Abstract
In honeybees fast and efficient exploitation of nectar and pollen sources is achieved by persistent endothermy throughout the foraging cycle, which means extremely high energy costs. The need for food promotes maximisation of the intake rate, and the high costs call for energetic optimisation. Experiments on how honeybees resolve this conflict have to consider that foraging takes place in a variable environment concerning microclimate and food quality and availability. Here we report, in simultaneous measurements of energy costs, gains, and intake rate and efficiency, how honeybee foragers manage this challenge in their highly variable environment. If possible, during unlimited sucrose flow, they follow an ‘investment-guided’ (‘time is honey’) economic strategy promising increased returns. They maximise net intake rate by investing both own heat production and solar heat to increase body temperature to a level which guarantees a high suction velocity. They switch to an ‘economizing’ (‘save the honey’) optimisation of energetic efficiency if the intake rate is restricted by the food source when an increased body temperature would not guarantee a high intake rate. With this flexible and graded change between economic strategies honeybees can do both maximise colony intake rate and optimise foraging efficiency in reaction to environmental variation.
Highlights
(D) Energy costs per stay. (A–D) 22 individuals of Apis mellifera carnica foraging 0.5 M sucrose provided in unlimited flow or at a rate of 15 μl/min, in shade or in sunshine, 504 visits, for legend see (D)
Symbols represent means with SD of individual stays shown in Supplementary Fig. S3; for radiation values see Supplementary Fig. S2, and for regression functions and statistics see Supplementary Table S1
Microclimatic conditions, to approach an empirical decision of how honeybee foragers master the challenge of balancing food intake rate and gains with energetic costs in reaction to their highly variable environment
Summary
It was a surprising finding that during unlimited sucrose flow (0.5 mol/l) bees foraging in shade kept the own heat production rather high and constant (~58–62 mW on average) up to an ambient temperature (Ta) of ~29 °C (Fig. 2A) despite a decreasing difference of the body surface temperature to Ta (see Supplementary Fig. S1). In sunshine, during unlimited sucrose flow the foragers even increased the own heat production at low Ta (
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