Rove beetles have come up with a truly novel way to ensure monogamy. After a male has mated with a female, he injects her with a chemical that makes her smell so bad that all other prospective suitors stay away.
We recently reported on one of the few actual aphrodisiacs ever discovered in nature. Well, the rove beetles use an anti-aphrodisiac, which makes other males lose interest in trying to mate with the female. For the original male, that's a huge reproductive benefit, as rove beetles are one of many insect species in which the female can theoretically be impregnated by multiple males simultaneously.
This stinky chemical prevents that, and it doesn't just benefit the original male. University of Frieburg researchers Jerry Schlechter-Helas, Thomas Schmitt, and Klaus Peschke explain in their abstract:
By reducing the attractiveness of their mating partner via an anti-aphrodisiac pheromone, males can prevent a remating of the female and thus reduce the risk of sperm competition. For females, the main benefit from allowing the chemical manipulation of their attractiveness is probably the avoidance of sexual harassments from rival males. While mating plugs generally constitute a physical barrier which hinders male mating attempts, chemical manipulations must trustfully inform the responding male of the female's reluctance to mate; otherwise, it would be beneficial to ignore the repellent information.
The researchers were able to isolate the specific chemical used by the males. When they artificially placed female test subjects in contact with the chemicals, males lost interest in mating with them, even though they weren't pregnant. The researchers describe how this mechanism works:
Coincident with the deposition of a spermatophore into the female genital chamber, an anti-aphrodisiac pheromone was transferred and readily spread onto the female surface, where it was subsequently perceived by rival males via parameres, the claspers of the male genitalia. Males aborted contact with the mated female to avoid further time- and energy-consuming elements of the mating sequence.