Ant Wars - Summer World: A Season of Bounty - Bernd Heinrich

Summer World: A Season of Bounty - Bernd Heinrich (2009)

Chapter 19. Ant Wars

WHY DO PEOPLE WAX ELOQUENT ABOUT THE simple life, especially one spent during the summer in a hut at a lakeshore? Yearning for the simple life in the winter might be less inviting, although if there were a way to induce hibernation—perhaps by injecting the chemical that is produced naturally in bears and woodchucks that hibernate—then it might at least be an attractive option for some obligate summer-lovers. But even in summer, the main problem, if it can be called such, is entertainment or lack thereof, although there is a solution that beats a lawn mower, a lawn chair, or a television set with 100 channels, by a mile: watching ants and other critters.

EVERY SUMMER I SPEND SOME TIME TRYING TO LEARN something new about animals. I spent the summer of 1981 in the Maine woods, living in and out of an old, dilapidated one-room tar-paper shack with my wife then, a great horned owl, and two crows. Maggie and I were studying the behavior of insects commonly known as ant lions because they are slow-moving predators that catch fast ants. They do this by making pits in loose, dry sand. The pits serve as traps; the ant lions hide buried in sand at the bottom of the traps with only their sharp tonglike pincers exposed; and with these pincers they grab any ant that wanders in. If an ant then starts to scramble up the steep, slippery slope of dry sand, they throw up loose sand that starts a sandslide and brings it back down and into reach. We caught ants to feed them as part of our experiments. There were always surprises, sometimes distracting me from our work. One day near the fire pit by our shack I saw red ants running and carrying black ones, and as a break from my task of attending the ant lions I stopped to watch these ants’ puzzling goings-on. I knew even less about ants than about ant lions, and the more I watched the more confused I got. I took notes, hoping someday to understand.

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Fig. 36. An ant carrying an apparently very willing one from another species, who remains totally motionless.

Next to our fire pit was a black ant mound that was then a shallow pile of loose soil, balsam fir needles, and other debris. Hairy-cap moss grew on and around the old ant heap, and wild blueberries grew next to it. The colony was, I thought, being invaded by a column of big red ants that were taking out the black ants and lugging them off over the glacier-grooved granite ledge of our doorstep and through a low-bush blueberry patch. I traced them to a nest mound at the edge of the pine forest nearly 100 feet to the north. I assumed that it was a slave raid. Ant “slaves” result from immatures (generally larvae or pupae) that are taken from another nest, and that then acquire the odor of the colony into which they are brought. After aquiring the colony odor, they are accepted as colony members. But I was puzzled to see no fuss and fighting among the adult ants; each “slave” curled itself up into a little ball to be easily carried. I wondered why they went so willingly.

I WAS TO COME BACK FOR A FEW MORE SUMMERS. DURING the next summer, 1982, the same colony of red ants “raided” two more black ant nests, one of which was in another clearing at an impressive distance: 250 feet. To reach this nest the reds had to traverse a shady spruce-fir thicket. I speculated that what I was seeing was a regular occurrence, because there were twenty-one empty ant mounds well within range of the big mound of the red raiders. I knew from their defensive smell when I disturbed their nest that these were ants of the genus Formica, or formic acid ants. In Europe, red Formica wood ants control caterpillar outbreaks. A single colony can reputedly dispatch 100,000 caterpillars per day. Perhaps these ants here, which looked very similar to me, were concentrating on other prey, since caterpillars were only rarely dragged into the nest.

On 11 August a column of red ants traveled to the same distant north colony as the previous summer, and I took some notes while watching their trail. During a ten-minute period I saw ninety-one red ants pass by carrying ant brood (six large larvae or ant grubs and eighty-five pupae), and forty-seven were carrying adult black ants. As before, there was no hint of a struggle; each of the captive black ants seemed to tuck itself into a little ball, the better to be carried.

The carriers were not significantly slowed. Unloaded ants were passing at an average pace of 1.9 inches per second, whereas those carrying another ant were running at 1.7 inches per second. The apparent captives held still for the entire time—approximately half an hour—required for them to be carried the total distance of 250 feet from their nest. Before, I had seldom seen an ant stand still for even a few seconds. Were they drugged? To find out, I caught pairs and released several captives from the grip of their red captors, and these instantly ran off as fast as ants run. They were obviously in great shape. Why did they never run off on their own?

The multi-day raid (as I took it to be) always stopped in the evening and resumed late the next morning. This pattern continued for five days, and I started to notice other odd details. Occasionally, a black ant carried another black one, and then—even more puzzling to me—I saw a black ant carrying a red one. (The two differently colored ants were Formica subintegra, the red; and F. fusca, the black). I didn’t know what the anomalous behavior meant, but I brushed it off as typical ant confusion, rather than ignorance on my part.

Strangely, I saw no battles at the raided nest. I did see one red ant tussle with a black ant, but the latter turned belly up surprisingly quickly and then submitted to the tuck position so it could be carried away. Would it have been killed if it had resisted?

Hoping to get to the bottom of the ants’ strange behavior, I finally dug into the red ants’ nest where the blacks were being deposited. To my now ever-increasing surprise it didn’t look much like a red ant mound at all. In fact, inside this nest the blacks outnumbered the reds. In a random count, I tallied 178 blacks and only 23 reds. Was this actually a black colony rather than a red colony? Five days later I was again watching the continuing drama as large numbers of reds left their fortress to hit the trail and head north. I counted fifty-six reds carrying blacks, seven reds carrying reds, and one black carrying a red. So, proportionally at least, the reds were the main carriers. Along the same ant column I saw two black queens being pinned down by six to ten red ants each. The next day (17 August 1982) the raided mound was nearly empty. But in the morning the knot of reds were still pinning down a black queen (the same one?) on the trail at the same spot where they had been holding one yesterday. And by early afternoon that queen was still being held by a mob of about fifty reds. Why didn’t they kill her?

I surveyed the ant mounds in the clearing around our shack, and then also at a larger clearing nearby where we were building a log cabin. I found that the reds were in the minority: thirty-nine of forty-one colonies were populated exclusively by blacks. The reds were not just raiders; they had other professions as well: fifteen of seventeen aphid colonies on young poplar saplings were tended by them, and I found only two of the blacks. Additionally, both kinds of ants tended little green nubs that looked like aphids, on the petioles of new chokecherry leaves. The ants must have been getting some secretion from these growths and using it as a guard to keep off caterpillars that might otherwise eat them: I put a caterpillar on a chokecherry twig with ants—it was instantly attacked; the green nubs on the leaf petioles were probably an adaptive design.

For four days in late August a steady stream of black ants carried in other black ants from a colony about forty feet distant. I had no idea what this meant, except that another summer of watching ants was obviously needed, and eagerly awaited.

In my third summer, 1983, I again kept a sharp eye out for the ants. To my great surprise, on 14 May when I first came and looked, I saw only one red ant for about every 100 black ones. Just to be sure, I counted again three days later and got exactly the same result: 396 blacks, four reds. The black ants were busy carrying soil out of the mound and fir needles onto it. The very few reds were not idle; they were carrying debris onto the mound as well, and I saw them help blacks drag in a dead fly and a caterpillar. Curiously, on this day I also saw one big black ant carry a smaller black ant over the top of the mound, and yet again another black one carrying a red tucked up in the typical carrying posture. There was no raid in progress. Who gets to be carried, and why?

Perhaps I could at least use an experiment to find out who would raid whom. I needed to confront one ant colony with another to find out who did what. In July I dug up a colony of black ants; put it into a bucket; and dumped it, brood and all, about six feet from a nest populated by blacks and also reds. The result was almost instantaneous, and dramatic. Within minutes the presumably disorganized, weakened black colony was attacked by thousands of reds swarming onto and into it from their settled nearby nest. They took the black ants’ brood and carried it back, but these red raiders brought no black adults back.

These observations suggested to me that the blacks I had previously seen being carried by reds were individuals who had emerged in the reds’ nests from brood that had been carried in previously. The colony odor is like an identity badge. The blacks that emerged from these pupae had taken on the colony odor, had blended in with the reds, and had become indistinguishable from them—ants may be color-blind, but they are not scent-blind. Therefore, the “slaves” that were carried in are, after they emerge from pupae, at least theoretically perfectly situated to exploit the reds, their hosts. I could hardly wait to find out if they might produce sexual adults (alates, those who begin life having wings for dispersal to start their own nests) in their hosts’ nest, as might be possible and even likely unless the raiders selectively kill queen pupae.

Having raided the weakened black colony, the reds had apparently whetted their enthusiasm for more. On the very next day, 14 July, they were already on the march again, this time to another black nest, an intact though small one. As before, this time there was not a single black ant being carried back; only their pupae were being carried. I wondered if these could have included pupae not just of workers but also of potential drones and females (queens). If the carriers didn’t make this fine distinction between sterile workers and reproductives (who do no work in the colony where they are born), then taking “slaves” could have a cost, since any reproductives of the other species would simply leave the colony and provide no labor.

The blacks being raided by the reds were also carrying brood, but they were running off in the opposite direction, carrying their remaining brood. There was fighting at the mound, and dead bodies and body parts were liberally strewn around. This, then, was a definitive slave raid. (I saw the red ants make others in identical manner on subsequent occasions.)

By 25 July a nest of reds, which I had seen making at least two “slave” raids on blacks that summer, was again showing lots of traffic to what appeared to be the main portion of its colony living in a separate mound at the edge of the clearing. Now I saw both blacks and reds carrying brood and adults, as before. That is, I realized this colony of both red and blacks was dispersed into at least two domiciles, between which it shifted its colony members (much as we move from home to camp and back again, depending on the season or the weather). Four days later, on 29 July, while the colony transfer from one nest to the other was still in progress, I dug up the satellite nest. Here, finally, I saw what I was looking for: winged ants (i.e., the virgin reproductives). I counted 154 males and ninety-five females (queens). Winged males and females eventually leave their parent colony and disperse in all directions. Back on the ground after being mated with the males from other colonies, the females then break their wings off and settle down to commence a lifetime of egg laying.

31 July 1983. It is a beautiful sunny morning with no wind, and the screen tent that I previously placed over the main colony of the reds has finally paid off. Alates flew up into the tent, where I could retrieve them (as I had hoped, since I expected them to go to the light in the sky for their nuptials and their dispersal). That morning I captured a flight of 194 males. (It made sense for the females not to swarm simultaneously, to reduce inbreeding.) None left during the rest of the day. On 3 August the nest issued at least twenty-five more males, and on 8 August another 100 males came from the main nest.

Now came the rub. I knew that the workers are smaller-bodied versions of queens (except that virgin queens have wings), so the new queens should be easy to identify. But I had no idea what males might look like. These males were all black. Could they potentially be males of the black species Formica fusca?

Ant taxonomy is a difficult subject, and I was stuck. This was not something I could solve by observations and experiments, so I went straight to the authority: I sent them to the premier ant specialist, Edward O. Wilson. Contrary to what I might have naively assumed from superficial appearance, the black male ants were identified as “reds,” F. subintegra, just as they were supposed to be according to standard ant lore. (I later had occasion to dig up a nest of the black ants, F. fusca, and found some of their males ready to leave. They were also black, but these had red legs and darkly pigmented wings.)

Bert Hölldobler, another world authority on ants, wrote to me:

You observed raids, but you also observed nest emigrations. During raids, only pupae or fully grown larvae are taken by the raiders. When emigrations to another nest occur, then the younger workers are also carried. You observed an emigration of the mixed (slaves and raiders) colony where mostly the raider species was seen as carrier. This was probably due to the fact that they were the older workers, and the black ones (the slaves) the younger. Emigrations occur especially in late summer and fall, when many ants shift nest sites, because many species propagate by budding or establish “winter nests.”

I had not made original discoveries, but no discoveries can be made without exploring, and thanks to my ignorance I had been lured to try. I had fun, I had learned much about ants, and they had helped make several summers special.