Increased investment in the defense of high-value offspring by a superorganism
Kevin Lee Haight, Kevin Lee Haight
Social Insect Research Group, School of Life Sciences, Arizona State University, Tempe, AZ, USA; Social Insect Research Group, School of Life Sciences, Arizona State University, Tempe, AZ, USA
Parental investment theory predicts parents should adjust their investment in offspring defense according to offspring value, i.e., the probability they will contribute to the next generation. Previous research has shown this offspring-value prediction is generally supported in unitary organisms, but colonies of advanced eusocial organisms, or ‘superorganisms’, have never been tested. Such a test would 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, to test whether superorganisms behave as predicted by the parental investment theory and invest more in defense (by delivering larger venom doses) when protecting higher-value offspring. Colonies delivered larger venom doses when defending nests containing the higher-value, sexual offspring. The altricial offspring of a superorganism (worker brood, sexual brood, & sexual adults) are intermixed within their ‘soma’ (the adult workers of the colony) so any threat to these offspring is a de facto threat to the parent as well, regardless of their value to it. Thus, in superorganisms, selection for optimized self-defense could be expected to swamp selection for optimized offspring defense, resulting in defensive behavior that does not vary in relation to secondary concerns such as offspring value. But, as shown, colonies of S. invicta conform to the offspring-value prediction of the parental investment theory, indicating the theory is applicable to this higher, superorganismal, level of organization. Furthermore, this finding underscores that the superorganism concept is deeper than simple anatomical/organizational analogy, but extends also to colony-level behaviour.