SUGAR PREFERENCES IN HUMMINGBIRDS - THE INFLUENCE OF SUBTLE CHEMICAL DIFFERENCES ON FOOD CHOICE

The nectar secreted by hummingbird-pollinated flowers is rich in sucrose, whereas nectar secreted by passerine-pollinated plants contains a mixture of glucose and fructose. To test the hypothesis that sugar preferences and nectar composition are correlated, I examined the sugar preferences of three species of Mexican hummingbirds (Amazilia rutila, Cynanthus latirostris and Chlorostilbon canivetii). As predicted, the three species preferred sucrose over glucose, fructose, and a mixture of glucose and furctose (hexose mixture) in paired preference tests. Preferences for simple sugars were ranked as: sucrose > hexose mixture > glucose > fructose. The preference of hummingbirds for sucrose was not reversed by feeding hummingbirds a hexose mixture as a sole diet for 20 days. The preference of hummingbirds for different sugars are puzzling because sucrose, glucose, and fructose have approximately the same energetic content. I hyposthesized that sugar preferences were correlated with differences in the efficiency with which hummingbirds assimilated different sugars and/or in the time they required to process these sugars in the digestive system. Sucrose, glucose, and fructose, however, were assimilated by hummingbirds with equally high efficiency (> 97%). Glucose solutions were processed by hummingbirds at a slower rate than hexose mixtures and sucrose solutions, and hence, were less profitable. Sucrose and hexose mixtures were processed at the same rate and therefore had the same profitability. Therefore, the preference of hummingbirds for sucrose over hexose mixtures cannot be explained by differences in assimilation efficiency or digestive handling time. Sucrose must by hydrolyzed into its monosaccharide components, glucose and fructose, before it can be absorbed in the intestine and used as an energy source. Relative to other birds, hummingbirds exhibit highly specialized digestive traits, such as very high rates of intestinal sucrose hydrolysis and glucose transport, which allow them to use sucrose as efficiently as mixtures of glucose and fructose. Many passerine species, in contrast, use the more easily absorbed monosaccharides, glucose and fructose, more efficiently than sucrose. The distribution of nectar sugars among bird-pollinated plants seems to be the result of the evolutionary response of plants to two sets of pollinators with different degrees of digestive specialization.