ABSTRACT 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.

To investigate the thermal characteristics of large reptiles living in water, temperature data were continuously recorded from 16 free-ranging loggerhead turtles, Caretta caretta , during internesting periods using data loggers. Core body temperatures were 0.7–1.7°C higher than ambient water temperatures and were kept relatively constant. Unsteady numerical simulations using a spherical thermodynamic model provided mechanistic explanations for these phenomena, and the body temperature responses to fluctuating water temperature can be simply explained by a large body mass with a constant thermal diffusivity and a heat production rate rather than physiological thermoregulation. By contrast, body temperatures increased 2.6–5.1°C in 107–152 min during their emergences to nest on land. The estimated heat production rates on land were 7.4–10.5 times the calculated values in the sea. The theoretical prediction that temperature difference between body and water temperatures would increase according to the body size was confirmed by empirical data recorded from several species of sea turtles. Comparing previously reported data, the internesting intervals of leatherback, green and loggerhead turtles were shorter when the body temperatures were higher. Sea turtles seem to benefit from a passive thermoregulatory strategy, which depends primarily on the physical attributes of their large body masses.

SUMMARY Northern elephant seals, Mirounga angustirostris , travel between colonies along the west coast of North America and foraging areas in the North Pacific. They also have the ability to return to their home colony after being experimentally translocated. However, the mechanisms of this navigation are not known. Visual information could serve an important role in navigation, either primary or supplementary. We examined the role of visual cues in elephant seal navigation by translocating three seals and recording their heading direction continuously using GPS, and acceleration and geomagnetic data loggers while they returned to the colony. The seals first reached the coast and then proceeded to the colony by swimming along the coast. While underwater the animals exhibited a horizontally straight course (mean net-to-gross displacement ratio=0.94±0.02). In contrast, while at the surface they changed their headings up to 360 deg. These results are consistent with the use of visual cues for navigation to the colony. The seals may visually orient by using landmarks as they swim along the coast. We further assessed whether the seals could maintain a consistent heading while underwater during drift dives where one might expect that passive spiraling during drift dives could cause disorientation. However, seals were able to maintain the initial course heading even while underwater during drift dives where there was spiral motion (to within 20 deg). This behavior may imply the use of non-visual cues such as acoustic signals or magnetic fields for underwater orientation.

SUMMARY Aerial flight and breath-hold diving present conflicting morphological and physiological demands, and hence diving seabirds capable of flight are expected to face evolutionary trade-offs regarding locomotory performances. We tested whether Kerguelen shags Phalacrocorax verrucosus , which are remarkable divers, have poor flight capability using newly developed tags that recorded their flight air speed (the first direct measurement for wild birds) with propeller sensors, flight duration, GPS position and depth during foraging trips. Flight air speed (mean 12.7 m s –1 ) was close to the speed that minimizes power requirement, rather than energy expenditure per distance, when existing aerodynamic models were applied. Flights were short (mean 92 s), with a mean summed duration of only 24 min day –1 . Shags sometimes stayed at the sea surface without diving between flights, even on the way back to the colony, and surface durations increased with the preceding flight durations; these observations suggest that shags rested after flights. Our results indicate that their flight performance is physiologically limited, presumably compromised by their great diving capability (max. depth 94 m, duration 306 s) through their morphological adaptations for diving, including large body mass (enabling a large oxygen store), small flight muscles (to allow for large leg muscles for underwater propulsion) and short wings (to decrease air volume in the feathers and hence buoyancy). The compromise between flight and diving, as well as the local bathymetry, shape the three-dimensional foraging range (<26 km horizontally, <94 m vertically) in this bottom-feeding cormorant.

SUMMARY To understand the foraging strategies of free-ranging diving animals, time series information on both foraging effort and foraging success is essential. Theory suggests that wing stroke frequency for aerial flight should be higher in heavier birds. Based on this premise, we developed a new methodology using animal-borne accelerometers to estimate fine-scale temporal changes in body mass of a pursuit-diving, piscivorous seabird, the European shag, Phalacrocorax aristotelis . We hypothesized that variations in body mass determined from changes in wing stroke frequency before and after a series of dives would be related to the amount of prey captured. The estimated net gain in body mass during a foraging trip was highly variable, ranging from–30 to 260 g, values that were extremely similar to food loads obtained from shags on the Isle of May in previous years using water-offloading and nest balances. Load sizes estimated using the wing stroke method were strongly and positively related to both cumulative flight time and return flight time. At the trip level, load size was unrelated to cumulative dive bout duration and the total amount of time spent underwater. However, highly significant relationships were apparent at the individual bout level, with birds showing bigger mass gains following longer dive bouts. Results from this study are therefore extremely encouraging and suggest that changes in body mass determined from changes in wing stroke frequency may provide a reliable method of obtaining short- to medium-term information on foraging effort and success of diving seabirds.

SUMMARY Buoyancy is one of the primary external forces acting on air-breathing divers and it can affect their swimming energetics. Because the body composition of marine mammals (i.e. the relative amounts of lower-density lipid and higher-density lean tissue) varies individually and seasonally,their buoyancy also fluctuates widely, and individuals would be expected to adjust their stroke patterns during dives accordingly. To test this prediction, we attached acceleration data loggers to four free-ranging Baikal seals Phoca sibirica in Lake Baikal and monitored flipper stroking activity as well as swimming speed, depth and inclination of the body axis(pitch). In addition to the logger, one seal (Individual 4) was equipped with a lead weight that was jettisoned after a predetermined time period so that we had a set of observations on the same individual with different body densities. These four data sets revealed the general diving patterns of Baikal seals and also provided direct insights into the influence of buoyancy on these patterns. Seals repeatedly performed dives of a mean duration of 7.0 min(max. 15.4 min), interrupted by a mean surface duration of 1.2 min. Dive depths were 66 m on average, but varied substantially, with a maximum depth of 324 m. The seals showed different stroke patterns among individuals; some seals stroked at lower rates during descent than ascent, while the others had higher stroke rates during descent than ascent. When the lead weight was detached from Individual 4, the seal increased its stroke rate in descent by shifting swimming mode from prolonged glides to more stroke-and-glide swimming, and decreased its stroke rate in ascent by shifting from continuous stroking to stroke-and-glide swimming. We conclude that seals adopt different stroke patterns according to their individual buoyancies. We also demonstrate that the terminal speed reached by Individual 4 during prolonged glide in descent depended on its total buoyancy and pitch, with higher speeds reached in the weighted condition and at steeper pitch. A simple physical model allowed us to estimate the body density of the seal from the speed and pitch(1027-1046 kg m -3 , roughly corresponding to 32-41% lipid content,for the weighted condition; 1014-1022 kg m -3 , 43-47% lipid content,for the unweighted condition).

SUMMARY Warm-blooded diving animals wintering in polar regions are expected to show a high degree of morphological adaptation allowing efficient thermal insulation. In stark contrast to other marine mammals and seabirds living at high latitudes, Arctic great cormorants Phalacrocorax carbo have very limited thermal insulation because of their partly permeable plumage. They nonetheless winter in Greenland, where they are exposed to very low air and water temperatures. To understand how poorly insulated diving endotherms survive the Arctic winter, we performed year-round recordings of heart rate,dive depth and abdominal temperature in male great cormorants using miniature data loggers. We also examined the body composition of individuals in the spring. Abdominal temperatures and heart rates of birds resting on land and diving showed substantial variability. However, neither hypothermia nor significantly lower heart rate levels were recorded during the winter months. Thus our data show no indication of general metabolic depression in great cormorants wintering in Greenland. Furthermore, great cormorants did not reduce their daily swimming time during the coldest months of the year to save energy; they continued to forage in sub-zero waters for over an hour every day. As birds spent extended periods in cold water and showed no signs of metabolic depression during the Arctic winter, their theoretical energy requirements were substantial. Using our field data and a published algorithm we estimated the daily food requirement of great cormorants wintering in Greenland to be 1170±110 g day -1 . This is twice the estimated food requirement of great cormorants wintering in Europe. Great cormorants survive the Arctic winter but we also show that they come close to starvation during the spring, with body reserves sufficient to fast for less than 3 days. Lack of body fuels was associated with drastically reduced body temperatures and heart rates in April and May. Concurrent, intense feeding activity probably allowed birds to restore body reserves. Our study is the first to record ecophysiological parameters in a polar animal on a year-round basis. It challenges the paradigm that efficient thermal insulation is a prerequisite to the colonization of polar habitats by endotherms.

SUMMARY Emperor penguins leap from the water onto the sea ice. Their ability to reach above-water height depends critically on initial vertical speed of their leaping, assuming that the kinetic energy is converted to gravitational potential energy. We deliberately changed the above-water heights of ice hole exits, in order to examine whether penguins adjusted swim speed in accordance with the above-water height of the ice. Penguins were maintained in a corral on the fast ice in Antarctica, and voluntarily dived through two artificial ice holes. Data loggers were deployed on the penguins to monitor under water behavior. Nine instrumented penguins performed 386 leaps from the holes during experiments. The maximum swim speeds within 1 s before the exits through the holes correlated significantly with the above-water height of the holes. Penguins adopted higher speed to exit through the higher holes than through the lower holes. Speeds of some failed exits were lower than the theoretical minimum values to reach a given height. Penguins failed to exit onto the sea ice in a total of 37 of the trials. There was no preference to use lower holes after they failed to exit through the higher holes. Rather, swim speed was increased for subsequent attempts after failed leaps. These data demonstrated that penguins apparently recognized the above-water height of holes and adopted speeds greater than the minimal vertical speeds to reach the exit height. It is likely, especially in the case of higher holes (>40 cm), that they chose minimum speeds to exit through the holes to avoid excess energy for swimming before leaping. However, some exceptionally high speeds were recorded when they directly exited onto the ice from lower depths. In those cases,birds could increase swim speed without strokes for the final seconds before exit and they only increased the steepness of their body angles as they surfaced, which indicates that the speed required for leaps by emperor penguins were aided by buoyancy, and that penguins can sometimes exit through the ice holes without any stroking effort before leaping.

SUMMARY It is generally assumed that air-breathing aquatic animals always choose the shortest route to minimize duration for transit between the surface and foraging depth in order to maximize the proportion of time spent foraging. However, empirical data indicate that the body angles of some diving animals are rarely vertical during descent and ascent. Why do they choose shallower body angles that result in longer descent and ascent durations? To investigate this question, we attached acceleration data loggers to eight female macaroni penguins, breeding on the Kerguelen Islands(48°45′–50°00′S,68°45′–70°58′E; South Indian Ocean), to record depth, two-dimensional acceleration (stroke cycle frequency and body angle)and temperature. We investigated how they controlled body angle and allocated their submerged time. The instrumented females performed multiple dives( N =6952) with a mean dive depth for each bird ranging from 24.5±28.5 m to 56.4±75.1 m. Mean body angles during descent and ascent were not vertical. There was large variation in mean descent and ascent angles for a given dive depth, which, in turn, caused large variation in descent and ascent duration. Body angles were significantly correlated with time spent at the bottom-phase of the dive. Birds that spent long periods at the bottom exhibited steep body angles during ascent and subsequent descent. By contrast, they adopted shallow body angles after they had short or no bottom phases. Our results suggest that macaroni penguins stay at the bottom longer after encountering a good prey patch and then travel to the surface at steep body angles. If they do not encounter prey, they discontinue the dive,without staying at the bottom, ascend at shallow body angles and descend at shallow body angles in a subsequent dive. A shallow body angle can increase the horizontal distance covered during a dive, contributing to the move into a more profitable area in the following dive. During the ascent, in particular,macaroni penguins stopped beating their flippers. The buoyantly gliding penguins can move horizontally with minimum stroking effort before reaching the surface.

SUMMARY Using a newly developed data logger to measure acceleration, we demonstrate that free-ranging king and Adélie penguins only beat their flippers substantially during the first part of descent or when they were presumed to be chasing prey at the bottom of dives. Flipper beating stopped during the latter part of ascent: at 29±9 % (mean ± S.D.) of dive depth(mean dive depth=136.8±145.1 m, N =425 dives) in king penguins,and at 52±20 % of dive depth (mean dive depth=72.9±70.5 m, N =664 dives) in Adélie penguins. Propulsive swim speeds of both species were approximately 2 m s -1 during dives; however, a marked increase in speed, up to approximately 2.9 m s -1 , sometimes occurred in king penguins during the passive ascending periods. During the prolonged ascending, oblique ascent angle and slowdown near the surface may represent one way to avoid the potential risk of decompression sickness. Biomechanical calculations for data from free-ranging king and Adélie penguins indicate that the air volume of the birds (respiratory system and plumage) can provide enough buoyancy for the passive ascent. When comparing the passive ascents for shallow and deep dives, there is a positive correlation between air volume and the depth of the dive. This suggests that penguins regulate their air volume to optimize the costs and benefits of buoyancy.

SUMMARY Heart rate and dive behaviour were monitored in double-crested cormorants ( Phalacrocorax auritus ) during shallow (1 m) and deep diving (12 m), after breathing different gas mixtures, to investigate the role of depth and the accompanying changes in blood gas levels in cardiac and behavioural control during voluntary diving. Pre-dive heart rate in both shallow- and deep-diving birds was approximately three times the resting heart rate (137.9±17.5 beats min –1 ; mean ± s.d. , N =5), falling abruptly upon submersion to around 200–250 beats min –1 . During shallow diving, the initial reduction in heart rate was followed by a secondary, more gradual decline, to around the resting level. In contrast, during deep diving, heart rate stabilised at 200–250 beats min –1 . In dives of similar duration, mean dive heart rate was significantly lower during shallow diving (163.2±14.0 beats min –1 ) than during deep diving (216.4±7.7 beats min –1 ), but in both cases was significantly above the resting value. The difference in cardiac response is probably due to an increase in arterial oxygen tension ( P a O 2 ) during the descent phase of deep dives (compression hyperoxia). Exposure to a hyperoxic gas mixture before shallow diving significantly increased mean dive heart rate, while exposure to a hypoxic gas mixture in both the shallow and deep dive tanks significantly reduced mean dive heart rate. In contrast, breathing hypercapnic gas before diving had no significant effect on dive heart rate. We suggest that the cardiac response to voluntary diving in double-crested cormorants is strongly influenced by changes in blood oxygen levels throughout the dive. Dive duration was unaffected by alterations in inspired gas composition, but surface interval duration decreased during hyperoxic gas exposure and increased during hypoxic gas exposure. The most efficient dive pattern (highest dive/pause ratio) was observed after hyperoxic exposure. Our study suggests that blood oxygen level is a powerful stimulus that facilitates the cardiac and behavioural adjustments during foraging that are important components of a strategy allowing double-crested cormorants to maximise the time spent under water and, hence, potential foraging time.