Pollinator behaviour and resource limitation maintain honest floral signalling

Abstract

In many communication systems, signal receivers profit from honest signals that indicate the signaller's quality, whereas low-quality signallers should profit from cheating. Under such a conflict of interests between signallers and signal receivers, the maintenance of honest signals presents a puzzle. In theory, honesty can represent an evolutionarily stable strategy, but the actual mechanisms have been studied in few systems only. Here, we investigate honest signalling in a plant species, Brassica rapa, that advertises nectar volumes to pollinators by two honest floral signals; corolla size and the floral volatile phenylacetaldehyde. In a series of experiments we tested for physiological constraints and pollinator behaviours related to honest floral signals and nectar volume, incorporated the result into a mathematical model, and verified its predictions experimentally. While honest floral signals attracted pollinators, the bees’ flower visitation time depended on nectar volume and was associated with the number of seeds that flowers developed. Furthermore, honest floral signals and seed set without pollen limitation both increased after soil fertilisation, indicating nutrient limitation in these traits. The mathematical model which incorporates these findings showed that honest signalling in B. rapa can be maintained by a combination of pollinator behaviour and resource limitation causing differential benefits of nectar production. The study demonstrates how honest floral signalling can evolve as a stable strategy in a plant species. A free Plain Language Summary can be found within the Supporting Information of this article.