It has become increasingly clear that life‐history patterns among the vertebrates have been shaped by the plethora and variety of immunological risks associated with parasitic faunas in their environments. Immunological competence could very well be the most important determinant of life‐time reproductive success and fitness for many species. It is generally assumed by evolutionary ecologists that providing immunological defences to minimise such risks to the host is costly in terms of necessitating trade‐offs with other nutrient‐demanding processes such as growth, reproduction, and thermoregulation. Studies devoted to providing assessments of such costs and how they may force evolutionary trade‐offs among life‐history characters are few, especially for wild vertebrate species, and their results are widely scattered throughout the literature. In this paper we attempt to review this literature to obtain a better understanding of energetic and nutritional costs for maintaining a normal immune system and examine how costly it might be for a host who is forced to up‐regulate its immunological defence mechanisms. The significance of these various costs to ecology and life history trade‐offs among the vertebrates is explored. It is concluded that sufficient evidence exists to support the primary assumption that immunological defences are costly to the vertebrate host.

SUMMARYThe prevalence and intensity of parasitic infection often increases in animals when they are reproducing. This may be a consequence of increased rates of parasite transmission due to reproductive effort. Alternatively, endocrine changes associated with reproduction can lead to immunosuppression. Here we provide support for a third potential mechanism: reduced immunocompetence as a consequence of adaptive reallocation of resources in times of increased energetic demand. In captive zebra finches Taeniopygia guttata, reproductive effort was manipulated through brood size. Enhanced effort was found to affect the production of antibodies towards sheep red blood cells. In addition, activity of zebra finches was manipulated independently of parental care. Experimentally increased daily workloads in activity reward schedules also suppressed antibody production. Thus, we show that not just the reproductive state, but the increased activity that accompanies reproduction is associated with immunocompetence. This mechanism may be sufficient to explain the increased parasitism observed in reproducing animals. We suggest that reduced immunocompetence as a consequence of increased reproductive effort may be an important pathway for the life history cost of reproduction.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. British Ecological Society is collaborating with JSTOR to digitize, preserve and extend access to Journal of Animal Ecology. Summary 1. Costs of reproduction have been assessed experimentally by measuring subsequent survival and reproduction of parent animals raising enlarged and reduced numbers of offspring. Reported effects on survival have so far always referred to local survival of marked individuals in the study population. They do not provide definitive proof of a cost of reproduction, since reduced local survival may be due either to reduced survival or to an increased tendency to emigrate from the study area. Therefore, it is important to assess mortality rates in connection with brood size experiments 2. We report an analysis of the time of death in 63 cases where kestrels, Falco tinnunculus L. had raised broods of manipulated size and were subsequently reported freshly dead. 60% of the parents raising two extra nestlings were reported dead before the end of the first winter, compared to 29% of those raising control or reduced broods. This result confirms our interpretation of the manipulation effects on local survival as due to mortality rather than emigration. The extra mortality occurred in the winter following the brood enlargement 3. Kestrel parents in these experiments have been shown to adjust their daily energy expenditure to the modified brood size. Increased parental effort in this species thus entails an increased risk of death half a year later.

The diet composition of Brent Geese Branta bernicla on a salt-marsh was quantified. Puccinellia maritima was the principal food species, while Plantago maritima and Triglochin maritima were less commonly taken. Festuca rubra only acted as a substitute for Puccinellia when production of the latter species dropped. The metabolizable energy of the food plants ranged from 5 to 11 kJ·g. By assessing the ingestion rates of geese feeding on different food species, the net intake rate could be derived. Plantago and Triglochin appeared to be the most profitable plants to eat. The proportion of these species in the diet was restricted by (1) the capacity of the alimentary tract, since high intake rates combined with high water contents of the food plants easily led to overfill; and (2) the limited distribution of these plants, in combination with their rapid depletion by grazing geese. These latter factors led to an unequal allocation among individual geese. Most Plantago and Triglochin was obtained by dominant pairs within the flocks. The high quality of Puccinellia allowed geese to gain mass in spring, but the metabolizable energy of this plant species declined during the staging period, and Plantago and Triglochin increased in importance in supplying the geese with components with which to build their body reserves. The timing of the onset of spring growth of the various food species differed between years, and plant phenology was shown to have a profound effect on the final body reserves of the geese.