Insights from venous oxygen profiles: oxygen utilization and management in diving California sea lions

The pattern and magnitude of depletion of elevated body O₂ stores underlie the dive capacity, feeding behavior and foraging ecology of marine mammals. The California sea lion [Zalophus californianus (Lesson 1828)] is an excellent model species for investigation of O₂ store depletion in an otariid (sea lions and fur seals) because both its O₂ stores and dive behavior have been extensively studied (Feldkamp et al., 1989; Kuhn, 2006; Weise and Costa, 2007). Forty-three percent of the 52 ml O₂ kg⁻¹ total body O₂ store in adult females is found in the blood (Weise and Costa, 2007). Consequently, investigation of blood O₂ depletion is especially crucial if we are to understand the physiological limits of their routine 3–5 min dives and the physiological strategies used in longer dives up to 14 min in duration (Feldkamp et al., 1989; Kuhn, 2006).

Venous blood O₂ depletion has been examined in two freely diving species, the emperor penguin (Aptenodytes forsteri) and the northern elephant seal (Mirounga angustirostris), both considered to be extreme breath-hold divers (Ponganis et al., 2007; Meir et al., 2009; Meir and Ponganis, 2009). The ability of the penguin and elephant seal to perform such long dives was attributed to extreme hypoxemic tolerance and near-complete depletion of the blood O₂ stores (Ponganis et al., 2007; Meir et al., 2009; Meir and Ponganis, 2009). During dives that exceeded 6 min in emperor penguins and 15 min in elephant seals, venous partial pressure of O₂ (P_O2) often reached values less than 10 mmHg [1.3 kPa, near 0% hemoglobin (Hb) saturation (S_O2)] in both species (Meir et al., 2009; Meir and Ponganis, 2009). Although both emperor penguins and elephant seals deplete venous blood O₂ to low values, the depletion patterns exhibited by emperor penguins were more variable (Ponganis et al., 2007; Meir and Ponganis, 2009). For example, at the 5.6 min aerobic dive limit (ADL; the dive duration associated with the onset of post-dive blood lactate accumulation) of emperor penguins (Ponganis et al., 1997b), end-of-dive S_O2 ranged from ~5 to 85% saturation (Meir and Ponganis, 2009). In contrast, for dive durations of 15 min (estimated ADL for juvenile elephant seals based on the ADL from the similar sized juvenile Weddell seal) (Kooyman et al., 1983), end-of-dive S_O2 ranged from 0 to 30% (Meir et al., 2009). Such differences are possibly attributed to differences in: (1) the relative size of the respiratory O₂ store (4% in the elephant seal versus 33% in the emperor penguin) (Kooyman and Ponganis, 1998; Sato et al., 2011), (2) heart rate regulation (degree and pattern of diving bradycardia), (3) pulmonary blood flow/gas exchange early in the dive and (4) peripheral organ perfusion, severity of peripheral vasoconstriction and degree of arterio-venous (a-v) shunting.

This study took advantage of the recurrent 1–10 day maternal foraging trips of California sea lions to measure venous blood O₂ depletion by utilizing a backpack P_O2 recorder to document venous P_O2 profiles during the routine dives of these animals at sea. The goals of this research were to: (1) obtain venous P_O2 profiles from freely diving animals, (2) characterize the sea lion O₂–Hb dissociation curve and apply it to convert P_O2 to S_O2 and (3) calculate the rate and magnitude of venous blood O₂ store depletion during both shallow and deep dives at sea.

We hypothesized that: (1) the O₂–Hb dissociation curve, the P₅₀ and the Bohr shift would be similar to those of other marine mammals; (2) venous blood O₂ depletion would be incomplete even in long-duration dives of sea lions because of the contribution of respiratory O₂ to blood and the probable isolation of muscle from the circulation due to slower heart rates during such dives (Ponganis et al., 1997b); (3) minimum venous P_O2, S_O2 and the magnitude of O₂ depletion would be negatively related to dive duration; and (4) depletion patterns would be highly variable in short dives (<3 min) as seen in emperor penguins due to the contribution of lung O₂ stores [16% of total body O₂ stores (Weise and Costa, 2007)], especially at shallow depths.

O₂–Hb dissociation curves were determined on fresh whole blood using the mixing technique at 37°C (Scheid and Meyer, 1978). Blood (30–60 ml) was collected in heparinized blood tubes from stranded adult sea lions rehabilitated at Sea World San Diego (N=7) and from healthy neutered male adult sea lions maintained by the US Navy Marine Mammal Program in San Diego, CA (N=4), during routine health procedures. Samples were placed on ice during transport. Analyses began immediately upon arrival to the laboratory (~30–60 min post collection) and were completed within 8 h of collection to prevent depletion of labile organic phosphates. Blood was tonometered (tonometer 237; Instrumentation Laboratory, Bedford, MA, USA) to create 0% saturated and 100% saturated blood at the desired pH (7.2, 7.3, 7.4 and 7.5) using an appropriate mix of N₂, O₂ and CO₂. Desaturated and 100% saturated blood was mixed to obtain S_O2 at various points along the curve (10, 20, 50, 70, 90 and 95% S_O2) and the P_O2 of the mix was then determined using an i-STAT blood gas analyzer (Abbott Point of Care, Princeton, NJ, USA). The use of the i-STAT also allowed verification of pH. The log[S_O2/(100–S_O2)] versus log(P_O2) relationship was plotted, and linear regression analysis was performed to determine the equation of the O₂–Hb dissociation curve at pH 7.2, 7.3, 7.4 and 7.5. Because of the limited quantities of blood, curves were not determined at each pH for all sea lions, but when possible additional P₅₀ values were obtained at each pH. The CO₂ Bohr coefficient was determined from the linear regression of log(P₅₀) on pH (averaged from all P₅₀ values at pH 7.2, 7.3 and 7.4) (Meir et al., 2009). The equipment and procedure used in this study were validated by determining the P₅₀ in species with previously published O₂–Hb binding data (rat, sheep and California sea lion).

The field study was conducted at San Nicolas Island, CA (22°14′12.3″N, 119°32′54.3″W), during August 2010 and 2011. Adult female California sea lions were captured using customized hoop nets and only lactating females (determined by manual expression of milk from the teat) were selected for instrumentation to increase the likelihood of recovering instruments. Females were anesthetized with isoflurane gas with O₂ using a portable field vaporizer anesthesia circuit (Gales and Mattlin, 1998). After mask induction and intubation on 5% isoflurane-O₂, anesthesia was maintained with 1–2% isoflurane.

Eleven females were instrumented with custom-built P_O2 data loggers, time depth recorders (TDRs) and radio transmitters. With ultrasound guidance over the caudal back, a P_O2 electrode (model Licox C1.1 Revoxode; Integra Life Sciences, Plainsboro, NJ, USA) and thermistor (model 554; Yellow Springs Instruments, Yellow Springs, OH, USA) were placed percutaneously through a Peel-Away catheter (5 Fr; Cook Medical, Bloomington, IN, USA) into the vena cava via the caudal gluteal vein. Catheterization, P_O2 electrode and thermistor procedures have been described previously (Ponganis et al., 1991; Ponganis et al., 1997a; Stockard et al., 2005; Ponganis et al., 2007; Meir et al., 2009). The P_O2 electrode and thermistor were connected to a custom-built microprocessor (3991 BioLog or UUB BioLog; UFI, Morro Bay, CA, USA) in a waterproof housing (5×17 cm, 570 g or 3.2×11.4 cm, 200 g; Meer Instruments, Palomar Mountain, CA, USA) that was mounted midline above the hips with 5 min epoxy (Loctite; Henkel Corp., Westlake, OH, USA). The logger recorded P_O2 and temperature every 1 s (two deployments), 5 s (eight deployments) or 15 s (one deployment) depending on the specific data logger used. An Mk9 TDR (Wildlife Computers, Redmond, WA, USA; 6.7×1.7×1.7 cm, 30 g, 1 s sampling interval, ±0.5 m resolution) and a radio transmitter (mm160B, 2.0×5.6 cm, 25 g; ATS, Isanti, MN, USA) were attached in front of the P_O2 logger. The time on the TDR and P_O2 logger were synchronized to the same internet-synced computer clock.

After instrumentation, females were weighed (±0.2 kg, MSI-7200 Dyna-link; Measurement Systems International, Seattle, WA, USA) and placed in a kennel to recover from anesthesia (25–60 min). Once recovered, females were released. After one to four trips to sea, females were recaptured for instrument recovery. Instruments were removed while the sea lion was manually restrained (entire capture ~10 min). All procedures were approved under a University of California, San Diego Animals Subjects Committee permit (no. S11303) and a National Marine Fisheries Service marine mammal permit (no. 14676).

We obtained S_O2 values by applying the linear regression equation determined by the dissociation curve analysis to the P_O2 data collected by the data loggers and solving for S_O2. Initial, maximum, minimum and end-of-dive S_O2 were estimated using the equation at pH 7.4 for most dives, but for dives greater than 3 min in duration, the minimum and end-of-dive S_O2 were estimated using the equation at pH 7.3. Although pH can shift the dissociation curve, previous work suggests that there is minimal change in pH even in forced dives of 30 min in Weddell seals (7.4 to 7.28) (Elsner et al., 1970). Therefore, we assumed little change in pH in the routine dives (<3 min) of sea lions. To be conservative, in dives longer than the calculated ADL (cADL; estimated ADL calculated from total body O₂ stores and metabolic rate) (Weise and Costa, 2007), the O₂ content at the end of dives >3 min was determined assuming a pH of 7.3.

Prior to deployment, P_O2 electrodes and thermistors were calibrated in the laboratory as previously described (Stockard et al., 2005; Ponganis et al., 2007). Thermistor connections frequently broke during the deployment so we were unable to correct electrode output for P_O2 data using in vivo temperatures (Stockard et al., 2005). However, continuous venous temperature profiles were successfully collected from three sea lions while at sea for a cumulative 165 h, including 2803 dives, surface intervals and subsurface swimming. Average temperature was 36.8±0.6°C while at sea (range 33.4–39.0°C, 93% of values were within 1°C of 37°C); therefore, we assumed that body temperature was 37°C when calculating P_O2 and S_O2. Although not ideal, this assumption would result in very small effects on P_O2. For example, at a P_O2 of 60 mmHg (8.0 kPa), a ±1°C in vivo temperature difference from 37°C would result in only a 2 mmHg (0.3 kPa) difference (1% S_O2). At low P_O2 values, the effect would be even smaller; e.g. the difference would only be 0.2 mmHg (0.1% S_O2) at a P_O2 of 5 mmHg (0.7 kPa).

TDR data were analyzed in MATLAB (The MathWorks, Natick, MA, USA) using a custom-written dive analysis program (IKNOS; Y. Tremblay), which calculates a zero offset correction at the surface and identifies dives on the basis of a minimum depth and duration. The minimum depth for defining a dive was set at 5 m and the minimum duration was 20 s.

P_O2 and TDR logger clock drifts were documented pre-deployment and, when possible, post-deployment, to allow data synchronization. A custom-written MATLAB code was used to obtain initial (P_O2 value before dive starts), maximum, minimum and end-of-dive P_O2 (and S_O2) and time to minimum P_O2 for every dive greater than 1 min in duration. Data were visually inspected to confirm processed results.

The relationship between dive duration and initial P_O2, maximum P_O2, minimum P_O2 and percent O₂ depleted during the dive were investigated using linear mixed effects models (Cran R 2.12.2, package nlme). Dives less than 2 min in duration were excluded from analysis due to the high variability of values in those dives. Dive duration was the fixed effect in all models, and to account for the lack of independence caused by repeatedly sampling the same individual over time, individual (sea lion ID) was included as a random effect. P_O2 data were log transformed and the proportion of O₂ depleted during a dive was arcsine transformed before analysis to meet model assumptions. Covariance and random effect structures of the full models were evaluated using Akaike's information criterion (AIC) and examination of residual plots (Zuur et al., 2009). AICs from all tested models are presented with the best model highlighted in bold.

P_O2 data were successfully obtained from eight females with trip durations ranging from 0.8 to 8.9 days (data loggers from one sea lion were not recovered and two deployments failed due to electrode failure before diving commenced). All but the two females with the shortest trip durations exhibited diving behavior typical of foraging trips (one of which did not perform any dives greater than 2 min in duration or 20 m in depth) (Feldkamp et al., 1989; Kuhn, 2006); however, an abnormal short trip following capture is not unusual in sea lions. The range in the number of profiles analyzed for each sea lion was secondary to trip duration, diving behavior and time until electrode failure (Table 1). We obtained 2596 venous P_O2 diving profiles from dives greater than 1 min in duration. Median dive duration was 1.7 min (range 1.0–8.1 min) and median depth was 29 m (range 5–377 m). Dive duration and depth data for each female exhibiting dives greater than 2 min in duration are given in Table 1 (seven females, 926 dives).

Blood O₂ depletion patterns differed between shallow dives and deep dives. In short-duration, shallow dives (<3 min, N=2252), P_O2 (and S_O2) depletion patterns were highly variable, and venous blood O₂ was not usually depleted [minimum P_O2 ≥30 mmHg (4 kPa; S_O2 ≥50%) in 60.7% of dives; Fig. 2, Fig. 3A,C]. P_O2 (and S_O2) typically increased during the initial part of the dive before decreasing until the end of the dive. In dives greater than 3 min in duration, there were two typical P_O2 profile shapes. The first profile type was similar in shape to the short, shallow dive profiles, with an initial increase before decreasing throughout most of the dive, but with lower minimum values than observed in short, shallow dives. However, in most of the deepest and longest dives, P_O2 also increased initially, but then rapidly declined to less than 10 mmHg (1.3 kPa; S_O2 <5%) halfway through the dive, remained low for a few minutes, and finally increased during the last minute of the dive as the sea lion was ascending (Fig. 3B,D). In most of the profiles from dives greater than 3 min in duration, there was an increase in P_O2 (and S_O2) prior to the end of the dive (88.1% of dives; Fig. 2, Fig. 4A). In general, P_O2 (and S_O2) continued, or quickly started, to increase after surfacing (Fig. 3); yet, because many post dive intervals were short and/or sea lions were sub-surface swimming, P_O2 did not always recover to resting values (~78% S_O2 assuming a 5 ml O₂ dl⁻¹ a-v difference at rest). In dives greater than 5 min, which typically were followed by a longer post-dive interval (median=4.3 min, range=1.8–126.7 min) with little evidence for sub-surface swimming during the post dive interval, median time to near-resting values (45 mmHg, 6 kPa, 75% S_O2) was 28 s (range=0–67 s).

Initial, maximum and minimum P_O2 and S_O2, percentage blood O₂ depletion and O₂ depletion rate are given for each sea lion exhibiting dives greater than 2 min in duration in Table 1. P_O2 (and S_O2) at the start of the dive was variable (Table 1). In 43.5% of all dives, initial venous P_O2 was greater than 49 mmHg (6.5 kPa, S_O2 >78%). This was seen in seven of the eight females and most commonly in sea lions that performed the deepest dives. There was a positive, but weak, relationship between initial and maximum P_O2 and dive duration, with high intra-class correlation indicating that each sea lion had a different relationship (Table 2, supplementary material Fig. S1).

In dives greater than 2 min in duration, there was a negative relationship between dive duration and minimum venous P_O2 (Table 2, Fig. 2A). In dives greater than 4 min in duration, minimum venous P_O2 routinely reached values below 15 mmHg (2.0 kPa, 15% S_O2; 79.3% of dives; Fig. 2A,C) and was occasionally as low as 5 mmHg (0.7 kPa, 1% S_O2; Fig. 2A,C, Fig. 3A,C). This resulted in near-complete blood O₂ depletion (Fig. 5A,B). Percentage venous O₂ depletion during a dive was positively related to dive duration in dives greater than 2 min (Table 2), with over 90% of venous blood O₂ being depleted in 69.6% of dives greater than 4 min in duration. Blood O₂ depletion rates were much more variable (Fig. 5C), with individual sea lions exhibiting different relationships between dive duration and depletion rate patterns. In general, depletion rates were highly variable in dives less than 3 min in duration, but much more consistent in longer duration dives (Fig. 5C).

As expected, the O₂–Hb dissociation curve for the California sea lion (Fig. 1) was similar to that of other pinnipeds and terrestrial mammals (Horvath et al., 1968; Lenfant, 1969; Meir et al., 2009). The mean P₅₀ of 28 mmHg (3.7 kPa) at pH 7.4 was slightly lower than the previously reported P₅₀ of 28.5 and 29.5 from two sea lions (Horvath et al., 1968; Lenfant, 1969). The sea lion P₅₀ falls within the range of most diving mammals (Lenfant, 1969; Meir et al., 2009). The relatively large Bohr effect was also similar to that of other pinnipeds and terrestrial mammals of similar size, and may facilitate offloading of O₂ at the tissues (Riggs, 1960; Lenfant, 1969; Lenfant et al., 1970; Snyder, 1983; Willford et al., 1990; Meir et al., 2009).

In the typical short shallow dives of California sea lions (<3 min), there was great variation in minimum and final venous P_O2 and S_O2 (10–90% S_O2 range; Fig. 2), and the rate and magnitude of O₂ depletion (Fig. 3A,C, Fig. 5). This variability may be secondary to differences in: (1) surface duration and the magnitude of venous blood O₂ repletion prior to a dive, (2) dive duration and (3) metabolic rate during an individual dive. For example, observed start-of-dive venous S_O2 values were as low as 15%, consistent with prior suggestions that the blood O₂ store is not necessarily restored prior to each dive (Fahlman et al., 2008b). And, in regard to metabolic rate, there has also been as much as fivefold variation in diving metabolic rate during short dives of another otariid, the Steller sea lion (Fahlman et al., 2008b).

In addition to highly variable venous blood O₂ depletion patterns in short dives, venous O₂ was not necessarily depleted at the end of dives at the cADL (3 min), with end-of-dive S_O2 values ranging from 10 to 80%. This is similar to the high variability in S_O2 at the end of dives at the ADL (5.6 min) in emperor penguins (range 5–85%) (Meir and Ponganis, 2009). Given that the venous O₂ store is not depleted at most estimates of the ADL (3 min) in sea lions, we hypothesize that, like the ADL of emperor penguins, the ADL of sea lions is determined by the depletion of the muscle O₂ store (Williams et al., 2011). Lack of depletion of the venous O₂ store at the ADL is not consistent with previous theoretical models of O₂ store management in which both the blood and muscle O₂ store are simultaneously depleted in order to maximize aerobic dive duration (Davis and Kanatous, 1999).

In all dives, there was a decrease between maximum and minimum venous blood O₂ during a dive, but in 15% of the dives (all <3 min in duration) end-of-dive venous blood O₂ was equal to or greater than the start-of-dive value. This indicates that pulmonary gas exchange can continue during these shallow dives, and that, as in emperor penguins, venous blood O₂ may even increase during the breath hold, perhaps due to a-v shunting during the dive (Ponganis et al., 2007). The lung O₂ store of the sea lion constitutes 16% of the total body O₂ store (Weise and Costa, 2007), making it a significant potential O₂ source during diving, especially in shallow dives when lungs do not collapse (Kooyman and Sinnett, 1982; McDonald and Ponganis, 2012).

In the longer-duration dives of California sea lions (>3 min), there was much less variability in minimum and final venous blood O₂, and in the rate and magnitude of O₂ depletion (Figs 2, 5). In contrast to shallow dives, there was near-complete O₂ depletion in most dives greater than 4 min in duration. We had hypothesized that venous blood O₂ depletion would be incomplete even in long dives because of the contribution of respiratory O₂ to blood and the likely isolation of muscle from the circulation during such dives. Yet, sea lions regularly depleted venous O₂ to levels equivalent to those recorded for both elephant seals and emperor penguins (Ponganis et al., 2007; Meir et al., 2009; Meir and Ponganis, 2009), and often much earlier in the dive (Fig. 3). In long-duration dives, elephant seals exhibited an increase in venous P_O2 at the beginning of a dive, as seen in sea lions, but then usually exhibited a steady decline throughout the remainder of the dive. In contrast, after the initial increase, sea lions showed a rapid decline in S_O2 during the first half of the dive, after which it remained low for a few minutes before increasing during the last 30–60 s of the dive during the ascent. The rapid decrease in venous S_O2 during the first half of the sea lion's dive may reflect increased blood O₂ extraction by perfused, working muscle. However, much of the decline is during descent, when sea lions are likely minimizing effort with a stroke and glide locomotory pattern (Williams, 2001; Fahlman et al., 2008a). Alternatively, this rapid decline in venous P_O2 observed in most long dives may be secondary to a severe bradycardia, low tissue perfusion and prolonged tissue transit time, resulting in near-complete extraction of blood O₂.

Initial venous S_O2 values greater than 78% (and sometimes as high as 95%) in over 40% of dives indicate that sea lions are able to enhance their blood O₂ stores by arterialization of venous blood (Fig. 3D). These elevated, pre-dive venous S_O2 values suggest the use of a-v shunts, as has been hypothesized for emperor penguins (Ponganis et al., 2007; Meir and Ponganis, 2009). This shunting could occur in the well-described a-v anastomoses in the skin of California sea lions (Bryden and Molyneux, 1978) or potentially in undescribed a-v shunts in other locations.

Alternatively, high initial venous S_O2 values could also be secondary to a lack of O₂ extraction in tissue (i.e. muscle) hyperperfused with well-oxygenated blood during the hyperventilation and tachycardia of the surface intervals. Artificial hyperperfusion through muscle can arterialize venous blood (Grassi et al., 1998); however, post-exercise hyperemia does not result in venous hyperoxia in healthy subjects (Bangsbo and Hellsten, 1998), and once the muscle is recovered, muscle blood flow decreases to resting rates (Walløe and Wesche, 1988; Bangsbo and Hellsten, 1998). Furthermore, hyperventilation at rest, at least in humans, is not associated with increased venous saturations (Huckabee, 1958). Therefore, the use of a-v shunts is the most plausible explanation for elevated S_O2 values observed during surface intervals, as it is unlikely that the arterialization of pre-dive venous S_O2 is due to hyperperfusion of resting muscle with already fully saturated myoglobin.

The magnitude of the a-v shunt before the dive can be estimated using the shunt equation: venous O₂ content = [arterial O₂ content × % a-v shunt] + [(arterial O₂ content – 5 ml O₂ dl⁻¹) × (1 – % a-v shunt)], assuming a 5 ml O₂ dl⁻¹ a-v O₂ difference at rest, using an initial arterial O₂ content of 24.07 ml O₂ dl⁻¹ (McDonald and Ponganis, 2012), and assuming that the vena caval O₂ content is close to the mixed venous value. For example, before a 7.4 min dive, one of the sea lions had an initial venous S_O2 of 90.7% (22.1 ml O₂ dl⁻¹). In order to obtain the elevated venous S_O2 observed, an a-v shunt of 60.4% is required. This magnitude of shunt is similar to what was calculated in an emperor penguin before a 23 min dive (Meir and Ponganis, 2009).

Sea lions appear to maximize venous blood O₂ utilization by arterializing venous blood before dives and then depleting venous O₂ to less than 1% saturation during deep dives. An initial venous S_O2 of 90%, in contrast to the 78% that would be used in a classic O₂ store calculation, allows sea lions to dive with an additional 3 ml O₂ dl⁻¹ in venous blood. This additional O₂ results in an approximate 16% increase in venous blood O₂. Thus, in addition to a role in thermoregulation (Bryden and Molyneux, 1978; Molyneux and Bryden, 1981), a-v anastomoses in the sea lion may contribute to enhanced blood O₂ storage and extension of the duration of aerobic metabolism in long-duration dives.

The increase in venous O₂ during the initial descent of a dive (Fig. 3) is inconsistent with O₂ extraction by working tissues, suggesting that diving sea lions are able to restrict blood flow to locomotory muscles. If sea lions were perfusing muscle during the initial descent, we would expect to see a decline in S_O2 because sea lions have a high stroke rate during the initial descent, presumably to overcome buoyancy (Fahlman et al., 2008a; Hindle et al., 2010). However, venous S_O2 often increased to values greater than 85% during early descent (Table 1, Fig. 3), implying that muscle is ischemic during the initial phase of the dive. This is also similar to venous S_O2 profiles obtained for both emperor penguins and elephant seals (Meir et al., 2009; Meir and Ponganis, 2009).

Although the increase in S_O2 suggests sea lions are not perfusing muscle early in the dive, the high variability in profile shape later in the dive, especially in shorter dives, may partially be explained by different patterns of muscle blood flow during diving as suggested in emperor penguins by different myoglobin saturation profiles (Williams et al., 2011). In addition, in deep dives of sea lions, resumption of some muscle blood flow may account for the rapid decline in venous S_O2 during the latter part of descent. If heart rate is low and muscle blood flow only partially resumes during the latter descent, the rapid decline in venous S_O2 may be due, at least partly, to enhanced muscle O₂ extraction to replace muscle O₂ consumed during the beginning of the dive. Increased O₂ demand in poorly perfused muscle could thus account for the rapid decline in venous S_O2 to extremely low levels during late descent.

As already mentioned, increases in venous S_O2 during early descent and net increases in venous S_O2 during some shallow dives are consistent with maintenance of gas exchange at shallow depths. The increases in venous S_O2 during ascent from deep dives (Fig. 3D, Fig. 4A) are also consistent with arterial P_O2 profiles of deep dives, which indicate resumption of gas exchange in re-expanded lungs during the ‘ascent tachycardia’ (Ponganis et al., 1997a; McDonald and Ponganis, 2012). The deep dives of California sea lions are associated with lung compression, and there appears to be complete or near-complete cessation of pulmonary gas exchange (~100% pulmonary shunt or near-complete ‘lung collapse’) at ~200 m depth (Kooyman and Sinnett, 1982; McDonald and Ponganis, 2012). We have proposed that in addition to minimization of nitrogen absorption, lung collapse preserves a pulmonary O₂ reservoir that supplements blood O₂ during ascent in the sea lion (McDonald and Ponganis, 2012). Our finding that end-of-dive venous S_O2 is usually greater than the minimum S_O2 during deep dives reinforces this hypothesis. Increased pulmonary blood flow, secondary to the ascent tachycardia routinely observed in diving sea lions (Ponganis et al., 1997a; Hindle et al., 2010), could facilitate O₂ uptake by the blood and maintain or even improve arterial S_O2. Similarly, increased blood flow secondary to the ascent tachycardia may result in blood passing through a-v shunts, thereby minimizing peripheral extraction of blood O₂ and supplementing venous blood O₂, thus accounting for the observed increases in end-of-dive venous S_O2. The possible use of shunts during ascent is supported by the positive relationship between initial S_O2 and the magnitude of increase in S_O2 at the end of the dive (Fig. 4B), indicating that when sea lions appear to be using a-v shunts during the surface interval, they may also use shunts during ascent. Alternatively, the increase in venous S_O2 near the end of a dive may be secondary to increased tissue blood flow with a constant or decreased a-v O₂ difference during the ascent tachycardia (i.e. a relative increase in muscle blood flow despite unchanged or decreased blood O₂ extraction by muscle). However, as already discussed in relation to pre-dive arterialization of venous blood, there is no evidence for this available in the literature.

The contribution of venous blood O₂ to diving metabolic rate was highly variable, especially in short dives. We were able to calculate the contribution of venous blood O₂ to metabolic rate while diving because the Hb concentration and the mass-specific blood volume of adult female California sea lions are known (18 g dl⁻¹, 110 ml kg⁻¹) (Weise and Costa, 2007). We used the following equation: venous blood O₂ contribution to metabolic rate (ml O₂ kg⁻¹ min⁻¹) = [(maximum O₂ content – minimum O₂ content) / dive duration] × (mass-specific blood volume) × (% venous blood), assuming two-thirds of the blood volume is venous.

Using Athena, the sea lion that demonstrated maximum O₂ loading and unloading, as an example, the average contribution of venous blood O₂ to diving metabolic rate for all dives was 1.9±1.0 ml O₂ kg⁻¹ min⁻¹. However, the majority of these dives (87% <3 min in duration) were 1–2 min in duration with minimal changes in venous S_O2, presumably due to continued pulmonary gas exchange and transfer of lung O₂ into the blood. We think that maintenance of such gas exchange masks the contribution of venous O₂ to metabolic rate in sea lions just as was proposed in emperor penguins (Meir and Ponganis, 2009). In longer dives, the contribution of venous O₂ to metabolic rate was higher. For example, the venous O₂ contribution to metabolic rate in Athena was 4.4 ml O₂ kg⁻¹ min⁻¹ for a 3 min dive, 3.3 ml O₂ kg⁻¹ min⁻¹ for a 5 min dive, and 2.4 ml O₂ kg⁻¹ min⁻¹ for a 7 min dive; ~43, 32 and 24%, respectively, of the metabolic rate of a sea lion resting at the surface (Hurley and Costa, 2001), and 133, 100 and 73%, respectively, of the allometrically predicted basal metabolic rate for a sea lion of this size (Kleiber, 1975). These contributions to diving metabolic rate do not include O₂ from arterial blood or from the respiratory and muscle O₂ stores. This venous O₂ contribution to metabolic rate in the sea lion for dives greater than 3 min is in the general range of the venous contribution of 3.4 ml O₂ kg⁻¹ min⁻¹ for an elephant seal (Meir et al., 2009), and much higher than the contribution of venous blood to metabolic rate calculated in emperor penguin (0.6 ml O₂ kg⁻¹ min⁻¹) (Meir and Ponganis, 2009).

Application of the O₂–Hb dissociation curve to arterial profiles from deep dives of sea lions (McDonald and Ponganis, 2012) allows comparison of the arterial S_O2 profile to the venous S_O2 profiles of this study (Fig. 6). As, we have previously emphasized, arterial S_O2 is well maintained during these long, deep dives (McDonald and Ponganis, 2012). It is notable that during the middle of these dives, arterial O₂ content is still near maximal values at 23 ml O₂ dl⁻¹, while venous O₂ content is almost 0 ml O₂ dl⁻¹, yielding an a-v O₂ content difference (Δa-v O₂) of 23 ml O₂ dl⁻¹, more than four times the typical value assumed in most animals at rest. As previously emphasized for a low venous S_O2, this large Δa-v O₂ is consistent with extreme hypoperfusion of tissue and complete blood O₂ extraction. Thus, these S_O2 profiles and the large Δa-v O₂ suggest that sea lions are extremely bradycardic during these segments of the dive.

During extreme bradycardia and tissue hypoperfusion, venous return is low and venous mixing is slow. Under such conditions, we raise the caveat that a P_O2 electrode in the distal posterior vena cava may not provide data representative of the entire venous blood O₂ store. Under such conditions, the position of the electrode in this study may only allow assessment of P_O2 in blood slowly draining peripheral tissues, but not in mixed venous blood (in the pulmonary artery). Therefore, there are limitations to our interpretations, and to our calculations of the contribution of venous O₂ to diving metabolic rate. Mixed venous blood P_O2 and S_O2 would be ideal, but were not feasible in this study because of the length of the P_O2 electrode and size of the animal.

The arterial and venous S_O2 profiles of deep dives also have other implications. If our venous S_O2 profiles truly reflect mixed venous S_O2, and if there is complete cessation of gas exchange as we have previously suggested due to lung collapse near 200 m, then it is unclear how arterial P_O2 and S_O2 can be maintained at such high levels during this middle portion of the deep dive. One would expect that arterial S_O2 should rapidly approach mixed venous S_O2 under conditions of complete lung collapse. Therefore, we conclude that either (1) mixed venous S_O2 is greater than the distal vena cava S_O2 during this segment of the dive, or (2) some degree of gas exchange still persists even at maximal depths, thus allowing for maintenance of arterial S_O2. It is again notable that, under either condition, a severe bradycardia is optimal. We hope to address these questions in future studies.

Lastly, the transient rises in venous S_O2 shown in Fig. 6, especially those in late ascent, are again consistent with intermittent large pulses of well-oxygenated arterial blood into the venous system. These transient increases in venous S_O2, as well as the near equivalence of arterial and venous S_O2 early in the dive (Fig. 6), again reinforce our hypothesis that the a-v shunts are utilized in the diving sea lion. Such ‘spikes’ in venous S_O2 were not uncommon (Fig. 3).

These are the first blood O₂ depletion data from a diving animal during natural foraging trips. We were able to take advantage of the foraging behavior of lactating sea lions to obtain data on how they manage O₂ during natural dives. Our results suggest that sea lions are optimizing the size of the venous blood O₂ store and the magnitude of its depletion during long dives by arterializing venous blood before a dive and then depleting it to extremely low levels, resulting in a net O₂ content depletion of up to 99%. In addition, the increase in venous O₂ at the end of deep dives supports the hypothesis that lung collapse preserves an O₂ reservoir that the sea lions can use as they ascend. The high variability in O₂ depletion patterns, both between and within individuals, suggests that the O₂ management strategy is variable and can be adapted to the current O₂ demands of the dive. Oxygen store depletion during shallow dives certainly differs from that of deep dives. Future work investigating diving heart rate and stroke rate during diving will further elucidate the mechanisms underlying O₂ store management in freely diving California sea lions.

We thank the US Navy (specifically G. Smith, J. Ugoretz and G. Smith), C. L. Williams, M. Fowler, P. W. Robinson, D. P. Costa, S. E. Simmons and J. U. Meir for exceptional logistical support and advice. We also thank Sea World staff (especially J. St Leger and E. Nelson) and the US Navy Marine Mammal Program for providing sea lion blood to determine the O₂–Hb dissociation curve. B. DeValle and S. Nelson (US Navy anesthesia residents) provided excellent anesthesia assistance. P. Thorson, M. Tift, R. Walsh, C. L. Williams, G. L. Kooyman and S. Tavoni provided valuable assistance in the field.

 
• a-v

  arterio-venous

 
• ADL

  aerobic dive limit

 
• cADL

  calculated aerobic dive limit

 
• Hb

  hemoglobin

 
• P_O2

  partial pressure of oxygen

 
• S_O2

  percent Hb saturation

 
• TDR

  time-depth recorder

 
• Δa-v O₂

  arterio-venous oxygen difference