Ecology and mechanisms of parasitism of Iridomyrmex by the thermophilic ant, Melophorus anderseni

Melophorus anderseni is a thermophilic ant species that nests near colonies of Iridomyrmex on which it conducts brood raids. We examined their colony founding behavior and foraging ecology to evaluate possible mechanisms that may facilitate interspecific nest raiding. Specifically, wee 1) examined colony establishment and survival, 2) patterns of worker activity (walking speed, foraging and raiding behavior), and 3) compared cuticular hydrocarbon profiles of M. anderseni and I. reburrus workers.Following nuptial flights, M. anderseni queens established colonies in clusters around I. reburrus colonies, with an average distance to the nearest Iridomyrmex nest of 133.1(± 8.9) cm (range 25 to 650 cm). The average number of M. anderseni queens per Iridomyrmex nest entrance was 8.4 (± 1.9), and the average distance to the nearest M. anderseni neighbor was 44.9 (± 5.3) cm (range 10 to 233 cm). Queen survival was lower in clusters that were farther from the nearest I. reburrus nest or to other M. anderseni queens. Iridomyrmex reburrus activity was negatively related to soil temperature, with no activity occurring above 50°C. In contrast, M. anderseni activity was positively related to temperature with peak activity occurring just above 50°C. Melophorus anderseni worker activity commenced when soil temperatures exceeded 37.4°C, and activity increased until mid-day. Activity ceased in the afternoon at temperatures lower than that for activity commencement, on average at 36.7°C. Activity of M. anderseni at I. reburrus nests followed that of activity at M. anderseni nests. Workers of M. anderseni had an average foraging duration of 63 seconds and a mean foraging distance of 249 cm. Of the 49 foragers followed, 42 (86%) entered an I. reburrus nest, and 32 (76.2%) of those entered the closest nest entrance. Of the 1,444 observations of M. anderseni exiting an I. reburrus nest, they stole an item in 122 (8.4%) instances, with 117 (95.9%) of these items being brood. Workers of M. anderseni had faster running speeds (mean 9.72 cm/s, peak 18 cm/s) than I. reburrus workers (7.05 cm/s). On average, the speed of M. anderseni exiting an I. reburrus nest was more than twice the speed of I. reburrus. Extracts of I. reburrus and M. anderseni worker cuticular hydrocarbon profiles were qualitatively identical in that no compounds were unique to either species. Saturated hydrocarbons with chain lengths between 25 and 29 carbons comprised the majority of hydrocarbons found on the cuticle of both species. The relative abundances of compounds within the profiles of both species were also similar. Two other sympatric Iridomyrmex and Melophorus species shared very few compounds with I. reburrus and M. anderseni .Our results support that M. anderseni specializes on brood raiding from I. reburrus nests. Both behavior (running speed) and chemical mimicry are likely used in combination to facilitate this specialized foraging. Additional research is still needed to determine the source (e.g. genetic versus environmental) of M. anderseni’s hydrocarbon profiles, and if geographic variation in host use and chemical mimicry exists.