Although myoglobin (Mb) is considered to contribute significantly to the oxygen and diving capacity of marine mammals, few data are available for cetaceans. Cetacean by-catch in the tuna driftnet fisheries in the Sulu Sea, Philippines, afforded the opportunity to examine Mb content and distribution, and to determine muscle mass composition, in Fraser's (Lagenodelphis hosei) and spinner (Stenella longirostris) dolphins and a pygmy killer whale (Feresa attenuata). Age was estimated by body length determination. Stomach contents were analyzed for the presence or absence of milk and solid foods. It was hypothesized (a) that Mb concentration ([Mb]) would be higher in Fraser's and spinner dolphins than in other small cetaceans because of the known mesopelagic distribution of their prey, (b) that [Mb] would vary among different muscles according to function during diving, and (c) that [Mb] would increase with age during development. The results were as follows. (1) Myoglobin concentrations of the longissimus muscle in adult Fraser's (6.8-7.2 g 100 g-1 muscle) and spinner (5–6 g 100 g-1 muscle) dolphins and in an immature pygmy killer whale (5.7 g 100 g-1 muscle) were higher than those reported previously for small cetaceans. (2) [Mb] varied significantly among the different muscle types in adult dolphins but not in calves; in adults, swimming muscles had significantly higher [Mb] than did non-swimming muscles, contained 82–86 % of total Mb, and constituted 75–80 % of total muscle mass. (3) Myoglobin concentrations in Fraser's and spinner dolphins increased with size and age and were 3–4 times greater in adults than in calves. The high Mb concentrations measured in the primary locomotory muscles of these pelagic dolphins are consistent with the known mesopelagic foraging behaviour of Fraser's and spinner dolphins and suggest that the pygmy killer whale is also a deep-diving species. The high Mb concentrations in epaxial, hypaxial and abdominal muscle groups also support the primary locomotory functions suggested for these muscles in other anatomical studies. As in other species, the increase in [Mb] during development probably parallels the development of diving capacity.

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