Foraging decisions in nectarivores: unexpected interactions between flower constancy and energetic rewards

Foraging decisions are based on a suite of choices that include energetics and physiological constraints. Although travelling farther to harvest a greater net energetic reward is beneficial, many animals opt for a smaller net reward that requires less travel. Recent discoveries of a visual basis for flower constancy in the honeybee, Apis mellifera, led us to examine older reports that colour cues are superceded by energetic considerations. Here we show that when individual bees foraged on pedicellate artificial flowers varying in colour and interfloral distance, their behaviour depended on the colours in the choice test. Colours of similar spectral reflectance (blue versus white), that would be clustered in the bee's visual colour space, elicited more visits to the closest flower when rewards were equal, but individuals travelled a greater distance to harvest a higher energetic reward when reward quality varied. Bees chose the closest flower more often when reward volume decreased while quality remained constant. Yet, even when all flowers were identical (morphology and reward), and only interfloral distance varied, bees did not always visit the closest flower. A dramatic difference was seen when the dimorphism was yellow-blue, colours quite separate in the bee colour space and known to elicit constancy behaviour. Here, bees visited the closest flower only 5% of the time, and varying reward volume did not elicit different behaviour. Animals thus display differential foraging behaviour with respect to environmental cues that must be considered when asking questions about other behavioural parameters. (C) 2001 The Association for the Study of Animal Behaviour.