Examining the Plasticity of the Dive Response in Relation to Dive Behavior of Northern Elephant Seals

Abstract

Searching and capturing prey are extraordinary challenges for marine mammals, as they must actively hunt during breath‐hold dives. This challenge is conquered by the dive response (the decreased heart rate and increased peripheral vasoconstriction associated with a breath hold), which lowers the rate of oxygen consumption and prolongs the duration of aerobic metabolism. The dive response is dynamic and potentially influenced by dive duration, depth, anticipation, and exercise. However, it is still unknown how these influences interact with each other to facilitate oxygen management pre‐ and post‐dive, during different dive phases, and at fine scale. We studied the dive response of one of the deepest diving marine mammals, the northern elephant seal (Mirounga angustirostris) across three years (2018, 2019, 2021) to determine interactions between these behavioral and physiological factors. Using a biologger that records electrocardiogram data (50‐100 Hz) and a Daily Diary time‐depth‐recorder that records acceleration and depth, we calculated heart rate, stroke rate, and minimum specific acceleration (MSA) in 7 seals in dives as deep as 460m. We examined the influence of dive depth, duration, and activity (MSA) on dive heart rate, pre‐ and post‐dive heart rate, and heart rate during different dive phases and at fine scale. We recorded heart rates as low as 20 bpm during natural dives, with the average heart rate at depth being 48.91 ± 25.93 bpm. The heart rate rapidly drops at the start of dives from ~100‐120 bpm to ~40‐50 bpm and continues to decline as the seal passively drifts to depth. At the bottom seals tend to stroke and glide while maintaining a relatively stable heart rate. On ascent heart rate gradually increases as the seals stroke and glide to the surface. We see a drastic increase in heart rate just before seals surface, likely to maximize offloading of carbon dioxide and onloading of oxygen upon surfacing. Our preliminary results reveal that both dive depth and MSA influence the dive heart rate at fine scale intervals of 20 s within the dive. These results suggest that the seals actively regulate their oxygen during dives. Our results will connect dive behavior and the dive response to understand how marine mammals perform deep‐dives while actively searching and capturing prey.