High diving metabolism results in a short aerobic dive limit for Steller sea lions (Eumetopias jubatus)

The diving capacity of marine mammals is typically defined by the aerobic dive limit (ADL) which, in lieu of direct measurements, can be calculated (cADL) from total body oxygen stores (TBO) and diving metabolic rate (DMR). To estimate cADL, we measured blood oxygen stores, and combined this with diving oxygen consumption rates (VO₂) recorded from 4 trained Steller sea lions diving in the open ocean to depths of 10 or 40 m. We also examined the effect of diving exercise on O₂ stores by comparing blood O₂ stores of our diving animals to non-diving individuals at an aquarium. Mass-specific blood volume of the non-diving individuals was higher in the winter than in summer, but there was no overall difference in blood O₂ stores between the diving and non-diving groups. Estimated TBO (35.9 ml O₂ kg^?1) was slightly lower than previously reported for Steller sea lions and other Otariids. Calculated ADL was 3.0 min (based on an average DMR of 2.24 L O₂ min^?1) and was significantly shorter than the average 4.4 min dives our study animals performed when making single long dives—but was similar to the times recorded during diving bouts (a series of 4 dives followed by a recovery period on the surface), as well as the dive times of wild animals. Our study is the first to estimate cADL based on direct measures of VO₂ and blood oxygen stores for an Otariid and indicates they have a much shorter ADL than previously thought.     

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.     

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.     

Physiological and behavioral determinants of the aerobic dive limit in Weddell seal (Leptonychotes weddellii) Pups

The aerobic dive limit, as defined by an increase in plasma lactate levels following dives, has to date only been determined in adult and juvenile Weddell seals (Leptonychotes weddellii). However, theoretical aerobic dive limits based on calculated total body oxygen stores, estimated metabolic rates, and dive duration frequencies have been published for several species. Using data collected over the past 3 years in McMurdo Sound. Antarctica, the aerobic dive limit of Weddell seal pups was determined by both the physiological and modeling methods. Time-depth diving recorders deployed on 36 pups between 2 and 14 weeks of age allowed the aerobic dive limit to be predicted from duration-frequency histograms. The aerobic dive limit was also calculated from estimates of total body oxygen stores and predicted diving metabolic rates. Finally, these two estimates were compared with aerobic dive limits determined from post-dive lactate levels in three pups between 5 and 7 weeks old. The aerobic dive limits of pups increased with age, but pup aerobic dive limits were still significantly shorter than those of yearlings and adults. In addition, the aerobic dive limits determined by the three methods were not equivalent for pups, yearlings, or adults, and indicate that care should be taken when modeling methods are used to estimate the aerobic dive limit in other species. Changes in hematocrit, plasma glucose, and plasma lactate levels during and between rest, diving, and recovery in pups were compared to known values for juveniles and adults. Plasma metabolite levels were more highly regulated in older pups, and together with the increasing aerobic dive limit, suggest that Weddell seal pups are not refined divers until after they are weaned, and that their diving ability continues to develop over several years.     

The influence of body size on the diving behaviour and physiology of the bimodally respiring turtle, Elseya albagula

In aquatic vertebrates that acquire oxygen aerially dive duration scales positively with body mass, i.e. larger animals can dive for longer periods, however in bimodally respiring animals the relationship between dive duration and body mass is unclear. In this study we investigated the relationships between body size, aquatic respiration, and dive duration in the bimodally respiring turtle, Elseya albagula. Under normoxic conditions, dive duration was found to be independent of body mass. The dive durations of smaller turtles were equivalent to that of larger individuals despite their relatively smaller oxygen stores and higher mass specific metabolic rates. Smaller turtles were able to increase their dive duration through the use of aquatic respiration. Smaller turtles had a relatively higher cloacal bursae surface area than larger turtles, which allowed them to extract a relatively larger amount of oxygen from the water. By removing the ability to respire aquatically (hypoxic conditions), the dive duration of the smaller turtles significantly decreased restoring the normal positive relationship between body size and dive duration that is seen in other air-breathing vertebrates.     

Body oxygen stores, aerobic dive limits, and the diving abilities of juvenile and adult muskrats (Ondatra zibethicus)

Intraspecific variability in body oxygen reserves, muscle buffering capacity, diving metabolic rate, and diving behavior were examined in recently captured juvenile and adult muskrats. Allometric scaling exponents for lung (b=1.04), blood (b=0.91), and total body oxygen storage capacity (b=1.09) did not differ from unity. The concentration of skeletal muscle myoglobin scaled positively with mass in 254-600-g juveniles (b=1.63) but was mass-independent in larger individuals. Scaling exponents for diving metabolic rate and calculated aerobic dive limit (ADL) were 0.74 and 0.37, respectively. Contrary to allometric predictions, we found no evidence that the diving abilities of muskrats increased with age or body size. Juveniles aged 1-2 mo exhibited similar dive times but dove more frequently than summer-caught adults. Average and cumulative dive times and dive&rcolon;surface ratios were highest for fall- and winter-caught muskrats. Total body oxygen reserves were greatest in winter, mainly due to an increase in blood oxygen storage capacity. The buffering capacity of the hind limb swimming muscles also was highest in winter-caught animals. Several behavioral indicators of dive performance, including average and maximum duration of voluntary dives, varied positively with blood hemoglobin and muscle myoglobin concentration of muskrats. However, none of the behavioral measures were strongly correlated with the total body oxygen reserves or ADLs derived for these same individuals.     

Changes in metabolism in response to fasting and food restriction in the Steller sea lion (Eumetopias jubatus)

Many animals lower their resting metabolism (metabolic depression) when fasting or consuming inadequate food. We sought to document this response by subjecting five Steller sea lions to periods of: (1) complete fasting; or (2) restricting them to 50% of their normal herring diet. The sea lions lost an average of 1.5% of their initial body mass per day (2.30 kg/d) during the 9-14-day fast, and their resting metabolic rates decreased 31%, which is typical of a "fasting response". However, metabolic depression did not occur during the 28-day food restriction trials, despite the loss of 0.30% of body mass per day (0.42 kg/d). This difference in response suggests that undernutrition caused by reduced food intake may stimulate a "hunger response", which in turn might lead to increased foraging effort. The progressive changes in metabolism we observed during the fasts were related to, but were not directly caused by, changes in body mass from control levels. Combining these results with data collected from experiments when Steller sea lions were losing mass on low energy squid and pollock diets reveals a strong relationship between relative changes in body mass and relative changes in resting metabolism across experimental conditions. While metabolic depression caused by fasting or consuming large amounts of low energy food reduced the direct costs from resting metabolism, it was insufficient to completely overcome the incurred energy deficit.     

Energetic cost of foraging in free-diving emperor penguins.

Hypothesizing that emperor penguins (Aptenodytes forsteri) would have higher daily energy expenditures when foraging for their food than when being hand-fed and that the increased expenditure could represent their foraging cost, we measured field metabolic rates (FMR; using doubly labeled water) over 4-d periods when 10 penguins either foraged under sea ice or were not allowed to dive but were fed fish by hand. Surprisingly, penguins did not have higher rates of energy expenditure when they dove and captured their own food than when they did not forage but were given food. Analysis of time-activity and energy budgets indicated that FMR was about 1.7 x BMR (basal metabolic rate) during the 12 h d(-1) that penguins were lying on sea ice. During the remaining 12 h d(-1), which we termed their "foraging period" of the day, the birds were alert and active (standing, preening, walking, and either free diving or being hand-fed), and their FMR was about 4.1 x BMR. This is the lowest cost of foraging estimated to date among the eight penguin species studied. The calculated aerobic diving limit (ADL(C)), determined with the foraging period metabolic rate of 4.1 x BMR and known O(2) stores, was only 2.6 min, which is far less than the 6-min ADL previously measured with postdive lactate analyses in emperors diving under similar conditions. This indicates that calculating ADL(C) from an at-sea or foraging-period metabolic rate in penguins is not appropriate. The relatively low foraging cost for emperor penguins contributes to their relatively low total daily FMR (2.9 x BMR). The allometric relationship for FMR in eight penguin species, including the smallest and largest living representatives, is kJ d(-1)=1,185 kg(0.705).     

The influence of depth on a breath-hold diver: Predicting the diving metabolism of Steller sea lions (Eumetopias jubatus)

Diving animals must endeavor to increase their dive depths and prolong the time they spend exploiting resources at depth. Results from captive and wild studies suggest that many diving animals extend their foraging bouts by decreasing their metabolisms while submerged. We measured metabolic rates of Steller sea lions (Eumetopias jubatus) trained to dive to depth in the open ocean to investigate the relationships between diving behaviour and the energetic costs of diving. We also constructed a general linear model to predict the oxygen consumption of sea lions diving in the wild. The resultant model suggests that swimming distance and depth of dives significantly influence the oxygen consumption of diving Steller sea lions. The predictive power of the model was tested using a cross-validation approach, whereby models reconstructed using data from pairs of sea lions were found to accurately predict the oxygen consumption of the third diving animal. Predicted oxygen consumption during dives to depth ranged from 3.37 L min^− 1 at 10 m, to 1.40 L min^− 1 at 300 m over a standardized swimming distance of 600 m. This equated to an estimated metabolic rate of 97.54 and 40.52 MJ day^− 1, and an estimated daily feeding requirement of 18.92 and 7.96 kg day^− 1 for dives between 10 and 300 m, respectively. The model thereby provides information on the potential energetic consequences that alterations in foraging strategies due to changes in prey availability could have on wild populations of sea lions.     

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.     

Seasonal changes in the oxygen storage capacity and aerobic dive limits of the muskrat (Ondatra zibethicus)

Summary The oxygen storage capacity and partitioning of body oxygen reserves were compared in summer-and winter-acclimatized muskrats (Ondatra zibethicus). Blood volume, blood oxygen capacity, and skeletal muscle myoglobin content were higher in December than in July (P<0.02). Total lung capacity increased only slightly in winter (P>0.05). The oxygen storage capacity of a diving muskrat was calculated at 25.2 ml O₂ STPD · kg^-1 in July, compared to 35.7 ml O₂ STPD · kg^-1 in December. Blood comprised the major storage compartment in both seasons, accounting for 57% and 65% of the total oxygen stores in summer and winter, respectively. Based on available oxygen stores and previous estimates of the cost of diving, the aerobic dive limit (ADL) increased from 40.9 s in July to 57.9 s in December. Concurrent behavioral studies suggested that most voluntary diving by muskrats is aerobic. However, the proportion of dives exceeding the calculated ADL of these animals was shown to vary with the context of the dive. Only 3.5% of all dives initiated by muskrats floating in the water exceeded their estimated ADL. Provision of a dry resting site and access to a submerged food source increased this proportion to 18–61%, depending on the underwater distance that foraging muskrats were required to swim. Serial dives exceeding the estimated ADL were not accompanied by extended postdive recovery periods.Abbreviations ADL acrobic dive limit - Hb hemoglobin - Hct hematocrit - Mb myoglobin - PaO₂ arterial O₂ tension - STPD standard temperature and pressure, dry     

A review of the multi-level adaptations for maximizing aerobic dive duration in marine mammals: from biochemistry to behavior

Marine mammals exhibit multi-level adaptations, from cellular biochemistry to behavior, that maximize aerobic dive duration. A dive response during aerobic dives enables the efficient use of blood and muscle oxygen stores, but it is exercise modulated to maximize the aerobic dive limit at different levels of exertion. Blood volume and concentrations of blood hemoglobin and muscle myoglobin are elevated and serve as a significant oxygen store that increases aerobic dive duration. However, myoglobin is not homogeneously distributed in the locomotory muscles and is highest in areas that produce greater force and consume more oxygen during aerobic swimming. Muscle fibers are primarily fast and slow twitch oxidative with elevated mitochondrial volume densities and enhanced oxidative enzyme activities that are highest in areas that produce more force generation. Most of the muscle mitochondria are interfibriller and homogeneously distributed. This reduces the diffusion distance between mitochondria and helps maintain aerobic metabolism under hypoxic conditions. Mitochondrial volume densities and oxidative enzyme activities are also elevated in certain organs such as liver, kidneys, and stomach. Hepatic and renal function along with digestion and assimilation continue during aerobic dives to maintain physiological homeostasis. Most ATP production comes from aerobic fat metabolism in carnivorous marine mammals. Glucose is derived mostly from gluconeogenesis and is conserved for tissues such as red blood cells and the central nervous system. Marine mammals minimize the energetic cost of swimming and diving through body streamlining, efficient, lift-based propulsive appendages, and cost-efficient modes of locomotion that reduce drag and take advantage of changes in buoyancy with depth. Most dives are within the animal’s aerobic dive limit, which maximizes time underwater and minimizes recovery time at the surface. The result of these adaptations is increased breath-hold duration and enhanced foraging ability that maximizes energy intake and minimizes energy output while making aerobic dives to depth. These adaptations are the long, evolutionary legacy of an aquatic lifestyle that directly affects the fitness of marine mammal species for different diving abilities and environments.     

REDUCTIONS IN OXYGEN CONSUMPTION DURING DIVES AND ESTIMATED SUBMERGENCE LIMITATIONS OF STELLER SEA LIONS (EUMETOPIAS JUBATUS)

Accurate estimates of diving metabolic rate are central to assessing the energy needs of marine mammals. To circumvent some of the limitations inherent with conducting energy studies in both the wild and captivity, we measured diving oxygen consumption of two trained Steller sea lions ( Eumetopias jubatus ) in the open ocean. The animals dived to predetermined depths (5–30 m) for controlled periods of time (50–200 s). Rates of oxygen consumption were measured using open-circuit respirometry before and after each dive. Mean resting rates of oxygen consumption prior to the dives were 1.34 (±0.18) and 1.95 (±0.19) liter/min for individual sea lions. Mean rates of oxygen consumption during the dives were 0.71 (±0.24) and 1.10 (±0.39) liter/min, respectively. Overall, rates of oxygen consumption during dives were significantly lower (45% and 41%) than the corresponding rates measured before dives. These results provide the first estimates of diving oxygen consumption rate for Steller sea lions and show that this species can exhibit a marked decrease in oxygen consumption relative to surface rates while submerged. This has important consequences in the evaluation of physiological limitations associated with diving such as dive duration and subsequent interpretations of diving behavior in the wild.     

Effects of increased swimming costs on foraging behavior and efficiency of captive Steller sea lions: Evidence for behavioral plasticity in the recovery phase of dives

A significant component of foraging energetics is the cost of locomotion, which for marine animals, is the cost of swimming. Increases in the cost of swimming may have significant impacts on foraging efficiency. Minimizing the cost of swimming can contribute to the optimization of foraging strategies by reducing the energetic cost of foraging. Results of several field studies suggest that an increase in the cost of locomotion may have comparable effects on foraging behavior and efficiency to a decrease in prey availability. We tested the hypothesis that an increased cost of swimming, brought on by increased hydrodynamic drag, has the same effect on dive behavior and efficiency as reduced prey availability under standard locomotion. Experiments were performed using two adult female Steller sea lions at the Alaska SeaLife Center in Seward, AK, using the same animals and general experimental design previously used to test the effects of reduced prey encounter rate on dive behavior and efficiency. Animals were fitted with a drag-inducing harness for half of the 500 simulated foraging dives in order to increase the cost of swimming. Individual dive duration and foraging time were significantly reduced in all cost-increased dives, comparable to the effects of reduced prey encounter rate. However, on a bout-by-bout basis, dive and foraging efficiency were only slightly reduced, which is likely due to an average 50% reduction in post-dive surface recovery duration during cost-increased dives. Increased heat flux across the body surface measured in a parallel study confirmed a significant increase in work during drag-increased dives. These results suggest that sea lions are able to compensate for changes in the cost of foraging and maintain their foraging efficiency by altering their dive strategy over an entire bout of dives when operating well within their aerobic scope.     

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     

Diving behaviour, dive cycles and aerobic dive limit in the platypus Ornithorhynchus anatinus

We investigated the diving behaviour, the time allocation of the dive cycle and the behavioural aerobic dive limit (ADL) of platypuses (Ornithorhynchus anatinus) living at a sub-alpine Tasmanian lake. Individual platypuses were equipped with combined data logger-transmitter packages measuring dive depth. Mean dive duration was 31.3 s with 72% of all dives lasting between 18 and 40 s. Mean surface duration was 10.1 s. Mean dive depth was 1.28 m with a maximum of 8.77 m. Platypuses performed up to 1600 dives per foraging trip with a mean of 75 dives per hour. ADL was estimated by consideration of post-dive surface intervals vs. dive durations. Only 15% of all dives were found to exceed the estimated ADL of 40 s, indicating mainly aerobic diving in the species. Foraging platypuses followed a model of optimised recovery time, the optimal breathing theory. Total bottom duration or total foraging duration per day is proposed as a useful indicator of foraging efficiency and hence habitat quality in the species.     

The diving paradox: new insights into the role of the dive response in air-breathing vertebrates

When aquatic reptiles, birds and mammals submerge, they typically exhibit a dive response in which breathing ceases, heart rate slows, and blood flow to peripheral tissues is reduced. The profound dive response that occurs during forced submergence sequesters blood oxygen for the brain and heart while allowing peripheral tissues to become anaerobic, thus protecting the animal from immediate asphyxiation. However, the decrease in peripheral blood flow is in direct conflict with the exercise response necessary for supporting muscle metabolism during submerged swimming. In free diving animals, a dive response still occurs, but it is less intense than during forced submergence, and whole-body metabolism remains aerobic. If blood oxygen is not sequestered for brain and heart metabolism during normal diving, then what is the purpose of the dive response? Here, we show that its primary role may be to regulate the degree of hypoxia in skeletal muscle so that blood and muscle oxygen stores can be efficiently used. Paradoxically, the muscles of diving vertebrates must become hypoxic to maximize aerobic dive duration. At the same time, morphological and enzymatic adaptations enhance intracellular oxygen diffusion at low partial pressures of oxygen. Optimizing the use of blood and muscle oxygen stores allows aquatic, air-breathing vertebrates to exercise for prolonged periods while holding their breath.     

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.     

Diving behaviour and energetics in breeding little penguins (Eudyptula minor)

We present data on the diving behaviour and the energetics of breeding little penguins in Tasmania, Australia. Using an 18 m long still water canal in conjunction with respirometry, we determined the energy requirements while diving. Using electronic devices measuring dive depth or swimming speed, we investigated the foraging behaviour at sea. Cost of Transport was calculated to be minimal at the speed the birds prefer at sea (1.8 m/s) and averaged 11.1 J/kg/m (power requirements at that speed: 20.0 W/kg). Metabolic rate of little penguins resting in water was found to be 8.5 W/kg. The externally-attached devices had no significant influence on the energy expenditure.
Foraging trips can be divided into four distinct phases with different diving behaviours. A mean of 500 dives was executed per foraging trip lasting about 18 hours with 60% of this time being spent swimming. The total distance travelled averaged 73 km per day, although foraging range was about 12km. Mean swimming speed of little penguins at sea was 1.8 m/s, maximum swimming speed was 3.3 m/s. More than 50% of all dives had maxima not exceeding 2 m. Maximum depth reached was 27 m. Mean dive duration was 21 s. There were inter-sex differences in diving behaviour as well as changes in foraging behaviour over the breeding period. Aerobic dive limits (ADL) in the wild were estimated between 42 and 50 s. From the swim canal experiments we derived an ADL of 44 s. Total oxygen stores were calculated to be 45 ml O₂/kg. Only 2% of all dives exceeded the ADL. FMRs at sea were calculated to be between 1280 and 1500 kJ/kg/d according to chick size. The yearly food requirements of a breeding little penguin amount to 114 kg.     

Development of foraging behavior in juvenile northern elephant seals

Rapid development of foraging ability is critical for phocids. In northern elephant seals Mirounga angustirostris , juvenile survivorship is low compared with adults and foraging difficulties are potentially associated with increased mortality. At Año Nuevo, California, foraging behavior of nine juvenile females during their third foraging migration and five juvenile females on their fourth foraging migration were documented using a variety of commercially available and custom time depth recorders. Foraging success, diving ability, time at depth, bouts of behavior and body composition changes were compared between trips to sea. There were no significant differences in foraging success measured as mass gain between the third and fourth trips to sea. There were differences in how energy was deposited between lean and adipose tissue compartments. Diving ability developed between trips to sea, reflected in significant increases in depth, dive duration and bottom time. Development also occurred within trips to sea. Depth, dive duration and bottom time increased with time at sea. Aerobic capacity appears to increase between the third and fourth trip, with a significantly increased percentage of total time submerged and a significantly lower diving rate. All juveniles on the fourth trip and four out of nine juveniles on the third trip followed marked diel patterns, foraging deep during the day and shallow at night. Like adults, juveniles appeared to stay primarily aerobic with surface intervals independent of dive durations. These results confirm that female juvenile northern elephant seals undergo important developmental changes in foraging behavior between the third and fourth trip, but these changes do not significantly impact foraging success.