Dive heart rate in harbour porpoises is influenced by exercise and expectations

The dive response, a decrease in heart rate (ƒ_H) and peripheral vasoconstriction, is the key mechanism allowing breath-hold divers to perform long duration dives. This pronounced cardiovascular response to diving has been investigated intensely in pinnipeds, but comparatively little is known for cetaceans, in particular in ecologically relevant settings. Here we studied the dive ƒ_H response in one the smallest cetaceans, the harbour porpoise (Phocoena phocoena). We used a novel multi-sensor data logger to record dive behaviour, ƒ_H, ventilations and feeding events in three trained porpoises, providing the first evaluation of cetacean ƒ_H regulation while performing a variety of natural behaviours, including prey capture. We predicted that tagged harbour porpoises would exhibit a decrease in ƒ_H in all dives, but the degree of bradycardia would be influenced by dive duration and activity, i.e., the dive ƒ_H response will be exercise modulated. In all dives, ƒ_H decreased compared to surface rates by at least 50% (mean maximum surface=173 beats min⁻¹, mean minimum dive=50 beats min⁻¹); however, dive ƒ_H was approximately 10 beats min⁻¹ higher in active dives due to a slower decrease in ƒ_H and more variable ƒ_H during pursuit of prey. We show that porpoises exhibit the typical breath-hold diver bradycardia during aerobic dives and that the heart rate response is modulated by exercise and dive duration; however, other variables such as expectations and individual differences are equally important in determining diving heart rate.