Insects play dead to avoid being eaten. Playing dead might help prey animals stay alive.
Nigel R. Franks at the University of Bristol in the United Kingdom, and his colleagues, were conducting a study on beetle-like larvae (grubs) of the flying Antlions (Euroleon nostras). When they dropped the 12-millimetre-long larvae onto a tray to weigh them, the insects froze.
Franks and his colleagues observed the behaviour repeatedly, noting that the insects would stay immobile for a few seconds to more than an hour.
The researchers thought that this behavioiur was a survival mechanism, imitating the times when birds accidentally dropped the Antlions after grabbing them out of their sandpits.
The researchers modelled the behaviour using computers to try to understand how playing dead keeps the insect alive. Instead of using the term ‘playing dead’ scientists call it post-contact immobility (PCI).
Their models considered various Antlion and bird behaviours, such as the number of pits (holes) in a patch of sand, the distance between them, the time it takes birds to travel between pits, aspects of the birds’ behaviour – and the likelihood that a bird will drop an Antlion, for instance – and the amount of time that the Antlion remains in PCI.
The models were also informed by the marginal value theorem, which describes the optimal way an animal should feed to maximise efficiency. For example, what are the benefits of an animal staying in one spot to eat every last morsel of food available there, or instead, to take the time to move to another food-rich spot when supplies at the initial location begin to run low?
Their results suggested that playing dead really can help an insect survive, if it lives in a patch with many other creatures of the same species. Having lots of creatures in one spot seems to confuse a bird about which one to choose. The researchers think that this might be due to the way birds hunt. If birds mainly look for movement, then a motionless ‘dead’ insect is a difficult target to see. Any nearby moving insect is easier for birds to detect and pick up, making the playing-dead strategy a winning one for the insect.
“I just find the phenomenon utterly entrancing and utterly bewildering, to tell you the truth,” says Franks.
This means birds can’t learn to anticipate exactly when a ‘dead’ insect will become ‘alive’ again by starting to move or to run away, leaving the birds even more confused and easily distracted by nearby moving insects.
For the insects, “it’s this concept of, ‘I’m going to hide from you in plain sight by keeping still, and you’re not going to be able to guess how long I’m going to do this for’, and it’s a really beautiful strategy,” says Franks.
He says the insect playing dead is relying on other insects to distract a bird rather than actively redirecting the bird’s attention.
However, there is no benefit in staying dead for too long. The models indicate that lengthy periods of immobilisation wouldn’t give better protection. Practically speaking, they would probably put the larvae at real risk of predation by scavengers.
“What we’re seeing here with the play-dead strategy is really an arms race between prey and predator, and the Antlions have carried this to an absolute extreme, beyond which they would gain no further advantage,” says Franks.
Journal reference: Biology Letters, DOI: 10.1098/rsbl.2020.0892
Photographer: Martina Nicolls
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