Interspecific territoriality in Calopteryx damselflies: the role of secondary sexual characters

Abstract

Interspecific territoriality is usually interpreted to result from interspecific interference competition, although it may also originate from mistaken species recognition. In the latter case, it may be based on similarity of secondary sexual characters. In the damselfly Calopteryx splendens , males have pigmented wing spots as a sexual character, and males with the largest spots resemble males of another species, Calopteryx virgo . Probably because of this resemblance, C. virgo males are more aggressive towards large- than small-spotted C. splendens males. We examined whether wing spot size of C. splendens males affects territorial interactions between the species. In a removal experiment, the number of territorial C. splendens increased after C. virgo males were removed. However, interspecific territoriality was incomplete since before the removal the territories of the species partially overlapped. Wing spot size of C. splendens seemed to affect interspecific territoriality: before the removal territorial and nonterritorial C. splendens males had similar spot sizes and the distance to the nearest C. virgo territory increased with wing spot size of C. splendens . In addition, after the removal of C. virgo , the relation between wing spot size and the distance to the nearest old C. virgo territory disappeared. Our results suggest that mistaken species recognition may account for the interspecific territorial behaviour in the two species. Furthermore, interspecific territoriality may cause negative selection on wing spot size and thus may explain character displacement in wing spot size of C. splendens males.