Acoustic measurements of post-dive cardiac responses in southern elephant seals (Mirounga leonina) during surfacing at sea

Abstract

Measuring physiological data in free-ranging marine mammals remains challenging, owing to their far-ranging foraging habitat. Yet, it is important to understand how these divers recover from effort expended underwater, as marine mammals can perform deep and recurrent dives. Among them, southern elephant seals (Mirounga leonina) are one of the most extreme divers, diving continuously at great depth and for long duration while travelling over large distances within the Southern Ocean. To determine how they manage post-dive recovery, we deployed hydrophones on four post-breeding female southern elephant seals. Cardiac data were extracted from sound recordings when the animal was at the surface, breathing. Mean heart rate at the surface was 102.4±4.9 beats min −1 and seals spent on average 121±20 s breathing. During these surface intervals, the instantaneous heart rate increased with time. Elephant seals are assumed to drastically slow their heart rate (bradycardia) while they are deep underwater, and increase it (tachycardia) during the ascent towards the surface. Our finding suggests that tachycardia continues while the animal stays breathing at the surface. Also, the measured mean heart rate at the surface was unrelated to the duration and swimming effort of the dive prior to the surface interval. Recovery (at the surface) after physical effort (underwater) appears to be related to the overall number of heart beats performed at the surface, and therefore total surface duration. Southern elephant seals recover from dives by adjusting the time spent at the surface rather than their heart rate.

Body condition influences ontogeny of foraging behavior in juvenile southern elephant seals

“…For both juveniles and adults, surface durations increased with the total swimming effort performed during the previous dive. Recent studies suggest that there is a positive relationship between surface duration relative to total dive duration and swimming effort during the previous dive in adult female elephant seals (Day, Joumaa, Bonnel, & Guinet, 2017;Génin et al, 2015;Jouma'a, 2016). Recent studies suggest that there is a positive relationship between surface duration relative to total dive duration and swimming effort during the previous dive in adult female elephant seals (Day, Joumaa, Bonnel, & Guinet, 2017;Génin et al, 2015;Jouma'a, 2016).…”

“…This was especially true for juveniles, where the slope of the relationship between the absolute swimming effort of a dive and the following surface duration was steeper than that of adults ( Figure 6). Day et al (2017) showed that This suggests that slight increases in swimming effort cause juveniles to increase their time at the surface in order to breathe and recover, even if they need to spend less time at the surface compared to adults.…”

“…This was especially true for juveniles, where the slope of the relationship between the absolute swimming effort of a dive and the following surface duration was steeper than that of adults ( Figure 6). Recent studies suggest that there is a positive relationship between surface duration relative to total dive duration and swimming effort during the previous dive in adult female elephant seals (Day, Joumaa, Bonnel, & Guinet, 2017;Génin et al, 2015;Jouma'a, 2016). This suggests that slight increases in swimming effort cause juveniles to increase their time at the surface in order to breathe and recover, even if they need to spend less time at the surface compared to adults.…”

“…This suggests that slight increases in swimming effort cause juveniles to increase their time at the surface in order to breathe and recover, even if they need to spend less time at the surface compared to adults. Day et al (2017) showed that…”

Three-dimensional assessment of hunting strategies in a deep diving predator, southern elephant seal Mirounga leonina

“…Yet, the investigation of the mechanisms connecting animal movement to these key processes is limited by our ability to track animals in sufficiently high spatial and temporal resolution at which they sense and respond to various environmental cues. The recent development of bio-logging technologies has significantly contributed to filling the gap in our knowledge, by relating animal movements to physiology (Butler & Jones 1997, Génin et al 2015, Day et al 2017, physical environment (Heupel & Simpfendorfer 2008, Ortega et al 2009 and exploited resources (Guinet et al 2014, Kuhn et al 2015, Goldbogen et al 2015, Wilmers et al 2015, Adachi et al 2017. In this context, high sampling-frequency tracking systems and loggers deployed on wild animals have made possible the recording of information that would have been impossible to get otherwise, without being restric ted by visibility, observer bias, or geographic scale (Brown et al 2013).…”