Termite raiding by the Ponerine ant Pachycondyla analis (Hymenoptera: Formicidae) : behavioural and chemical ecology

Abstract

The ant Pachycondyla analis (formerly Megaponera foetens, commonly known as the Matabele ant) is a widespread ponerine in sub-Saharan Africa. It feeds solely on termites of economic importance belonging to the sub-family Macrotermitinae. These termites are captured during organised raids on their nests and galleries. Previous studies mostly concentrated on certain aspects of the raiding behaviour and trail laying pheromones in this species. Thus the detailed raiding behaviour and chemically-mediated communication between P. analis and its prey are virtually unknown. The aim of this study was to undertake detailed behavioural studies on termite raiding behaviour of P. analis, and to investigate whether P. analis uses olfactory cues for intra-specific communication during termite raids, and for detecting its prey. Termite raiding behaviour of P. analis was monitored at Mpala, a Kenyan savannah for six months (April to September, 2007). During this period, raids were found to occur mainly in the mornings and evenings, with late night raids occurring during dry periods. P. analis at Mpala mainly nests under rocks and in deserted termite mounds. Microtermes and Odontotermes were the main preyed termite genera, and ant raiding behaviour was synchronised with termite prey behaviour, and was influenced by foraging costs, prey defences and rewards. Olfactometric assays showed that P. analis workers used olfactory cues in their intra-specific chemical communication, with workers responding more to volatiles of individuals of the same size class (major to major and minor to minor) than between groups. Major workers discriminated more between the volatiles of the two groups than minor workers. GC-MS analysis of volatiles from major and minor workers revealed a cocktail of 48 compounds, majority of which were hydrocarbons. Volatile compounds were colony specific and quantitative analysis showed that major and minor workers alone released 2.5 fold more volatiles than the mixed stages. This suggests that ants have the innate ability to regulate the levels of the colony odour which they make up for with higher release levels when separated from each other. Using a Mandible Opening Response (MOR) bioassay, ants were able to distinguish between nestmates and non-nestmates based on cuticular hydrocarbon (CHC) profiles. This suggests that P. analis uses CHCs as short range contact recognition cues within the nest in traditional nest protection and during raids on termite species. GC-MS analyses revealed hydrocarbons of chain lengths in the range C8-C31 in the CHC profiles, comprising mainly alkanes, alkenes and methyl-branched alkanes. The CHCs were colony and individual worker specific. Nestmate recognition in P. analis may be encoded in the alkenes and methyl-branched alkanes. Dual choice olfactometric assays revealed that P. analis uses olfactory cues in locating potential termite sources with an average of 65% of workers choosing odours against the blank (clean air). When termite odours were offered to both major and minor workers, their choices were biased towards the termite odours, with minor workers attracted more to the odours than were major workers. Although ants responded to odours from the soil obtained for the termite gallery, overall, odours from termites inside their galleries were the most attractive to ants. These results suggest that the combined odours from both the termites and gallery components (in particular soil), serves as an effective nest location cue for the ants. Comparative GC-MS analyses showed that the composition of the volatiles from the gallery soil was richer than that released by the termites. Consistent with previous studies, the volatiles of the gallery soil were found to contain hydrocarbons, naphthalene and derivatives of this compound. In conclusion, these studies have revealed the rich diversity of chemical communication cues used by this ant species for nestmate recognition and for prey location during raids in search for its food source.