Guest Blog: Yuval Sapir and Iris SROs

Dr. Yuval Sapir is Director of the Tel Aviv University Botanical Gardens

Royal Irises are among the most prominent flowers to bloom in the Israeli Spring. This group of species includes the largest flowers in the Middle East, with some as large as Mike Tyson's boxing gloves. Not only large, these flowers are oddly dark-colored. For example, the Gilboa Iris (Iris haynei [picture]) is purple, and other species can be as dark as black. How did this conspicuous dark color evolve?

These huge flowers double as SROs (single room occupancy units) for solitary male bees that sleep overnight in the flowers. These poor males are not allowed in the safe borrows of the females, who nest in the ground. Instead, they are banished for the night to look for a safe refuge from their predators. Iris flowers provide the perfect hospice. Following sunrise, rays from the morning sun heat the dark-colored flowers.  The inside of the flower quickly heats up, and with it, the resident sleeping bee. The warmed-up bee can then start its daily activities earlier than colder bees which slept outside. On its way out of the Iris, the bee picks up some pollen, which it carries around with him in its search for food (nectar) and sex (female bees).

The following evening, tired from his day and still carrying the Iris pollen, the male remembers to go back to his safe and warm Iris-shelter. Upon entering the flower, the pollen-carrying male brushes against the flower's stigma, pollinating the flower.

So here we have the answer to the evolution of the dark-colored irises - the dark color was selected for by the pollinators who preferred a night shelter that promise them a warm place to wake-up in.