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 a nd pollen almost exclusively from Alkanna orientalis (Boraginaceae). The be e and plant are found together in the early spring, living in the bottom of steep-sided 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 ne sts. This study analyses the following in order to identify the factors str ucturing this daily pattern: thermal aspects of the bee and its environment , temporal patterns of resource provision by the plant, and female nectar a nd pollen foraging behaviour. 3. Although A. pauperata can generate substantial heat endothermically, mor ning and evening ambient temperatures well below 10 degrees C defined a the rmal window within which foraging occurred. Maximum air temperatures were m oderate (25-30 degrees C), and examination of the physiology and behaviour of A. pauperata suggests that the midday reduction in flight activity was n ot due to thermal constraints. 4. Alkanna orientalis produces protandrous hermaphroditic flowers. Female A . pauperata collected pollen from male-phase flowers and harvested nectar p referentially from female-phase flowers. Although the nectar standing crop was relatively constant throughout the day, pollen availability peaked stro ngly in the early afternoon. 5. Female A. pauperata visited young male-phase flowers as soon as they ope ned, generating an early afternoon peak in pollen foraging activity and dep leting the pollen standing crop rapidly. A morning peak in pollen foraging occurred when females gleaned remnant pollen from flowers that had opened t he previous day. Pollen availability in the morning was far lower than in t he early afternoon, and the time taken to collect a full pollen load in the morning was significantly longer. Collection of pollen in the morning desp ite very low resource availability suggests that pollen may be a limiting r esource for A. pauperata. 6. In contrast to many existing examples of bimodal activity patterns in hi ghly 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.