Heart rate during breath-holding and diving in the unrestrained harbor seal (Phoca vitulina richardi)

Heart rate and breathing movements have been monitored in three unrestrained harbor seals. On voluntary submergence heart rate fell markedly in all seals, and after 2–3 s submergence stabilized at about 40–50% of the predive level. Heart rate increased before the animals broke surface at the end of the dive and, when breathing began again, a postdive tachycardia was observed. Two of the three seals frequently showed anticipation of the dive as judged from their heart-rate response. Heart rate during feeding dives was generally more variable; in fact one seal exhibited no bradycardia in 20% of its feeding dives, although another seal showed a significantly greater bradycardia than was seen in routine dives. When breathing rate was low (less than five breaths min −1) respiratory variations in the heart rate occurred, although the onset of bradycardia was much more rapid during diving than during breath-holding. Because of the flexibility of the response it is concluded that the generation of diving bradycardia in the seal is a complex phenomenon which, aside from any responses set in train by peripheral receptors, must also involve some form of associative learning.