Increased investment in the defence of high-value offspring by a superorganism

Parental investment theory predicts that parents should adjust their investment in offspring defence according to offspring value (i.e. the probability they will contribute to the next generation). Although previous research has shown this offspring value prediction is generally supported in unitary organisms, the 'superorganisms' formed by advanced colonial organisms have rarely been tested, and never by direct manipulation of offspring value. As an important complement to correlational studies, such a test would help better illuminate the applicability of the parental investment theory to that higher level of biological organization, as well as provide insight into the depth of the superorganism analogy. Here I use colonies of the fire ant, Solenopsis invicta, in a manipulative experiment to test whether superorganisms behave as predicted by the parental investment theory and invest more in defence (deliver larger venom doses) when protecting higher-value offspring. In addition, to test a proposed mechanism for observed increases, I conducted an experiment to investigate whether S. invicta alarm pheromone modulates venom dose. I found that colonies delivered larger venom doses when defending nests containing higher-value (sexual) offspring than when defending worker-destined brood, but I found no venom dose increase as a result of additional alarm pheromone. The conformity of S. invicta colonies to the offspring value prediction of the parental investment theory supports the theory's applicability to their higher (superorganismal) level of animal organization. It furthermore underscores that the superorganism concept is deeper than simple anatomical/organizational analogy, but extends also to colony-level behaviour.