Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Video: Hero ant jumps off cliff to remove intruders

Cliffhanger (Image: April Nobile/cc-by-sa-3.0)

Species: Malagidris sofina

Habitat: Clay cliffs in forest fragments in north-west Madagascar, fortunately not suffering from vertigo


If an intruder enters your home, wrap your arms around them and throw yourself out of a top-floor window. Sure it’s a drop of several metres, but you’ll get rid of the intruder and the fall probably won’t kill you.

Assuming you’re a human being with a typical attitude to risk, and not Batman, you probably aren’t convinced by this strategy. You’re thinking the fall will probably break a bone, at the very least, and that there are less drastic ways of dealing with burglars. You could punch them, for instance.

But if you were a social ant belonging to one little-known species from Madagascar, your attitude would be very different. The nest you share with your siblings needs to be defended against predators, and anyway you can safely drop many times your own height without injury. That strategy qualifies you for membership of the only known species to jump off cliffs to get rid of unwelcome visitors.

Cliffs and funnels

Malagidris sofina ants are only found in a few forest fragments in north-west Madagascar. They had never been studied in detail, until Jackson Helms of the University of Oklahoma in Norman and colleagues visited the area in February 2013.

Helms found that the ants build their nests into the vertical sides of boulders and rocky outcrops, or in clay banks, between 10 centimetres and 3 metres above the ground. The ants dig into their chosen cliff and excavate a single chamber around 7 cm deep. Then they build a funnel-shaped entrance that sticks out around 2 cm from the cliff. The finished nest looks like one of those plastic wall plugs.

Several other ant species also build funnel-shaped nests, but it’s not clear why. It was while trying to answer that question via experiments on M. sofina nests that Holms stumbled across their outrageous nest defence tactics.

There were three possible explanations for the funnels. They could help the worker ants defend the nest, improve air circulation, or divert water to ensure that the nest doesn’t get too soggy.

Helms found that water did run around the funnels, but the same would be true of simple tubes, so water defence probably isn’t their main function. However, he calculated that there were enough ants in the nest to produce dangerous levels of carbon dioxide, and that the funnel should increase the flow of air six-fold, reducing the risk.

The funnels don’t seem to help the ants defend their nest. Helms dropped 78 intruder ants, either from rival nests or from different species, onto the funnels. All of them were able to walk on them without falling off. But then the worker ants got involved.

Tombstoning

If they could be bothered, that is. In 45 of the 78 tests, the M. sofina workers ignored the intruders or retreated before them, allowing them to enter the main nest chamber. In particular, they never attacked ants of their own species.

However, in 33 cases the workers fought back – especially when faced with predatory ants that might pose more of a threat. A single worker would grab the intruder in its jaws and try to sting it. About half the time, the worker then dragged the intruder ant to the edge of the funnel, and hurled itself off taking the intruder with it. Once the pair hit bottom the worker let go of the intruder and climbed back up to its nest.

Cliff-jumping “appears to be a very effective nest defence strategy”, says Stephen Yanoviak of the University of Louisville in Kentucky. “I do not know of any other soil-dwelling ants that jump like this species.”

Jumping off cliffs is not unique to this species: many tree-living ants will jump off branches and glide down to the ground to escape predators. Yanoviak says these gliding ants position their bodies carefully to control their descent, which M. sofina don’t seem to do. “The really cool part is that they use jumping to evict intruders.”

Journal reference: Insectes Sociaux, DOI: 10.1007/s00040-014-0360-8