Does sharing slaves preclude sharing space for socially parasitic slave-making ants?

Background/Question/Methods

Western slave-making ants (Polyergus mexicanus) are obligate social parasites that raid heterospecific ant nests for “slave” workers. Slave-makers parasitize several Formica species, but each slave-making colony uses a single host species. Slave-makers are known to be aggressive towards conspecifics to the point of exclusion within their raiding territories, but we report a high density of slave-making colonies that coexist within each other’s raiding territories, often with little aggression. Variation in intraspecific competition for hosts may explain the variation in aggression among colonies, so we expected pairs of colonies that parasitize different hosts to be more mutually tolerant than those that parasitize the same host. We tested this prediction by measuring the area of territory overlap between pairs of slave-maker nests that use the same slave species compared to those that used two different slave species. Additionally, to identify nests that appear to be avoiding each other, we generated simulated raids by using the length of observed raids but in random directions and compared the resulting simulated area of territory overlap to the observed area of overlap.

Results/Conclusions

Contrary to expectations, slave-making ant raiding territories share more area when neighbors use the same slave species than when they use different slave species. However, when considered in the context of the randomly generated raiding territories, there were no obvious differences in patterns of territory overlap between pairs of nests that used the same slaves compared to those that used different slaves. Interestingly, within both of these two groups (pairs of nests that have the same slave species and pairs of nests that do not) some individual pairs of nests’ raiding territories do show significant signs of avoidance while other pairs of nests show significantly more overlap than predicted by the random raiding simulation.  Slave-making ant colonies appear to be sensitive to the existence of conspecific nests in some cases but it remains to be understood why some nests seem to respond to each other’s presence while others do not. Future work will examine the role of kinship, relative colony size, and the timing of raiding in shaping slave-making ant raiding territories.