Parasite scouting and host defence behaviours are influenced by colony size in the slave-making ant Protomognathus americanus

In the slave-making ant Protomognathus americanus, scout workers leave their colony, discover host colonies, and initiate slave raids. Captured host pupae subsequently emerge in the slavemaker colony and replenish the slave workforce. The course of these antagonistic encounters can be influenced by the species, aggressivity, or size of the host colony. We asked how the demography of parasite and host colonies influences the initial raiding phase by observing the scouting behaviour of P. americanus slavemakers during 48 raiding attempts. Experiments were performed under controlled laboratory conditions in a Y-shaped experimental arena. The number of active scouts increased with increasing slavemaker worker numbers, but was unaffected by the slave to slavemaker ratio, showing that slavemaker worker numbers are a good indicator for the scouting workforce. Colonies with fewer slaves discovered host colonies faster (colonies with 15 or less slaves: median 9:53 min, colonies with 42 or more slaves: median 18:55 min), suggesting that small slave workforces lead to intensified scouting behaviour. The more scouts were active, the faster a host colony was discovered, but the time between discovery and trial completion was unaffected by slavemaker colony demography. Host colonies were successfully attacked in 79.2 % of the trials, and they fought off an intruding scout only once. Yet host aggression towards slavemaker scouts increased with host colony size, and higher aggression rates delayed a subsequent attack. Our study demonstrates that colony size influences the behaviour and the course of crucial interspecific interactions of a social parasite and its host.