If your favourite snack is fish or octopus, then most animals only have one choice; hold your breath and dive. And with only a lungfull of air to get you through the excursion, it would seem to make sense to get back to the surface quickly. But when Katsufumi Sato began monitoring the diving behaviour of foraging penguins, he realised that instead of returning vertically to the surface, the birds took a more leisurely route; they returned along a gentler incline. Intrigued by this unexpected finding, Sato decided to investigate the bird's underwater behaviour, to find out why macaroni penguins ascend gently(p. 4057).
Travelling to Kerguelen Island in the South Indian Ocean, Sato teamed up with his French collaborator, Jean-Benoît Charrasin, ready to tag macaroni penguins and track their dives. But the duo had to find volunteers to accompany them to the penguin's nesting site before their base research facility would allow them to venture into the wilderness alone. Fortunately Amélie Lescroel, Laurent Mely and Florent Colin were free to accompany them to Cape Cotter Colony.
Setting up camp near the beach, Sato attached tiny data loggers to the penguins. Luckily the birds were unphased by the scientists, and didn't seem to notice the data loggers once secured on their backs. It was Sato who was in for a stressful time, waiting for the intrepid voyagers to return from their two week round trip. Would the data loggers remain attached? Would they collect good data? The team kept a round-the-clock watch on the penguins'nests ready to retrieve the data loggers when the birds returned. And two weeks latter, their patience was rewarded when all eight penguins returned their data loggers safely, packed full of acceleration and depth recordings from their foraging dives.
Undaunted by the amount of data from almost 7000 dives, Sato calculated the bird's angles of ascent and descent from the acceleration records. Surprisingly, the birds seemed to use different diving strategies depending on how long they remained at depth. When the birds dived and spent only a brief time at depth, they returned to the surface at a shallow angle, and followed with another dive also at a shallow body angle. But when the penguins spent a lengthy period at depth, their surfacing angle was much steeper, and subsequent redives were also steep. Sato also analysed the birds' wing beat pattern as they swam and found that, just like other penguins, the macaronis swam hard during the descent phase of the dive, and later during the first half of their ascent. However, while the birds were at depth, they glided along without swimming, and also allowed their natural buoyancy to complete their return to the surface without taking a stroke.
But why use different dive strategies depending on the amount of time spent at depth? Sato suspects that the birds use shallow angled dives when the pickings are meagre and they need to cover greater distances to find a decent meal. But when they happen upon a tasty shoal, they use steeper ascents to get back to the surface quickly, before plunging steeply again to make the most of the awaiting feast.