Female pheromonal chorusing in an arctiid moth, Utetheisa ornatrix

We report an unusual case of communal sexual display in the arctiid moth Utetheisa ornatrix that we designate female pheromonal chorusing. As in most moths, female U. ornatrix release a long-distance sexual advertisement pheromone during a nightly activity period. We arranged U. ornatrix females in 2 types of signaling conditions: grouped and solitary. When the females were grouped with neighboring signaling females (grouped), they initiated pheromone release sooner, continued release with less interruption and over a longer total period, and performed the release with faster abdominal pumping than observed in isolated females (solitary). This differs from the usual form of sexual communication in moths: female (chemical) signalers, male receivers, and a general lack of interaction among females. At mating, male U. ornatrix transfer a large spermatophore that may enhance female reproductive success and which represents either mating effort or paternal investment. This action results in an extended postmating male refractory period leading to a female-biased operational sex ratio. We argue that this biased sex ratio generates intrasexual competition among females, to which they respond by elevating signaling effort such that the likelihood of at least matching their neighbors' signals is increased. In the field, U. ornatrix are clustered around patches of host plants, and we also explore the possibility that pheromonal chorusing is driven by cooperation among groups of related-or nonrelated-females.