To fool predators, antlion larvae play dead for unpredictable amounts of time

A belly-up antlion larva stays completely still, playing dead, to avoid the attention of a hungry predator. Photo by Nigel R. Franks/University of Bristol
A belly-up antlion larva stays completely still, playing dead, to avoid the attention of a hungry predator. Photo by Nigel R. Franks/University of Bristol

July 8 (UPI) -- Researchers consider playing dead, or post-contact immobility, a strange phenomenon.

Many animals play dead when faced with a predator, and researchers don't understand how the ruse functions -- have predators evolved to avoid dead prey because of potential dangers?


A new study, published Wednesday in the journal Proceedings of the Royal Society B, didn't answer the why, but did find that varying the time prey spends motionless, at the very least keeps, predators perplexed.

In the case of the new study, a fierce group of insects called antlions do exactly that.

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"[Antlion larvae] are voracious sit-and-wait predators that build exquisite pits in sandy soil," lead study author Nigel Franks, professor of animal behavior and ecology at the University of Bristol, told UPI in an email.

Franks and his colleagues didn't actually set out to study the acting abilities of antlion larvae. Instead, they were interested in the physics of their sand pit structures.


But when scientists went to weigh antlion larvae as part of their research efforts -- a normally difficult process -- they found the insects were quite obliging.

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"Once gently dropped from a small height on to the pan of the balance they remained stationary for very long periods of time. So, it was a piece of cake to weigh them," Franks said. "We'd seen such post-contact immobility in other organisms, such as woodlice, but the antlion larvae seemed to be taking it to an extreme. So, we thought we'd better investigate."

According to Franks, the investigation was rather simple. Researchers took antlion larvae and dropped them individually onto a substrate. The researchers then timed the insects as they stayed motionless, playing dead.

"One of our first investigations of this had the three of us, each with a stop watch in hand, staring at an individual antlion," Franks said. "Unbelievably, it remained completely motionless for over an hour. It sounds peculiar because absolutely nothing was happening but at the same time it was strangely thrilling."

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Researchers found that on the individual level, antlion larvae played dead for an unpredictable amount of time. Scientists calculated the "half-life" of antlion larvae playing dead.


"At the level of individual atoms, radio-active decay is entirely unpredictable but a population of radio-active atoms will decay with a predictable half-life," Franks said. "The half-life is the time taken for 50 percent of the population to change state."

The half-life for antlion larvae playing dead is eight minutes, according to the study. But some antlion larvae play dead for brief periods of time, while others remain motionless for half-an-hour or more. If a predator doesn't know how long the wait is going to be, the predator might not wait at all, researchers said.

The novel research suggests the use of quantitative and analytical methods, such as the half-life method, could be used to better understand the defensive strategies of other animals -- perhaps even by schoolchildren.

"It would be very interesting indeed if lots of organisms showed such half-life behavior. We really like this phenomenon because it does not require a lot of scientific kit, just care and patience and one can return the animals to the field completely unharmed," Franks said.

"Thus, we think that this kind of work would be ideal for school projects and getting young people really interested in the science of animal behavior," Franks said.


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