1. In order to understand fully the evolution of a behavioural trait one must not only consider whether it is adaptive in its present environment but also whether it originated as an adaptation to existing selective forces or as a fortuitous consequence of selection for a different role in other environments (i.e., as a pre-adaptation) or of selection for different traits (e.g., as a pleiotropic effect). In this paper interspecific territorialism is examined in species of humming-birds, sun-birds, tropical reef fishes, stingless bees, stomatopods, crayfish, and limpets as a means of determining its adaptiveness and its origins.

2. Humming-birds form complex assemblages with species sorted out among the available resources. Dominant species establish feeding territories where flowers provide sufficient nectar. A few large, dominant species, usually uncommon, are marauders on others' territories. Subordinate species establish territories where flowers are more dispersed or produce less nectar, or they fly a circuit from nectar source to nectar source when flowers are even more dispersed, a foraging pattern called ‘traplining’, or they steal nectar from the territorial species by being inconspicuous while foraging. Two species, Amazilia saucerottei and Selasphorus sasin, subordinate in one-to-one encounters, are able to take over rich resources by establishing several small territories within a territory of a dominant and forcing it to forage elsewhere.

3. Among humming-birds, territorial individuals attacked not only subordinate competitors but marauding humming-birds and some insects, which stayed in the territory and foraged at will, and seemingly inappropriate targets, such as non-competitors. This suggests that the stimulus for aggression is ‘any flying organism near the food resources’, regardless of its appearance. The behaviour rather than the identity of the intruder is the stimulus.

4. Sun-birds resemble humming-birds to the extent that dominants establish territories on rich nectar sources and subordinates establish territories on less rich nectar sources or steal from the territories of dominants. The diversity of foraging patterns is not so great as in humming-birds, perhaps because so few species of sun-birds have been studied. However, the advantage of territorialism has been measured in the sun-bird Nectarinia reichenowi. Individuals with territories lose much less nectar to competitors than do those without territories.

5. Field work on three species of tropical reef fishes involved a single aggressive species whose individuals attacked a wide range of species intruding on their territories. The stimulus for aggression in Pomacentrus jenkinsi seemed to be an “object moving through [its] territory”. As suggested for humming-birds, the stimulus is the behaviour rather than the identity of the intruder.

6. The relationships found in stingless bees, stomatopods, crayfish, and limpets are simpler. The dominant and subordinate species divide the resources in their habitat, the dominants' aggression preventing the subordinates from using resources that were otherwise available to them.

7. A general pattern emerges. Mutual interspecific territorialism occurs between species that (i) have different geographic ranges, (ii) occupy different habitats, or (iii) use different resources within the same habitat. Examples of two species holding separate territories on the same resources within the same habitat are rare and occur when the dominant species is rare relative to the available resources. These observations are contrary to the usual view that interspecific territorialism is an adaptation that permits co-existence of potential competitors within the same habitat.

8. Interspecific territorialism is sometimes adaptive and sometimes maladaptive, depending upon the species and the situation.

9. The general pattern of occurrence of the behaviour and the general nature of the stimulus for aggression, i.e., the behaviour rather than the identity of the intruder, suggest that interspecific territoriality is a fortuitous consequence of selection for intraspecific territorialism, the latter being not only an adaptation to the presence of conspecific competitors but a pre-adaptation to the presence of competitors of other species, should they occur.