395 Effects of Plagiorhynchus cylindraceus (Acanthocephala) on the energy metabolism of adult starlings, Sturnus vulgaris V. A. CONNORS* and B. B. NICKOL School of Biological Sciences, University of Nebraska - Lincoln, Lincoln, NE 68588-0118, USA (Received 13 February 1991; revised 23 April 1991; accepted 28 April 1991) SUMMARY Although the relationship between intestinal parasitism, the ingestion and use of energy, and host survival is expected, little work has been done to outline the effect of such organisms upon their host's nutritional requirements in an ecological context. This study is the first to demonstrate that an intestinal helminth previously reported to be of little or no histopathological consequence, Plagiorhynchus cylindraceus, has a significant detrimental impact upon the flow of food energy through a definitive host, the European starling, Sturnus vulgaris. Within both male and female adult European starlings reductions in standard metabolic rates occurred as the result of initial infection, indicating that the host's basal metabolism/thermal regulatory abilities were altered. Moreover, initially infected male starlings, but not females, had an increased consumption and excretion of energy and maintained lower average daily body weights versus controls when temperature stressed. These results appear to be due to either a parasite-mediated alteration in host activity and/or to the disruption of host-digestive abilities. Additionally, these data indicate that, overall, male and female 5. vulgaris respond differently to infection and that intestinal helminths normally thought to be of little or no pathological consequence to the host are factors that should be addressed in future studies regarding animal energetics, ecology, and behaviour. Key words: Acanthocephala, Plagiorhynchus cylindraceus, energetics, starling, Sturnus vulgaris, pathophysiology. INTRODUCTION There is now ample theoretical and empirical evidence to support the contention that parasites with the potential to harm their hosts also have the potential to influence their host's population density (Anderson & May, 1978, 1979; May & Anderson, 1978, 1979; Lemly & Esch, 1984; Scott, 1987a, b). Theoretically, even relatively slightly pathogenic parasites may influence host abundance or density (Anderson, 1979; May, 1985) and it is likely that the interplay of helminth pathogenicity and host nu- trition is an important factor contributing to this effect in nature (Rees, 1968; Anderson, 1979). Although parasites are often recognized as stressers of their hosts, the energetic or nutritional effects of infection are not well defined and data relating to these effects are sparse (Holmes, 1987). Studies involving the administration of gastro- intestinal helminths possessing distinct migratory and/or severely pathogenic stages have demon- strated that such parasites have a significantly detri- mental effect upon their domestic or laboratory host (Castro & Olson, 1967; Stephenson et al. 1980; Coop, 1982; Ovington, 1985 ; Entrocasso et al. 1986). However, the conclusions drawn from these studies are difficult to extrapolate to many naturally occur- ring intestinal parasite—host systems because: (1) except under rare circumstances (Maclean, Gous & * Present address: Division of Biological Sciences, University of Montana, Missoula, Montana 59812. Bosman, 1973; Schmidt, Walley & Wijek, 1974), many intestinal parasites evoke only a highly local- ized and apparently inconsequential pathological response within their definitive host (Bullock, 1963; Hine & Kennedy, 1974; Insler & Roberts, 1976); (2) many helminths lack migratory phases within their definitive hosts; and (3) the results of such studies do not account for the increased energy expenditure of the host as the result of environmental stress (Coop, 1982). Because few hosts, if any, need die as the result of infection in order that parasitism be considered an important affector of a host's population in nature (May, 1985), it is important to recognize if, when, where, and how these organisms, particularly those parasites thought to evoke only minor pathological responses, might have a measurable effect upon the nutritional status of their hosts. Given that a major aspect of any host's nutrition is the acquisition and use of energy, determination of energy consumption and use by a definitive host would allow the estimation of the effects of infection upon the allocation of ingested energy to all major bodily processes (i.e. basal metabolism, activity, thermo- regulation, growth, reproduction etc.) in a form directly transferable from the level of the individual to that of the host's population (Robbins, 1983). The acanthocephalan Plagiorhynchus cylindraceus, although implicated as a cause of death in both American robins (Turdus migratorius) and Western bluebirds (Sialia mexicana), has been reported to be of little pathological consequence within its definitive Parasitology (1991) 103, 395-402 Printed in Great Britain