Neutral buoyancy is optimal to minimize the cost of transport in horizontally swimming seals

Downsizing a heavyweight: factors and methods that revise weight estimates of the giant fossil whale Perucetus colossus

Extremes in organismal size have broad interest in ecology and evolution because organismal size dictates many traits of an organism's biology. There is particular fascination with identifying upper size extremes in the largest vertebrates, given the challenges and difficulties of measuring extant and extinct candidates for the largest animal of all time, such as whales, terrestrial non-avian dinosaurs, and extinct marine reptiles. The discovery of Perucetus colossus, a giant basilosaurid whale from the Eocene of Peru, challenged many assumptions about organismal extremes based on reconstructions of its body weight that exceeded reported values for blue whales (Balaenoptera musculus). Here we present an examination of a series of factors and methodological approaches to assess reconstructing body weight in Perucetus, including: data sources from large extant cetaceans; fitting published body mass estimates to body outlines; testing the assumption of isometry between skeletal and body masses, even with extrapolation; examining the role of pachyostosis in body mass reconstructions; addressing method-dependent error rates; and comparing Perucetus with known physiological and ecological limits for living whales, and Eocene oceanic productivity. We conclude that Perucetus did not exceed the body mass of today's blue whales. Depending on assumptions and methods, we estimate that Perucetus weighed 60-70 tons assuming a length 17 m. We calculated larger estimates potentially as much as 98-114 tons at 20 m in length, which is far less than the direct records of blue whale weights, or the 270 ton estimates that we calculated for body weights of the largest blue whales measured by length.

Downsized: gray whales using an alternative foraging ground have smaller morphology

Describing individual morphology and growth is key for identifying ecological niches and monitoring the health and fitness of populations. Eastern North Pacific ((ENP), approximately 16 650 individuals) gray whales primarily feed in the Arctic/sub-Arctic regions, while a small subgroup called the Pacific Coast Feeding Group (PCFG, approximately 212 individuals) instead feeds between northern California, USA and British Columbia, Canada. Evidence suggests PCFG whales have lower body condition than ENP whales. Here we investigate morphological differences (length, skull, and fluke span) and compare length-at-age growth curves between ENP and PCFG whales. We use ENP gray whale length-at-age data comprised of strandings, whaling, and aerial photogrammetry (1926-1997) for comparison to data from PCFG whales collected through non-invasive techniques (2016-2022) to estimate age (photo identification) and length (drone-based photogrammetry). We use Bayesian methods to incorporate uncertainty associated with morphological measurements (manual and photogrammetric) and age estimates. We find that while PCFG and ENP whales have similar growth rates, PCFG whales reach smaller asymptotic lengths. Additionally, PCFG whales have relatively smaller skulls and flukes than ENP whales. These findings represent a striking example of morphological adaptation that may facilitate PCFG whales accessing a foraging niche distinct from the Arctic foraging grounds of the broader ENP population.

Deep dives and high tissue density increase mean dive costs in California sea lions (Zalophus californianus)

Diving is central to the foraging strategies of many marine mammals and seabirds. Still, the effect of dive depth on foraging cost remains elusive because energy expenditure is difficult to measure at fine temporal scales in wild animals. We used depth and acceleration data from eight lactating California sea lions (Zalophus californianus) to model body density and investigate the effect of dive depth and tissue density on rates of energy expenditure. We calculated body density in 5 s intervals from the rate of gliding descent. We modeled body density across depth in each dive, revealing high tissue densities and diving lung volumes (DLVs). DLV increased with dive depth in four individuals. We used the buoyancy calculated from dive-specific body-density models and drag calculated from swim speed to estimate metabolic power and cost of transport in 5 s intervals during descents and ascents. Deeper dives required greater mean power for round-trip vertical transit, especially in individuals with higher tissue density. These trends likely follow from increased mean swim speed and buoyant hinderance that increasingly outweighs buoyant aid in deeper dives. This suggests that deep diving is either a 'high-cost, high-reward' strategy or an energetically expensive option to access prey when prey in shallow waters are limited, and that poor body condition may increase the energetic costs of deep diving. These results add to our mechanistic understanding of how foraging strategy and body condition affect energy expenditure in wild breath-hold divers.

Kinematics and energetics of foraging behavior in Rice's whales of the Gulf of Mexico

Rorqual foraging behavior varies with species, prey type and foraging conditions, and can be a determining factor for their fitness. Little is known about the foraging ecology of Rice's whales (Balaenoptera ricei), an endangered species with a population of fewer than 100 individuals. Suction cup tags were attached to two Rice's whales to collect information on their diving kinematics and foraging behavior. The tagged whales primarily exhibited lunge-feeding near the sea bottom and to a lesser extent in the water-column and at the sea surface. During 6-10 min foraging dives, the whales typically circled their prey before executing one or two feeding lunges. Longer duration dives and dives with more feeding-lunges were followed by an increase in their breathing rate. The median lunge rate of one lunge per dive of both animals was much lower than expected based on comparative research on other lunge-feeding baleen whales, and may be associated with foraging on fish instead of krill or may be an indication of different foraging conditions. Both animals spent extended periods of the night near the sea surface, increasing the risk for ship strike. Furthermore, their circling before lunging may increase the risk for entanglement in bottom-longline fishing gear. Overall, these data show that Rice's whale foraging behavior differs from other lunge feeding rorqual species and may be a significant factor in shaping our understanding of their foraging ecology. Efforts to mitigate threats to Rice's whales will benefit from improved understanding of patterns in their habitat use and fine-scale ecology.

The Functional and Ecological Significance of Deep Diving by Large Marine Predators

Many large marine predators make excursions from surface waters to the deep ocean below 200 m. Moreover, the ability to access meso- and bathypelagic habitats has evolved independently across marine mammals, reptiles, birds, teleost fishes, and elasmobranchs. Theoretical and empirical evidence suggests a number of plausible functional hypotheses for deep-diving behavior. Developing ways to test among these hypotheses will, however, require new ways to quantify animal behavior and biophysical oceanographic processes at coherent spatiotemporal scales. Current knowledge gaps include quantifying ecological links between surface waters and mesopelagic habitats and the value of ecosystem services provided by biomass in the ocean twilight zone. Growing pressure for ocean twilight zone fisheries creates an urgent need to understand the importance of the deep pelagic ocean to large marine predators.

The locomotion of extinct secondarily aquatic tetrapods

The colonisation of freshwater and marine ecosystems by land vertebrates has repeatedly occurred in amphibians, reptiles, birds and mammals over the course of 300 million years. Functional interpretations of the fossil record are crucial to understanding the forces shaping these evolutionary transitions. Secondarily aquatic tetrapods have acquired a suite of anatomical, physiological and behavioural adaptations to locomotion in water. However, much of this information is lost for extinct clades, with fossil evidence often restricted to osteological data and a few extraordinary specimens with soft tissue preservation. Traditionally, functional morphology in fossil secondarily aquatic tetrapods was investigated through comparative anatomy and correlation with living functional analogues. However, in the last two decades, biomechanics in palaeobiology has experienced a remarkable methodological shift. Anatomy-based approaches are increasingly rigorous, informed by quantitative techniques for analysing shape. Moreover, the incorporation of physics-based methods has enabled objective tests of functional hypotheses, revealing the importance of hydrodynamic forces as drivers of evolutionary innovation and adaptation. Here, we present an overview of the latest research on the locomotion of extinct secondarily aquatic tetrapods, with a focus on amniotes, highlighting the state-of-the-art experimental approaches used in this field. We discuss the suitability of these techniques for exploring different aspects of locomotory adaptation, analysing their advantages and limitations and laying out recommendations for their application, with the aim to inform future experimental strategies. Furthermore, we outline some unexplored research avenues that have been successfully deployed in other areas of palaeobiomechanical research, such as the use of dynamic models in feeding mechanics and terrestrial locomotion, thus providing a new methodological synthesis for the field of locomotory biomechanics in extinct secondarily aquatic vertebrates. Advances in imaging technology and three-dimensional modelling software, new developments in robotics, and increased availability and awareness of numerical methods like computational fluid dynamics make this an exciting time for analysing form and function in ancient vertebrates.

Aerial photogrammetry and tag-derived tissue density reveal patterns of lipid-store body condition of humpback whales on their feeding grounds

Monitoring the body condition of free-ranging marine mammals at different life-history stages is essential to understand their ecology as they must accumulate sufficient energy reserves for survival and reproduction. However, assessing body condition in free-ranging marine mammals is challenging. We cross-validated two independent approaches to estimate the body condition of humpback whales (Megaptera novaeangliae) at two feeding grounds in Canada and Norway: animal-borne tags (n = 59) and aerial photogrammetry (n = 55). Whales that had a large length-standardized projected area in overhead images (i.e. whales looked fatter) had lower estimated tissue body density (TBD) (greater lipid stores) from tag data. Linking both measurements in a Bayesian hierarchical model to estimate the true underlying (hidden) tissue body density (uTBD), we found uTBD was lower (-3.5 kg m^-3) in pregnant females compared to adult males and resting females, while in lactating females it was higher (+6.0 kg m^-3). Whales were more negatively buoyant (+5.0 kg m^-3) in Norway than Canada during the early feeding season, possibly owing to a longer migration from breeding areas. While uTBD decreased over the feeding season across life-history traits, whale tissues remained negatively buoyant (1035.3 ± 3.8 kg m^-3) in the late feeding season. This study adds confidence to the effectiveness of these independent methods to estimate the body condition of free-ranging whales.

In Vivo Measurements of Lung Volumes in Ringed Seals: Insights from Biomedical Imaging

Marine mammals rely on oxygen stored in blood, muscle, and lungs to support breath-hold diving and foraging at sea. Here, we used biomedical imaging to examine lung oxygen stores and other key respiratory parameters in living ringed seals (Pusa hispida). Three-dimensional models created from computed tomography (CT) images were used to quantify total lung capacity (TLC), respiratory dead space, minimum air volume, and total body volume to improve assessments of lung oxygen storage capacity, scaling relationships, and buoyant force estimates. Results suggest that lung oxygen stores determined in vivo are smaller than those derived from postmortem measurements. We also demonstrate that-while established allometric relationships hold well for most pinnipeds-these relationships consistently overestimate TLC for the smallest phocid seal. Finally, measures of total body volume reveal differences in body density and net vertical forces in the water column that influence costs associated with diving and foraging in free-ranging seals.

Direct measurement of swimming and diving kinematics of giant Atlantic bluefin tuna (Thunnus thynnus)

Tunas possess a range of physiological and mechanical adaptations geared towards high-performance swimming that are of considerable interest to physiologists, ecologists and engineers. Advances in biologging have provided significant improvements in understanding tuna migrations and vertical movement patterns, yet our understanding of the locomotion and swimming mechanics of these fish under natural conditions is limited. We equipped Atlantic bluefin tuna (Thunnus thynnus) with motion-sensitive tags and video cameras to quantify the gaits and kinematics used by wild fish. Our data reveal significant variety in the locomotory kinematics of Atlantic bluefin tuna, ranging from continuous locomotion to two types of intermittent locomotion. The tuna sustained swimming speeds in excess of 1.5 m s-1 (0.6 body lengths s-1), while beating their tail at a frequency of approximately 1 Hz. While diving, some descents were entirely composed of passive glides, with slower descent rates featuring more gliding, while ascents were primarily composed of active swimming. The observed swimming behaviour of Atlantic bluefin tuna is consistent with theoretical models predicting such intermittent locomotion to result in mechanical and physiological advantages. Our results confirm that Atlantic bluefin tuna possess behavioural specializations to increase their locomotory performance, which together with their unique physiology improve their capacity to use pelagic and mesopelagic habitats.

Physical trade-offs shape the evolution of buoyancy control in sharks

Buoyancy control is a fundamental aspect of aquatic life that has major implications for locomotor performance and ecological niche. Unlike terrestrial animals, the densities of aquatic animals are similar to the supporting fluid, thus even small changes in body density may have profound effects on locomotion. Here, we analysed the body composition (lipid versus lean tissue) of 32 shark species to study the evolution of buoyancy. Our comparative phylogenetic analyses indicate that although lean tissue displays minor positive allometry, liver volume exhibits pronounced positive allometry, suggesting that larger sharks evolved bulkier body compositions by adding lipid tissue to lean tissue rather than substituting lean for lipid tissue, particularly in the liver. We revealed a continuum of buoyancy control strategies that ranged from more buoyant sharks with larger livers in deeper ecosystems to relatively denser sharks with small livers in epipelagic habitats. Across this eco-morphological spectrum, our hydrodynamic modelling suggests that neutral buoyancy yields lower drag and more efficient steady swimming, whereas negative buoyancy may be more efficient during accelerated movements. The evolution of buoyancy control in sharks suggests that ecological and physiological factors mediate the selective pressures acting on these traits along two major gradients, body size and habitat depth.

High diving metabolic rate indicated by high-speed transit to depth in negatively buoyant long-finned pilot whales

To maximize foraging duration at depth, diving mammals are expected to use the lowest cost optimal speed during descent and ascent transit and to minimize the cost of transport by achieving neutral buoyancy. Here, we outfitted 18 deep-diving long-finned pilot whales with multi-sensor data loggers and found indications that their diving strategy is associated with higher costs than those of other deep-diving toothed whales. Theoretical models predict that optimal speed is proportional to (basal metabolic rate/drag)^1/3 and therefore to body mass^0.05 The transit speed of tagged animals (2.7±0.3 m s^-1) was substantially higher than the optimal speed predicted from body mass (1.4-1.7 m s^-1). According to the theoretical models, this choice of high transit speed, given a similar drag coefficient (median, 0.0035) to that in other cetaceans, indicated greater basal metabolic costs during diving than for other cetaceans. This could explain the comparatively short duration (8.9±1.5 min) of their deep dives (maximum depth, 444±85 m). Hydrodynamic gliding models indicated negative buoyancy of tissue body density (1038.8±1.6 kg m^-3, ±95% credible interval, CI) and similar diving gas volume (34.6±0.6 ml kg^-1, ±95% CI) to those in other deep-diving toothed whales. High diving metabolic rate and costly negative buoyancy imply a 'spend more, gain more' strategy of long-finned pilot whales, differing from that in other deep-diving toothed whales, which limits the costs of locomotion during foraging. We also found that net buoyancy affected the optimal speed: high transit speeds gradually decreased during ascent as the whales approached neutral buoyancy owing to gas expansion.

Diving behavior in a free-living, semi-aquatic herbivore, the Eurasian beaver Castor fiber

Semi-aquatic mammals have secondarily returned to the aquatic environment, although they spend a major part of their life operating in air. Moving both on land, as well as in, and under water is challenging because such species are considered to be imperfectly adapted to both environments. We deployed accelerometers combined with a depth sensor to study the diving behavior of 12 free-living Eurasian beavers Castor fiber in southeast Norway between 2009 and 2011 to examine the extent to which beavers conformed with mass-dependent dive capacities, expecting them to be poorer than wholly aquatic species. Dives were generally shallow (<1 m) and of short duration (<30 s), suggesting that the majority of dives were aerobic. Dive parameters such as maximum diving depth, dive duration, and bottom phase duration were related to the effort during different dive phases and the maximum depth reached. During the descent, mean vectorial dynamic body acceleration (VeDBA-a proxy for movement power) was highest near the surface, probably due to increased upthrust linked to fur- and lung-associated air. Inconsistently though, mean VeDBA underwater was highest during the ascent when this air would be expected to help drive the animals back to the surface. Higher movement costs during ascents may arise from transporting materials up, the air bubbling out of the fur, and/or the animals' exhaling during the bottom phase of the dive. In a manner similar to other homeotherms, beavers extended both dive and bottom phase durations with diving depth. Deeper dives tended to have a longer bottom phase, although its duration was shortened with increased VeDBA during the bottom phase. Water temperature did not affect diving behavior. Overall, the beavers' dive profile (depth, duration) was similar to other semi-aquatic freshwater divers. However, beavers dived for only 2.8% of their active time, presumably because they do not rely on diving for food acquisition.

Physostomous channel catfish, Ictalurus punctatus, modify swimming mode and buoyancy based on flow conditions

The employment of gliding in aquatic animals as a means of conserving energy has been theoretically predicted and discussed for decades. Several studies have shown that some species glide, whereas others do not. Freshwater fish species that widely inhabit both lentic and lotic environments are thought to be able to adapt to fluctuating flow conditions in terms of locomotion. In adapting to the different functional demands of lentic and lotic environments on fish energetics, physostomous (open swim bladder) fish may optimise their locomotion and activity by controlling their net buoyancy; however, few buoyancy studies have been conducted on physostomous fish in the wild. We deployed accelerometers on free-ranging channel catfish, Ictalurus punctatus, in both lentic and lotic environments to quantify their swimming activity, and to determine their buoyancy condition preferences and whether gliding conserves energy. Individual comparisons of swimming efforts between ascent and descent phases revealed that all fish in the lentic environment had negative buoyancy. However, all individuals showed many descents without gliding phases, which was contrary to the behaviour predicted to minimise the cost of transport. The fact that significantly fewer gliding phases were observed in the lotic environment, together with the existence of neutrally buoyant fish, indicated that channel catfish seem to optimise their locomotion through buoyancy control based on flow conditions. The buoyancy optimisation of channel catfish relative to the flow conditions that they inhabit not only reflects differences in swimming behaviour but also provides new insights into the adaptation of physostome fish species to various freshwater environments.

Flipper strokes can predict energy expenditure and locomotion costs in free-ranging northern and Antarctic fur seals

Flipper strokes have been proposed as proxies to estimate the energy expended by marine vertebrates while foraging at sea, but this has never been validated on free-ranging otariids (fur seals and sea lions). Our goal was to investigate how well flipper strokes correlate with energy expenditure in 33 foraging northern and Antarctic fur seals equipped with accelerometers, GPS, and time-depth recorders. We concomitantly measured field metabolic rates with the doubly-labelled water method and derived activity-specific energy expenditures using fine-scale time-activity budgets for each seal. Flipper strokes were detected while diving or surface transiting using dynamic acceleration. Despite some inter-species differences in flipper stroke dynamics or frequencies, both species of fur seals spent 3.79 ± 0.39 J/kg per stroke and had a cost of transport of ~1.6–1.9 J/kg/m while diving. Also, flipper stroke counts were good predictors of energy spent while diving (R² = 0.76) and to a lesser extent while transiting (R² = 0.63). However, flipper stroke count was a poor predictor overall of total energy spent during a full foraging trip (R² = 0.50). Amplitude of flipper strokes (i.e., acceleration amplitude × number of strokes) predicted total energy expenditure (R² = 0.63) better than flipper stroke counts, but was not as accurate as other acceleration-based proxies, i.e. Overall Dynamic Body Acceleration.

Body density and diving gas volume of the northern bottlenose whale (Hyperoodon ampullatus)

Diving lung volume and tissue density, reflecting lipid store volume, are important physiological parameters that have only been estimated for a few breath-hold diving species. We fitted 12 northern bottlenose whales with data loggers that recorded depth, 3-axis acceleration and speed either with a fly-wheel or from change of depth corrected by pitch angle. We fitted measured values of the change in speed during 5 s descent and ascent glides to a hydrodynamic model of drag and buoyancy forces using a Bayesian estimation framework. The resulting estimate of diving gas volume was 27.4±4.2 (95% credible interval, CI) ml kg(-1), closely matching the measured lung capacity of the species. Dive-by-dive variation in gas volume did not correlate with dive depth or duration. Estimated body densities of individuals ranged from 1028.4 to 1033.9 kg m(-3) at the sea surface, indicating overall negative tissue buoyancy of this species in seawater. Body density estimates were highly precise with ±95% CI ranging from 0.1 to 0.4 kg m(-3), which would equate to a precision of <0.5% of lipid content based upon extrapolation from the elephant seal. Six whales tagged near Jan Mayen (Norway, 71°N) had lower body density and were closer to neutral buoyancy than six whales tagged in the Gully (Nova Scotia, Canada, 44°N), a difference that was consistent with the amount of gliding observed during ascent versus descent phases in these animals. Implementation of this approach using longer-duration tags could be used to track longitudinal changes in body density and lipid store body condition of free-ranging cetaceans.

The foraging benefits of being fat in a highly migratory marine mammal

Foraging theory predicts that breath-hold divers adjust the time spent foraging at depth relative to the energetic cost of swimming, which varies with buoyancy (body density). However, the buoyancy of diving animals varies as a function of their body condition, and the effects of these changes on swimming costs and foraging behaviour have been poorly examined. A novel animal-borne accelerometer was developed that recorded the number of flipper strokes, which allowed us to monitor the number of strokes per metre swam (hereafter, referred to as strokes-per-metre) by female northern elephant seals over their months-long, oceanic foraging migrations. As negatively buoyant seals increased their fat stores and buoyancy, the strokes-per-metre increased slightly in the buoyancy-aided direction (descending), but decreased significantly in the buoyancy-hindered direction (ascending), with associated changes in swim speed and gliding duration. Overall, the round-trip strokes-per-metre decreased and reached a minimum value when seals achieved neutral buoyancy. Consistent with foraging theory, seals stayed longer at foraging depths when their round-trip strokes-per-metre was less. Therefore, neutrally buoyant divers gained an energetic advantage via reduced swimming costs, which resulted in an increase in time spent foraging at depth, suggesting a foraging benefit of being fat.

Unexpected Positive Buoyancy in Deep Sea Sharks, Hexanchus griseus, and a Echinorhinus cookei

We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Sharks, for example, rely on oil-filled livers instead of gas-filled swim bladders to increase their buoyancy, but are nonetheless ubiquitously regarded as either negatively or neutrally buoyant. Deep-sea sharks have particularly large, oil-filled livers, and are believed to be neutrally buoyant in their natural habitat, but this has never been confirmed. To empirically determine the buoyancy status of two species of deep-sea sharks (bluntnose sixgill sharks, Hexanchus griseus, and a prickly shark, Echinorhinus cookei) in their natural habitat, we used accelerometer-magnetometer data loggers to measure their swimming performance. Both species of deep-sea sharks showed similar diel vertical migrations: they swam at depths of 200–300 m at night and deeper than 500 m during the day. Ambient water temperature was around 15°C at 200–300 m but below 7°C at depths greater than 500 m. During vertical movements, all deep-sea sharks showed higher swimming efforts during descent than ascent to maintain a given swimming speed, and were able to glide uphill for extended periods (several minutes), indicating that these deep-sea sharks are in fact positively buoyant in their natural habitats. This positive buoyancy may adaptive for stealthy hunting (i.e. upward gliding to surprise prey from underneath) or may facilitate evening upward migrations when muscle temperatures are coolest, and swimming most sluggish, after spending the day in deep, cold water. Positive buoyancy could potentially be widespread in fish conducting daily vertical migration in deep-sea habitats.

Gait switches in deep-diving beaked whales: biomechanical strategies for long-duration dives

Diving animals modulate their swimming gaits to promote locomotor efficiency and so enable longer, more productive dives. Beaked whales perform extremely long and deep foraging dives that probably exceed aerobic capacities for some species. Here, we use biomechanical data from suction-cup tags attached to three species of beaked whales (Mesoplodon densirostris, N=10; Ziphius cavirostris, N=9; and Hyperoodon ampullatus, N=2) to characterize their swimming gaits. In addition to continuous stroking and stroke-and-glide gaits described for other diving mammals, all whales produced occasional fluke-strokes with distinctly larger dorso-ventral acceleration, which we termed ‘type-B’ strokes. These high-power strokes occurred almost exclusively during deep dive ascents as part of a novel mixed gait. To quantify body rotations and specific acceleration generated during strokes we adapted a kinematic method combining data from two sensors in the tag. Body rotations estimated with high-rate magnetometer data were subtracted from accelerometer data to estimate the resulting surge and heave accelerations. Using this method, we show that stroke duration, rotation angle and acceleration were bi-modal for these species, with B-strokes having 76% of the duration, 52% larger body rotation and four times more surge than normal strokes. The additional acceleration of B-strokes did not lead to faster ascents, but rather enabled brief glides, which may improve the overall efficiency of this gait. Their occurrence towards the end of long dives leads us to propose that B-strokes may recruit fast-twitch fibres that comprise ∼80% of swimming muscles in Blainville's beaked whales, thus prolonging foraging time at depth.

Drift diving by hooded seals (Cystophora cristata) in the Northwest Atlantic Ocean

Many pinniped species perform a specific dive type, referred to as a 'drift dive', where they drift passively through the water column. This dive type has been suggested to function as a resting/sleeping or food processing dive, and can be used as an indication of feeding success by calculating the daily change in vertical drift rates over time, which reflects the relative fluctuations in buoyancy of the animal as the proportion of lipids in the body change. Northwest Atlantic hooded seals perform drift dives at regular intervals throughout their annual migration across the Northwest Atlantic Ocean. We found that the daily change in drift rate varied with geographic location and the time of year and that this differed between sexes. Positive changes in buoyancy (reflecting increased lipid stores) were evident throughout their migration range and although overlapping somewhat, they were not statistically associated with high use areas as indicated by First Passage Time (FPT). Differences in the seasonal fluctuations of buoyancy between males and females suggest that they experience a difference in patterns of energy gain and loss during winter and spring, associated with breeding. The fluctuations in buoyancy around the moulting period were similar between sexes.

From physiology to physics: are we recognizing the flexibility of biologging tools?

The remote measurement of data from free-ranging animals has been termed ‘biologging’ and in recent years this relatively small set of tools has been instrumental in addressing remarkably diverse questions – from ‘how will tuna respond to climate change?’ to ‘why are whales big?’. While a single biologging dataset can have the potential to test hypotheses spanning physiology, ecology, evolution and theoretical physics, explicit illustrations of this flexibility are scarce and this has arguably hindered the full realization of the power of biologging tools. Here we present a small set of examples from studies that have collected data on two parameters widespread in biologging research (depth and acceleration), but that have interpreted their data in the context of extremely diverse phenomena: from tests of biomechanical and diving-optimality models to identifications of feeding events, Lévy flight foraging strategies and expanding oxygen minimum zones. We use these examples to highlight the remarkable flexibility of biologging tools, and identify several mechanisms that may enhance the scope and dissemination of future biologging research programs.

Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour

Mature female southern elephant seals come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring descent rate during drift dives. However, relatively few drift dives are performed resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were instrumented with time-depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed consequences of density change on the swimming effort of individuals while diving and effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One percent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.