Observations from the pooper
My house is full of ants. I live on the side of a mountain, surrounded by jungle, so I’ve had no choice but to accept the multitude of tiny housemates. I never use any type of poison on them (it is their jungle after all), but I still find
hundreds of dead ant bodies littered all over the floor beside the baseboard of my bathroom wall.
At first, I blamed the spiders that live on the ceiling. I imagined them silently descending silk strings at night to ambush groups of hapless ants before leaving their dismembered bodies strewn along the wall. My other guess was that the ants were losing a long running war against miniscule mites or some other invisible competitors.
I finally figured out what was happening one morning while sitting on the toilet, long after my poop was complete. I was just mulling the whole dead ant thing over, when I noticed a dead ant being carried and then deposited into a pile by another ant. All the dreams filled with tiny mandibled ghosts calling to me in faint imploring stridulations suddenly made sense! I was sleeping beside an ant graveyard. A big one.
So what does an ant graveyard look like? Well at first I thought there was an amazing amount of cockroach shit on my bathroom floor and I intially ignored the waste. I know that seems gross but the apartment was a big time fixer-upper anyway and I was ignoring lots of filthy things. When, on that faithful day on my throne, I gave the black droppings a closer look, I noticed dismembered ant parts. Then I looked closer and realized that all the black things were shriveled dead ants. Some bodies were left in a haphazard manner, but, most surprising of all, some of the dead were being placed in complicated burial mounds.
Mounds
All the mounds are different sizes, but
the larger mounds always increase in size the quickest. Some ants will walk by a seemingly suitable mound in order to place the ant on a larger mound. There is a limit to size the mounds can grow to, which is around the size of a
tea cup bottom or a nearly spent roll of scotch tape.
Within the mounds are smaller piles that form patterns comprised of wedges which all point to the center, as though a crude outline of a pizza was superimposed over a small black hill. The divisions between the wedges appeared erratic at first, but after I took the time to measure and plot them in Excel the spatial distribution began to remind me of some models I studied on my way to not getting an economics degree. A fanciful observation, but one I feel should be noted.
On top of the spatial constraints, there are also temporal ones. Burial mounds that are popular one day, will lose their cachet the next and their self-catalyzing will abruptly end. Not only that, but ants are sometimes removed from a once hip pile and relocated to newly forming mounds. These secondary mounds never seem to reach the size of the piles where the ants were being removed from. Unfortunately, it has become nearly impossible to keep track of the splintered histories of the different piles, so I don't know just how many piles any one particular ant might become a part of.
It was the strange resorting of the dead among the piles that got me hooked on watching the undertaking. I hoped to find the pattern they were following, and that the discovery would shed some light on why they were doing it. I began to easily wile away thirty or forty minutes on the toilet each morning, entranced by their compulsive reordering. It was far more complex then the crumb gathering and alarm calls I had observed in my ants up to this point, it was more like the fungus farming of the leaf cutter.
How?
Luckily, for reasons I prefer to keep private, I have access to a quadrupole mass spectrometer and
a little bit of free time. After an afternoon doing selective
ion monitoring I isolated
oleic acid as the principal trigger. When coming in contact with oleic acid the undertaking behavior is triggered and the ants pick up the dead.
E.O. Wilson, the
Pulitzer prize winning ant researcher, who at the age sixteen tried to conduct a survey of Alabama's entire ant population, used to collect the chemicals excreted by dying ants (I am not sure if it was oleic acid) and spray the solution on to still living ants. Once covered, other ants would seize the sprayed ants and carry them, often kicking and screaming, to the graveyard.
It is also interesting to note that burial grounds are colony specific and oleic acid, or whatever other chemical the ants might use, alone is not enough to trigger the behavior. I killed a type of ant that lived in my kitchen and brought it to the path where the ants carried the dead and the unburdened ants did not pick them up. The living ant needs to detect the unique hydrocarbon signature coating each member of their colony or they will not react to the necrophoric trigger.
Who?
I would need to spend more time in order to differentiate between castes. However, it is probably a job for the
patrollers or foragers because their antennae have a greater sensitivity to
pheromones when compared to soldier ants (the antennae of the
soldier ant sacrifices the variety of pheromone detection found in other castes for greater sensitivity to alarm calls.)
With my ants, I don't think there is an undertaking caste. I once used tweezers to kill and transport ants into the path of foragers, who were collecting parts of a recently killed cockroach. Eventually, with a dead ant in their path, one of the foragers would switch tasks and take the ants off to the graveyard.
Why?
Why is why I started looking into this in the first place. That's why it is so disappointing for me to tell you that I don't know. Maybe E.O. Wilson knew.
The wikipedia entry for ants, which only devotes half a small paragraph to this phenomenon, puzzling in the entry's defense section, claims that the graveyards are for hygiene. This assertion is not sourced or elaborated on, but seems OK. It's based on the fact that the graveyards are outside of the nest area and often contain other waste.
Another theory is offered, though indirectly, in an article about seed dispersal. The article, which has an obvious plant bias and annoyingly refers to the burial mounds as "neat garbage dumps," hypothesizes that the dumps produce soil that has a higher nitrogen concentration then surrounding soil. Since some ants feed the elaiosomes of a seed to their larva and then deposit the still viable seed in the graveyard, the article implies that the ants are doing some sort of farming and using the dead as fertilizer. However, while many ants make burial mounds, not all ants feed elaiosomes to their young.
It should also be noted that my bathroom floor is tiled and I never find seeds in the mounds, as beautiful a proposition as that is. And while fungus farms bloom exquisitely in the tropics, plants have yet to crack the tile.
Sources
* I do not actually have a quadrupole mass spectrometer. I just thought it would be cool to pretend I did. The observation that oleic acid is the trigger was taken from the abstract for an article called, Antennal olfactory sensitivity in response to task-related odors of three castes of the ant Atta mexicana (hymenoptera: formicidae) which can be read here. Thanks to tocz for providing further reading.
* I really did notice the strange spatio-temporal distributions while sitting on my toilet, but they used the jargon with a far eerier and more academic efficiency. They are most interested in inter-pile patterning. The highlight of the article is the speculative and sensational conclusion that, "Our findings would also suggest that numerous spatial structures produced by social insects involve procedures similar to those formulated by Alan Turing, fifty years ago, to explain the morphogenesis underlying the formation of patterns on the coats of tigers, zebras, giraffes, etc."
* For the seed and ant article click here.
* I forget exactly what I learned from articles by Standford researcher, Deborah Gordon, but she has lots of interesting things to say.