CLASSIFICATION OF WEDDELL SEAL DIVING BEHAVIOR

Most studies of pinniped diving behavior have manually grouped dives according to similarities in the depth, duration, and appearance of the dive profile. Dives of 15 adult female Weddell seals ( Leptonychotes weddellii ) were recorded with time-depth recorders and 39, 119 dives were classified manually and statistically (principal components analysis, discriminant function analysis, cluster analysis, and shape-fitting algorithms). Four dive types, common to all classification methods, and a fifth dive type, common to two of the methods, represented most of the observed diving behavior. However, a few variations of these dive types, specifically a flat-bottomed dive determined manually, may have also represented important behavior. Using a combination of these methods, all dives were classified into six dive types, Inspection of dive variables (mean maximum depth, mean duration, and frequency) over time for each dive type, as well as comparisons to previous studies of pinniped diving behavior, indicated different behaviors that the dive types may represent. Hypothesized functions for the dive types were pelagic foraging, benthic foraging, exploration, and traveling. The results indicate that there are strong similarities in diving behavior across various phocid species, that statistical analyses of diving behavior are useful in the analysis of a large data set, and that these analyses reduced human subjective bias in interpreting diving behavior.     

Assessment of scale-dependent foraging behaviour in southern elephant seals incorporating the vertical dimension: a development of the First Passage Time method

1. Identifying the spatial scales at which top marine predators forage is important for understanding oceanic ecosystems. Several methods quantify how individuals concentrate their search effort along a given path. Among these, First-Passage Time (FPT) analysis is particularly useful to identify transitions in movement patterns (e.g. between searching and feeding). This method has mainly been applied to terrestrial animals or flying seabirds that have little or no vertical component to their foraging, so we examined the differences between classic FPT and a modification of this approach using the time spent at the bottom of a dive for characterizing the foraging activity of a diving predator: the southern elephant seal. 2. Satellite relayed data loggers were deployed on 20 individuals during three successive summers at the Kerguelen Islands, providing a total of 72 978 dives from eight juvenile males and nine adult females. 3. Spatial scales identified using the time spent at the bottom of a dive ( = 68.2 +/- 42.1 km) were smaller than those obtained by the classic FPT analysis ( = 104.7 +/- 67.3 km). Moreover, foraging areas identified using the new approach clearly overlapped areas where individuals increased their body condition, indicating that it accurately reflected the foraging activity of the seals. 4. These results suggest that incorporating the vertical dimension into FPT provides a different result to the surface path alone. Close to the Antarctic continent, within the pack-ice, sinuosity of the path could be explained by a high sea-ice concentration (restricting elephant seal movements), and was not necessarily related to foraging activity. 5. Our approach distinguished between actual foraging activity and changes in behaviour induced by the physical environment like sea ice, and could be applied to other diving predators. Inclusion of diving parameters appears to be essential to identify the spatial scale of foraging areas of diving animals.     

Dangerous dive cycles and the proverbial ostrich

Data rarely are available to address the level of predation risk faced by diving animals in different parts of the water column. Consequently, most published research on diving behaviour implicitly assumes – like the proverbial ostrich – that 'unseen' predators are functionally unimportant. We argue that failure to consider diving in a predation risk framework may have precluded many insights into the ecology of aquatic foragers that breathe air. Using existing literature and a simple model, we suggest that fear from submerged predators in several systems might be influencing patch residence time, and therefore the duration of other dive cycle components. These analyses, along with an earlier model of predation risk faced by diving animals at the surface, suggest that dive cycle organisation can be modified to increase safety from predators, but only at the cost of reduced energy gain. Theoretical arguments presented here can seed hypotheses on factors contributing to population declines of diving species. For instance, adjustments to the dive cycle that reduce predation risk might be unaffordable if resources are scarce. Thus, if animals are to avoid imminent starvation or substantial loss of reproductive potential, resource declines might indirectly increase predation rates by limiting the extent to which dive cycles can deviate from those that would maximize energy gain. We hope that ideas presented in this paper stimulate other researchers to further develop theory and test predictions on how predation risk might influence diving behaviour and its ecological consequences.     

The scaling of diving time budgets: insights from an optimality approach

Simple scaling arguments suggest that, among air-breathing divers, dive duration should scale approximately with mass to the one-third power. Recent phylogenetic analyses appear to confirm this. The same analyses showed that duration of time spent at the surface between dives has scaling very similar to that of dive duration, with the result that the ratio of dive duration to surface pause duration is approximately mass invariant. This finding runs counter to other arguments found in the diving literature that suggest that surface pause duration should scale more positively with mass, leading to a negative scaling of the dive-pause ratio. We use a published model of optimal time allocation in the dive cycle to show that optimal decisions can predict approximate mass invariance in the dive-pause ratio, especially if metabolism scales approximately with mass to the two-thirds power (as indicated by some recent analyses) and oxygen uptake is assumed to have evolved to supply the body tissues at the required rate. However, emergent scaling rules are sensitive to input parameters, especially to the relationship between the scaling of metabolism and oxygen uptake rate at the surface. Our results illustrate the utility of an optimality approach for developing predictions and identifying key areas for empirical research on the allometry of diving behavior.     

Recording the free-living behaviour of small-bodied, shallow-diving animals with data loggers

1. Time-depth data recorders (TDRs) have been widely used to explore the behaviour of relatively large, deep divers. However, little is known about the dive behaviour of small, shallow divers such as semi-aquatic mammals. 2. We used high-resolution TDRs to record the diving behaviour of American mink Mustela vison (weight of individuals 580-1275 g) in rivers in Oxfordshire (UK) between December 2005 and March 2006. 3. Dives to > 0.2 m were measured in all individuals (n = 6). Modal dive depth and duration were 0.3 m and 10 s, respectively, although dives up to 3 m and 60 s in duration were recorded. Dive duration increased with dive depth. 4. Temperature data recorded by TDRs covaried with diving behaviour: they were relatively cold (modal temperature 4-6 degrees C across individuals) when mink were diving and relatively warm (modal temperature 24-36 degrees C across individuals) when mink were not diving. 5. Individuals differed hugely in their use of rivers, reflecting foraging plasticity across both terrestrial and aquatic environments. For some individuals there was < 1 dive per day while for others there was > 100 dives per day. 6. We have shown it is now possible to record the diving behaviour of small free-living animals that only dive a few tens of centimetres, opening up the way for a new range of TDR studies on shallow diving species.     

To what extent is the foraging behaviour of aquatic birds constrained by their physiology?

Aquatic birds have access to limited amounts of usable oxygen when they forage (dive) underwater, so the major physiological constraint to their behaviour is the need to periodically visit the water surface to replenish these stores and remove accumulated carbon dioxide. The size of the oxygen stores and the rate at which they are used (V dot o2) or carbon dioxide accumulates are the ultimate determinants of the duration that aquatic birds can remain feeding underwater. However, the assumption that the decision to terminate a dive is governed solely by the level of the respiratory stores is not always valid. Quantification of an optimal diving model for tufted ducks (Aythya fuligula) shows that while they dive efficiently by spending a minimum amount of time on the surface to replenish the oxygen used during a dive, they dive with nearly full oxygen stores and surface well before these stores are exhausted. The rates of carbon dioxide production during dives and removal during surface intervals are likely to be at least as important a constraint as oxygen; thus, further developments of optimal diving models should account for their effects. In the field, diving birds will adapt to changing environmental conditions and often maximise the time spent submerged during diving bouts. However, other factors influence the diving depths and durations of aquatic birds, and in some circumstances they are unable to forage sufficiently well to provide food for their offspring. The latest developments in telemetry have demonstrated how diving birds can make physiological decisions based on complex environmental factors. Diving penguins can control their inhaled air volume to match the expected depth, likely prey encounter rate, and buoyancy challenges of the following dive.     

Artifacts arising from sampling interval in dive depth studies of marine endotherms

Most depth recorders used to study the diving behaviour of polar marine endotherms record depth data at specific time intervals. The length of recording interval can have potentially profound implications for the interpretation of the data. We used data acquired on the diving behaviour of king penguins, Aptenodytes patagonicus, to examine the validity of various analyses routinely conducted on depth data. In our experiments, increasing the sampling interval led to an underestimation of the number of dives performed, an overestimation in mean dive duration and substantial changes in the form of the dive profile. Our analysis indicates that depth data should be recorded at a minimum rate corresponding to 10% of the total dive duration and that conventional dive profile categorization may be inappropriate. Alternatives that are less subjective, and based on curve fits of dive depth versus time, are proposed.     

The comparative biology of diving in two genera of European Dytiscidae (Coleoptera)

Surfacing behaviour is fundamental in the ecology of aquatic air-breathing organisms; however, it is only in vertebrates that the evolutionary ecology of diving has been well characterized. Here, we explore the diving behaviour of dytiscid beetles, a key group of surface-exchanging freshwater invertebrates, by comparing the dive responses of 25 taxa (Deronectes and Ilybius spp.) acclimated at two temperatures. The allometric slopes of dive responses in these dytiscids appear similar to those of vertebrate ectotherms, supporting the notion that metabolic mode shapes the evolution of diving performance. In both genera, beetles spend more time submerged than on the surface, and surface time does not vary with the temperature of acclimation. However, presumably in order to meet increased oxygen demand at higher temperatures, Deronectes species increase surfacing frequency, whereas Ilybius species decrease dive time, an example of 'multiple solutions.' Finally, widespread northern species appear to possess higher diving performances than their geographically restricted southern relatives, something which may have contributed to their range expansion ability.     

Haematological and rheological characteristics of blood in seven marine mammal species: physiological implications for diving behaviour

The haeniatological and rheological characteristics of blood from seven marine mammal species have been examined to determine the relationship between increased haematocrit. which is correlated with the ability to increase aerobic dive limits. and blood viscosity. The species examined reflect adaptations to a variety of marine niches ranging from coastal to pelagic to iceedge environments. and exhibit a wide range of diving behaviours. Average haematocrits ranged from43–45% in bottlenose dolphins. killer whales and California sea lions to more than 60% in the deeper diving species (beluga whales and northern elephant seals). Whole blood viscosity () increased exponentially with haematocrit (= 0.96*e^0-0335*Hct). representin a two-fold increase from 4.1 cP for killer whale blood to 8.9 cP for northern elephant seal. There was no apparent compensatory mechanism to reduce viscosity at any shear rate. The optimal haematocrit for oxygen transport was calculated to be40–50% for all species tested. The species with lower haematocrits were within optimal values for oxygen transport. while the two species with the highest haematocrits (beluga whales and northern elephant seals) were above predicted optimal oxygen transport values. On the basis of comparisons of the diving behaviour of these seven species, we suggest that marine mammal species with the greatest adaptation for increased oxygen stores via increased haematocrit have the capacity for deep, long-duration dives, but a limited oxygen transport capacity. We predict that this compromise precludes fast sustainable swimming behaviour in these species.     

Long-term monitoring of leatherback turtle diving behaviour during oceanic movements

The diving behaviour of four leatherback turtles (Dermochelys coriacea) was recorded for periods of 0.5-8.1 months during their postnesting movements in the Indian and Atlantic Oceans, when they covered 1569-18,994 km. Dive data were obtained using satellite-linked transmitters which also provided information on the dive depths and profiles of the turtles. Turtles mainly dove to depths < 200 m, with maximum dive durations under 30-40 min and exhibited diel variations in their diving activity for most part of the routes, with dives being usually longer at night. Diurnal dives were in general quite short, but cases of very deep (> 900 m) and prolonged (> 70 min) dives were however recorded only during daytime. The three turtles that were tracked for the longest time showed a marked change in behaviour during the tracking, decreasing their dive durations and ceasing to dive deeply. Moreover, diel variations disappeared, with nocturnal dives becoming short and numerous. This change in turtle diving activity appeared to be related to water temperature, suggesting an influence of seasonal prey availability on their diving behaviour. The turtle diving activity was independent on the shape of their routes, with no changes between linear movements in the core of main currents or looping segments in presence of oceanic eddies.     

The diving behaviour of the Manx Shearwater Puffinus puffinus

The diving capabilities of the Procellariformes remain the least understood component of avian diving physiology. Due to their relatively small size, shearwaters may have high oxygen consumption rates during diving relative to their available oxygen stores. Dive performance in this group should be strongly limited by the trade‐off between oxygen consumption and oxygen stores, and shearwaters could be a good model group for testing predictions of dive theory. Many earlier measurements of shearwater dive behaviour relied on observations from the surface or potentially biased technology, and it is only recently that diving behaviour has been observed using electronic recorders for many of the clades within the family. The diving behaviour of Manx Shearwaters Puffinus puffinus breeding in Wales, UK, was studied on a large sample of birds using time–depth–temperature recorders deployed on chick‐rearing shearwaters in July and August over 3 years (2009–2011). Light availability apparently limited diving as dives only occurred between 04:00 and 19:00 h GMT. All individuals routinely dived deeper than traditionally assumed, to a mean maximum depth of 31 m and occasionally down to nearly 55 m. We compiled all available data for a comparison of the dive depth across shearwater species. There was a positive allometric relationship between maximum dive depth and body mass across Puffinus and Ardenna shearwater species, as expected, but only if samples of fewer than two individuals were excluded. The large intra‐specific range in maximum dive depth in our study illustrates that apparent diversity in diving performance across species must be interpreted cautiously.     

Utilisation of Intensive Foraging Zones by Female Australian Fur Seals

Within a heterogeneous environment, animals must efficiently locate and utilise foraging patches. One way animals can achieve this is by increasing residency times in areas where foraging success is highest (area-restricted search). For air-breathing diving predators, increased patch residency times can be achieved by altering both surface movements and diving patterns. The current study aimed to spatially identify the areas where female Australian fur seals allocated the most foraging effort, while simultaneously determining the behavioural changes that occur when they increase their foraging intensity. To achieve this, foraging behaviour was successfully recorded with a FastLoc GPS logger and dive behaviour recorder from 29 individual females provisioning pups. Females travelled an average of 118 ± 50 km from their colony during foraging trips that lasted 7.3 ± 3.4 days. Comparison of two methods for calculating foraging intensity (first-passage time and first-passage time modified to include diving behaviour) determined that, due to extended surface intervals where individuals did not travel, inclusion of diving behaviour into foraging analyses was important for this species. Foraging intensity ‘hot spots’ were found to exist in a mosaic of patches within the Bass Basin, primarily to the south-west of the colony. However, the composition of benthic habitat being targeted remains unclear. When increasing their foraging intensity, individuals tended to perform dives around 148 s or greater, with descent/ascent rates of approximately 1.9 m•s^-1 or greater and reduced postdive durations. This suggests individuals were maximising their time within the benthic foraging zone. Furthermore, individuals increased tortuosity and decreased travel speeds while at the surface to maximise their time within a foraging location. These results suggest Australian fur seals will modify both surface movements and diving behaviour to maximise their time within a foraging patch.     

Sex differences in the diving behaviour of a size-dimorphic capital breeder: the grey seal

Both body size dimorphism and sex differences in the relative costs and benefits associated with acquiring energy for reproduction have been advanced to explain the evolution of sex differences in foraging behaviour. We examined the extent to which these factors influenced sex differences in the diving behaviour of a size-dimorphic, capital breeder, the grey seal, Halichoerus grypus. Using time-depth data loggers, we examined the diving behaviour of 46 male and 49 female grey seals for 7 months before parturition and mating. Males and females showed significantly different seasonal patterns in the characteristics of individual dives and dive effort. Compared with males, females showed significantly higher levels of dive effort immediately following moult and in the 3 months before parturition. Females also had longer dives (5.5 versus 4.9 min) and spent more time at depth (3.4 versus 2.7 min), whereas males dived deeper (57 versus 49 m). Males dived consistently throughout the day, whereas females showed strong diurnal patterns in dive depth, duration and frequency. The diving behaviour and rates of mass gain by females suggested a pattern of foraging consistent with early accumulation of body energy to support pregnancy and the subsequent lactation period during which females fast. Males, on the other hand, showed diving behaviour and rates of mass gain consistent with a more gradual accumulation of energy stores. Our results suggest that sex differences in the seasonal patterns of diving behaviour reflect sex differences in the costs and benefits of stored energy for reproduction rather than the influence of body size dimorphism alone.     

Inter-population differences in diving behaviour of adult male southern elephant seals (Mirounga leonina)

Access to different environments may lead to inter-population behavioural changes within a species that allow populations to exploit their immediate environments. Elephant seals from Marion Island (MI) and King George Island (KGI) (Isla 25 de Mayo) forage in different oceanic environments and evidently employ different foraging strategies. This study elucidates some of the factors influencing the diving behaviour of male southern elephant seals from these populations tracked between 1999 and 2002. Mixed-effects models were used to determine the influence of bathymetry, population of origin, body length (as a proxy for size) and individual variation on the diving behaviour of adult male elephant seals from the two populations. Males from KGI and MI showed differences in all dive parameters. MI males dived deeper and longer (median: 652.0?m and 34.00?min) than KGI males (median: 359.1?m and 25.50?min). KGI males appeared to forage both benthically and pelagically while MI males in this study rarely reached depths close to the seafloor and appeared to forage pelagically. Model outputs indicate that males from the two populations showed substantial differences in their dive depths, even when foraging in areas of similar water depth. Whereas dive depths were not significantly influenced by the size of the animals, size played a significant role in dive durations, though this was also influenced by the population that elephant seals originated from. This study provides some support for inter-population differences in dive behaviour of male southern elephant seals.     

Breathing hypoxic gas affects the physiology as well as the diving behaviour of tufted ducks

We measured the effects of exposure to hypoxia (15% and 11% oxygen) and hypercapnia (up to 4.5% carbon dioxide) on rates of respiratory gas exchange both between and during dives in tufted ducks, Aythya fuligula, to investigate to what extent these may explain changes in diving behaviour. As found in previous studies, the ducks decreased dive duration (t(d)) and increased surface duration when diving from a hypoxic or hypercapnic gas mix. In the hypercapnic conditions, oxygen consumption during the dive cycle was not affected. Oxygen uptake between dives was reduced by only 17% when breathing a hypoxic gas mix of 11% oxygen. However, estimates of the rate of oxygen metabolism during the foraging periods of dives decreased nearly threefold in 11% oxygen. Given that tufted ducks normally dive well within their aerobic dive limits and that they significantly reduced their t(d) during hypoxia, it is not at all clear why they make this physiological adjustment.     

Diving in shallow water: the foraging ecology of darters (Aves: Anhingidae)

Diving birds have to overcome buoyancy, especially when diving in shallow water. Darters and anhingas (Anhingidae) are specialist shallow-water divers, with adaptations for reducing their buoyancy. Compared to closely-related cormorants (Phalacrocoracidae), darters have fully wettable plumage, smaller air sacs and denser bones. A previous study of darter diving behaviour reported no relationship between dive duration and water depth, contrary to optimal dive models. In this study I provide more extensive observations of African darters Anhinga melanogaster rufa diving in water<5 m deep at two sites. Dive duration increases with water depth at both sites, but the relationship is weak. Dives were longer than dives by cormorants in water of similar depth (max 108 s in water 2.5 m deep), with dives of up to 68 s observed in water<0.5 m deep. Initial dives in a bout were shorter than expected, possibly because their plumage was not fully saturated. Dive efficiency (dive:rest ratio) was 5–6, greater than cormorants (2.7±0.4 for 18 species) and other families of diving birds (average 0.2–4.3). Post-dive recovery periods increased with dive duration, but only slowly, resulting in a strong increase in efficiency with dive duration. All dives are likely to fall within the theoretical anaerobic dive limit. Foraging bouts were short (17.8±4.3 min) compared to cormorants, with birds spending 80±5% of time underwater. Darters take advantage of their low buoyancy to forage efficiently in shallow water, and their slow, stealthy dives are qualitatively different from those of other diving birds. However, they are forced to limit the duration of foraging bouts by increased thermoregulatory costs associated with wettable plumage.     

Diving behaviour, aquatic respiration and blood respiratory properties: a comparison of hatchling and juvenile Australian turtles

Australia has a number of bimodally respiring freshwater turtle species that use aquatic respiration to extend their aerobic dive limit. While species variations in reliance on aquatic respiration are reflected in the diving behaviour and ecology of adults, it is unknown whether these relationships also occur in hatchling and juvenile turtles. This study compared the diving behaviour, aquatic respiration and blood respiratory properties of hatchling and juveniles from five species of Australian freshwater turtles: Rheodytes leukops , Elusor macrurus , Elseya albagula , Elseya latisternum and Emydura signata . Both diving behaviour and physiology differed significantly between species as well as age classes. Dive duration in R. leukops was 17 times longer than the other species, with two hatchlings remaining submerged for the entire 72 h recording period. The long dive duration recorded for R. leukops was supported by a high reliance on aquatic respiration (63–73%) and high blood oxygen affinity ( P ₅₀=17.24 mmHg). A correlation between dive duration, aquatic respiration and blood respiratory properties was not observed in the remaining turtle species where, despite the longer dive duration of Els. albagula and Elu. macrurus compared with Em. signata and Els. latisternum , there was no difference observed in per cent aquatic respiration or blood oxygen affinity between these species. When compared with adult individuals (data from previous studies), dive duration was positively correlated with body size in Em. signata , Els. albagula and R. leukops , but a negative relationship occurred in Els. latisternum and Elu. macrurus .     

First records of oceanic dive profiles for leatherback turtles, Dermochelys coriacea, indicate behavioural plasticity associated with long-distance migration

We used Satellite Relay Data Loggers to obtain the first dive profiles for critically endangered leatherback turtles outside the nesting season. As individuals moved from the Caribbean out into the Atlantic, key aspects of their diving behaviour changed markedly, in line with theoretical predictions for how dive duration should vary with foraging success. In particular, in the Atlantic, where foraging success is expected to be higher, dives became much longer than in the Caribbean. The deepest-ever dive profile recorded for a reptile was obtained in the oceanic Atlantic, with a 54-min dive to 626 m on 26 August 2002. However, dives were typically much shallower (generally <200 m) and shorter (<40 min). These results highlight the suitability of this species for testing models of dive performance.     

A phylogenetic analysis of the allometry of diving

The oxygen store/usage hypothesis suggests that larger animals are able to dive for longer and hence deeper because oxygen storage scales isometrically with body mass, whereas oxygen usage scales allometrically with an exponent <1 (typically 0.67-0.75). Previous tests of the allometry of diving tend to reject this hypothesis, but they are based on restricted data sets or invalid statistical analyses (which assume that every species provides independent information). Here we apply information-theoretic statistical methods that are phylogenetically informed to a large data set on diving variables for birds and mammals to describe the allometry of diving. Body mass is strongly related to all dive variables except dive:pause ratio. We demonstrate that many diving variables covary strongly with body mass and that they have allometric exponents close to 0.33. Thus, our results fail to falsify the oxygen store/usage hypothesis. The allometric relationships for most diving variables are statistically indistinguishable for birds and mammals, but birds tend to dive deeper than mammals of equivalent mass. The allometric relationships for all diving variables except mean dive duration are also statistically indistinguishable for all major taxonomic groups of divers within birds and mammals, with the exception of the procellariiforms, which, strictly speaking, are not true divers.     

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.