Ascent exhalations of Antarctic fur seals: a behavioural adaptation for breath-hold diving?

Abstract

Novel observations collected from video, acoustic and conductivity sensors showed that Antarctic fur seals consistently exhale during the last 50-85% of ascent from all dives (10-160 m, n > 8000 dives from 50 seals). The depth of initial bubble emission was best predicted by maximum dive depth, suggesting an underlying physical mechanism. Bubble sound intensity recorded from one seal followed predictions of a simple model based on venting expanding lung air with decreasing pressure. Comparison of air release between dives, together with lack of variation in intensity of thrusting movement during initial descent regardless of ultimate dive depth, suggested that inhaled diving lung volume was constant for all dives. The thrusting intensity in the final phase of ascent was greater for dives in which ascent exhalation began at a greater depth, suggesting an energetic cost to this behaviour, probably as a result of loss of buoyancy from reduced lung volume. These results suggest that fur seals descend with full lung air stores, and thus face the physiological consequences of pressure at depth. We suggest that these regular and predictable ascent exhalations could function to reduce the potential for a precipitous drop in blood oxygen that would result in shallow-water blackout.

Figure 1

Ascent exhalations recorded by three data sources. (a) Salinity record from a dive to 125 m, showing anomalies apparent in the results during the later portion of ascent. (b) Digital photograph taken from a neck-mounted camera showing another ascending seal. (c) Spectrograms showing the bubble sounds heard during ascent from a deep dive to 125 m (i), and a shallow dive to 35 m (ii). Insets show time and depth profiles for each dive from which data are shown, either for the whole dive (a), with position of record shown by dot (b), or section of dive highlighted from which spectrogram is plotted (c).

Figure 2

The depth of bubble production versus depth of dive for data recorded by camera and acoustic tags. Open diamonds, camera 1; squares, camera 2; triangles, camera 3; crosses, acoustic camera; closed diamonds, D-tag.

Figure 3

Intensity of bubble sounds (acoustic pressure squared) recorded by D-tag versus depth for (a) deep (more than 69 m) and (b) shallow (less than 44 m) dives. Note the strong pattern of increased bubble sound intensity as the seal ascended into shallower depths. The model described in equation (2.1) is shown (line) to fit observations at 20 m for deep dives and at 10 m for shallow dives. (c) The intensity of bubble sounds versus 1/(d+10). The slope of the linear regression for each dive was used to estimate V_c. (d) The slope of the increase in acoustic intensity, or estimate of V_c from (c), versus 1/P_B, where P_B is the pressure at which bubbles were first released. The strong linear relationship between these values is predicted from equation (2.1) if the seal inhaled equal amounts of air at the surface for all dives, and the rate of air release is determined by the maintenance of lung collapse (see text and Appendix A for details). trianges, deep dives; circles, shallow dives.

Figure 4

Intensity of thrusting movement, measured as variance in the z-axis accelerometer signal of the D-tag produced by the seal during (a) descent and (c) ascent in the top 13 m below the surface, calculated as the seal travelled through four depth bins (1–4, 4–7, 7–10 and 10–13 m) and also summed over the total depth range (1–13 m). The variance measure was normalized by the distance travelled in each depth bin. The overall variance in the z-axis of the accelerometer was greater for descent than ascent across this depth range. Note the decreasing thrusting intensity as the seal descends from the surface, and the converse for ascent. (b) Total variance in z-axis accelerometer (1–13 m) for descent is plotted against the maximum dive depth. Equal thrusting intensity regardless of dive depth suggests equal buoyancy (i.e. lung volume) for all dives. (d) Total variance in z-axis accelerometer (13–1 m) for ascent is plotted against the depth at which bubbles were first released. This metric of thrusting intensity was greater during ascents from dives in which bubbles were first produced at greater depths, reflecting the energetic cost of releasing air during ascent. Triangles, deep dives; circles, shallow dives.