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Effects of sugar concentration for nectar selection by birds: coevolutionary effects of a physiological mechanism.
Schondube, Jorge*,1, Martinez del Rio, Carlos2, 1 University of Arizona, Tucson, Arizona2 University of Wyoming, Laramie, Wyoming
ABSTRACT- Sugar composition of nectars secreted by bird-pollinated plants reveals a perplexing pattern: flowers pollinated by passerines secrete nectars dominated by glucose and fructose, whereas the nectar secreted by hummingbird pollinated flowers are richer in sucrose. In sugar selection trials hummingbirds show a preference for sucrose, while passerines have shown different levels of aversion for this sugar. Diglossa baritula, a small nectarivorous tanager, does not appear to display preferences for sucrose over a glucose-fructose mixture. When we offered them a choice between nectar containing the same caloric concentration (20% weight/total volume) of sucrose and a 1:1 glucose and fructose mixture, they showed no preferences (preference index = 0.49 SE = 0.08, t = 0.303, p >0.7). However, sugar preferences of Diglossa baritula depended on sugar concentration. At low concentrations (10%), a mixture of glucose and fructose was preferred over sucrose. As concentration increased birds switched their preferences, preferring sucrose over glucose and fructose. We found the same pattern for hummingbirds. Why would birds shift in sugar preferences with nectar concentration? We hypothesize that differences in osmotic pressure caused by the different sugar molecules may limit the consumption of the glucose-fructose mixture at high concentrations. Our observations may help to elucidate why some plants produce hexose nectars whereas others secrete sucrose ones. Our results may have revealed an unexpected link between sugar concentration, nectar composition, and pollinator type.
KEY WORDS: bird pollination, nectar selection, sugar concentration, animal-plant coevolution