Success in territorial defence by male tarantula hawk wasps Hemipepsis ustulata: the role of residency

1. Some male tarantula hawk wasps Hemipepsis ustulata defend landmark shrubs growing on mountain tops, while others only visit a succession of territory sites, generally fleeing from territorial males. To determine whether residency asymmetries contribute to the short duration of most interactions between territorial males and visiting intruders, established residents were held captive for varying periods while other males replaced them at their territories. 2. Replacement males immediately assumed resident status and began to defend the shrubs and trees with success against non-resident intruders, many of which were larger. Interactions between replacements and non-resident intruders rarely escalated into pursuits leading to ascending flights. 3. In contrast, when the original resident was released, its subsequent encounter with the replacement male almost always resulted in one or more ascending flights, even when the replacement had been on territory for less than 10 min. The longer the period of territory occupation by replacements, the longer on average the interaction and the more ascending flights, showing that the duration of time on a territory affects male motivation to defend the site. Despite strong resistance from long-term replacements, however, returning residents almost always regained their territories. 4. The strong residency effect in H. ustulata may be an adaptive proximate control mechanism for male behaviour that arises because established residents typically have high resource holding power, which makes them difficult to displace.