In early September of 2004, I was in my front yard in Tucson, Arizona when I noticed a swarm of large flying insects. They were centered just above a real-estate sign that was out in the middle of the road. Hmm, I thought…odd.
As I got closer, I saw that the ground beneath the flying insects was covered with mating ants. Oh, how marvelous – this was a mating congregation of Acromyrmex versicolor, the Desert Leafcutter Ant.
There were at hundreds of ants flying in the cluster. Ants would fly into the swirling cluster, grapple with each other, and fall to the ground.
The coupled males and females would be surrounded by waiting male ants. They would cluster into into dense balls of ants. The ants would be so entangled that it was difficult to count exactly how many ants were in each mating ball.
The bodies of males were spindly in comparison with the females. That makes sense, since the males would die shortly after the mating flight, while the females would go on to found new colonies.
Eventually the females would try to break away from the mating congregation. Sometimes they would still have males mating with them as they attempted to make their getaways.
The only time in their lives that the females mate is on their mating flight. Reichert and Wheeler (1996) found that the females mate with an average of three males, even though the sperm from a single male is sufficient to fill the female’s spermatheca (the sperm-storage organ). The genetics of all of the males that mated successfully with the queen are reflected in the queen’s offspring – she doesn’t seem to preferentially produce workers from a ‘favorite’ male.
The reason that’s interesting is that the multiple paternity of the worker ants means the worker females are less related to each other than they would otherwise be. In many ant species, the workers are more closely related to each other than they would be to their own offspring. That’s due to the genetics of sex determination in wasps, bees and ants. William Hamilton proposed that the reason eusociality could develop in the Hymenoptera is because, genetically, it favors the genes of the workers more to raise sisters than to raise their own offspring. That degree-of-relatedness argument begins to fall apart in the case of multiply-mated queens, though.
Like most things, though, it’s probably not that simple.
Some people (see Reichardt and Wheeler paper) hypothesize that the multiple matings contribute to greater genetic diversity in the workers, thus favoring the queen and the colony. There would be a tension between that and having a single-single mated queen, increasing the worker relatedness.
Additionally, Cahan etal found that in another multiply-mated species, Pogonomyrmex barbatus, the genetics of the workers and the sexuals was quite different. P. barbatus hybridizes with P. rugosus. The worker ants showed the genetics of the hybrid matings, while the sexuals (the winged males and females) did not.
So – It’s complicated.
Sources:
Antwiki.org’s wonderful Acromyrmex versicolor page.
Cahan SH, Parker JD, Rissing SW, Johnson RA, Polony TS, Weiser MD, Smith DR. (2002). Extreme genetic differences between queens and workers in hybridizing Pogonomyrmex harvester ants. Proc Biol Sci. 2002 Sep 22;269(1503):1871-7
Johnson, Robert A and Stephen Rissing. (1993) Breeding Biology of the Desert Leafcutting Ant Acromyrmex versicolor (Pergande) (Hymenoptera:Formicidae). Journal of the Kansas Entomological Society 66(1), pp. 127-128.
Reichardt, AK and Wheeler DE. (1996) Multiple mating in the ant Acromyrmex versicolor: a case of female control. Behavioral Ecology and Sociobiology volume 38, pages 219–225.
