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Exposure to potentially cannibalistic conspecifics induces an increased immune response (2020) Murray et al., Ecological Entomology, https://doi.org/10.1111/een.12806

The Crux

Plasticity is a powerful force in nature that allows organisms to change the way they look, the way they act, and even their own physiological processes. Prey species commonly exhibit plastic responses when they are exposed to predators, and recent studies have shown that these predator-induced effects can affect the immune function of the prey species. Because of this, predators have the potential to modify disease dynamics, either increasing disease/parasite infection by reducing the prey’s immune function, or decreasing disease by increasing immune function.

Interestingly, predators are not the only organisms that consume prey species. Some prey species eat both members of their own trophic level (an intraguild predator, see Did You Know) and members of their own species (a cannibal). Because they act like a predator (by eating a prey organism), there’s a possibility that these cannibalistic individuals may have the same effect on their potential victims. Today’s authors used larval dragonflies to investigate that exact question.

What They Did

To examine the effects of potential cannibalism on host immune function the authors used larvae of the dot-tailed whiteface dragonfly (Leucorrhinia intacta). Because other studies have shown that simply being around more members of your own species (called “conspecifics”) can increase immune function, the authors first tested for this effect (called “density dependent prophylaxis”). Larval dragonflies were injected with a thin piece of plastic to simulate parasitic attack and then placed into an aquarium either alone, with one other larva, with three other larvae, or with five other larvae. If the immune response to the simulated parasitic attack increased with the density of larvae, this would be evidence for density-dependent prophylaxis.

Next the authors tested for an effect of potentially cannibalistic conspecifics on immune function. Dragonflies were injected with the thin plastic piece and then placed into an aquarium with an empty cage (control) or into an aquarium with a cage containing two conspecifics. Differences in immune function between the two treatments would indicate that the presence of potentially cannibalistic conspecifics would be enough to induce a greater immune response.

Lastly, they tested for an effect of recently cannibalistic conspecifics on immune function. To do this they once again injected dragonfly larvae with the thin piece of plastic to induce an immune response, after which they exposed the dragonfly to one of three treatments: being alone in an aquarium (control), placed into an aquarium with another dragonfly that has been eating other insects (non-cannibalism treatment), or with another dragonfly that recently cannibalized other dragonflies (cannibalism treatment). Differences in immune function between these treatments would be evidence for an effect of cannibalism on immune function, instead of just an effect of the conspecific itself.

Did You Know: Intraguild predation

Whenever you hear the word “predator” there’s a good chance that the first animals to come to mind are wolves, tigers, lions, or even sharks. These are classic examples of organisms at higher levels of the food web that feed on those below them. However, there are also organisms that eat other members of their own level of the food web. Because they are eating animals that are on the same level (guild) as them, they are known as intraguild predators.

The dragonflies used in today’s paper are a great example of intraguild predators. During their larval stage dragonflies are voracious predators and will consume just about any animal smaller than they are. If another dragonfly happens to be smaller than a hungry dragonfly, well that smaller dragonfly is about to become an intraguild meal.

What They Found

For the density experiment, dragonfly immune function increased with conspecific density, providing evidence for density-dependent prophylaxis. For the second experiment the authors found that being exposed to potentially cannibalistic conspecifics was enough to induce a heightened immune function.

In the third experiment they found that there was not an effect of recent cannibalism on immune function. Much like in the second experiment, simply being around other larval dragonflies was enough to increase immune function, regardless of whether or not they had recently cannibalized a conspecific.

Problems

The experimental design did not allow the authors to distinguish between the effects of cannibalism risk and the risk of infection. A key part of density dependent prophylaxis is that immune function increases with density because the risk of encountering a parasite or disease increases with host density (that’s why social-distancing is so important!). Cannibalism, at least in dragonflies, also increases with density. So it was unclear if the heightened immune function seen in this study was due to either the increased risk of cannibalism, the increased risk of infection, or some combination of the two.

So What?

The results of this study add an interesting layer of complexity to the already hyper-complex web of interactions that make up food webs. Conspecifics have the potential to actually reduce the level to which a given parasite establishes in a population, due to their “immune priming” effect on host organisms. Additionally, this study highlights that even a relatively solitary organism like a dragonfly can exhibit density dependent prophylaxis, a phenomenon usually seen/studied in other, less solitary organisms.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.