Optimal concentrations in nectar feeding

Abstract

Nectar drinkers must feed quickly and efficiently due to the threat of predation. While the sweetest nectar offers the greatest energetic rewards, the sharp increase of viscosity with sugar concentration makes it the most difficult to transport. We here demonstrate that the sugar concentration that optimizes energy transport depends exclusively on the drinking technique employed. We identify three nectar drinking techniques: active suction, capillary suction, and viscous dipping. For each, we deduce the dependence of the volume intake rate on the nectar viscosity and thus infer an optimal sugar concentration consistent with laboratory measurements. Our results provide the first rationale for why suction feeders typically pollinate flowers with lower sugar concentration nectar than their counterparts that use viscous dipping.biomechanics | biocapillarity | optimal concentration M any insects and birds feed primarily or opportunistically on floral nectar. There are three principal techniques employed by nectar feeders: active suction, capillary suction, and viscous dipping. 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). 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). Most bees (except orchid bees) and some ants ingest nectar by dipping their tongue into, then extracting it from, the viscous nectar (4, 5). It is advantageous for creatures to ingest energy rapidly due to the threat of predation during feeding. Optimal conditions might thus be sought to maximize their energy intake rate. 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. 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. Our viscous dipping model indicates an optimal sugar concentration of 52%, which is higher than that for suction feeding, 33%. 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).The sugar concentration that maximizes energy intake rate has been evaluated for a variety of nectar feeders in previous experimental studies (Fig. 1). 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. Roughly speaking, the optimal concentration for active or capillary suction feeders is 30-40% while that for creatures using viscous dipping is 50-60%. Optimal sugar concentrations for s...

Optimal concentrations in transport systems

“…The manner in which biological constraints determined the dependence of the pressure Dp on nectar viscosity has been treated elsewhere rsif.royalsocietypublishing.org J R Soc Interface 10: 20130138 [8,9]. Active suction feeders are typically constrained by constant work rate W ¼ QDp ¼ pa 4 /(8hl)Dp 2 , so the pressure Dp ¼ (8Wl/(pa 4 )) 1/2 h 1/2 depends on viscosity and, hence, concentration.…”

“…Two different suction mechanisms are typically used by nectar feeders: active suction and capillary suction [7][8][9]11]. Active suction feeders such as butterflies use muscle contraction to suck nectar through their roughly cylindrical proboscises.…”

“…We can thus express the constraint as f ¼ (8Wl/(pa 4 )) 1/2 m 1/2 (table 1). Capillary suction feeders such as hummingbirds use surface tension to draw nectar along their tongue, during repeated cycles of tongue insertion and retraction [7,9]. If the duration of a cyclic motion is the sum of the nectar loading time T and unloading time T 0 , the average nectar mass flow rate J s can be expressed as J s ¼ rcpa 2 lðTÞ=ðT þ T 0 Þ; where pa 2 l(T ) is the time-dependent nectar volume extracted during the loading.…”

“…Additionally, most transport systems are subject to a set of limiting constraints. For example, nectar feeders are typically constrained by a constant work rate, which in turn is a function of flow impedance and hence concentration [7][8][9]. These transport systems may thus be characterized in terms of an optimization problem subject to appropriate constraints.…”

“…These transport systems may thus be characterized in terms of an optimization problem subject to appropriate constraints. This approach has been used to rationalize observed concentrations in a wide range of natural systems [4,[8][9][10]. For example, the observed volume fraction of erythrocytes (red blood cells), typically approximately 40-50% in humans, has been shown to maximize oxygen transport [4,10].…”