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Evidence of a state-dependent trade-off between energy intake and parasite avoidance in Steller's eiders

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We examined the hypothesis that Steller's eiders, Polysticta stelleri (Pallas, 1769), in good body condition avoided nutritious and abundant prey that were intermediate hosts of acanthocephalans, while birds in poor condition accepted the long-term costs of parasitism to minimize the short-term risk of starvation. We predicted that the intensity of the acanthocephalan Polymorphus phippsi (Kostylev, 1922) should be positively related to the intake of intermediate hosts and that the intake of such prey should be negatively related to body condition. All Steller's eiders were infected (2–1142 parasites/bird). Only the intake of the amphipod Gammarus oceanicus (Segerstråle, 1947), a known intermediate host of P. phippsi, was significantly related to parasite intensity. Juvenile Steller's eiders were in poorer condition than adults and fed more on amphipods (44% vs. 9% of total biomass). On the contrary, adults preferred isopods (26% of total biomass vs. 12% for juveniles), which were less abundant than littoral amphipods but were not intermediate hosts of P. phippsi. Moreover, in juveniles there was a negative relationship between body condition and the proportion of amphipods in the diet. Hence, juveniles in poor body condition consumed potentially infected prey while adults and juveniles in good condition avoided such prey. The cost of avoiding littoral amphipods was probably a lower energy return per unit feeding effort.

Nous avons examiné l'hypothèse selon laquelle les eiders de Steller, Polysticta stelleri (Pallas, 1769), en bonne condition physique évitent les proies nutritives et abondantes qui sont les hôtes intermédiaires d'acanthocéphales, alors que les oiseaux en mauvaise condition acceptent le coût à long terme du parasitisme afin de minimiser le risque à court terme de mourir de faim. Nous avons prédit que l'intensité du parasitisme par l'acanthocéphale Polymorphus phippsi (Kostylev, 1922) devrait être en corrélation positive avec l'ingestion d'hôtes intermédiaires et que cette ingestion serait en corrélation négative avec la condition physique. Tous les eiders de Steller étaient infectés (2–1142 parasites/oiseau). Seule l'ingestion de Gammarus oceanicus (Segerstråle, 1947), un hôte intermédiaire connu de P. phippsi, est en corrélation significative avec l'intensité du parasitisme. Les jeunes eiders de Steller sont en moins bonne condition que les adultes et ils se nourrissent plus d'amphipodes (44 % contre 9 % de la biomasse totale). Au contraire, les adultes préfèrent les isopodes (26 % de la biomasse totale contre 12 % chez les jeunes) qui sont moins abondants que les amphipodes littoraux, mais qui ne servent pas d'hôtes intermédiaires à P. phippsi. De plus, chez les jeunes, il existe une corrélation négative entre la condition physique et la proportion des amphipodes dans le régime alimentaire. Les jeunes en mauvaise condition physique consomment donc des proies qui sont peut-être infectées, alors que les adultes et les jeunes en bonne condition évitent de telles proies. Le coût de l'évitement des amphipodes littoraux est sans doute un bénéfice énergétique réduit par unité d'effort d'alimentation.[Traduit par la Rédaction]

Document Type: Research Article

Publication date: October 1, 2004

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  • Published since 1929, this monthly journal reports on primary research contributed by respected international scientists in the broad field of zoology, including behaviour, biochemistry and physiology, developmental biology, ecology, genetics, morphology and ultrastructure, parasitology and pathology, and systematics and evolution. It also invites experts to submit review articles on topics of current interest.
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