Diving pattern and performance in the macaroni penguinEudptes chrysolophus

Arnould J, Poupart T, Bost CA. Influence of oceanographic structures on foraging strategies: Macaroni penguins at Crozet Islands

BACKGROUND

In the open ocean, eddies and associated structures (fronts, filaments) have strong influences on the foraging activities of top-predators through the enhancement and the distribution of marine productivity, zooplankton and fish communities. Investigating how central place foragers, such as penguins, find and use these physical structures is crucial to better understanding their at-sea distribution. In the present study, we compared the travel heading and speed of the world's most abundant penguin, the Macaroni penguin (Eudyptes chrysolophus), with the distribution of surface physical structures (large-scale fronts, eddies and filaments).

RESULTS

The study was performed during December 2012 in the Crozet Archipelago (46.42° S; 51.86° E), South Indian Ocean. Six males at incubation stage were equipped with GPS loggers to get their trajectories. We used Eulerian and Lagrangian methods to locate large-scale fronts, mesoscale eddies (10-100 km) and part of the sub-mesoscale structures (<10 km, filaments) at the surface of the ocean. By comparing the positions of birds and these structures, we show that Macaroni penguins: i) target the sub Antarctic Front; ii) increase their foraging activity within a highly dynamic area, composed of eddy fields and filamentary structures; and iii) travel in the same direction as the predominant currents.

CONCLUSIONS

We show that penguins adjust their travel speed and movement during their whole trips in relation with the oceanographic structures visited. At a large scale, we hypothesize that Macaroni penguins target the sub Antarctic Front to find profitable patches of their main prey. At finer scale, Macaroni penguin may adopt a horizontal drifting behavior in strong currents, which could be a way to minimize costs of displacement.

Why do macaroni penguins choose shallow body angles that result in longer descent and ascent durations?

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.

Optimal diving under the risk of predation

Many air-breathing aquatic foragers may be killed by aerial or subsurface predators while recovering oxygen at the surface; yet the influence of predation risk on time allocation during dive cycles is little known in spite of numerous studies on optimal diving. We modeled diving behavior under the risk of predation at the surface. The relationship between time spent at the surface and the risk of death is predicted to influence the optimal surface interval, regardless of whether foragers accumulate energy at a constant rate while at the food patch, deplete food resources over the course of the dive, or must search for food during the dive. When instantaneous predation risk during a single surface interval decreases with time spent at the surface, a diver should increase its surface interval relative to that which maximizes energy intake, thereby increasing dive durations and reducing the number of surfacings per foraging bout. When instantaneous risk over a single surface interval does not change or increases with increasing time at the surface, divers should decrease their surface interval (and consequently their dive duration) relative to that which maximizes energy intake resulting in more dives per foraging bout. The fitness consequences of selecting a suboptimal surface interval vary with the risk function and the way divers harvest energy when at depth. Finally, predation risk during surface intervals should have important consequences for habitat selection and other aspects of the behavioral ecology of air-breathing aquatic organisms.

The influence of oxygen and carbon dioxide on diving behaviour of tufted ducks, Aythya fuligula

While optimal diving models focus on the diver's oxygen (O(2)) stores as the predominant factor influencing diving behaviour, many vertebrate species surface from a dive before these stores are exhausted and may commence another dive well after their O(2) stores have been resaturated. This study investigates the influence of hypoxia and also hypercapnia on the dive cycle of tufted ducks, Aythya fuligula, in terms of surface duration and dive duration. The birds were trained to surface into a respirometer box after each dive to a feeding tray so that rates of O(2) uptake (VO2) and carbon dioxide output (VCO2) at the surface could be measured. Although Vco2 initially lagged behind Vo2, both respiratory gas stores were close to full adjustment after the average surface duration, indicating that they probably had a similar degree of influence on surface duration. Chemoreceptors, which are known to influence diving behaviour, detect changes in O(2) and CO(2) partial pressures in the arterial blood. Thus, the need to restore blood gas levels appears to be a strong stimulus to continue ventilation. Mean surface duration coincided with peak instantaneous respiratory exchange ratio due to predive anticipatory hyperventilation causing hypocapnia. For comparison, the relationship between surface duration and O(2) uptake in reanalysed data for two grey seals indicated that one animal tended to dive well after fully restocking its O(2) stores, while the other dived at the point of full restocking. More CO(2) is exchanged than O(2) in tufted ducks during the last few breaths before the first dive of a bout, serving to reduce CO(2) stores and suggesting that hypercapnia rather than hypoxia is more often the limiting factor on asphyxia tolerance during dives. Indeed, according to calculations of O(2) stores and O(2) consumption rates over modal diving durations, a lack of O(2) does not seem to be associated with the termination of a dive in tufted ducks. However, factors other than CO(2) are also likely to be important, and perhaps more so, such as food density and rate of food ingestion. Because some predictive success has been demonstrated for optimal diving models, they should continue to incorporate O(2) stores as a variable, but their validity is likely to be improved by also focusing on CO(2) stores.

Energetics of diving in macaroni penguins

Heart rate (fH), abdominal temperature (T(ab)) and diving depth were measured in thirteen free-ranging breeding female macaroni penguins. Measurement of these variables allowed estimation of the mass-specific rate of oxygen consumption (V(O(2))) while diving and investigation of the physiological adjustments that might facilitate the diving behaviour observed in this species. In common with other diving birds, macaroni penguins showed significant changes in fH associated with diving, and these variables accounted for 36% of the variation in dive duration. When V(O(2)) was calculated for dives of different durations, 95.3% of dives measured were within the calculated aerobic dive limit (cADL) for this species. Mean fH for all complete dive cycles was 147+/-6 beats min(-1). When this fH is used to estimate (O(2)) of 26.2+/-1.4 ml min(-1) kg(-1) then only 92.8% of dives measured were within the cADL. Significant changes in abdominal temperature were not detected within individual dives, though the time constant of the measuring device used may not have been low enough to record these changes if they were present. Abdominal temperature did decline consistently during bouts of repeated diving of all durations and the mean decrease in T(ab) during a diving bout was 2.32+/-0.2 degrees C. There was a linear relationship between bout duration and the magnitude of this temperature drop. There was no commensurate increase in dive duration during dive bouts as T(ab) declined, suggesting that macaroni penguins are diving within their physiological limits and that factors other than T(ab) are important in determining the duration of dives and dive bouts. Lowered T(ab) will in turn facilitate lower metabolic rates during diving bouts, but it was not possible in the present study to determine the importance of this energy saving and whether it is occurs actively or passively.

Impact of externally attached loggers on the diving behaviour of the king penguin

The impact of relatively small externally attached time series recorders on some foraging parameters of seabirds was investigated during the austral summer of 1995 by monitoring the diving behaviour of 10 free-ranging king penguins (Aptenodytes patagonicus) over one foraging trip. Time-depth recorders were implanted in the abdominal cavities of the birds, and half of the animals also had dummy loggers attached on their backs. Although most of the diving behaviour was not significantly affected by the external loggers (P>0.05), the birds with externally attached loggers performed almost twice as many shallow dives, between 0 and 10 m depth, as the birds without external loggers. These shallow dives interrupted more frequently the deep-diving sequences in the case of birds with external loggers (percentage of deep dives followed by deep dives: 46% for birds with implants only vs. 26% for birds with an external attachment). Finally, the distribution pattern of the postdive durations plotted against the hour of the day was more heterogeneous for the birds with an external package. In addition, these penguins had extended surfacing times between two deep dives compared to birds without external attachments (P<0.0001). These results suggest the existence of an extra energy cost induced by externally attached loggers.

Diving physiology of birds: a history of studies on polar species

Our knowledge of avian diving physiology has been based primarily on research with polar species. Since Scholander's 1940 monograph, research has expanded from examination of the 'diving reflex' to studies of free-diving birds, and has included laboratory investigations of oxygen stores, muscle adaptations, pressure effects, and cardiovascular/metabolic responses to swimming exercise. Behavioral and energetic studies at sea have shown that common diving durations of many avian species exceed the calculated aerobic diving limits (ADL). Current physiological research is focused on factors, such as heart rate and temperature, which potentially affect the diving metabolic rate and duration of aerobic diving.

Comparative diving behaviour and segregation of the marine habitat by breeding Royal Penguins, Eudyptes schlegeli, and eastern Rockhopper Penguins, Eudyptes chrysocome filholi, at Macquarie Island

Comparative use of the water column by Royal Penguins, Eudyptes schlegeli, and eastern Rockhopper Penguins, Eudyptes chrysocome filholi, was examined by comparing their diving behaviour at Macquarie Island during the 1993-1994, 1994-1995, and 1995-1996 breeding seasons. Fifty-eight deployments of time-depth recorders measured depth and duration of dives, time spent submerged, bottom time, occurrence of wiggles, and descent and ascent rates. Both species dived predominantly during daylight hours (4:00-21:00 local time), with shallower dives around midday. Royal and Rockhopper penguins spent 38.9 ± 8.9 and 36.6 ± 9.3% of a 24-h period under water, respectively, but Rockhopper Penguins performed more dives (14.8 ± 9.4/h) of shorter duration (1.2 ± 0.7 min) than did Royal Penguins (11.1 ± 6.9 dives/h; 1.7 ± 0.6 min). Although both could dive to over 100 m, they rarely did so, with Royal and Rockhopper penguins making 79 ± 0.13 and 91 ± 0.08% of their dives to depths of less than 60 m, respectively. Although the difference was not significant, Royal Penguins dived in deeper water (32.9 ± 25.6 m) than did Rockhopper Penguins (27.3 ± 20.3 m). However, Royal Penguins performed wiggles (assumed foraging activity) in water significantly deeper (47.7 ± 24.3 m) than did Rockhopper Penguins (41.3 ± 19.0 m). Royal Penguins also performed more dives with wiggles than Rockhopper Penguins, suggesting differences in foraging technique. The amount of time both spent at the bottom of dives increased across the breeding season from incubation to chick rearing. As dive durations and ascent and descent rates did not change during this time, dive angles must have changed. There were no interannual differences in the diving behaviour of Royal Penguins, but Rockhopper Penguins exhibited differences in dive depths and durations and in the amount of bottom time. Royal Penguins, unlike Rockhopper Penguins, performed fewer dives on the first day of foraging trips, indicating more travelling and less foraging, which reflects differences in foraging zones between the two. The estimated foraging efficiency of Rockhopper Penguins was lower than that of Royal Penguins, probably making them more vulnerable to changes in prey availability and abundance. The two species exhibited some differences in diving behaviour but overlapped substantially in their use of the water column. Therefore, for minimising competition for resources, segregation in this aspect of their habitat is far less important than differences in diet and foraging zone.

Body mass and dive duration in alcids and penguins

Interspecific allometric equations for dive duration were calculated for two groups of wing-propelled divers: penguins, which specializing in diving, and alcids, which balance demands for aerial flying with those of diving. The equations for maximum dive duration (min) were 1.433M0.702 and 3.612M0.735 (where M is body mass in kilograms) for penguins (10 species) and alcids (9 species), respectively, hence did not support a simple oxygen store/usage hypothesis based on the prediction that the mass exponent of aerobic dive limit is close to 0.25. Equations for feeding dives were 0.569M0.712 and 1.094M0.391 in penguins (9 species) and alcids (10 species), respectively. The allometric exponent for the duration of feeding dives for penguins did not match the predicted value of 0.25, but that for alcids did not differ significantly from this value. Alcids exhibited a maximum dive duration 2.5 times longer than that for penguins after mass effects were controlled for. The size of oxygen stores and metabolic rates based on laboratory studies of penguins and alcids failed to explain the longer dive duration in alcids than in penguins.

Dive durations in pinnipeds and seabirds

Many endothermic divers regularly exceed the theoretical limit to the duration of aerobic dives, suggesting that assumptions about either the oxygen storage capacity of tissues or the metabolic rate of divers are wrong. This study examined the frequency distributions of dive durations in five species of endothermic divers from the island of South Georgia, South Atlantic. The theoretical aerobic dive time (TADT), calculated from average field metabolic rates, was exceeded regularly by all species except the Antarctic fur seal (<6% of dives). In contrast, the gentoo penguin exceeded its TADT in 69% of dives and the elephant seal in 91%. The frequency distributions of dive durations were bimodal, most especially in the penguins, with one mode below the TADT and another above the TADT, suggesting two different physiological strategies for diving. Interspecific allometric comparisons of dive durations in endothermic divers showed that, in general, relative dive durations in seabirds were greater than in pinnipeds. Dive durations in pinnipeds scaled approximately to field metabolic rate, whereas in seabirds they scaled most closely to flipper surface area, suggesting that they may be partly limited by the rate and degree of conduction of heat to the water.