Why female ants battle for foreign sperm

Michael Herrmann and Sara Helms Cahan

Why female ants battle for foreign sperm

Once a year, after the summer monsoon rains, two species of harvester ants (Pogonomyrmex) join in a mad mating frenzy, as in the photo above. The queens aren’t making a mistake by hooking up with the wrong species, scientists report in the Proceedings of the Royal Society B. Instead, they use the sperm from the foreign males to produce sterile worker ants. And they use the sperm from their own species to produce their daughter queens—which the sterile workers then care for. Of course, there’s no evolutionary payoff for those males who father the sterile workers, so it would seem that the guys would be good at distinguishing among the queens. Yet males that hook up with the wrong queens don’t realize that they’ve made a bad choice until they begin to copulate, say the scientists who studied these pairings in a series of closely controlled experiments. The males try to rectify their error by reducing the rate at which they transfer sperm, but the queens respond by holding on. The longer copulation forces the male to continue releasing his sperm, until he’s given the same amount as he would to a gal of his own kind. Sexual bondage may seem a strange evolutionary tactic, but if the female ants weren’t “sperm parasites,” the scientists say, the harvester ant colonies would collapse.

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