Tracheal compression delays alveolar collapse during deep diving in marine mammals.

Marine mammals have very compliant alveoli and stiff upper airways; an adaptation that allows air to move from the alveoli into the upper airways, during breath-hold diving. Alveolar collapse is thought occur between 30 and 100 m and studies that have attempted to estimate gas exchange at depth have used the simplifying assumption that gas exchange ceases abruptly at the alveolar collapse depth. Here we develop a mathematical model that uses compliance values for the alveoli and upper airspaces, estimated from the literature, to predict volumes of the respiratory system at depth. Any compressibility of the upper airways decreases the volume to contain alveolar air yielding lung collapse pressures 2x that calculated assuming an incompressible upper airway. A simple relationship with alveolar volume was used to predict relative pulmonary shunt at depth. The results from our model agree with empirical data on gas absorption at depth as well as the degree of tracheal compression in forced and free diving mammals.

Resource requirements of the Pacific leatherback turtle population.

The Pacific population of leatherback sea turtles (Dermochelys coriacea) has drastically declined in the last 25 years. This decline has been linked to incidental capture by fisheries, egg and meat harvesting, and recently, to climate variability and resource limitation. Here we couple growth rates with feeding experiments and food intake functions to estimate daily energy requirements of leatherbacks throughout their development. We then estimate mortality rates from available data, enabling us to raise food intake (energy requirements) of the individual to the population level. We place energy requirements in context of available resources (i.e., gelatinous zooplankton abundance). Estimated consumption rates suggest that a single leatherback will eat upward of 1000 metric tonnes (t) of jellyfish in its lifetime (range 924-1112) with the Pacific population consuming 2.1×10(6) t of jellyfish annually (range 1.0-3.7×10(6)) equivalent to 4.2×10(8) megajoules (MJ) (range 2.0-7.4×10(8)). Model estimates suggest 2-7 yr-old juveniles comprise the majority of the Pacific leatherback population biomass and account for most of the jellyfish consumption (1.1×10(6) t of jellyfish or 2.2×10(8) MJ per year). Leatherbacks are large gelatinous zooplanktivores with consumption to biomass ratios of 96 (up to 192 if feeding strictly on low energy density Cnidarians); they, therefore, have a large capacity to impact gelatinous zooplankton landscapes. Understanding the leatherback's needs for gelatinous zooplankton, versus the availability of these resources, can help us better assess population trends and the influence of climate induced resource limitations to reproductive output.

The genetic component of the forced diving bradycardia response in mammals.

We contrasted the forced diving bradycardia between two genetically similar (inbred) rat strains (Fischer and Buffalo), compared to that of outbred rats (Wistar). The animals were habituated to forced diving for 4 weeks. Each animal was then tested during one 40 s dive on each of 3 days. The heart rate (f(H)) was measured before, during, and after each dive. Fischer and Buffalo exhibited marked difference in dive bradycardia (Fischer: 120.9 ± 14.0 beats min(-1) vs. Buffalo: 92.8 ± 12.8 beats min(-1), P < 0.05). Outbred rats showed an intermediate response (103.0 ± 30.9 beats min(-1)) but their between-animal variability in mean dive f(H) and pre-diving resting f(H) were higher than the inbred strains (P < 0.05), which showed no difference (P > 0.05). The decreased variability in f(H) in inbred rats as compared with the outbred group indicates that reduced genetic variability minimizes variability of the diving bradycardia between individuals. Heritability within strains was assessed by the repeatability (R) index and was 0.93 ± 0.05 for the outbred, 0.84 ± 0.16 for Buffalo, and 0.80 ± 0.12 for Fischer rats for f(H) during diving. Our results suggest that a portion of the mammalian diving bradycardia may be a heritable trait.

Behaviour and physiology: the thermal strategy of leatherback turtles.

BACKGROUND: Adult leatherback turtles (Dermochelys coriacea) exhibit thermal gradients between their bodies and the environment of ≥8°C in sub-polar waters and ≤4°C in the tropics. There has been no direct evidence for thermoregulation in leatherbacks although modelling and morphological studies have given an indication of how thermoregulation may be achieved. METHODOLOGY/PRINCIPAL FINDINGS: We show for the first time that leatherbacks are indeed capable of thermoregulation from studies on juvenile leatherbacks of 16 and 37 kg. In cold water (< 25°C), flipper stroke frequency increased, heat loss through the plastron, carapace and flippers was minimized, and a positive thermal gradient of up to 2.3°C was maintained between body and environment. In warm water (25 - 31°C), turtles were inactive and heat loss through their plastron, carapace and flippers increased. The thermal gradient was minimized (0.5°C). Using a scaling model, we estimate that a 300 kg adult leatherback is able to maintain a maximum thermal gradient of 18.2°C in cold sub-polar waters. CONCLUSIONS/SIGNIFICANCE: In juvenile leatherbacks, heat gain is controlled behaviourally by increasing activity while heat flux is regulated physiologically, presumably by regulation of blood flow distribution. Hence, harnessing physiology and behaviour allows leatherbacks to keep warm while foraging in cold sub-polar waters and to prevent overheating in a tropical environment.

Validation of the use of doubly labeled water for estimating metabolic rate in the green turtle (Chelonia mydas L.): a word of caution.

Marine turtles often have extremely high water turnover accompanied by a low field metabolic rate (FMR), a combination that can contraindicate the use of doubly labelled water (DLW). Therefore, we conducted a validation study to assess the suitability of the DLW technique for determining FMR of marine turtles. Six green turtles (22.42+/-3.13 kg) were injected with DLW and placed in a tank of seawater with a respirometer for continuous monitoring of oxygen consumption (MR) over a 5-day period. Trials were conducted for turtles in both fed and fasted states. Respiratory exchange ratio (RER) was determined in a dry respirometer and used to calculate energy expenditure. For fed and fasted turtles, total body water (TBW) was 66.67+/-3.37% and 58.70+/-7.63% of body mass, and water flux rates were 9.57+/-1.33% and 6.14+/-0.65% TBW day(-1), respectively. Water turnover in fasted turtles was 36% lower than that of fed turtles but MR (from oxygen consumption) of fasted turtles (13.77+/-1.49 kJ kg(-1) day(-1)) was 52% lower than in fed turtles (28.66+/-5.31 kJ kg(-1) day(-1)). Deuterium to oxygen-18 turnover rate (k(d):k(o)) ratios averaged 0.91+/-0.02 for fed turtles and 1.07+/-0.16 for fasted turtles. Fed turtles had a mean group difference of 8% and a mean individual difference of 53% between DLW and respirometry. The DLW method gave negative MR values in fasted turtles and could not be compared with respirometry data. Researchers should use caution when applying the DLW method in marine reptiles, especially when high water flux causes >90% of the labeled oxygen turnover to be due to water exchange.

Exercise warms adult leatherback turtles.

Leatherback sea turtles (Dermochelys coriacea) can maintain body temperature (T(B)) up to 18 degrees C above that of the surrounding sea water (T(W)) which allows leatherbacks to enter cold temperate waters and have the largest global range of any reptile. Using a cylindrical model of a leatherback we investigated the extent to which heat production through variation of swim speed could be used in a leatherback's thermal strategy. Drag force of a full scale cast of a leatherback was measured in a low velocity wind tunnel to obtain an estimate of the metabolic cost needed to offset drag. Heat released in the core of a turtle as a byproduct of the metabolic cost of locomotion is conducted from the core of the turtle to the surrounding water through its insulation layer. By keeping insulation thickness constant, we highlight the effectiveness of swim speed in maintaining T(B)-T(W). Our model, when tested against published data at a given T(W), showed a close correlation between predicted and measured swimming speed at a given T(B). We conclude that the ability to maintain a large T(B)-T(W) is an interplay between mass, insulation thickness and water temperature selection but behavioural control of swimming speed predominates.