Why do macaroni penguins choose shallow body angles that result in longer descent and ascent durations?

Abstract

SUMMARY It is generally assumed that air-breathing aquatic animals always choose the shortest route to minimize duration for transit between the surface and foraging depth in order to maximize the proportion of time spent foraging. However, empirical data indicate that the body angles of some diving animals are rarely vertical during descent and ascent. Why do they choose shallower body angles that result in longer descent and ascent durations? To investigate this question, we attached acceleration data loggers to eight female macaroni penguins, breeding on the Kerguelen Islands(48°45′–50°00′S,68°45′–70°58′E; South Indian Ocean), to record depth, two-dimensional acceleration (stroke cycle frequency and body angle)and temperature. We investigated how they controlled body angle and allocated their submerged time. The instrumented females performed multiple dives(N=6952) with a mean dive depth for each bird ranging from 24.5±28.5 m to 56.4±75.1 m. Mean body angles during descent and ascent were not vertical. There was large variation in mean descent and ascent angles for a given dive depth, which, in turn, caused large variation in descent and ascent duration. Body angles were significantly correlated with time spent at the bottom-phase of the dive. Birds that spent long periods at the bottom exhibited steep body angles during ascent and subsequent descent. By contrast, they adopted shallow body angles after they had short or no bottom phases. Our results suggest that macaroni penguins stay at the bottom longer after encountering a good prey patch and then travel to the surface at steep body angles. If they do not encounter prey, they discontinue the dive,without staying at the bottom, ascend at shallow body angles and descend at shallow body angles in a subsequent dive. A shallow body angle can increase the horizontal distance covered during a dive, contributing to the move into a more profitable area in the following dive. During the ascent, in particular,macaroni penguins stopped beating their flippers. The buoyantly gliding penguins can move horizontally with minimum stroking effort before reaching the surface.

Mechanics, hydrodynamics and energetics of blue whale lunge feeding: efficiency dependence on krill density

“…maximize the horizontal area covered and increase the chance of locating a higher quality prey patch (Sato et al, 2004). For these reasons, we attributed the lunge frequency patterns we observed (Fig.3B) to a general decrease in prey density with decreasing depth.…”

N-dimensional animal energetic niches clarify behavioural options in a variable marine environment

“…Fig.5), a pattern that has also been observed in Adélie penguins feeding on Antarctic krill (Euphausa superba) (Ropert-Coudert et al, 2001) and is probably what accounts for inter-dive variation within individual penguins (Fig.5). Sato et al and Goldbogen et al showed for penguins and whales, respectively, that swim angle can have a graded response with respect to patch quality and distribution (Sato et al, 2004;Goldbogen et al, 2008). The effect of swim angles during the descent, modulated by prey capture rate (Table4), on net harvesting rate can be examined by applying the energy envelope model to data gathered from the IMASEN-equipped birds.…”

Pedalling downhill and freewheeling up; a penguin perspective on foraging

“…higher descent and ascent rates with increasing maximum depth reached. Work on other penguin species, where either measured speed allows access to descent and/or ascent angles or where the body angle can be measured directly via static acceleration, indicates that rates of descent and ascent are indeed modulated primarily by angle rather than speed (Ropert-Coudert et al 2001b, Sato et al 2004). In accordance with this, ODBA, and therefore metabolic power, was higher in birds diving deeper in all phases of the descent (Fig.…”