Abstract
Deep diving air-breathing species by necessity must balance submergence time and level of exercise during breath-holding: a low activity level preserves oxygen stores and allows longer duration submergence whereas high activity levels consume oxygen quickly and shorten submergence time. In this study, we combined high-resolution multi sensor animal-borne tag data to investigate diving behavior and locomotion styles of the narwhal (Monodon monoceros) (n= 13, mean record length 91 h)–a deep diving Arctic species. Narwhals in this study dove down to >800 m but despite the deep diving abilities, one-third of the dives (33%) were shallow (>100 m) and short in duration (<5 min). Narwhals utilized energy saving measures such as prolonged gliding during descent with increasing target depth but stroked actively throughout the ascent indicating excess oxygen storages. Foraging behavior, as detected by the presence of buzzes, was a key factor influencing dive depth and spinning behavior—the rolling movement of the animal along its longitudinal axes. Narwhals in East Greenland utilized two foraging strategies, while transiting and while stationary, with different target depths and buzzing rates. The first targeted deep-dwelling, possibly solitary prey items and the latter, more schooling prey closer to the surface. The buzzing rate during stationary foraging was on average twice as high as during transiting foraging. Spinning was an integrated part of narwhal swimming behavior but the amount of spinning was correlated with foraging behavior. The odds for spinning during all dive phases were 2–3 times higher during foraging than non-foraging. Due to the spinning behavior, stroking rate might be better suited for estimating energy consumption in narwhals than ODBA (overall dynamic body acceleration). The narwhal is considered as one of the most sensitive species to climate change–the results from this study can act as a baseline essential for evaluating changes in the behavior and energy usage of narwhals caused by stressors evolving in the Arctic.
Highlights
Marine mammals are forced to move between the surface and the deep in order to gain access to the two life sustaining resources– air at the surface and food at depth
Live-capture of narwhals was conducted from a field station at Hjørnedal inside Scoresby Sound in East Greenland in collaboration with local Inuit hunters as part of a larger ongoing study on narwhal ecology, physiology and behavior (HeideJørgensen et al, 2020; Figure 1)
The longest continuous stroking bout lasted 84.7 min and was performed by a non-foraging transiting whale. Narwhals in this relatively short duration study conducted in August 2013–2017 and 2019 dove regularly to depths below 600 m with the deepest dives extending below 800 m, which agrees with previous findings from this and other populations (HeideJørgensen and Dietz, 1995; Watt et al, 2015; Ngô et al, 2019)
Summary
Marine mammals are forced to move between the surface and the deep in order to gain access to the two life sustaining resources– air at the surface and food at depth. Due to the energetic cost of locomotion, marine mammals often utilize energy saving measures during a dive such as prolonged gliding (Williams et al, 2000), relatively low mean swimming speeds (Williams, 2009) and a gait of alternating gliding and stroking (Williams et al, 1992, 2000; Skrovan et al, 1999) In this gait, the forward propelling energy created by a few strokes is utilized to the maximum where the body of the animal glides through the water with minimal effort (Williams et al, 2000). High speed, uninterrupted stroking is sometimes required to escape from predators (Ford et al, 2006), or from anthropogenic disturbance (e.g., DeRuiter et al, 2017; Williams et al, 2017a,b; van Beest et al, 2018)
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