![]() ![]() A cornerstone of biomechanics is that the force that a creature of characteristic size l can generate ( 10) F ∼ l 2 thus, one expects the suction pressure generated by muscles, Δ P ∼ F/ l 2 ∼ l 0, to be independent of scale and to be of comparable magnitude for all creatures. For active suction, Δ P is mainly generated by cibarial muscles ( 1, 2), while for capillary suction, Δ P ∼ σ/ a results from curvature pressure, where σ is the surface tension ( 3). Where g is the gravitational acceleration and Δ P the pressure difference applied at the height h of the nectar. ![]() The result is a global physical picture that describes all nectar feeders, and indicates that the optimal concentration depends exclusively on drinking style. We proceed by briefly reviewing the active suction model developed by Pivnick and McNeil ( 2) and the capillary suction model of Kingsolver and Daniel ( 3), then developing a dynamic model for viscous dipping. However, for larger uptake volumes ( Fig. 1), this suction model is no longer expected to be valid and fails to rationalize the higher optimal concentrations of 50–60% reported, for example, for bees ( 8, 9). Owing to its reliance on capillary suction, their model implies an optimal concentration of 30–40%, identical to that for suction feeders. Kingsolver and Daniel ( 4) also suggested a dynamic model for bees that relies on capillary imbibition into the hairs of the tongue, a physical picture expected to be valid only in the limit of small nectar uptake rates. further demonstrated how the optimal concentration of 35% emerges for active suction when muscular mechanics is considered ( 7). Pivnick and McNeil ( 2) advanced the active suction model by introducing the assumption of constant power output for the suction pump, and so predicted an optimal concentration of approximately 35%, consistent with that observed. ![]() Optimal sugar concentrations for suction feeders have been previously rationalized by Kingsolver and Daniel, who established dynamic models for both active ( 1) and capillary ( 3) suction. Roughly speaking, the optimal concentration for active or capillary suction feeders is 30–40% while that for creatures using viscous dipping is 50–60%. Careful consideration of all of these results indicates that this so-called “optimal concentration” depends exclusively on feeding mechanism but not on body size, quantity of intake, or species. The sugar concentration that maximizes energy intake rate has been evaluated for a variety of nectar feeders in previous experimental studies ( Fig. 1). This result suggests a rationale for the fact that the nectar concentration of flowers pollinated by viscous dippers such as bees (35%) is typically higher than that of those pollinated by suction feeders such as hummingbirds or butterflies (20–25%) ( 6). Our viscous dipping model indicates an optimal sugar concentration of 52%, which is higher than that for suction feeding, 33%. We here rationalize the different optimal concentrations reported for the different drinking strategies by developing a dynamic model for viscous dipping and comparing it to existing models of suction feeding. While the sweetest nectar offers the greatest energetic rewards, the exponential increase of viscosity with sugar concentration ( 2) also makes it the most difficult to transport. Optimal conditions might thus be sought to maximize their energy intake rate. It is advantageous for creatures to ingest energy rapidly due to the threat of predation during feeding. Most bees (except orchid bees) and some ants ingest nectar by dipping their tongue into, then extracting it from, the viscous nectar ( 4, 5). Nectar-feeding birds (e.g., hummingbirds * and sunbirds) employ capillary suction, in which capillary pressure drives flow along the tongue once its tip touches the nectar ( 3). Lepidopterans (e.g., butterflies and moths) employ the former, sucking nectar through their probosci, along which a pressure gradient is generated by cibarial muscles ( 1, 2). There are three principal techniques employed by nectar feeders: active suction, capillary suction, and viscous dipping. Many insects and birds feed primarily or opportunistically on floral nectar. ![]()
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