Water loss, not overheating, limits the activity period of an endothermic Sonoran Desert bee

Desert animals must manage the physiological stresses of heat and
desiccation; evaporative heat loss mitigates overheating but exacerbates
water stress. Small endothermic flying insects may be particularly
vulnerable to overheating and water stress as a result of high surface
area to volume ratios, but we lack quantitative understanding of the
relative magnitude of these abiotic stressors in flying desert
invertebrates, despite their ecological importance. During the hottest and
driest weeks of the year, many thousands of males of the Sonoran Desert
digger bee (Centris caesalpiniae) flew near-continuously at elevated
thorax temperatures for hours at mating aggregation sites, while
occasionally fighting other males and digging for females. To determine
whether incapacitating high temperatures or water loss limited the
activity period of male C. caesalpiniae, we assessed wet and dry body mass
and water content through the activity period, crop volume and sugar
content, microclimate selection, water loss rate and metabolic water
production during flight, critical water content, and maximum critical
temperature. Body masses and sizes of males declined through the morning
and smaller bees had higher fractional water contents. Crop volume and
sugar content did not vary through the day or with bee size. Maximum
critical temperature during flight was 51°C, similar to those measured for
other bees, and well above temperatures reached in the field, suggesting
that avoidance of over-heating does not limit activity in this desert bee.
The critical water content of Centris bees averaged 50%. Measures of net
water loss rate indicated that males approached lethal water loss limits
within four hours, suggesting that desiccation tolerance limits activity.
Remarkably, male C. caesalpiniae were not observed to forage at floral or
water sources during the activity period, and foraged over multiple days,
suggesting selection to maintain reproductive success and that these males
have a mechanism to rehydrate when not at the mating aggregation.