Windows of opportunity and the temporal structuring of foraging activity in a desert solitary bee

1. Females of the desert solitary bee Anthophora pauperata collect nectar and pollen almost exclusively from Alkanna orientalis (Boraginaceae). The bee and plant are found together in the early spring, living in the bottom of steepsided wadis (dry river valleys) at an altitude of 1500 m in Egyptian Sinai. 2. Female A. pauperata showed clear morning and afternoon peaks in foraging activity, separated by a 2-3 h midday period spent in their underground nests. This study analyses the following in order to identify the factors structuring this daily pattern: thermal aspects of the bee and its environment, temporal patterns of resource provision by the plant, and female nectar and pollen foraging behaviour. 3. Although A. pauperata can generate substantial heat endothermically, morning and evening ambient temperatures well below 10°C defined a thermal window within which foraging occurred. Maximum air temperatures were moderate (25-30°C), and examination of the physiology and behaviour of A. pauperata suggests that the midday reduction in flight activity was not due to thermal constraints. 4. Alkanna orientalis produces protandrous hermaphroditic flowers. Female A. pauperata collected pollen from male-phase flowers and harvested nectar preferentially from female-phase flowers. Although the nectar standing crop was relatively constant throughout the day, pollen availability peaked strongly in the early afternoon. 5. Female A. pauperata visited young male-phase flowers as soon as they opened, generating an early afternoon peak in pollen foraging activity and depleting the pollen standing crop rapidly. A morning peak in pollen foraging occurred when females gleaned remnant pollen from flowers that had opened the previous day. Pollen availability in the morning was far lower than in the early afternoon, and the time taken to collect a full pollen load in the morning was significantly longer. Collection of pollen in the morning despite very low resource availability suggests that pollen may be a limiting resource for A. pauperata. 6. In contrast to many existing examples of bimodal activity patterns in highly endothermic bees, the bimodal activity patterns of female A. pauperata appear to be driven not by thermal considerations but by daily patterns of pollen release from its principal food source.