Ants and body odor

While staying at a camp in the Peruvian Amazon between 26 Dec 2016 and 5 Jan 2017, I think I stumbled upon a possible evolutionary purpose for human body odor. Story time!

During my stay, I used a tree-tent to sleep in and store my items: suspended 1-2 meters off the ground, attached to three trees by straps coming from the three lateral corners of the tent, and covered by a tarp to keep out rain. It was bouncy. There was a triangle-shaped entrance at the bottom in the center, through I which I would pull myself up as quickly as possible to keep the entrance zipped closed as much as possible so the mosquitoes and sand flies would not follow me inside. It is pictured below, from below:

Only a day after I moved into it, a large group of ants had appeared on the bottom in the daytime, coming from one of the corner-straps towards the center; they got there by climbing the tree to which the strap was attached. They were medium-sized (~1 cm long), stingless, black ants, probably Camponotus sp. One is pictured below:

I swiped them off with gloved ants in several minutes; at that point, none had gotten inside the tent. When I returned again later that day, the ants had returned: there were easily hundreds of them covering almost the whole bottom of the tent, from corner to corner to corner, most dense at the moist spots and at the center (covering the entrance). (It is a great shame that I did not take a picture of that. That might have been during a period when my phone was out of power.)

They seemed to be interested in the moisture that had collected in the floor of the tent beneath my large, heavy duffel bag (which was inside the tent), and a few other places, for I saw them gathered around those spots licking at the surface. The moisture was there because it was raining strongly while we were setting up the tent, and since the tent's roof without tarp is a mesh, rainwater got the inside of the tent wet before we could put the tarp on. By the time I slept in it, it had dried off to an acceptable extent, but not absolutely.

It was clear that even if I were to clear them off manually in order to enter the tent, if I were to stay in the tent for as much as half an hour the entrance would be covered again, which I guessed would result in me being unpleasantly covered by ants and letting them inside (at that point, I had not determined whether they had stings or not). I borrowed someone's DEET (I use a different repellent normally because I enjoy handling animals without killing them) and sprayed only the strap from which the ants arrived, hoping that might deter them... It seemed to work for a little while, but later they returned from other straps, and also I had not swiped all of them off that time anyway. Still, since they were not inside the tent yet, I allowed myself to sleep there that night, and I was fine. The ants did not seem to move as much during the night.

The next morning, they were not yet active, so I could get out unscathed, but they returned later, albeit in smaller numbers than before. By then, they had started getting into the tent through a small opening allowed by the entrance's zipper. Either that day or the next, I applied more repellent at almost all the possible entrances to the tent; by the time I did so, the ants seemed to be coming along the original DEET'd strap again. The next time I went inside the tent, they had become noticeably plentiful in there, and I tried scooping some out down the entrance; but that could not be done so easily as the bottom since my items were in the way, and some were on the ceiling. After that, and maybe one or two sessions more of swiping ants off the bottom, I did nothing else about the ants.

The next time I slept, there were some ants inside the tent, fewer than before due to my attempted sweeping. But, I noticed that I was not disturbed at all while in my sleeping bag, not while conscious and waiting for sleep to happen, nor was I awoken by ants crawling on me in the middle of the night. The same was true of the next night, and the next, when the ant numbers inside the tent had increased greatly such that I did regularly get a few on me (or step on them) while entering/exiting the tent or sleeping bag. Since they never touched me when I was not moving, my initial worry - that their crawling on me would keep me from sleeping - was moot. This picture should give an idea of their density during this period:

Those ants are at the base of my duffel bag (the other thing on the left is a rubber boot). Indeed, they were quite willing to crawl on the bag, as well as the garbage bag holding my laundry (but not inside it), and some other stray items; my sleeping bag was on an inflatable mattress pad, and if the bag slid off the pad, the ants would readily walk on the uncovered pad. However, even after I was absent from the tent for a while, I never or scarcely saw any ants crawling on/in my sleeping bag, my backpack, or my recently-used clothes and boots.

Surely there was some reason the ants were consistently avoiding these things and myself. What the avoided items all have in common was that they were often in close contact with my body, and would therefore retain some of my odorous residue (I wear my backpack very frequently); whereas the other things were not. Furthermore, I know that ants generally are highly dependent on chemical sensation, usually more so than other senses. Therefore, I am inclined to believe that the ants were repelled by some smell emitted by me - my body odor - to such an extent that they would actively avoid me and the things my smell has rubbed off onto.

After noticing this and thinking about it, I realize that this situation makes much sense evolutionarily, given the ubiquity of various ants in any human habitation. Suppose ants were not repelled by human smell, and hence would be willing to crawl around on a motionless one at night unprovoked. Then the sensation of crawling on one's bare skin would almost surely wake up the human, forcing it to lose sleep and possibly have to deal with ants biting and/or stinging, which would be unhealthy for the human. Hence, I would expect human evolution to favor some smelliness that deters ants. Also, although the human's direct killing of the invading ants would probably not affect their evolution much since they are typically non-reproductives, the whole event might make a line of ants lose track of where they are going and then be less likely to get whatever food source they are seeking, or to return held items to the colony, or somethings along those lines which would be detrimental. At the least, they would be wasting a bunch of energy from all that excitement.

So, the main lesson to be gotten from all this is that if you are camping in a tent in the Amazon rainforest, and your sleeping quarters get invaded by ants (or at least Camponotus), have no fear! for you can sleep without being bothered because you stink.

Posted by eccentric_entomophile eccentric_entomophile, January 18, 2017 07:47

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