The **O _{2} consumption of marine mammals** is of large ecological interest as these values directly relate to the food requirement of the animals. Studies examining the oxygen uptake, i.e. the metabolic rate (MR), have been carried out since the early 1940s when Scholander performed forced submersion experiments on restrained animals. Since then the focus has shifted towards the study of volunteer dives both from animals in captivity and from free-ranging animals. Several strategies have been used to calculate the total energy budgets, the

**field metabolic rate**(

**FMR**), of marine mammals. These include the doubly labeled water method, extrapolating from the activity level or the heart rate and mathematical simulations using available literature data. Another approach has been to calculate the

**oxygen extraction coefficient**(volume fraction of O

_{2}extracted per breath) and to multiply it by the ambient oxygen concentration, the ventilation rate, and the tidal volume.

My study builds on this last approach. In order to calculate the oxygen extraction coefficient, I use the alveolar gas equation to calculate the alveolar oxygen fraction. For this, the **arterial partial pressure of CO _{2}** (

**P**) is needed, which in turn can be calculated from the CO2 binding properties of blood samples.

_{a}CO_{2}In blood, a tight relationship exists between the pH, the P_{a}CO_{2} and the concentration of bicarbonate as is described by the Henderson-Hasselbalch equation. By equilibrating blood samples at set PCO_{2} steps (tonometer/gas mixing pumps) and concomitantly measuring both the pH and the bicarbonate concentration (Cameron method), I can then calculate the P_{a}CO_{2} from the resulting buffer line and the *in vivo* arterial pH.

My study aims to include four cetacean species, **harbor porpoise** (*Phocoena phocoena*), **beluga whale** (*Delphinapterus leucas*), **bottlenose dolphin** (*Tursiops truncates*), **killer**** whale** (*Orcinus orca*) and two pinniped species **south american sea lion** (*Otaria flavescens)* and **walrus** (*Odobenus rosmarus*). These are all drawn from animals in captivity kindly provided by Oceanographic Valencia (Spain), Dolphinarium Harderwijk (Netherlands) and Marineland Côte d’Azur (France). Preliminary results reveal how two of the larger cetaceans included (the beluga whale and the bottlenose dolphin) has higher values of P_{a}CO_{2} compared to terrestrial mammals, while the smallest of the cetaceans (harbor porpoise) and one pinniped (south american sea lion) shows values near those of terrestrial mammals. Since the predicted FMR depends heavily on the oxygen extraction coefficient, and so on the P_{a}CO_{2}, these results highlight the importance of determining correct values of P_{a}CO_{2} when using this approach.

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