Abstract
Heterothermic insects like honeybees, foraging in a variable environment, face the challenge of keeping their body temperature high to enable immediate flight and to promote fast exploitation of resources. Because of their small size they have to cope with an enormous heat loss and, therefore, high costs of thermoregulation. This calls for energetic optimisation which may be achieved by different strategies. An ‘economizing’ strategy would be to reduce energetic investment whenever possible, for example by using external heat from the sun for thermoregulation. An ‘investment-guided’ strategy, by contrast, would be to invest additional heat production or external heat gain to optimize physiological parameters like body temperature which promise increased energetic returns. Here we show how honeybees balance these strategies in response to changes of their local microclimate. In a novel approach of simultaneous measurement of respiration and body temperature foragers displayed a flexible strategy of thermoregulatory and energetic management. While foraging in shade on an artificial flower they did not save energy with increasing ambient temperature as expected but acted according to an ‘investment-guided’ strategy, keeping the energy turnover at a high level (∼56–69 mW). This increased thorax temperature and speeded up foraging as ambient temperature increased. Solar heat was invested to increase thorax temperature at low ambient temperature (‘investment-guided’ strategy) but to save energy at high temperature (‘economizing’ strategy), leading to energy savings per stay of ∼18–76% in sunshine. This flexible economic strategy minimized costs of foraging, and optimized energetic efficiency in response to broad variation of environmental conditions.
Highlights
Honeybees are heterothermic insects which change from the ectothermic to the endothermic state for foraging
Inserts: if not given, axes labelings as in main graphs; insert in (C): Ta, the sucrose solution (Ts), the abdomen (Tab), the head (Thd), the thorax (Tth) refer to x-axis temperature to be used, respectively. doi:10.1371/journal.pone.0105432.g002
Under our experimental conditions with unlimited flow of 1.5 M sucrose solution, a very high-quality resource, the foraging bees displayed a high energy turnover under most environmental conditions, in shade amounting to about 55–70 mW on average (Figure 2A), which is higher than the 57–60 mW reported by Stabentheiner et al [6]
Summary
Honeybees are heterothermic insects which change from the ectothermic to the endothermic state for foraging. More forward directed; ‘investment-guided’ strategy would be to invest heat production and external heat gain to optimize physiological parameters like body temperature which might speed up foraging Though this would mean an instantaneous increase of costs it might optimize foraging indirectly in the longer term by speeding up feeding, pollen gathering and flight, which in turn would decrease costs of a foraging trip. To answer these questions we here present a novel approach of simultaneous measurement of body temperature and energy turnover (from CO2 production) of bees foraging sucrose solution from an artificial flower under outdoor conditions, in a broad range of environmental temperature and radiation variation
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