High heart rates in hunting porpoises

The impressive breath-hold capabilities of marine mammals are facilitated
by both enhanced O2 stores and reductions in the rate of O2 consumption
via peripheral vasoconstriction and bradycardia, coined the dive response.
Many studies have focused on the extreme role of the dive response in
maximizing dive duration in marine mammals, but few have addressed how
these adjustments may compromise the capability to hunt, digest and
thermoregulate during routine dives. Here we use DTAGs which record heart
rate together with foraging and movement behaviour to investigate how O2
management is balanced between the need to dive and forage in five wild
harbour porpoises that hunt thousands of small prey daily during
continuous shallow diving. Dive heart rate was moderate (median minimum
47-69 bpm) and relatively stable across dive types, dive duration (0.5-3.3
min), and activity. A moderate dive response, allowing for some perfusion
of peripheral tissues, may be essential for fuelling the high field
metabolic rates required to maintain body temperature and support
digestion during diving in these small, continuously-feeding cetaceans.
Thus, despite having the capacity to prolong dives via a strong dive
response, for these shallow-diving cetaceans, it appears to be more
efficient to maintain circulation while diving: extreme heart-rate
gymnastics are for deep dives and emergencies, not everyday use.