Thursday, 14 May 2015

Evolution In Progress - The Great Ant Escape

Trap-door ant (Odontomachus brunneus)
Great escape: ant uses spring-loaded jaws to jump away from predators | Science | The Guardian.

Here is a nice example of two components of evolution: the arms race between predator and prey and co-opting a structure for a new purpose, showing the opportunist nature of evolution by descent with modification. It involves a trap-door ant and how it tries to escape from a predator, in this case an antlion.

First a little about trap-door ants and antlions:

Trap-door ants have massive jaws which they normally hold wide open as they forage around the forest floor until they come up against something which triggers the sensory hairs between the jaws, when the jaws swing forward and clamp onto the prey at an amazing 140 miles an hour - the fastest known movement of an appendage in a living creature.

Antlion lava
Credit: Johnathan Number, Wikipedia
This speed is achieved with a pair of massive muscles inside the ant's head and jaws which have a latch mechanism which prevents the jaws closing before the muscles are exerting enough pull to snap them shut at this speed. These jaws have clearly evolved for the purpose of catching, holding onto and killing prey as quickly as possible.

Antlions are the lava of a damselfly-like insect. They prey on small crawling creatures, especially ants but rather than actively hunting they make a pit of sand and hide buried in the bottom of it, waiting for something to fall in. They will even throw grains of sand at the victim to make it fall to the bottom of the pit. This behaviour probably evolved from making burrows and hiding inside to rush out and grab passing prey. A neat solution where the ground is too dry and sandy to make good borrows, so enabling antlions to exploit new areas.

In this video we can see a trap-door ant using a surprising method to escape from an antlion.

This mechanism has been known about for a long time but the question was whether it actually improved the ant's survival rate. To test this, researchers at the University of Illinois, USA, glued the jaws of trap-door ants together and compared their escape success to that of a control group with glue applied but the jaws not glued shut. They found that ants still able to use their jaws had a greatly improved escape rate.


Animals use a variety of escape mechanisms to increase the probability of surviving predatory attacks. Antipredator defenses can be elaborate, making their evolutionary origin unclear. Trap-jaw ants are known for their rapid and powerful predatory mandible strikes, and some species have been observed to direct those strikes at the substrate, thereby launching themselves into the air away from a potential threat. This potential escape mechanism has never been examined in a natural context. We studied the use of mandible-powered jumping in Odontomachus brunneus during their interactions with a common ant predator: pit-building antlions. We observed that while trap-jaw ant workers escaped from antlion pits by running in about half of interactions, in 15% of interactions they escaped by mandible-powered jumping. To test whether escape jumps improved individual survival, we experimentally prevented workers from jumping and measured their escape rate. Workers with unrestrained mandibles escaped from antlion pits significantly more frequently than workers with restrained mandibles. Our results indicate that some trap-jaw ant species can use mandible-powered jumps to escape from common predators. These results also provide a charismatic example of evolutionary co-option, where a trait that evolved for one function (predation) has been co-opted for another (defense).

So, not the most important piece of scientific research, coming into the category of "doing silly things to animals to figure out what they're doing", in lead researcher, Fredrick Larabee's own words, but a neat example of how a pair of spring-loaded jaws have been co-opted for use as an escape mechanism in an arms race with a predator. This shows how natural selection works on whatever is to hand and, especially where an arms race between prey and predator involved so the stakes are high, can be a powerful driver of evolution.

Now, project this a long way into the future to some descendants of these ants that have maybe evolved some other mechanism for capturing and killing their prey. We could expect to see these powerful jaws with their spring mechanism retained and fully exapted for an entirely new purpose and apparently having no obvious evolutionary pathway - the sort of structure 'Intelligent Design' creationists point to as examples of 'irreducible complexity' and the subject of another book aimed at people who prefer to believe in simplistic notions like magic performed by an imaginary special friend because only that makes them feel important enough.

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