Even the smallest bees warm up endothermically before flight, although at relatively low rates (Stone and Willmer 1989b). Andrenid bees weighing only 29 mg, Andrena bicolor, bask inside flowers and fly at Tth of 22-31oC, with an average Tth during free flight of 27oC (Herrera 1995a). These values are about 10oC lower than those reported for many species of Apidae and Anthophoridae, so A. bicolor appears to be ectothermic; but many small solitary bees use their endothermic warm-up abilities rather infrequently (Willmer 1991b).
The phylogenetically based comparative study of Stone and Willmer (1989b) has already been discussed in the context of preflight warm-up in bees. The best endothermic abilities are seen in larger species and those active in cool climates. Bees from cooler environments fly at lower Ta and have higher flight temperatures and warm-up rates. For warm climate bees, tolerance of high Tth and Ta is more important, which means that warm-up at low Ta and activity at high Ta are not compatible. The superior thermoregulation of bumblebees compared to solitary bees is largely, but not entirely, explained by size differences (Bishop and Armbruster 1999). Bombus has a long history of specialization to cold climates and the richness of this group is greatest in the high latitudes of the Northern Hemisphere (Williams 1994).
Anthophora plumipes is active early in the European spring and can fly at low Ta by tolerating low Tth (Stone and Willmer 1989b). Males are smaller than females and both sexes vary considerably in size. Male-female differences in endothermic physiology, and the relationship to foraging and reproductive behaviour, have been considered in detail by Stone and colleagues (Stone 1994a; Stone et al. 1995). Provisioning activity in female A. plumipes involves long working days, and the mass of the nectar and pollen loads collected increases with Ta. At low Ta only the larger females are able to collect pollen, and thus complete provisioning of cells. They are also able to forage earlier in the morning, and these factors influence the body size of their offspring. Reproductive success in male A. plumipes is also dependent on weather and body size: larger males can forage for longer and have competitive advantages in conflicts with other males. There is no difference in endothermic abilities between males and females of the same mass, but the sexes show marked differences in thermal biology.
In A. plumipes the slope of the relationship between voluntary flight temperature (VFT—when a bee initiated tethered flight) and Ta is lower than that between stable flight temperature (SFT— measured after a period of continuous flight) and Ta, which indicates that there was significant cooling of the thorax during tethered flight at low Ta (Fig. 6.15) (Stone 1993). Decreases in Tth after the initiation of flight have been reported in many insects. Thermoregulation is thus more effective before than during flight, and comparison of Tth/Ta gradients before and during flight, where available, shows that this is true of bees, moths, and beetles (Stone 1993,1994b). Apparently endothermic insects are unable to compensate fully for convective losses at low Ta or MHP at high Ta. Voluntary flight temperature and SFTare also significantly correlated
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