The Energetics of Bee Foraging Contributed by: Elizabeth Newell, Dept. of Biology, Hobart and William Smith Colleges, Geneva, NY 14456 Wiser far than human seer, Yellow-breeched philosopher! Seeing only what is fair, Sipping only what is sweet, Thou dost mock at fate and care, Leave the chaff, and take the wheat. Emerson, The Humble-Bee   Do bees make wise choices as they visit flowers? Nectar is both the individual bee's and the hive's main source of energy. Do bees forage optimally for this energy source - leaving the "chaff" and taking the "wheat"? In lab this week we'll try to find out. Natural selection is expected to favor efficient foraging patterns and this expectation forms the basis of a large body of optimal foraging theory. Optimal foraging models begin by specifying a set of possible foraging behaviors (e.g. choice of food items, patchiness of the food) and then solve for the behavior that will maximize the fitness of the individual practicing it. Because it is difficult to measure fitness directly, optimal foraging models are generally solved for the behavior that will maximize the animal's net rate of energy intake. The net rate of energy intake is defined as the energy gained while foraging minus the energy spent on foraging, divided by time spent foraging. The logic is that an animal that maximizes its net rate of energy intake can spend the least time foraging (which is a risky business if there are predators around) and can gain the maximum surplus of energy to devote to survival and reproduction. Optimality models produce quantitative or qualitative predictions that can be tested. If the predictions are upheld, we can be more confident in the assumptions of the model. What behaviors should we expect to see if bees do forage optimally for nectar? Nectar is basically a sugar solution but the concentration of sugars and the total volume of nectar produced per flower may vary among species. Given this information, we might predict that bees should prefer flowers producing the most nectar with the highest concentration of sugar. However, flowers differ in shape and in the accessibility of their nectaries. Maybe bees should visit the flowers from which they can remove nectar most quickly. In this lab we will begin the way most scientists do (though not usually the way labs begin), by observing bee behavior. Based on our observations we'll come up with questions about their behavior, develop hypotheses, predictions, and experiments to test our predictions. Then we'll carry out the experiments and analyze the results to see if our predictions are correct. To get you thinking about bee behavior in terms of optimal foraging, I want to mention one behavior that has been noted by other bee observers. The observation is that bees visiting the vertical inflorescences of foxglove (Digitalis) begin foraging at the bottom-most flower and work their way up the inflorescence. They usually leave the inflorescence before reaching the top flower. Is this optimal behavior? To answer that question, researchers investigated the pattern of nectar availability in the flowers. In foxglove, a new flower opens each day at the top of the inflorescence as a flower at the bottom withers. The lower flowers contain more nectar than the upper flowers. If bees are foraging optimally, they should begin to forage at the bottom where there's more nectar per flower, then move upwards, without revisiting just-emptied flowers, and leave when the "profits" become too low. Bees do seem to behave in this manner on inflorescences that haven't already been visited by bees. When visiting "empty" inflorescences (those already visited by bees), most bees leave after visiting only one flower. These results indicate that foraging patterns change in response to nectar availability in a way that improves foraging efficiency. This serves as just one example of how observed behavior can be examined in terms of optimal foraging theory.   Background on bees Most of the bees we'll see will be honey bees. A honey bee colony consists of a queen, many drones (males) and thousands of workers (non-reproductive females). The worker bees build honeycomb, rear the young, clean the colony, feed the queen and drones, guard the hive, and collect food. The sole function of the drones is to mate with the queen and during the fall they are ejected from the colony. The queen's job is to lay eggs and to start new colonies. Nectar is the bees' energy source. Bees can carry more than their weight in nectar in the extendible "honeycrop" located in their abdominal cavity. Despite their large nectar-holding capacity, it may take as many as 2000 foraging trips with visits to close to 100 flowers each trip to produce an ounce of honey. Workers turn nectar into honey by adding an enzyme that converts the sugars to a chemical form that bacteria and many molds can't metabolize. Bees also collect pollen and it is their main protein source, used primarily to feed the queen and the colony's young. During the winter, colonies go into semi-dormancy. The bees cluster together and generate heat by shivering their wing muscles. The energy for this activity comes from honey stored during the previous season. A typical colony will burn about 25 kg of honey to keep warm during a winter in our area. The energy budget of individual foraging honey bees can be calculated by determining the proportion of time spent in various activities and then measuring the energetic cost of each activity. Consider flight in foraging bees. Bees cannot fly unless their thorax temperature is above 27"C. On cool days, bees must use energy to "shiver" in order to increase their temperature before they can fly. Thus, the energetic cost of flight is greater on cool days than on warm days. As you observe foraging bees, consider how the ambient temperature may influence the optimality of different foraging behaviors. Flowers We are likely to see the following kinds of flowers this week: Goldenrods (Solidago spp.) Touch-me-not (Impatiens capensis) Thistles (Cirsium spp.) Purple loosestrife (Lythrum salicaria) Queen Anne's Lace (Daucus carota) Joe-pye weed (Eupatorium sp.) To summarize: Our goal this week is to carry out experiments to test predictions about bee behavior that relate to the optimality of their nectar foraging. You will be responsible for recording all observations, questions, hypotheses, predictions, and data collected. You will need to describe the methods you used in your experiment(s) and to discuss your results in light of your original predictions and hypotheses.   References for this lab: Gould, James L. and Carol Grant Gould. 1988. The Honey Bee. Scientific American Library, distributed by W. H. Freeman, New York, NY. Heinrich, Bernd. 1979. Bumblebee Economics. Harvard University Press, Cambridge, MA. Kearns, Carol Ann and David William Inouye. 1993. Techniques for Pollination Biologists. University Press of Colorado, Niwot, CO. Seeley, Thomas D. 1985. Honeybee Ecology. A study of adaptation in social life. Princeton University Press, Princeton, NJ. Thomson, James D. Unpublished laboratory handouts. Waser, Nicholas M. Unpublished laboratory handouts.