Dangerous neighbors: Factors influencing cannibalism in antlion larvae

Background/Question/Methods

A population is an ensemble of individuals differing in their age, stage or size class. Variation in demographic rates among different classes, related to life history, biotic interactions or both, can strongly influence population dynamics. One such biotic interaction is cannibalism, usually involving the consumption of small individuals by larger ones. We aimed at exploring if antlion cannibalism, is influenced by differences in developmental stage, con-specific density, sand depth (serving as a refuge from predators) and hunger level. As a model system, we used larvae of the pit-building antlion Myrmeleon hyalinus (Neuroptera: Myrmeleontidae), frequently found at high densities ("antlion zones") under trees or bushes. Antlion larvae develop through three instar stages, co-occurring in the same antlion zone. We performed two complementary experiments: the first aimed at testing if the rate of cannibalism within and between developmental stages (six possible pairwise instar combinations) is influenced by the combined effects of sand depth (0.5 cm and 3 cm) and con-specific density (one individual per 10 cm² or 30 cm²). The second experiment focused on exploring if cannibalism within and between developmental stages is affected by hunger level of the larvae prior to encountering each other.

Results/Conclusions

The frequency of cannibalism was higher between than within developmental stages, with late feeding on early instar larvae, while also increasing as the difference in body mass between individuals was larger. Increased density caused an increase in the rate of cannibalism. Sand depth brought about a reduction in the rate of cannibalism; however, this pattern was mainly evident at low density. There was a substantial increase in the rate of cannibalism when at least one of the two larvae experienced starvation prior to the encounter. The identity of the cannibalistic predator was largely determined by the difference in body mass between individuals, rather than by their physiological state. Our results demonstrate that pit-building antlions larvae can be exposed to predation not only by other predators, but also by larger con-specifics. Therefore, the incorporation of cannibalistic interactions in models aiming to predict population and community dynamics in such systems is highly essential.