A species of butterfly discovered some 100 years ago has just been found to have a curious, lifelong relationship with ants. Like cunning knaves, the butterflies not only manage to steal the ants' food without trouble, but also convince them to look after their young.


A lengthy stint in the Amazon often means getting wet, muddy and stung – but for scientists hoping to unveil the secrets of the most elusive rainforest inhabitants, it's a venture well worth the trouble. That's how entomologist Phil Torres first made the butterfly-ant discovery near Peru's Tambopata Research Center.

While hiking on a nearby trail, Torres spotted an adult metalmark butterfly (Adelotypa annulifera) drinking nectar from the tips of bamboo shoots. There was nothing unusual about the scene – until he noticed something else. The ants that call the shoots home, and rely on their sweet secretions for survival, were doing nothing to defend their territory from the butterfly interloper. 

After snapping a quick photograph, Torres moved along, but curiosity came calling and he returned to the site for a second look the following morning. "The next day, that one butterfly turned into three butterflies all doing the same thing and I thought, OK, this is going to be something cool," recalls Torres. 

Determined to find out just what was going on, he tagged in biologist and colleague Aaron Pomerantz, who would be spending time in the area later that year. "I was really mind-blown when I first saw the pictures," says Pomerantz. "Despite years of being involved in entomology, I had never heard or seen anything like this before.

While ants might not seem like particularly threatening adversaries, some species in the Peruvian Amazon – like the infamous bullet ant – can inflict an intense, venom-packed sting. The experience has been likened to walking over flaming charcoal with a three-inch nail embedded in your heel. And that's the level of pain the ants can inflict on adult humans. For tinier recipients, a sting could certainly do damage. 

"How does this butterfly not get torn to shreds by the ants?" Pomerantz wondered.



Lifetime liaisons 

Several months later, Pomerantz  visited the site in the hope of witnessing similar behaviour from the adult metalmark butterflies – but he found something more.

Hidden beneath a dead leaf on the tip of one of the bamboo stalks were dozens of metalmark larvae – and the ants were tending to them like a horde of willing wet nurses. Combined with Torres's previous observations, this led the scientists to suspect that the butterflies interacted with ants throughout their entire life history: from egg to egg-laying adult. 

The million-dollar question: what's in it for the ants? In the early stages of life, butterfly larvae are basically defenceless protein sausages, and would make a fine meal. But even the ornery bullet ants showed no interest in a buffet-style binge. Torres and Pomerantz explain that the caterpillars' saving grace comes in the form of a nutritious goo they secrete, which the ants can feed on. 

The substance, expelled from a specially evolved "nectary" organ, is rich in both sugar and amino acids. And because bamboo is not a particularly good source of energy, it's possible that the caterpillars' offerings surpass that of the host plant.

Food for defence is a trade-off we've long observed in the natural world, and in fact, some species take it one step further. The secretions of the Lycaenid butterfly caterpillar, for example, have powers of mind control. Chemicals within the goo interfere with dopamine production in the ants that eat it, turning them into armoured rage-heads. 

Various species, including bullet ants, tend to the metalmark young. Image: Phil Torres/Aaron Pomerantz

The caterpillar-ant interaction Torres and Pomerantz observed in Peru appears to be mutual  but that doesn't explain how the adult butterflies manage to stay in the ants' good graces. The team even observed a butterfly stealing a droplet of nectar straight from the mandibles of an ant, the first time this has been documented.

"This is most likely achieved by some sort of pheromone signal," explains Pomerantz. "Ants aren't very visual, but they communicate primarily through chemicals. Somehow the adult butterfly may have evolved the right chemistry to 'trick' the ants into not attacking them, perhaps by mimicking the ants' own chemical signature or by recycling the chemistry from its larval stage that says 'hey I produce good stuff, don't eat me'."

At this point, it's anyone's guess, but the scientists are hungry (no pun intended) to get back in the field to discover the method behind the metalmark's madness.


A dirty affair

Torres pauses for a selfie after hours of watching butterfly butts. Image: Phil Torres

While spending weeks or even months beneath the lush canopies of the rainforest sounds like a great adventure, discoveries like this one often involve a lot of pretty monotonous grunt work. We're talking waiting for hours for a single butterfly to move from one leaf to another. 

"The most memorable waiting period for me was when I had to watch the butterflies' butts to see what happens when they pee," recalls Torres. 

Because the ants would occasionally tap their antennae on the butterflies' backsides, Torres was convinced that – if he waited long enough – he would see whether the adult butterflies were offering the ants some nutritious liquid, like their caterpillars.

"But after hours of waiting and witnessing three 'urination events', I concluded that the butterflies would just pee whenever they wanted, and the ants wouldn’t respond to it whatsoever. I went a little crazy being eaten up by mosquitoes and sitting cramped staring at butterfly butts. I took a selfie to remember it by."


Puzzles abound 

The rainforests surrounding Peru's Tambopata Research Center are some of the most remote and pristine in the world, holding the promise of many a future discovery. Yet this wilderness, along with the rest of Amazon, is at risk of vanishing before we can uncover its secrets. 

Since 1978, over 750,000 square kilometres (289,000 square miles) of rainforest have been destroyed across Brazil, Peru, Colombia, Bolivia, Venezuela, Suriname, Guyana and French Guiana. More than three-quarters of that cleared land is now devoted to cattle ranching. 

As humans continue to expand their reach, roads have also been opening up previously inaccessible swathes of land to illegal logging and poachers.

All of this makes protected areas like the vast Tambopata reserve increasingly precious. The region's old-growth lowland forests have existed mostly untouched by human activity for some 30 to 50 million years

Beyond life in the undergrowth, the park shelters over 170 species of mammals, 670 birds and some 20,000 different plants – and that's just what we've found so far. For conservationists like Torres and Pomerantz, it's vital that this haven remains protected.

Image: Phil Torres/Aaron Pomerantz

The team will return to the jungle in the coming years to delve deeper into their discovery – and there are many questions still waiting for answers.

Are interactions between butterflies and ants more common than we thought, and simply overlooked because the rainforest is so big, and these critters so small? Do the butterflies' red spots mimic the appearance of the dangerous ants they rely on, making them less appealing to predators? And just what kind of chemical trickery is used to fool the ants? 

"This butterfly-ant relationship was a puzzle that literally kept me up at night," says Torres. "I would browse through the images telling myself I must be missing something. And eventually, I found those missing somethings. Photography helps pause your natural history observations and allows you to really soak in what nature shows you."

Torres and Pomerantz head into the field. Image: Lucas M. Bustamante

For stunning images of wildlife and updates on their adventures, keep up with both Torres and Pomerantz on Instagram, and the Jungle Diaries YouTube channel. The A. annulifera life history paper can be found here

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Top header image: Phil Torres