Abstract
Blood parasites (Haemosporidia) are thought to impair the flight performance of infected animals, and therefore, infected birds are expected to differ from their non-infected counterparts in migratory capacity. Since haemosporidians invade host erythrocytes, it is commonly assumed that infected individuals will have compromised aerobic capacity, but this has not been examined in free-living birds. We tested if haemosporidian infections affect aerobic performance by examining metabolic rates and exercise endurance in migratory great reed warblers (Acrocephalus arundinaceus) experimentally treated with Plasmodium relictum pGRW04 and in naturally infected wild birds over consecutive life-history stages. We found no effect of acute or chronic infections on resting metabolic rate, maximum metabolic rate or exercise endurance in either experimentally treated or free-living birds. Oxygen consumption rates during rest and while undergoing maximum exercise as well as exercise endurance increased from breeding to migration stages in both infected and non-infected birds. Importantly, phenotypic changes associated with preparation for migration were similarly unaffected by parasitaemia. Consequently, migratory birds experiencing parasitaemia levels typical of chronic infection do not differ in migratory capacity from their uninfected counterparts. Thus, if infected hosts differ from uninfected conspecifics in migration phenology, other mechanisms besides aerobic capacity should be considered.
Highlights
Parasites are thought to adversely affect the physical performance of their hosts [1] and to be key factors in shaping life history, with effects cascading to communities and ecosystems [2]
We have examined the metabolic consequences of blood parasite infections over three consecutive stages of the annual cycle in great reed warblers (Acrocephalus arundinaceus), which are long-distance Palaearctic migrants with non-breeding sites in tropical Africa [16]
Our results consistently showed that low parasitaemia during the acute or chronic phases of infection did not alter the maintenance metabolism, suggesting that the immune defences in low-parasitaemia settings are energetically frugal, unlike the significant rises in resting metabolic rate (RMR) displayed by strong immune responses [38]
Summary
Parasites are thought to adversely affect the physical performance of their hosts [1] and to be key factors in shaping life history, with effects cascading to communities and ecosystems [2]. Haemosporidian parasites, the causative agents of malaria in the broader sense, commonly occur in temperate and tropical regions and infect a variety of host species including humans, other mammals, reptiles and birds. Depending on lineage and host species, the parasites invade their hosts’ inner organs and, red blood cells [3]. The infection follows a typical pattern with an acute (symptomatic) phase with high levels of parasites in the blood ( parasitaemia) followed by a chronic phase with low (or zero) parasitaemia [3]. Acute infections (with high parasitaemia) can result in substantial declines in red blood cell content [5] owing to haemosporidians targeting haemoglobin as a major nutrient [6]
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