Differential fitness returns in relation to spatial position in groups.

1. A general overview of position-related fitness differences in group-living animals is given for the first time. 2. Differential fitness returns in groups are often related to competition for resources among group members due to resource limitation. If resources are very scarce, then competition may finally lead to the disbandment of groups. The encounter-dilution effect predicts that grouping confers a benefit if the conspicuousness of large groups does not outweigh the dilution effect and each predator or parasite attacks only one (or a few) prey per encounter (Uetz & Hieber, 1993). While the encounter-dilution effect limits the number of predators and parasites a group is faced with at a given time, marginal predation and communal defence are probably the most important factors for position-related differences in predation risk. 3. Limitation of resources increases competition between groups members and should widen the gap between fitness returns of dominant and subordinate group members. In some group-living animals such as schooling fish, however, predator pressure has apparently selected for uniformity in morphology and behaviour which presumably restricts the potential for resource competition (Magurran & Seghers, 1991). 4. A general problem of assessing individual fitness returns in that different short-term strategies may achieve the same long term goals (Magurran, 1993). The use of feeding rates and predation risks as currencies for fitness returns over short time periods is therefore questionable and can only be a first step. So far, hardly any information is available on long-term effects of differences in positioning behaviour. 5. Food availability and food intake rates tend to be higher in edge positions, whereas energy expenditure does not differ significantly with group position in non-roosting mobile groups. This suggests that edge positions achieve a higher net-energy pay-off and should therefore be preferred by individuals with low energy reserves. 6. Differential predation risks are well documented for stationary and colony-breeding species, but there is a lack of data for mobile, non-breeding species. The reason for the latter is that predation is rarely observed (in the field) and often difficult to relate to group position in fast-moving species such as flocks of birds. Further studies, especially in the field, are needed. 7. Vigilance rates and attack rates do not necessarily give a true reflection of mortality risk. Vigilance is not exclusively related to predation and can be confounded by competition effects and hunger. Attack rates are difficult to translate into per capita risks if hardly any mortality is observed and therefore little information is available about the ratio of successful attacks/total attacks. 8. Most of the literature up to date consists of descriptive studies. There is a need for experimental work that investigate (a) the influence of predator attack mode on differential predation risks; (b) the influence of internal states (such as hunger) on the tradeoff between predation risk and foraging behaviour and its consequences for the positioning behaviour in groups; (c) the 'egalitarian' nature of groups with respect to group position. Do all individuals continuously rotate positions? Or do dominant individuals monopolize certain positions?, and (d) that separate the effects of nutritional state, vigilance and food availability on feeding rates in different group positions. 9. Reproductive success is often related to group position in breeding colonies and lekking animal species. Females probably prefer the safer territories in the group centre, but more empirical data are required to test this hypothesis. 10. Position-related differences in parasitism rate between group members are well documented with per capita rate of parasitism being lower in the centre of groups than at the periphery.(ABSTRACT TRUNCATED AT 400 WORDS)