Soldier deployment in ants: Do colonies respond optimally to changing defensive needs?

Background/Question/Methods

Many ant species have soldier castes, which specialize in defending resources such as nesting cavities.  The number of soldiers that are available to respond to any new opportunity or threat is generally constrained by the relatively slow rate of soldier production; however, the deployment of soldiers among heterogeneous, changing resources seems to be much more flexible.  How does the optimal allocation of soldiers among cavities depend on their relative quality and defensibility?  How does the optimal balance between expansion into new cavities and defense of old cavities depend on the level of threat in the environment?  We use a mathematical model to explore how the optimal deployment pattern—the one that maximizes the expected number of nesting cavities held for an entire season—depends on resource quality, defensibility, and level of threat.  We parameterize the model for the Arizona turtle ant (Cephalotes rohweri) from field collections as well as field experiments looking at nest cavity survival rate.  We qualitatively compare the results of the model to results from lab experiments in which cavity quality, defensibility, and level of threat are manipulated.

Results/Conclusions

For each cavity, we find a threshold number of soldiers needed, which increases with the level of threat and the difficulty of defense.  The optimal deployment pattern commits no soldiers to a new cavity unless that threshold can be reached.  Beyond the threshold, more soldiers are allocated to cavities that are more difficult to defend regardless of the threat level.  Higher quality cavities should be occupied first, but neither the threshold defense level nor the relative allocation beyond that depends on quality.  

As predicted by the model, lab experiments showed that among new cavities, those of high quality were more likely to be defended; the level of defense did not depend on quality.  Also in agreement with the model, we found that soldiers defended original cavities more heavily when under threat.  However, while the model predicts that this extra defense should come at the expense of fewer new cavities defended, we found instead that they defend the same number of new cavities, but less well.  We discuss implications of these findings.