Context-dependent defences in turtle ants: resource defensibility and threat level induce dynamic shifts in soldier deployment

Scott Powell, Matina Donaldson‐Matasci, Augustus Woodrow‐Tomizuka, Anna Dornhaus

How do you best defend yourself in a dangerous world, when attacks may occur in many places simultaneously and you only have a limited supply of defensive resources? This is a fundamental problem faced by all organisms, and a common solution is the evolution of sophisticated strategies to induce defenses whenever and wherever enemies strike. We see exactly these kinds of strategies in our own immune system when pathogens attack, and in the physical and chemical defenses of plants when herbivores feed on them. In some complex insect societies, morphologically distinct soldiers provide the specialized defenses of the colony. Nevertheless, we know little about how colonies deploy their limited supply of soldiers across the various locations that they need to defend, and how deployment may shift when conditions become more dangerous. We address how environmental danger induces the dynamic deployment of soldiers across different locations with contrasting levels of defensibility and enemy threat. We do this using a species of turtle ant, a group known for their morphologically elaborate soldiers that use their armored heads to block nest entrances against enemy breaches. These ants live in tree cavities, and soldiers play a critical role in securing additional cavities so that the colony can grow and produce more offspring. By combining field and laboratory experiments, we show that colonies deploy fewer soldiers to less defensible nests when the level of danger in the environment increases. They also reduce the total number of new nests they defend, and commit fewer of their soldier reserves to expansion when under greater threat. These findings therefore reveal an unusual risk-limiting strategy at multiple levels of the colony’s response to danger: when the level of danger increases, the colony puts less on the line during the critical process of expanding into new nests for colony growth. This work provides the new insight that the evolution of morphologically specialized soldiers in complex societies is coupled with the evolution of sophisticated defense strategies, to deal with ever-changing environmental threats. By exploring these kinds of defensive dynamics in complex societies, where they have been previously overlooked, we open up new opportunities for exploring commonalities in the evolution of defense strategies across a greater variety of organisms.

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Image provided by authors.

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