Temperature and Optimal Nectar Foraging

Environmental temperature and nectar sugar concentration interact to influence both the energetic costs or foraging and the rate of energy intake during feeding. Nectar viscosity increases at colder temperatures, and the dependence of viscosity on temperature increases with increasing sugar concentration [26]. Consequently, one general prediction is that nectar intake rates should decline at cooler temperatures, a prediction that has been confirmed in experiments with both butterflies [126] and euglossine bees (B.J. Borrell, unpublished). Thus, foraging insects would do well to forage in sunny patches [26,111,127] or at inflorescences with endogenous heat sources [128]. The relevant behavioral experiment would involve independently controlling nectar temperature and air temperature to partition thermoregulatory costs from feeding costs.

Some researchers have argued that nectars are less concentrated at high elevations owing to temperature effects on viscosity [89,103]. However, Heyneman [26]

showed that optimal concentrations should shift no more than 1 to 2% for a 10°C decrease in air temperature. Indeed, for the butterfly Thymelicus lineola, optimal nectar sugar concentrations lie at approximately 40% sucrose at both 25 and 35°C. At the cooler temperature, however, energy intake rate exhibited a less well-defined peak, remaining equally rewarding between 25 and 45% sucrose [126].

For endotherms such as hummingbirds, hawkmoths, or large bees, temperature can also have a direct influence on the energetic cost of foraging. The energetic cost of preflight warm-up and shivering during flower visits is substantially higher at colder temperatures [69]. As noted above, endothermic flowers have the potential to offset these costs by providing pollinators a heat reward [128]. Neotropical euglossine bees are known to regulate heat production during flight: A 10°C decline in air temperature results in a 30% increase in metabolic power requirements [129]. Consequently, an increase in transport costs at lower temperatures may have a greater effect on optimal nectar sugar concentrations than changes in nectar physical properties [26]. Contrary to this hypothesis, Borrell [91] found that euglossine bees harvest nectars of the same concentration in both dry and wet forests in both the lowlands and highlands of Costa Rica. For hummingbirds, Tamm [130] demonstrated a preference for more concentrated nectars as transport costs increased, and it would be interesting to see if the same relation holds true for temperature-mediated changes in flight costs. One final note is that the metabolic cost of warming nectar on a cold day cannot be ignored in examining thermal effects on foraging choice [131].

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