PATTERNS OF EVOLUTION OF WARM-UP RATES AND BODY TEMPERATURES IN-FLIGHT IN SOLITARY BEES OF THE GENUS ANTHOPHORA

1. Energetic considerations of preflight warm-up in bees predict that bees should warm-up as fast as they can and that larger species should be able to warm-up more rapidly. Larger species should also be able to maintain higher thoracic temperatures during flight. These predictions are examined for 19 species in the solitary bee genus Anthopora (Hymenoptera; Apoidea; Anthophoridae).

2. The genus Anthophora has uniformly high warm-up rates and body temperatures in flight, and the highest warm-up rates measured in any heterotherm.

3. Under standard conditions, both warm-up rates and thoracic temperatures during flight increase with body mass in Anthophora, in keeping with the prediction that heterotherms should minimize the energetic cost of warm-up.

4. Having controlled for body mass effects, Anthophora species able to maintain flight activity at lower ambient temperatures have higher warm-up rates and body temperatures in flight, showing that even in the smallest endotherms there may still be room for selective modification within the limits imposed by body size.

5. Differences in warm-up rates between subgenera within Anthophora suggest that endothermic abilities have diverged in response to differences in the thermal environments which are the centres of diversity for the subgenera.

6. Similar importance of both body mass and the thermal environment in which the species is active is demonstrated over 40 bee species in six families. Because species over a wide taxonomic range do not constitute independent data points, phylogenetic effects in these analyses are controlled for using Grafen's phylogenetic regression.

7. The evolution of high levels of endothermy in Anthophora is discussed with reference to patterns of nectar secretion found in the arid environments which are current centres of diversity for the genus.