Oecologia. 2006 Feb;147(1):53-9. Epub 2005 Oct 5.

Morning floral heat as a reward to the pollinators of the Oncocyclus irises.
Sapir Y, Shmida A, Ne'eman G.


Relationships between flowering plants and their pollinators are usually affected by the amount of reward, mainly pollen or nectar, offered to pollinators by flowers, with these amounts usually positively correlated with floral display. The large Oncocyclus iris flowers, despite being the largest flowers in the East Mediterranean flora, are nectarless and have hidden pollen. No pollinators visit the flowers during daytime, and these flowers are pollinated only by night-sheltering solitary male bees. These iris flowers are partially or fully dark-colored, suggesting that they gather heat by absorbing solar radiation. Here we test the hypothesis that the dark-colored flowers of the Oncocyclus irises offer heat reward to their male solitary bee pollinators. Floral temperature was higher by 2.5 degrees C than ambient air after sunrise. Solitary male bees emerged earlier after sheltering in Oncocyclus flowers than from other experimental shelter types. Pollination tunnels facing east towards the rising sun hosted more male bees than other aspects. We suggest that floral heat reward can explain the evolution of dark floral colors in Oncocyclus irises, mediated by the pollinators' behavior.

Removal of the standards had no effect on floral temperature after sunrise relative to intact flowers (Fig. 4). Increase in floral temperature faster than the ambient air seems to be the result of the dark color of other floral parts, even in the light colored Oncocyclus irises. Given that heat is the major reward to the night-sheltering solitary male bees, we propose that the standards are not an adaptation for increasing the temperature within the flowers in the morning, and are not a target for pollinator-mediated selection. Some differences were found in the pattern of temperature increase between light- and dark-colored standards (compare a to b in Fig. 2), but the small sample size does not allow further inference. Note that such a difference was not found in comparisons of intact flowers in the botanical garden (Table 1). This could be due to the difference in temperature/wind regimes in the coastal (I. atropurpurea) and inland (I. hermona) regions in the morning (Jaffe 1988>), whereas in the botanical garden conditions were identical for each pair of flowers in any given day.