These Ants Think They're Spartans

"This is SPARTA!" It's a line that nearly everyone you know is likely familiar with. Now a new study has found that societies of ants in the rainforests of Malaysia should be shouting this chant as well - if ants had voices - as they regularly throw invading armies from their tall tree-side homes.

If you haven't seen the 2006 blockbuster hit 300, it's probably worth the two hours you'll spend glued to the screen. You can watch overly dramatic men in capes shouting loudly and doing many manly things - one of which involves kicking a Persian liaison into a seemingly bottomless well (what? Do you not have one of those in the middle of your town?) after screaming the aforementioned line.

That kick, from Sparta's King Leonidas, was a declaration of war on Persia and its massive invading army, and was some hefty foreshadowing for what was to come  - a lot of Spartans throwing a lot more Persians off cliffs (just as seen on the box office poster).

Now, a study recently published in the journal Ecology Letters has shown how Leonidas and his 300 Spartans may have a lot in common with fern-dwelling ant colonies in the Malaysian rainforests.

The study details how when a new invading any colony was introduced into a fern, competition-driven war often ensued "with ants sometimes being thrown from the edge of the fern," according to the authors.

That alone isn't exactly an unusual discovery. War between ant colonies is actually pretty common, as they compete for the same local resources. One species in northwest Madagascar, Malagidris sofina, may even be more fearless than the Spartans - as past studies found that its members will physically grab hold of prospective invaders and leap off the edge of their cliff-side homes. They sometimes fall as far as 10 feet - quite a distance for insects that are fractions of an inch in length.

So what makes these Malaysian fern-dwellers even more impressive than that? The authors of the new study were fascinated not only by the way in which these ants ejected invaders, but also by who they let stay.

Epiphytic birds' nest ferns (Asplenium spp.) are large and wide plants that trap leaf litter from tropical canopies. Over time, this creates multiple layers of prospective ant homes - a virtual city for insects and even frogs. The researchers were intrigued by the revelation that these ferns could often support up to 12 completely separate ant communities which (for the most part) do not actively try to evict one another. (Scroll to read on...)

More intriguing still was that surveys of 86 epiphytes revealed how these cities usually boasted a stunning variety of ant species, in which the ferns more often supported ants of all different sizes than ones of similar size.

And this was unexpected. Reason would suggest that specific local ecosystems would only attract similar ants, as their size could reflect specific behaviors and foraging patterns suited to the area. These similarities could make cooperation within the 'fern city' easier, but the researchers quickly determined this was not the case.

After collecting a wide variety of local ants, the researchers set out to recreate what they saw in the field. In a controlled lab setting, the scientists inoculated epiphytes with single or multi-host communities of ants. Two days later, they introduced the same epiphytes to an "invader" species.

It quickly became apparent that if the invaders were of the same size as the current fern occupants, they were quickly driven away Spartan style - with the ejected party falling into containers the researchers had placed below the fern. However, if the invaders were vastly different in size, compared to the current occupants, those containers remained empty, as no conflict ensued.

A lot of this can be explained by a simple rule: you don't pick fights with those who are bigger than you (a lesson Leonidas never learned). In addition, when an unwanted party showed up, it was not the largest ants who shooed them away, but those of equal size, perhaps because they are the ones who recognize the invaders as a threat. Likewise, smaller ants could also get a free pass into the fern when its larger occupants outright ignored them.

"Similar-sized species are expected to be more likely to share resources or natural enemies and therefore [are expected] to experience more competition," the researchers added, explaining for this phenomenon.

The team was also quick to point out that it remains unclear what the size threshold is for when one species recognizes another as a prospective threat.

"Understanding how species assemble into communities is a key goal in ecology," they wrote, explaining that much more work should be done in this area if experts hope to understand how and why these little 'Spartan city-states' form.

For more great nature science stories and general news, please visit our sister site, Headlines and Global News (HNGN).

- follow Brian on Twitter @BS_ButNoBS