Aquatic and terrestrial heart rates in fur seals: evidence for delayed metabolic processing

A seal's heart rate is affected by their physiological adaptation to aquatic and terrestrial environments. At-sea, heart rate during dive bouts cycles between bradycardia during dives and tachycardia for oxygen replenishment on the sea-surface. While onshore, heart rate reflects apnoea and eupnoea. Intriguingly, complete sea-and-onshore heart rate traces and any relationship between sea and subsequent onshore heart rate, including any immediate or delayed physiological adaptation, remain unexamined. In this study, sea-and-onshore heart rate traces from female Cape fur seals (Arctocephalus pusillus pusillus; CFS, N = 4) and Australian fur seals (A. p. doriferus; AUFS, N = 8) revealed expected at-sea cycles of bradycardia (mean minimum beats·min^-1, CFS: 14.8 ± 1.3 SE; AUFS: 5.7 ± 1.6) and tachycardia (mean maximum beats·min^-1, CFS: 161.1 ± 1.5; AUFS:163.8 ± 2.6), with interspaced periods where heart rate stabilized as the seal swam at the sea surface. Following haul-out, heart rate traces revealed peaks approximately 20 to 40 beats·min^-¹ above the apparent minimum, reaching a maximum 6 to 8 hours following the seal's return to land. Potentially, this onshore heart rate trace reflects a physiological response attributable to delayed compensation for at-sea debt, a scenario explored with multiple linear analyses of area under the heart rate curve, the results of which were all significant (p < 0.05) but varied in strength (R² range from 0.61 to 0.86). These findings underscore the complex interplay of heart rate across aquatic and terrestrial environments, highlighting the benefits of examining holistic physiological traces, and potentially revealing evidence for delayed metabolic processing.