Summer World_ A Season of Bounty - Bernd Heinrich [89]
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