A praying mantis (Tenodera sinensis). Photo credit: David Bertsch.

A praying mantis (Tenodera sinensis). Photo credit: David Bertsch.

There is something eerie about the gently swaying motion of a praying mantis poised in wait for a tasty snack to saunter past. ‘The praying mantis is an incredibly motivated organism when hungry’, says David Bertsch from Case Western Reserve University, USA, describing how the famished predators are even partial to picking off members of their own species. However, when his colleague Josh Martin noticed that the voracious insects might begin to lose interest in pouncing on food when their appetites are satisfied, coworkers Gavin Svenson and Roy Ritzmann began wondering why the full insects lose motivation.

‘We know that metabolism is critical to an animal's daily life, but how metabolism affects animal behaviour is not well understood’, says Bertsch, who needed a year-round supply of motivated mantises to find out how their hunger affects their hunting style. ‘[But] living in Cleveland, Ohio, we only had access to wild mantises during the warmer months’, explains Bertsch, who became guardian to a colony of the aggressive insects, which had to be housed individually for their own safety. ‘Cockroaches are infinitely easier to raise’, he chuckles. Next, Bertsch filmed the ravenous insects’ actions when they were offered four lively cockroach nymphs – relieving the successful female hunters of their trophies to keep them on their toes in order to film further encounters – before eventually allowing the starving animals to dine. Then, he filmed their next encounters – on a partially full stomach – repeating the process until the mantises had eaten all four of their cockroach snacks.

Intriguingly, the mantises’ strike style never varied over the course of each filming session; each mantis fixed a nymph in its gaze until the tasty treat was within 1–2 cm range, when it lunged forward, grabbing the unsuspecting cockroach youngster with its forelimbs. However, the mantises’ attention range shifted depending on how well fed they were. The hungriest animals seemed to be interested in victims that were as far as 20 cm away. However, as the mantises consumed more cockroaches, they lost interest in the more distant nymphs, with only the closest cockroaches catching their attention. In short, they switched from actively pursuing prey to simply waiting for a snack to saunter within range as they became fuller. But what was driving the change?

‘Insulin is a hugely important hormone in animals ranging from C. elegans nematode worms to humans’, says Bertsch, explaining that the hormone is released when blood is flooded with glucose after feeding. Could the hormone be responsible for triggering the change in the mantis's behaviour? This time, Bertsch injected ravenous praying mantises with the hormone and filmed their reactions to a mini-intrusion of cockroaches. Initially, the famished insects were keen to focus on any cockroach, no matter how distant. However, as the insulin took hold over a matter of minutes, the mantises’ interest waned until they were only interested in cockroaches that scuttled right past. The hormone could drive the switch from active predator to couch potato, suggesting that it might be a key driver in the insects’ behavioural change as their appetites became more satisfied: ‘Something that we as humans can relate to’, smiles Bertsch.

So, praying mantises change their behaviour depending on how well fed they are and insulin is one hormone that seems to drive the behavioural switch. Bertsch adds that he is keen to find out how the hormone alters the insects’ behaviour.

References

Bertsch
,
D. J.
,
Martin
,
J. P.
,
Svenson
,
G. J.
and
Ritzmann
,
R. E.
(
2019
).
Predatory behavior changes with satiety or increased insulin levels in the praying mantis (Tenodera sinensis)
.
J. Exp. Biol.
222
,
jeb197673
.