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Journal of Experimental Biology partnership with Dryad

Kathryn Knight

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For their size, birds have a relatively long lifespan. Tiny birds routinely outlive similarly sized mammals, which means that they continue reproducing into relatively old age. Maryline Le Vaillant and colleagues from several European institutions explain that all reproducing animals have to trade off investing in their own survival against that of their offspring, possibly allowing longer lived species to indulge more in their own survival during their early years at the cost of their own young. However, Le Vaillant and her co-workers suggest a possible alternative reason for older king penguins' reproductive success: they may simply be better at foraging and providing for their chicks because they are better divers. Explaining that no one had compared the diving performances of mature and less experienced king penguins, the team travelled south to the Crozet Archipelago in the Indian Ocean. Recording the dive depths and acceleration patterns of 5-year-old (young breeders) and 8/9-year-old (experienced breeders) penguins during their vast foraging odysseys, the team was able to estimate the penguins' diving costs (p. 3685).

Having found that the duration and depth of the old and young penguins' dives were essentially the same, the team focused on the descending and ascending leg of each bird's dive. They noticed that the older birds had to work harder than the youngsters early during their descents, but had a relatively easy return to the surface. However, the younger birds had to work harder than the older animals as they swam deeper, and they continued swimming hard during the ascent until the final 50 m, when the two groups put in the same amount of effort.

Explaining that king penguins often dive in groups led by older – more experienced – animals and that penguins adjust the amount of air they take down depending on their predicted dive strategy, the team suspects that experienced penguins inhale more than novice foragers before a dive and so have to work harder during the descent because of their increased buoyancy.

However, the old-timers get an almost free ride back to the surface. In contrast, the younger penguins seem to be working hard against drag during most of their dives and Le Vaillant and colleagues suggest that this could be due to several factors, including scruffier plumage, poor dive posture and less direct dive trajectories, resulting from their inexperience.

‘The dive strategy adopted by older king penguins ostensibly contributes to reduce their swim efforts and should decrease their foraging effort’, the team concludes, potentially allowing older parents to invest more in their chick's success than novice mums and dads.