Response of pit-building antlions to repeated unsuccessful encounters with prey

1. What they did

Antlion larvae are small insects that dig cone-shaped pits in sand and wait at the bottom for prey, mainly ants, to fall in. Predators that sit and wait were long thought to behave in fixed, automatic ways. The authors asked whether antlions change their behaviour when they detect prey but fail to catch them, which tells them that prey is around.

They collected larvae of the antlion Myrmeleon hyalinus from a sandy area south of Beer-Sheva, Israel, fed them, starved them for 14 days, weighed them and placed each in a cup of sand in the laboratory.

• Response times and mass loss (81 larvae). Larvae were split into three groups of 27: one repeatedly offered a small flour beetle larva (about 5 mg), one offered a large one (about 27 mg), and a control group offered nothing. The prey was placed in the centre of the pit and taken away with forceps as soon as the antlion responded (moving, flicking sand at it or grabbing at it with its jaws) or after 30 seconds. This was repeated on days 2, 5, 6 and 7, and the time to respond was recorded. Larvae were weighed on day 8, given two more failed encounters on days 12 and 13, then allowed to catch and eat prey on day 14 and weighed again on day 15. Finally, larvae from the two prey groups were offered prey once more to see how a meal changed their response.
• Pit size (92 larvae). Larvae were split into three groups: one repeatedly offered small prey that was taken away, one allowed to catch and eat similar prey, and an unfed, undisturbed control group. Pit diameter was measured the day after each of two treatments on successive days.

2. What they found

Response to prey

• Antlions responded within 30 seconds in 88.5% of trials.
• They responded significantly faster to small prey than to large prey.
• Response times did not get shorter over the four failed encounters: repeated experience with the same prey did not make them quicker.
• After a real meal, antlions responded more slowly than before it. After feeding, larger antlions responded faster than smaller ones to small prey; before feeding, small prey was attacked very fast whatever the antlion’s size.

Body mass and growth

• All the larvae lost mass while unfed, but the rate of loss did not differ between larvae that kept meeting prey and the control group, against the authors’ expectation.
• How efficiently the meal on day 14 was turned into body mass did not differ between groups.

Pit size

• Pits grew at a steady rate in the group that kept failing to catch prey and in the group that caught and ate prey. In the unfed, undisturbed group, pit growth slowed over time.

3. What they concluded

• A failed encounter with prey is enough to make an antlion invest more in foraging by enlarging its pit, just as a successful capture does. The authors read this as antlions judging their patch: they invest more where there are signs of prey nearby. Without such signs, an antlion keeps only its first small enlargement, waits, and may eventually move.
• How quickly antlions respond to prey depends on context: smaller prey, hungrier antlions and larger antlions gave faster responses. Large prey may be treated as a disturbance rather than a meal, since overly large prey can damage the pit or the predator.
• Experience with a prey type did not speed up responses, perhaps because these opportunistic predators are limited more by prey size than prey type.
• The lack of extra mass loss in the prey-encounter groups may be because antlion metabolic rates are low and the experiment was too short to show a difference.
• Together with other recent work, the study shows antlions have complex, context-dependent behaviour, much more than was expected.

6. Other technologies, skills or innovations

• A simple laboratory design for “unsuccessful prey encounters”: prey placed in the pit and removed with forceps on the antlion’s first response, so the predator detects prey without eating it.
• Pit diameter used as a measure of how much the predator invests in foraging.