Larval behaviour of a giant damselfly: territoriality or size-dependent dominance?

Behavioural interactions and habitat use by larvae of the giant damselfly, Megaloprepus coerulatus, were studied in Panama. These larvae live in water-filled holes in fallen and living trees. Despite potential advantages for larval territoriality in this species, M. coerulatus did not exhibit exclusive use of resources, probably because their prey were not spatially clumped. Both well-fed and starved larvae initiated agonistic displays typical of other damselfly species, and even small differences in relative size affected their behaviour towards conspecifics. Habitat use in M. coerulatus varied with levels of food and cover. When cover was abundant, as it is in natural holes, larvae were hyperdispersed. A larva rarely showed exclusive use of a given area from one day to the next, however, even when both cover and motile prey were experimentally limited. The ability of larger individuals to displace smaller ones as they move around the habitat is best described as size-dependent (and site-independent) dominance rather than territoriality. The results suggest that it is premature to characterize an animal as ‘territorial’ on the basis of its behaviour in the laboratory, unless the abundance and distribution of resources that are provided are realistic, and the experimental design is sufficient to differentiate between size-dependent dominance and site-specific exclusion of conspecifics from a critical resource. ? 1996 The Association for the Study of Animal Behaviour Territoriality may be defined as occupancy of a defended area that is used exclusively by the defending individual (e.g. Brown 1975; Morse 1980; Wittenberger 1981). This type of interference competition should evolve if limiting resources are spatially, but not temporally, clumped, such that the benefits gained from a territory outweigh the energetic costs of its defence (Emlen & Oring 1977). In contrast to territoriality, dominance implies ‘priority acquired by past or present aggressive behaviour’ (Brown 1975, page 85), and is often found in species in which the home ranges of individuals overlap. Distinguishing between territoriality and dominance is not easy, particularly if the behaviour of individuals is difficult to observe under field conditions. Both types of competition are often established by agonistic behaviour, and the criteria that determines successful competitors in each case may be identical (e.g. size and/or age). For example, several authors have demonstrated that larval odonates use agonistic behaviour to exclude conspecifics from a resource and concluded that larval odonates exhibit territoriality (Rowe 1980; Harvey & Corbet 1985; Convey 1988). Nevertheless, their results are equally consistent with dominance of some individuals over others. Although resource defence is widespread among adult vertebrates and invertebrates as a means of competing with conspecifics for mates (reviewed by R. R. Baker 1983), territoriality has rarely been well documented among juvenile vertebrates or larval invertebrates. Where it has been found, territoriality typically functions in defence of food resources (e.g. Hart 1987; Stamps & Eason 1989). Hart (1987) predicted that territoriality should evolve in aquatic insect larvae that are resource-limited when (1) body size is large relative to the available foraging area and (2) standing crop is large or readily renewable. Furthermore, because variation in larval growth of insects may result in considerable variation in adult body size Correspondence: O. M. Fincke, Department of Zoology, University of Oklahoma, Norman, OK 73019, U.S.A. (email: [email protected]). 0003–3472/96/010077+11 $12.00/0 ? 1996 The Association for the Study of Animal Behaviour