there was no difference between swimming speed. As we found a positive relationship between the body length and age, we considered body length as an index of growth. Therefore, we conclude that pups gradually acquire the ability to glide with utilizing positive buoyancy during ascending toward the end of lactation.     

Postnatal development of muscle biochemistry in nursing harbor seal (Phoca vitulina) pups: limitations to diving behavior?

Adult marine mammal muscles rely upon a suite of adaptations for sustained aerobic metabolism in the absence of freely available oxygen (O₂). Although the importance of these adaptations for supporting aerobic diving patterns of adults is well understood, little is known about postnatal muscle development in young marine mammals. However, the typical pattern of vertebrate muscle development, and reduced tissue O₂ stores and diving ability of young marine mammals suggest that the physiological properties of harbor seal (Phoca vitulina) pup muscle will differ from those of adults. We examined myoglobin (Mb) concentration, and the activities of citrate synthase (CS), β-hydroxyacyl coA dehydrogenase (HOAD), and lactate dehydrogenase (LDH) in muscle biopsies from harbor seal pups throughout the nursing period, and compared these biochemical parameters to those of adults. Pups had reduced O₂ carrying capacity ([Mb] 28–41% lower than adults) and reduced metabolically scaled catabolic enzyme activities (LDH/RMR 20–58% and CS/RMR 29–89% lower than adults), indicating that harbor seal pup muscles are biochemically immature at birth and weaning. This suggests that pup muscles do not have the ability to support either the aerobic or anaerobic performance of adult seals. This immaturity may contribute to the lower diving capacity and behavior in younger pups. In addition, the trends in myoglobin concentration and enzyme activity seen in this study appear to be developmental and/or exercise-driven responses that together work to produce the hypoxic endurance phenotype seen in adults, rather than allometric effects due to body size.     

Remarkable development of diving performance and migrations of hooded seals (Cystophora cristata) during their first year of life

Newborn hooded seals (Cystophora cristata) have smaller weight-specific oxygen stores than adults, but nothing is known about how this affects their diving behaviour. Here, we present data on the diving behaviour and migrations of seven weaned hooded seal pups of the Greenland Sea stock during their first year of life, as collected by use of satellite telemetry. The pups started diving 1–2 days after tagging, and during a tracking period of 25–398 days they dispersed over vast areas of the Greenland and Norwegian Seas in a manner similar to adults. The initial development of diving depths and durations in April–May was rapid, and pups reached depths of >100 m and dived for >15 min within 3 weeks of age. During early summer (May–June) this development was temporarily discontinued, to be resumed throughout autumn and winter, during which time maximum depths and durations of >700 m and >30 min, respectively, were reached. Depths and durations were significantly related to age/season, location and time of day. The dive behaviour in early summer, with relatively shallow and short dives without diurnal variations, resembles that of adults and probably reflects the vertical distribution of prey rather than physiological constraints. Dives of pups were nevertheless shallower and shorter than those of adults, but relative to body mass both hooded seal pups and adults display a remarkable diving capacity which makes the species particularly suited for studies of defence mechanisms against hypoxia insult in mammals.     

Development of the aerobic dive limit and muscular efficiency in northern fur seals (Callorhinus ursinus)

Northern fur seal (Callorhinus ursinus; NFS) populations have been declining, perhaps due to limited foraging ability of pups. Because a marine mammal’s proficiency at exploiting underwater prey resources is based on the ability to store large amounts of oxygen (O₂) and to utilize these reserves efficiently, this study was designed to determine if NFS pups had lower blood, muscle, and total body O₂ stores than adults. Pups (<1-month old) had a calculated aerobic dive limit only ~40% of adult females due to lower blood and, to a much greater extent, muscle O₂ stores. Development of the Pectoralis (Pec) and Longissimus dorsi (LD) skeletal muscles was further examined by determining their myosin heavy chain (MHC) composition and enzyme activities. In all animals, the slow MHC I and fast-twitch IIA proteins typical of oxidative fiber types were dominant, but adult muscles contained more (Pec ~50%; LD ~250% higher) fast-twitch MHC IID/X protein characteristic of glycolytic muscle fibers, than pup muscles. This suggests that adults have greater ability to generate muscle power rapidly and/or under anaerobic conditions. Pup muscles also had lower aerobic and anaerobic ATP production potential, as indicated by lower metabolically scaled citrate synthase, β-hydroxyacyl CoA dehydrogenase, and lactate dehydrogenase activities (all P values ≤0.001). In combination, these findings indicate that pups are biochemically and physiologically limited in their diving capabilities relative to adults. This may contribute to lower NFS first year survival.     

Aerobic dive limit: how often does it occur in nature?

Diving animals offer a unique opportunity to study the importance of physiological constraint in their everyday behaviors. An important component of the physiological capability of any diving animal is its aerobic dive limit (ADL). The ADL has only been measured in a few species. The goal of this study was to estimate the aerobic dive limit from measurements of body oxygen stores and at sea metabolism. This calculated ADL (cADL) was then compared to measurements of diving behavior of individual animals of three species of otariids, the Antarctic fur seal, Arctocephalus gazella, the Australian sea lion, Neophoca cinerea, and the New Zealand sea lion, Phocarctos hookeri. Antarctic fur seals dove well within the cADL. In contrast, many individuals of both sea lion species exceeded the cADL, some by significant amounts. Australian sea lions typically dove 1.4 times longer than the cADL, while New Zealand sea lions on average dove 1.5 times longer than the cADL. The tendency to exceed the cADL was correlated with the dive pattern of individual animals. In both Antarctic Fur Seals and Australian sea lions, deeper diving females made longer dives that approached or exceeded the cADL (P<0.01, r(2)=0.54). Australian and New Zealand sea lions with longer bottom times also exceeded the cADL to a greater degree. The two sea lions forage on the benthos while the fur seals feed shallow in the water column. It appears that benthic foraging requires these animals to reach or exceed their aerobic dive limit.     

Brain size in neonatal and adult Weddell seals: Costs and consequences of having a large brain

Little is known about the ontogeny of brain size in pinnipeds despite potential functional implications of brain substrate (glucose, oxygen) requirements for diving, fasting, growth, and lactation strategies. We measured brain mass (brM) and cranial capacity (CC) in newborn and adult Weddell seals. Neonatal Weddell seals had brM that represented ~70% of adult brM. Weddell seals have the largest neonatal brain, proportional to adult brain, reported for any mammal to date, which is remarkable considering the relatively small size of Weddell seal pups at birth (6%–7% of maternal body mass) compared to neonates of other highly precocial mammals. Provision of sufficient glucose to maintain the large, well‐developed brain of the neonatal Weddell seal has a nontrivial metabolic cost to both pup and mother. We therefore hypothesize that this phenomenon must have functional significance, such as allowing pups to acquire complex under‐ice navigation skills during the period of maternal attendance.     

Different thermoregulatory strategies in nearly weaned pup, yearling, and adult Weddell seals (Leptonychotes weddelli)

Mammals balance heat dissipation with heat production to maintain core body temperatures independent of their environment. Thermal balance is undoubtedly most challenging for mammals born in polar regions because small body size theoretically results in high surface-area-to-volume ratios (SA:V), which facilitate heat loss (HL). Thus, we examined the ontogeny of thermoregulatory characteristics of an ice-breeding seal (Weddell seal Leptonychotes weddelli). Morphology, blubber thickness, rectal temperature (T(r)), muscle temperature (T(m)), and skin temperatures on the trunk (T(s)) and flipper (T(f)) in 3-5-wk-old pups, yearlings, and adults were measured. Adults maintained the thickest blubber layers, while yearlings had the thinnest; T(r) and T(m) fell within a narrow range, yet T(r) and T(m) decreased significantly with body length. All seals maintained skin temperatures lower than T(r), our index of core body temperature. The T(s)'s were positively correlated with environmental temperatures; conversely, T(f)'s were not. Although pups had the greatest proportion of blubber, their greater SA:V and limited ability to minimize body-to-environment temperature gradients led to the greatest calculated mass-specific HL. This implies that pups relied on elevated metabolic heat production to counter HL. Heat production in pups and yearlings may have been aided by nonshivering thermogenesis in the skeletal muscle via the enhanced muscle mitochondrial densities that have been observed in these segments of this population.     

Foraging by deep-diving birds is not constrained by an aerobic diving limit: a model of avian depth-dependent diving metabolic rate

The theoretical aerobic diving limit (tADL) specifies the duration of a dive after which oxygen reserves available for diving are depleted. The tADL has been calculated by dividing the available oxygen stores by the diving metabolic rate (DMR). Contrary to diving mammals, most diving birds examined to date exceed the tADL by a large margin. This discrepancy between observation and theory has engendered two alternative explanations suggesting that dive duration is extended either anaerobically or by depressing aerobic metabolism. Current formulations of tADL uncritically assume that DMR is independent of depth. However, diving birds differ from other vertebrate divers by having a larger respiratory system volume and by retaining air in their plumage while diving, thereby elevating buoyancy. Because air compresses with depth, diving power requirement decreases with depth. Following this principle, we modeled DMR to depth for Adelie and little penguins and reformulated the tADL accordingly. The model's results suggest that < approximately 5% of natural dives by Adelie penguins exceed the reformulated tADL(d), or maximal aerobic depth, and none in the more buoyant little penguin. These data suggest that, for both small and large species, deep diving birds rarely if ever exceed tADL(d).     

Ontogeny of diving behaviour in the Australian sea lion: trials of adolescence in a late bloomer

1. Foraging behaviours of the Australian sea lion (Neophoca cinerea) reflect an animal working hard to exploit benthic habitats. Lactating females demonstrate almost continuous diving, maximize bottom time, exhibit elevated field metabolism and frequently exceed their calculated aerobic dive limit. Given that larger animals have disproportionately greater diving capabilities, we wanted to examine how pups and juveniles forage successfully. 2. Time/depth recorders were deployed on pups, juveniles and adult females at Seal Bay Conservation Park, Kangaroo Island, South Australia. Ten different mother/pup pairs were equipped at three stages of development (6, 15 and 23 months) to record the diving behaviours of 51 (nine instruments failed) animals. 3. Dive depth and duration increased with age. However, development was slow. At 6 months, pups demonstrated minimal diving activity and the mean depth for 23-month-old juveniles was only 44 +/- 4 m, or 62% of adult mean depth. 4. Although pups and juveniles did not reach adult depths or durations, dive records for young sea lions indicate benthic diving with mean bottom times (2.0 +/- 0.2 min) similar to those of females (2.1 +/- 0.2 min). This was accomplished by spending higher proportions of each dive and total time at sea on or near the bottom than adults. Immature sea lions also spent a higher percentage of time at sea diving. 5. Juveniles may have to work harder because they are weaned before reaching full diving capability. For benthic foragers, reduced diving ability limits available foraging habitat. Furthermore, as juveniles appear to operate close to their physiological maximum, they would have a difficult time increasing foraging effort in response to reductions in prey. Although benthic prey are less influenced by seasonal fluctuations and oceanographic perturbations than epipelagic prey, demersal fishery trawls may impact juvenile survival by disrupting habitat and removing larger size classes of prey. These issues may be an important factor as to why the Australian sea lion population is currently at risk.     

Diving physiology and winter foraging behavior of a juvenile leopard seal (Hydrurga leptonyx)

Diving physiology and at-sea behavior of a juvenile leopard seal (Hydrurga leptonyx) were opportunistically measured in the Antarctic Peninsula during winter 2002. Total body oxygen stores were estimated from measures of hematocrit, hemoglobin, myoglobin, and total blood volume and were used to calculate an aerobic dive limit (ADL). Movement patterns and diving behavior were measured by equipping the seal with a Satellite Relay Data Logger that transmitted data from 8–31 August 2002. The seal remained in a focal area, in contrast to crabeater seals tracked simultaneously. The seal displayed short, shallow dives (mean 2.0±1.4 min, 44±48 m) and spent 99.9% of its time within the estimated ADL of 7.4 min. The shallow diving behavior contradicts previous diet research suggesting Antarctic krill (Euphausia superba) is the primary prey of leopard seals during the winter months as krill were found at deeper depths during this period. These measurements of diving and movement of a leopard seal provide valuable preliminary data necessary to develop future research on the at-sea behavior of an apex predator in the Antarctic ecosystem.