The use of landmarks to define territorial boundaries

Numerous anecdotal reports suggest that members of many territorial species use naturally occurring landmarks to define the boundaries of their territories. In the work reported here, we first tested whether artificial landmarks would be adopted as boundaries by territorial male cicada killer wasps, Sphecius speciosus. To perform this test, we set out wooden dowels on a flat, grassy lawn on which male wasps were defending mating territories. The dowels were situated so that they did not coincide with any existing territorial boundary. After we provided the dowels, the wasps established new territorial boundaries coinciding with the dowels. We hypothesized that using visual landmarks as territorial borders might lower defensive costs, and indeed, focal samples on territorial wasps revealed that borders that were defined by dowels cost less to defend than did borders that were not defined by any landmark. This result suggests that the use of natural landmarks as territorial boundaries may have evolved as a result of the reduced defensive costs that accrue to these boundaries. Furthermore, defensive costs may not depend directly on territory size: territory owners may be able to reduce defensive costs by selecting sites with high tactical defensibility. (C) 1999 The Association for the Study of Animal Behaviour.