Variation in body condition during the post-moult foraging trip of southern elephant seals and its consequences on diving behaviour

Abstract

Mature female southern elephant seals (Mirounga leonina) come ashore only in October to breed and in January to moult, spending the rest of the year foraging at sea. Mature females may lose as much as 50% of their body mass, mostly in lipid stores, during the breeding season due to fasting and lactation. When departing to sea, post-breeding females are negatively buoyant, and the relative change in body condition (i.e. density) during the foraging trip has previously been assessed by monitoring the descent rate during drift dives. However, relatively few drift dives are performed, resulting in low resolution of the temporal reconstruction of body condition change. In this study, six post-breeding females were equipped with time-depth recorders and accelerometers to investigate whether changes in active swimming effort and speed could be used as an alternative method of monitoring density variations throughout the foraging trip. In addition, we assessed the consequences of density change on the swimming efforts of individuals while diving and investigated the effects on dive duration. Both descent swimming speed and ascent swimming effort were found to be strongly correlated to descent rate during drift dives, enabling the fine-scale monitoring of seal density change over the whole trip. Negatively buoyant seals minimized swimming effort during descents, gliding down at slower speeds, and reduced their ascent swimming effort to maintain a nearly constant swimming speed as their buoyancy increased. One per cent of seal density variation over time was found to induce a 20% variation in swimming effort during dives with direct consequences on dive duration.

The foraging benefits of being fat in a highly migratory marine mammal

“…increased bottom duration per dive cycle, shown in the electronic supplementary material, figure S8) suggests that seals gained a foraging benefit from achieving neutral buoyancy by being fat. A similar foraging benefit has also been suggested in a study [37] that assessed the relationship among stroking effort and dive duration in southern elephant seals, a species that undergoes similarly significant changes in their body density (buoyancy) during oceanic migrations [27].…”

“…However, recording at high sampling rates limits the recording period owing to limitations in logger memory or battery power [36]. Only one study, which used accelerometry, showed a change in stroking effort of southern elephant seals over their post-breeding foraging migrations (less than 80 days), but the study mainly focused on the validation of estimating seal density using stroking effort and swim speed [37]. The paucity of long-term acceleration data has hindered further understanding of the relationships among buoyancy, swimming costs and foraging behaviour in long distance migrants such as highly pelagic marine animals.…”

Southern Elephant Seals Replenish Their Lipid Reserves at Different Rates According to Foraging Habitat

“…[19]. First, dives where SESs stopped actively swimming at the bottom (as indicated by a low vertical speed and null lateral acceleration, i .…”

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

“…Static acceleration is caused by the Earth's gravitational pull whereas dynamic acceleration results from the animal's movements (body waves, tail strokes, head motions). The static component corresponds to low frequencies and the dynamic one to higher frequencies (Génin et al, 2015;Richard et al, 2014).…”

“…The lateral axis of the accelerometer contains information on putative turning and rolling movements (static acceleration) and on flipper stroke (dynamic acceleration) (Richard et al, 2014). Swimming effort was obtained by summing the absolute values of the local extrema of the lateral axis of the acceleration filtered with a band-pass filter which cuts frequencies bellow 0.44 Hz and above 1 Hz (Jouma'a et al, 2016;Richard et al, 2014).…”