The role of landmarks in territory maintenance by the black saddlebags dragonfly, Tramea lacerata

Abstract

Territoriality can reduce competition for resources, but territorial defense can be costly; therefore, any behavior that reduces territorial costs may increase the net benefit of territoriality. Some species will align their territory boundaries with conspicuous landmarks that may serve to reduce defense costs. Dragonflies, including black saddlebags (Tramea lacerata), defend territories at breeding sites, keeping rival males away to allow themselves access to females. We used three treatments to investigate whether T. lacerata used landmarks: constraining landmarks (an object that provided a physical barrier to flight), non-constraining landmarks (an object of the same dimensions and construction that did not impede flight), and a control without landmarks. We observed patrolling male black saddlebags and recorded the locations of turns at their territory boundary and interactions with other dragonflies. When either type of landmark was present, individuals placed their boundary at the landmark far more often than any other location. In addition, individuals that used landmarks had a significantly narrower range of turn locations than those that did not. Unlike other studies, the use of a landmark did not seem to reduce defense costs, and interestingly not all individuals used landmarks when they were provided. We hypothesize that in this species, landmarks may only reduce costs during territory establishment, rather than during territory maintenance. Alternatively, landmarks may serve as part of a spatial reference system that aids male dragonflies in efficiently searching for females, and thus may be more important in increasing benefits rather than decreasing the costs of territoriality.