Correlation of depth and heart rate in harbour seal pups

Abstract

Harbour seal (Phoca vitulina L., 1758) pups are aquatically active from birth and have been shown to develop increased cardiac control throughout the nursing period. In an attempt to quantify and compare these developmental changes, data previously collected on pups in the St. Lawrence River estuary, Quebec, Canada, were analyzed. Time–depth recorders and heart-rate recorders were employed on eight pups to obtain depth and heart-rate measurements simultaneously. Analyses involved partitioning the data into intervals of surface–dive–surface. These intervals were then allocated into nine consecutive segments: presurface, predive surface, descent, prebottom, bottom, postbottom, ascent, postdive surface, and postsurface. Mean heart rate for each segment was then correlated with the mean depth per segment and overall dive duration. With increasing dive depth, a decrease in heart-rate variability with age was observed. There was no apparent relationship between mean heart rate during the dive and overall dive duration. The proportion of time spent in the lower heart-rate mode was observed to increase with age during most phases of a dive. Relative changes in mean heart rate between consecutive dive segments indicated an initial decrease in mean heart rate prior to submersion and an increasing trend before surfacing. These findings indicate that harbour seal pups develop increased cardiac control prior to weaning and that anticipatory cardiac responses to diving and surfacing (bradycardia and tachycardia, respectively) may be evident.