Morphology and physiology in some small pelagic cetaceans: Is Dall's porpoise a deep diver and a thoroughbred of the sea?

Heart size and blood oxygen (O-2) in Dall's porpoises (Phocoenoides dalli), Pacific white-sided dolphins (Lagenorhynchus obliquidens), and common bottlenose dolphins (Tursiops truncatus) correlate with reported swim speeds and dive behavior. Deep-dive capacity is often associated with smaller relative lung masses, larger "inexpensive" tissue masses (locomotor muscle + blubber + bone masses), and larger total O-2 stores. To gain insight into dive behaviors and surface travel of small pelagic cetaceans, we compared body compositions of Dall's porpoises, Pacific white-sided dolphins, and northern right whale dolphins (Lissodelphis borealis) to the bottlenose dolphin. Relative lung masses, inexpensive tissue masses, and total O-2 stores of Dall's porpoises and Pacific white-sided dolphins were consistent with deeper dive capacities than in the coastal bottlenose dolphin. In contrast to known low myoglobin concentrations in northern right whale dolphins, inexpensive tissue mass suggested intermediate dive capacity. The estimated body O-2 store of Dall's porpoise was among the highest of any cetacean. The relatively large hearts of Dall's porpoises, Pacific white-sided dolphins, and northern right whale dolphins are hypothesized as especially important during high-speed surface travel. Based on similarities in locomotory performance, morphology, and physiology to those of the horse, we consider Dall's porpoise a thoroughbred of the sea.