© 2001 Int. Union Physiol. Sci./Am. Physiol. Soc.
Diving Beyond the Limits
Some free-ranging birds and mammals dive for periods that substantially exceed those for which their usable O2 stores are estimated to last. The mechanisms that extend the duration of aerobic diving include marked reductions in blood flow (and hence O2 delivery) to certain organs and tissues, passive gliding and, most probably, regional hypothermia.
To exploit the rich abundance of food that is normally available in the aquatic, and in particular in the marine, environment, aquatic birds and mammals have to spend varying proportions of their lives underwater, where they do not have access to O2. Although these animals have re-evolved many of the morphological and anatomic features of the early aquatic vertebrates (fish), they have not re-evolved gas exchange organs (gills) that function in water. The reasons for this lie in the physical properties of water and O2 and because birds and mammals are endothermic homeotherms with a high metabolic rate. At 10°C, the metabolic rate of an ectotherm, such as a fish, is only ~1% of that of an endotherm at its normal body temperature (37–40°C; Ref. 1). The solubility of O2 is so low in water that its concentration is ~20–40 times less than that in air, and the thermal conductivity of water is ~25 times greater than that of air.
So, for a given rate of O2 consumption (•VO2), an aquatic bird or mammal would have to move 20–40 times more water over its gas exchange surface than a terrestrial animal, and it would be impossible for it to maintain its body temperature above that of the water unless it possessed heat retention mechanisms far more effective than those present in some species of tuna. Thus diving birds and mammals are obliged to visit the surface periodically to replenish their O2 stores and to remove the accumulated CO2. This places a restriction on their feeding behavior and limits the duration for which they can remain submerged and hence the depth to which they can penetrate the water column.
An associated problem with the retention of the lungs is the compressibility of the gas contained within them (see Refs. 7, 11, and 12). For every 10-m descent in the water column, hydrostatic pressure increases by 1 atmosphere, and for gas within a fully compliant structure, a given increase in pressure is accompanied by a proportional decrease in volume (Boyle's Law). If the air at the increased pressure in the lung is in contact with the gas exchange surfaces, the partial pressure (P) of all of the constituent gases (O2, CO2, and N2) will increase in the blood. Nitrogen is metabolically inert, may have a narcotic effect during submersion, and will come out of solution as the animal surfaces and the pressure in the lung decreases, thus forming bubbles in the blood ("the bends"). Also, PO2 in the lungs and blood will decrease on ascent and may fall to dangerously low levels, thus causing loss of consciousness ("shallow water blackout"). As a diver descends, the lung volume and, in fact, the volume of all air-containing spaces in the body, must be compressed to maintain normal pressure differences between the air and fluid-filled compartments of the body. If such compression is not possible, the increased pressure differences that occur will result in extravasation of blood and rupture of blood vessels, a situation commonly known as "the squeeze." In addition, air has ~1/800th the density of water, so that it will increase the buoyancy of an aquatic bird or mammal and, therefore, increase the energy cost of diving, at least during the first few meters, before the air becomes sufficiently compressed.
This review will primarily discuss the various ways in which diving birds and mammals maximize and manage their O2 stores so as to optimize their dive duration and, therefore, their foraging time. It will also briefly investigate how some species of diving birds and mammals have avoided the problems associated with diving to great depths. Much of the data in this article are discussed in the cited reviews (6, 7, 8, 11, 12).
O2 can be stored in three major regions of the body in birds and mammals: in the respiratory system, combined with the myoglobin (Mb) in skeletal muscles, and combined with the hemoglobin (Hb) in the circulatory system. The muscles of many diving birds and mammals have much (up to 10 times) greater concentrations of Mb than those of their nondiving relatives, and blood volume and Hb concentration ([Hb]) are also greater in aquatic birds and mammals than in terrestrial species (8). Because of the presence of air sacs, the respiratory system of birds, including many diving birds, has a greater volume per unit of body mass than that of mammals. The respiratory system of the tufted duck (Aythya fuligula) may contribute >60% of the total usable O2 stores. On the other hand, there is no evidence that the mass-specific volume of the respiratory system in marine mammals is any greater than that in terrestrial mammals. In fact, it seems that some of the deeper-diving species, such as the Weddell seal (Leptonychotes weddellii) exhale on submersion.
It is not simple to determine the amount of O2 stored in each of these compartments before submersion and less so to ascertain what proportion of these stores are available to the metabolizing cells. For example, because of the problems associated with diving to great depths listed above, air in the lungs may not be in contact with the gas exchange surfaces while diving, particularly during deep diving, and, because of the high affinity of Mb for O2, the O2 stored in muscle may not be available to other tissues. Despite these uncertainties, there is general consensus that the circulatory system and the muscles accommodate most (70–95%) of the available O2 in aquatic mammals, whereas the respiratory system of most birds contributes 35–60% of the usable O2 stores (Table 1). One notable exception is the king penguin (Aptenodytes patagonicus), in which the respiratory system only contributes ~17% to body O2 stores and the muscles ~50%. In fact, in the emperor penguin, (A. forsteri) which dives for longer and to greater depths than the king penguin, the increase in the concentration of Mb is, together with body mass, an important factor in the increase in diving capacity of juveniles (14).
Aerobic vs. anaerobic metabolism during diving
During steady-state exercise in air, birds and mammals use aerobic metabolism, whereby the metabolic substrate, normally carbohydrate and/or fat, is completely oxidized in the slow oxidative (SO) and fast oxidative glycolytic (FOG) muscle fibers via the Krebs cycle and the electron transport chain to produce ATP and waste products, namely CO2 and water. During such exercise, there are no changes in blood gases or in blood pH from their resting values. In other words, the rate at which O2 is being consumed by the tissues, especially by the active muscles, is matched by its increased rate of provision by the respiratory and cardiovascular systems. However, above a certain level of exercise, the SO and FOG muscle fibers are not able to produce ATP at a sufficiently high rate, and the fast glycolytic (FG) fibers become active. These fibers produce ATP anaerobically from carbohydrate, with lactic acid as a waste product. Such high levels of exercise are not sustainable; the blood becomes acidotic and fatigue soon sets in. During increasing exercise intensity, the point at which blood lactate begins to increase markedly above the resting level is known as the lactate threshold.
When aquatic birds and mammals submerge themselves, they only have the O2 they take with them, so the size and availability of these stores and the rate at which they are utilized, to overcome buoyancy and drag and to thermoregulate, will determine the duration for which an animal can remain submerged and metabolize aerobically. Ever since the classic experiments of Per Scholander (see Ref. 7) on restrained aquatic birds and mammals, it has been known that these animals can survive for long periods of breath-hold by constricting the blood vessels in tissues that can withstand periods of O2 lack, thus conserving O2 for the O2-dependent organs, such as the central nervous system (CNS) and heart. The selective vasoconstriction is accompanied by a reduction in heart rate (bradycardia), and the underperfused tissues metabolize anaerobically, thus producing lactic acid. The extent to which anaerobic metabolism is used by naturally diving birds and mammals is unclear, although the general consensus is that most dives are basically aerobic in nature (8).
Jerry Kooyman and his colleagues (see Ref. 7), working on free-living Weddell seals, coined the term aerobic dive limit (ADL) to describe the dive duration up to which there was no increase in postdive blood lactate concentration (that is, the duration up to which all dives were completely aerobic). On the basis of measurements of postdive blood lactate in freely diving, adult Weddell seals and of their diving behavior, 97% of their dives were found to be aerobic. Unfortunately, it has been possible to collect similar data from only one other aquatic animal, the emperor penguin. However, many authors have obtained a calculated ADL (cADL) by dividing calculated usable O2 stores by the calculated, estimated, or measured •VO2 during diving (8). This involves the uncertainties associated with the calculations of both the numerator (see above) and denominator (see below) of this equation and implies that at cADL all of the usable O2 stored in the body has been consumed. However, we know that Weddell seals can remain submerged for up to approximately three times their ADL based on the measurements of postdive blood lactate concentrations. This indicates that there is some O2 remaining for the CNS and heart, even after lactate begins to accumulate in the blood. A further complication in the calculation of cADL is the possible role of phosphocreatine as a source of phosphorus for the production of ATP (8). All of these shortcomings of cADL must be borne in mind when reading what follows. Despite these shortcomings, cADL is a useful tool with which to indicate the extent to which some aquatic species have to employ extreme adaptations to maximize their foraging duration.
To avoid possible confusion between ADL and cADL, Butler and Jones (8) recommended that the dive duration at which postdive blood lactate concentration increases should be termed the diving lactate threshold (DLT). This makes it analogous to the lactate threshold during increasing exercise intensity in terrestrial birds and mammals. I shall follow this recommendation in the remainder of this article.
Determination of •VO2 during submersion
It is not easy to determine •VO2 when a bird or mammal is diving, and all of the methods used have their shortcomings. It has been possible to use respirometry with some species, such as ducks, in the laboratory and with others, such as Weddell seals, in the field, but such studies are confined to relatively short durations. Data over a longer time period have been obtained from a number of species in the field by using doubly labeled water, but this method only gives an average value over the whole of the experimental period, which is itself limited by the biological half-life of the stable isotope, 18O (see Ref. 8). Heart rate has also been used to estimate •VO2 in free-ranging aquatic birds and mammals by way of an implanted data logger (3, 5). With the exception of studies on the tufted duck, none of these methods has enabled the rate at which O2 is consumed by the animal when it is actually under water to be estimated. They all give an average of •VO2 when the animal is at the surface and when it is submerged, so they may not give an accurate estimate of the amount of O2 consumed during the act of diving itself. In other studies, •VO2 has been determined in animals swimming under water in a static water channel, and these values have been used as measures of •VO2 while diving in the field. The data obtained from some species suggest, at first sight, that the animals are routinely diving beyond the limits of their O2 stores.
Factors affecting •VO2 during submersion
I shall begin by discussing three species that do appear to remain submerged for periods that are well within their cADL. These are tufted ducks, Antarctic fur seals (Arctocephalus gazella), and bottlenose dolphins (Tursiops truncatus). Tufted ducks feed on benthic invertebrates, preferring the freshwater mussel (Dreissena polymorpha) when in abundance, and dive predominantly to depths up to 5 m. These birds are positively buoyant, and their •VO2 when foraging under water is over three times that when they are resting on water and, incidentally, not significantly different from that when they are swimming on the surface at their maximum sustainable speed. However, they remain submerged for such relatively short durations that all of their dives are within their cADL (Table 2) and their heart rates do not remain below the resting value after the first few seconds of submersion, i.e., there is no diving bradycardia. In fact, heart rate during diving is, on average, ~50% greater than that in birds resting in water. Despite this, they do undergo a quite dramatic redistribution of blood flow when diving such that the active leg muscles, heart, and CNS receive an adequate supply to enable aerobic metabolism to continue, whereas the remainder of the body receives a reduced supply.
The Antarctic fur seal and bottlenose dolphin are hydrodynamically much better adapted to an aquatic existence than the tufted duck, which, no doubt, contributes to the fact that their •VO2 when foraging at sea is similar to that when they are resting in water (Table 2). By attaching time/depth recorders to female Antarctic fur seals during their breeding season, it has been demonstrated that they feed at night when their prey [krill (Euphausia superba)] are close to the surface. Thus 65% of their dives are to <20 m in depth and ~95% are less than the cADL of 2.1 min (Table 2). During submersion, heart rate falls to ~80% of the value recorded when the animals are ashore (5).
Cetaceans do not come ashore to breed, so it is not so easy to study these animals in the wild. Studies on captive animals have shown that, when swimming at speeds up to 2 m/s, •VO2 is similar to that when the animals are resting in water, but beyond that speed •VO2 increases dramatically. This means that the cost of transport (the amount of O2 required to move a given mass of an animal a given distance) is lowest at around this speed. However, it appears that these animals are also able to use changes in their buoyancy associated with changes in lung volume as they descend and ascend to move passively through the water column. Underwater video sequences indicate that they save energy by gliding during the last part of their descent (7 m for dives to 16 m and 44 m for dives to 100 m) and during the last few meters of their ascent (15). During both relatively shallow (60 m) and deep (210 m) dives, heart rate falls to ~20% of its predive value (17).
There are few data on the behavior of freely diving bottlenose dolphins, and those we have are for animals diving in relatively shallow (<50 m deep) water. Under these conditions, it appears that all of their dives are within their cADL (Table 2). For an Atlantic spotted dolphin, Stenella frontalis, diving in water 33 m deep, 93.3% of its dives were of <2 min in duration and 99.9% were <4 min.
More is known about the behavior and physiology of the Weddell seal than of any other marine bird or mammal. Around McMurdo Sound in Antarctica, where most of the studies have taken place, these animals feed predominantly on small notothenid fish at depths between 100 and 350 m. We know from the studies of Kooyman and his colleagues that the DLT of large (~450 kg) males is between 20 and 25 min. However, even taking into account the very large O2 stores of these animals, the cADL is only 19 min (see Table 2). As mentioned earlier, the cADL implies that all of the usable O2 is consumed, although it does not take into account the use of phosphocreatine, and yet we know that the large males referred to above can remain submerged under water for up to 60 min and must, therefore, have sufficient O2 available for the CNS and heart. As also mentioned above, the value for •VO2 during diving was obtained from animals returning to a respirometer over an ice hole and is, therefore, an average value of •VO2 while the animal is under water plus that at the subsequent period at the surface. Thus the actual value of •VO2 during diving may be lower than that used to calculate cADL. Certainly, Weddell seals also make use of changes in their buoyancy to save energy when diving, and if the postdive •VO2 is related to the extent of gliding during the preceding dive, postdive •VO2 associated with gliding dives is, on average, 28% lower than that associated with nongliding (stroking) dives (Fig. 1; Ref. 16). Greater savings occur with deeper dives. Such behavior could, therefore, increase the value of cADL for Weddell seals.
Heart rate and the associated cardiovascular responses of Weddell seals during diving are related to the duration of the dives. For those within the DLT, heart rate is ~60% of the predive rate and both renal and hepatic blood flows are maintained. For those dives that are longer than the DLT, heart rate is ~50% of the predive rate and renal blood flow is dramatically reduced. There is no change in hepatic blood flow.
It is the larger penguins, e.g., gentoos and kings, and mammals such as the elephant seals that appear to perform the impossible. Gentoo penguins around South Georgia feed on krill, but unlike the fur seals of the area (see above), they feed during the day when the krill are deeper in the water column, so they dive to depths of 80–90 m. King penguins feed on myctophid fish at depths of 100–250 m. A large proportion of the natural dives of both of these species are longer than their cADLs (Table 2). As indicated above, at least part of this discrepancy is probably related to inaccuracies in determining •VO2 during diving. For example, if the value of •VO2 recorded from gentoo penguins swimming in a water channel (40.9 ml•min–1 •kg–1; Ref. 6) is used to calculate cADL, then >55% of the natural dives exceed this duration (Table 2). During natural dives, heart rate does not fall below the mean value when ashore in either of these species of penguin (2, 3).
Bevan et al. (3) have used the heart rate method to deter-mine •VO2 and obtained an estimate of 27 ml•min–1•kg–1 for gentoo penguins foraging at sea. This is 67% of the value obtained when birds were swimming in a water channel and only 20% above the value recorded from gentoos resting in water. Nonetheless, there are still 40% of natural dives that exceed the resultant cADL (Table 2). Like respirometry, the heart rate method gives an average estimate of •VO2 for dives and the subsequent periods at the surface, so the actual rate at which O2 is being consumed by the tissues while the bird is under water may be lower than the estimate. That this may well be the case is suggested by the fact that the temperature in the abdominal cavity (Tab) of both gentoo and king penguins drops during diving bouts and returns to normal when diving behavior ceases (Fig. 2). For gentoo penguins, Tab over the whole of a dive bout of average duration was, on average, 2.4°C lower than when the birds were not diving (3). The lowest Tab recorded for each penguin was, on average, 33.6°C for the gentoos and 29.7°C for the kings. So if the birds allow temperature to fall by a certain amount in some parts of the body (i.e., they do not increase metabolic rate in an attempt to return Tab to normal), there will be a saving in energy both in terms of thermoregulatory costs and in terms of a Q10 effect. (Q10 is the factorial change in the rate of a chemical reaction, i.e., metabolic rate, associated with a 10°C change in temperature.) An indication of the extent of such a saving in energy can be calculated.
If the metabolic rate of the gentoo penguin during diving is the same as that when it is resting on water (22.7 ml•min–1• kg–1), almost 21% of the dives would be longer than the cADL. If there was an average temperature drop of 2.4°C throughout the whole body during a dive bout and with an apparent Q10 of 3, cADL would be extended by almost 40 s and all dives would be within the cADL (3). The data we have from king penguins are similar to those from gentoos. Taking the lowest value of •VO2 calculated for king penguins swimming in a water canal (see Ref. 6), ~40% of natural dives are longer than the cADL (Table 2). If it is assumed that the •VO2 of king penguins while diving is the same as when they are resting in water, then cADL would be 4.1 min and, as for gentoos, ~20% of dives would exceed the cADL.
Thus, in both species of penguin, hypothermia during diving could well be associated with a reduction in metabolic rate below that of birds resting in water so as to enhance their cADL (10). What we do not know is precisely in which regions of the body (in addition to the abdominal cavity) and to what extent the hypothermia exists during diving in these animals. The deeper-diving penguins may, like some of the marine mammals, also use passive gliding as a means of reducing their •VO2 during diving.
How available are the O2 stores to diving birds and mammals?
The derivation of cADL is based on the assumption that a large proportion of the O2 contained within the lung/air sac system of birds, in the circulatory system of birds and mammals, plus virtually all of that bound to the Mb in birds and mammals is available for use by the metabolizing tissues. Measures of expired gas from Humboldt penguins (Pygoscelis humboldti) surfacing at the end of a dive indicate that there is substantial extraction of O2 from the respiratory system during submersion. In both tufted ducks (4) and little blue penguins (Eudyptula minor), it seems that movement of the limbs (legs in the ducks, flippers in the penguins) cause pressure differences between the anterior and posterior air sacs, which could drive the movement of gases between these sacs via the tertiary bronchi, where gas exchange would occur. The significance of O2 in the respiratory system of the deeper-diving penguins, such as the kings and emperors, is not so clear (see below).
Davis and Kanatous (9) produced a mathematical model to investigate how Weddell seals could ensure maximum use of both their blood (Hb-bound) and muscle (Mb-bound) O2 stores so as to produce the greatest possible cADL at different levels of muscular exercise. The problem is that Mb has a much greater affinity for O2 [PO2 for half-saturation (P50) = 0.3–0.4 kPa] than Hb (P50 = 3.6 kPa), and the O2 stores in the Mb are insufficient to support muscle O2 uptake and, at the same time, maximize cADL. The trick, therefore, is to ensure that both Hb-bound O2 and Mb-bound O2 are depleted at the same time, and this requires careful matching of cardiac output (and, therefore, of heart rate) and perfusion of the active muscles. The predicted outcome is that convective transport of O2 by the blood to the muscles progressively decreases and they become increasingly reliant on their own O2 stores. However, even toward the end of a dive, the muscles would still be receiving ~25% of their O2 from the blood while simultaneously desaturating the Mb. Thus the "classic" dive response of bradycardia and selective peripheral perfusion is a key component of this model. As we have seen with animals such as the tufted duck, selective perfusion is possible without a reduction in heart rate below the resting level. However, at the same level of •VO2, heart rate during diving in this species is ~25% lower than it is during swimming at the surface.
Adaptations to deep diving
There is evidence from forcibly submerged and free-diving Weddell seals and from free-diving bottlenose dolphins that gas exchange between the lungs and the blood is impaired at depths between 20 and 70 m. The lower airways are reinforced with cartilage or connective tissue and smooth muscle, and the thorax is very compliant. Thus, as the hydrostatic pressure increases as the animal dives deeper and the lungs progressively collapse, air is forced into the upper airways where gas exchange cannot occur. Although bottlenose dolphins can be trained to dive to over 100 m, it is not clear to what extent they routinely do so in the wild, whereas we know for certain that Weddell seals routinely dive in excess of 100 m when foraging (see above). Thus the lungs will not be much use as a store for O2 in these animals; hence their greater reliance on the circulatory system in this respect (Table 1).
Virtually all birds have relatively large respiratory systems (Table 1) as a result of the presence of the air sacs. In fact, the lungs accommodate a relatively small proportion (10%) of air in the respiratory system. Early studies on gentoo penguins indicated that, during simulated dives to 68 m, some gas exchange did occur and PN2 in their arterial blood was somewhat higher than that in Weddell seals during free dives to 89 m (350 and 200 kPa, respectively). It has been suggested, however, that the duration and depth of natural dives performed by gentoo penguins are sufficiently short and shallow to provide adequate protection against dangerously high values of PN2 in the blood. However, king penguins dive much deeper and for somewhat longer durations than gentoos (Table 2). Recent experiments have demonstrated that, even during simulated dives to 136 m, their PN2 was, at <280 kPa, lower than that in gentoos during simulated dives to 68 m (13). An important contribution to this seems to be the relatively small volume of the respiratory system in this species (see Table 1), which will reduce the amount of N2 available for absorption in the body fluids. In addition, there may be a reduction in cardiac output, which would limit the amount of N2 available for absorption, and a possible engorgement of blood capillaries in the gas exchange regions of the lungs, thus preventing or reducing gas exchange. Thus, as in deeply diving mammals, a reduction in the size of the respiratory system and, therefore, in its importance as a store of O2, seems to be an important adaptation to deep diving in penguins.
Because of editorial restrictions, many relevant papers could not be cited.
My work on this topic has been supported by the Natural Environmental Research Council of the United Kingdom.
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