Abstract
Unique in the insect world for their extremely sedentary predatory behavior, pit-dwelling larval antlions dig pits, and then sit at the bottom and wait, sometimes for months, for prey to fall inside. This sedentary predation strategy, combined with their seemingly innate ability to detect approaching prey, make antlions unlikely candidates for learning. That is, although scientists have demonstrated that many species of insects possess the capacity to learn, each of these species, which together represent multiple families from every major insect order, utilizes this ability as a means of navigating the environment, using learned cues to guide an active search for food and hosts, or to avoid noxious events. Nonetheless, we demonstrate not only that sedentary antlions can learn, but also, more importantly, that learning provides an important fitness benefit, namely decreasing the time to pupate, a benefit not yet demonstrated in any other species. Compared to a control group in which an environmental cue was presented randomly vis-à-vis daily prey arrival, antlions given the opportunity to associate the cue with prey were able to make more efficient use of prey and pupate significantly sooner, thus shortening their long, highly vulnerable larval stage. Whereas “median survival time,” the point at which half of the animals in each group had pupated, was 46 days for antlions receiving the Learning treatment, that point never was reached in antlions receiving the Random treatment, even by the end of the experiment on Day 70. In addition, we demonstrate a novel manifestation of antlions' learned response to cues predicting prey arrival, behavior that does not match the typical “learning curve” but which is well-adapted to their sedentary predation strategy. Finally, we suggest that what has long appeared to be instinctive predatory behavior is likely to be highly modified and shaped by learning.
Highlights
Pit-digging antlions (Neuroptera: Myrmeleontidae; see Fig. 1), the larvae of winged adult insects, are thought to be the most sedentary of insect predators [1,2,3,4]
One member of each pair was randomly assigned to the Learning treatment; its pairmate was assigned to the Random treatment
Our results show that antlions are capable of anticipating prey arrival through associative learning, which provides important fitness benefits but which does not manifest itself behaviorally with the frequency of a typical ‘‘learning curve.’’ Our findings expand our understanding of how learning benefits animals, and force us to modify current models for the evolution of learning [35,40,41]
Summary
Pit-digging antlions (Neuroptera: Myrmeleontidae; see Fig. 1), the larvae of winged adult insects, are thought to be the most sedentary of insect predators [1,2,3,4]. After larvae emerge from their eggs and find a shady location that offers protection from wind and rain, they construct a funnel-shaped pit in sandy soil by spiraling backwards, excavating the sand with their head and mandibles [5,6,7]. Once their pits are completed, antlions position themselves at the vertex, covered either partially or entirely by the substrate, and wait motionless unless disturbed, for prey to stumble inside. Antlions’ egg, pupal and adult stages last 30 days or less
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