Respiration rate has been used as an indicator of metabolic rate and associated cost of transport (COT) of free-ranging cetaceans, discounting potential respiration-by-respiration variation in O 2 uptake. To investigate the influence of respiration timing on O 2 uptake, we developed a dynamic model of O 2 exchange and storage. Individual respiration events were revealed from kinematic data from 10 adult Norwegian herring-feeding killer whales (Orcinus orca) recorded with high-resolution tags (DTAGs). We compared fixed O 2 uptake per respiration models with O 2 uptake per respiration estimated through a simple 'broken-stick' O 2 -uptake function, in which O 2 uptake was assumed to be the maximum possible O 2 uptake when stores are depleted or maximum total body O 2 store minus existing O 2 store when stores are close to saturated. In contrast to findings assuming fixed O 2 uptake per respiration, uptake from the broken-stick model yielded a high correlation (r 2 >0.9) between O 2 uptake and activity level. Moreover, we found that respiration intervals increased and became less variable at higher swimming speeds, possibly to increase O 2 uptake efficiency per respiration. As found in previous studies, COT decreased monotonically versus speed using the fixed O 2 uptake per respiration models. However, the broken-stick uptake model yielded a curvilinear COT curve with a clear minimum at typical swimming speeds of 1.7-2.4 m s −1. Our results showed that respiration-by-respiration variation in O 2 uptake is expected to be significant. And though O 2 consumption measurements of COT for free-ranging cetaceans remain impractical, accounting for the influence of respiration timing on O 2 uptake will lead to more consistent predictions of field metabolic rates than using respiration rate alone.

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