Flower display and temperature in royal irises (Iris section Oncocyclus, Iridaceae)

Large flowers are typically more conspicuous to pollinators and are associated with more nutritional rewards than small flowers. Flower size can also determine flower temperature, which can be a reward for flower-visiting insects. Nectarless Royal irises offer overnight shelter and morning warmth to male Eucera bee pollinators. A dark spot on their lower tepals (black patch) may act as a visual cue and contribute to flower heating. Here, we examined the association of floral display (i.e. flower size and black patch size) with flower temperature and female fitness (i.e. seed set) in Royal irises populations across an aridity gradient. First, we tested pollinator preference using artificial flowers of varying sizes. Next, we assessed associations between flower and black patch size, flower warming rate and female fitness. Finally, we manipulated flowers in the field to determine if the black patch influences heating. Pollinators preferred larger artificial flowers for overnight shelter. However, selection for larger flowers was found only in the population with the smallest flowers. No association was found between flower/black patch size and floral heating in natural populations, and the black patch did not affect flower heating. Flowers reached the temperature threshold for bees to start flying (18 °C) 10–35 min faster than ambient air. We conclude that the large flower size in the Royal irises serves as a visual signal, advertising for night shelter and flowers—independently of the size—heat up faster in the morning than ambient air. Flowers thus, potentially offer a ‘head start’ to the flower-dwelling bees, allowing them to warm up more quickly than in outside conditions.