Brown Food Web Friday–in praise of litter ants



How’re y’all doing this fine Friday? I’m slogging through a cold once thought vanquished. Now seems intent on hanging over me like a stale chain mail party dress. Bleah.

    In today’s BFWF we contemplate one of the great biological systems on earth–the litter ant nest. Litter ants live on the forest floor in small hollow twigs, empty acorns, or even between leaves. The whole colony may consist of only 100 or so ants, just enough to cover the tip of your pinky. This small size allows litter ants to be incredibly abundant: in a tropical rainforest there may be 5-10 species living together in a meter square plot. Yes, I will admit it, litter ants changed my life.

Every scientist has the occasional “aha!” moment. Mine came sitting on my butt on the forest floor at La Selva, cracking twigs with Margaret Byrne, a graduate student at the University of Florida at the time. I was in the middle of a PhD project happily placing bits of seed and bird poop across the forest floor to see what ants arrived, who consumed what, and if they preferred some bits of habitat and climate more than others. I only saw the ants when they emerged from the leaf litter to crawl on my baits, but that was fine. I was getting my data, and every night at the microscope I would empty my vials and see the biodiversity.

Margaret was collecting litter ant colonies for her research and she offered to show me how. Turns out, it wasn’t hard.

Step 1: Sit on you butt in the litter (a black plastic bag is useful here, unless you want your tush to become chiggerflesh).

Step 2: Grab a handful of litter and put it in the tray on your lap. Some folks wear gloves but for me the “litterfeel” is an important part of the process. And what’s an occasional chiggerbite or scorpion encounter in the big picture anyway?

Step 3: Inspect every leaf, and crack every twig. This is where it gets very Zen. There is a lot of litter out there in a tropical forest, and a meter-square plot may occupy you for 2 or three hours until you scraping mineral soil. That’s a lot of “pick up leaf, look at leaf, toss leaf over your shoulder” action. As I sat along the Atajo trail with Margaret, I began to wonder if skipping through litter laying out bird turds wasn’t, in fact, the superior method to study ants.

Then it happened. I cracked a perfectly normal looking twig (you actually, I learned later, gently twist the twig for the best effect). In the darkness of the forest understory, its the light parts of a colony you see first. There was a small twiggy cavity full of tiny pupae, attached to much darker tiny ants that were slowly moving deeper into the uncracked portion of the twig. No big rush, but a certain urgency.

Pheidole nigricula, a common litter ant of the wet rainforest in CR


Most looked like this, a tiny Pheidole nigricula worker, a chocolate brown, shiny ant only about 1.5 mm long.




Pheidole nigricula, a common litter ant of the wet rainforest in CRHowever, a handful were larger, more robust, with big honkin’ heads. These, I recognized, were the majors (sometimes called the soldiers). Pheidole it turns out, is one of handful of ant genera that show this distinctive worker dimorphism–two forms with the more massive form rare. To this day, a comprehensive understanding of this dimorphism remains elusive but it was (and is) generally thought that one role played by majors is colony defense. As I peered more intently, I could make out the colony’s queen (lucky thing too: as the genetic future of any ant colony, a litter queen is likely to jump ship the minute she sees the glassy eyes of a myrmecologist) as well as a few winged versions of the queen, and a couple strange, pinheaded things that I later figured out to be males.

And then the “aha!” moment.

I saw this Pheidole colony as carrying within it a little tally sheet as to how much it had allocated to growth and colony maintenance (the number of workers), how much it had allocated to defense (the majors), and how much it had allocated to reproduction (the winged males and queens). Furthermore, if I looked very carefully, I could see the ants carrying pupae of workers, majors, and reproductives: a record of where the colony wanted to go in the future! The mind raced. I was surrounded by thousands of litter ant colonies, hundreds of species (of which, scads were Pheidole) all waiting to be harvested and tallied toward understanding a big question in EEB: “by what rules do individuals invest their limited time and resources into different activities–all of them critical to the colony’s fitness?


Note added in proof: Any resemblence to this earlier obsession and the topic of this blog is likely not coincidental.


This led to a nice little paper in Behavioral Ecology and Sociobiology and a lifelong interest in litter ants in general, and Pheidole in particular.

That was 1999. Margaret moved from Ecology to Economics (same greek root, after all) and is on the faculty at the University of Miami.

cz-mikelitterplot.gifI have since spent countless delightful hours sitting by myself in the lotus position in one tropical rainforest or the other with a panful of litter on my lap. While I quietly twist apart twigs, I’ve had ocelots walk by, startled countless armadillos, been harassed by white faced monkeys, heard the song of the nightingale wren, and enjoyed tens of other “aha!” moments, as the mind drifts while the brain switches to full analague mode. And every new colony is a small jewel, a tiny gift from the brown food web.


h/t to Kari at The Ant Room



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