Infrared footage of dung beetle's attack on a millipede
It was rooted at the rear end of the food chain, but now the humble dung beetle is biting back.
A ferocious scarab species has been filmed in Peru attacking and eating millipedes 10 times its length.
D. valgum no longer dines on faeces. Instead, the nocturnal predator prefers to decapitate live prey with its armour "teeth" and then devour their insides.
It is a rare example of a scavenger species turning carnivore, say US scientists in a Royal Society journal.
Dung beetles (Scarabaeidae) are not renowned for their predatory instincts.
It seems like such a huge jump - from a scavenger to a hunter-predator - so how did it get from A to Z?
Dr Adrian Forsyth,
Blue Moon Fund
They feast on fresh animal faeces which they gather into balls and roll with their hind legs.
But after the species Deltochilum valgum was seen grappling with millipedes several times its size, Dr Trond Larsen of Princeton University decided to find out whether it could actually be preying on the creatures.
Working in the Peruvian rainforest, his team set up more than 1,000 traps containing different treats to tempt the beetles.
These included a traditional menu of dung, fungus and fruit, as well as millipedes, which were live, injured or dead.
They found D. valgum fed exclusively on the millipedes, preferring prey which were alive but injured.
The millipedes coiled up in defence against the beetles
Using infrared cameras, they filmed one adult beetle attacking and killing an injured millipede, which dwarfed it in size, by decapitating it with its body armour.
"This is a remarkable transition," wrote Dr Larsen, a tropical ecologist, in the journal Biology Letters.
"Despite its close relationships with dung feeding species, D. valgum has entirely abandoned its ball-rolling behaviour.
"This is the first known case of an obligate predatory dung beetle species."
The carnivorous beetles use the same mouthparts their cousins use to extract bacteria from dung and carrion as weapons to assault and kill their much larger prey.
I find it amazing that what we thought was just another dung beetle turned out to be something rather exceptional
Dr Adrian Forsyth,
Blue Moon Fund
When placed together in observation chambers, the beetles began their attacks by grasping a millipede's body with its mid- and hind-legs.
The millipedes - up to 110mm long - responded by coiling or flailing their bodies to resist the grip of beetles just 7-8mm wide.
After waiting until the flailing slowed down, the beetles inserted their armour teeth between body segments, prying upwards with their head while simultaneously sawing with their fore-teeth.
"During one kill we observed, the force of the beetle's prying severed the millipede's head from the rest of its body," said Dr Larsen.
One giant leap
The team say their discovery demonstrates how small changes in the physiology of a species can lead to giant leaps in its behaviour.
The beetles were never seen rolling dung balls. Instead, they used their powerful hind legs to drag a killed millipede to a safe site and then begin devouring it.
Most dung beetle species feed on balls of faeces
Dung beetles' heads are usually flat and wide like a shovel in order to roll balls of dung but D. valgum has a narrow, pointy head which it uses to get right inside the millipede's body and feed on its insides.
It also has sharper "teeth", which are used to prise open the body and sever it into smaller pieces.
And unlike most dung beetle species, which bury their food, the remains of dead millipedes were left lying under leaves, entirely cleaned of their soft inner tissue.
"It seems like such a huge jump - from a scavenger to a hunter-predator - so the real story is, how did it get from A to Z?" said Dr Adrian Forsyth of the Blue Moon Fund, a co-author on the paper.
"We knew plenty of dung beetles which are attracted to dead insects - drawn by their potent cyanide-rich odours. And now we find a species which just couldn't wait.
"This is a beetle which says: 'It doesn't matter if it's dead or alive, I'm going to eat it'.
"It's a nice example of how you can take an apparently big step - to become a carnivore."
The scientists believe this unusual evolutionary transition was driven by high levels of competition for food.
Instead of rolling dung balls, the beetle used its hind legs to drag millipedes
Dung beetles undergo fierce competition for resources, both between species and within species.
Adults are seen to aggressively defend their dung balls from competitors by pushing and chasing each other.
The same behaviour was seen in beetles which had captured millipedes.
The dramatic evolutionary leap could help to explain the coexistence of so many different types of insects, researchers say.
"I find it amazing that what we thought was just another dung beetle turned out to be something rather exceptional," said Dr Forsyth.
However, this is not the first time the dung beetle has been shown to be more sophisticated than its name suggests.
In 2006, researchers in Kuwait won an Ig Nobel Prize for a study showing that "dung beetles are fussy eaters".
When offered dung from three herbivorous animals - horse, camel and sheep - the beetles preferred the more fluid horse dung to the others.
Furthermore, the African dung beetle was the first animal found to navigate using moonlight, enabling it to orientate itself and make a hasty retreat from competitors when rolling a ball of dung.
So while they may be the butt of many jokes, it would be foolish to pooh-pooh their talents.
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