Drinking made easier: honey bee tongues dip faster into warmer and/or less viscous artificial nectar

ABSTRACTOptimal concentrations for nectar drinking are limited by the steep increase in the viscosity of sugar solutions with concentration. However, nectar viscosity is inversely related to temperature, which suggests there are advantages to foraging from flowers that are warmer than the surrounding air. The honey bee (Apis mellifera L.) dips nectar using a hairy tongue. However, the microscopic dynamics of the tongue while the bee ingests nectar of varying concentration, viscosity and temperature are unknown. In this study, we found that honey bees respond to the variation of nectar properties by regulating dipping frequency. Through high-speed imaging, we discovered that the honey bee traps warmer sucrose solutions with a quicker tongue. The honey bee dips the warmest and most dilute solution (40°C and 25% w/w sucrose) 1.57 times as fast as the coldest and thickest solution (20°C and 45% w/w sucrose). When the viscosity of different sucrose concentrations was kept constant by adding the inert polysaccharide Tylose, honey bees dipped nectar at constant frequency. We propose a fluid mechanism model to elucidate potential effects on sucrose intake and show that higher dipping frequency can increase the volumetric and energetic intake rates by 125% and 15%, respectively. Our findings broaden insights into how honey bees adapt to foraging constraints from the perspective of tongue dynamics, and demonstrate that elevated intrafloral temperatures and lower nectar viscosity can improve the volumetric and energetic intake rates of pollinators.

Traveling to thermal refuges during stressful temperatures leads to foraging constraints in a central-place forager

Central-place foragers can be constrained by the distance between habitats. When an organism relies on a central place for thermal refuge, the distance to food resources can potentially constrain foraging behavior. We investigated the effect of distance between thermal refuges and forage patches of the cold-intolerant marine mammal, the Florida manatee (Trichechus manatus latirostris), on foraging duration. We tested the alternative hypotheses of time minimization and energy maximization as a response to distance between habitats. We also determined if manatees mitigate foraging constraints with increased visits to closer thermal refuges. We used hidden Markov models to assign discrete behaviors from movement parameters as a function of water temperature and assessed the influence of distance on foraging duration in water temperatures above (> 20°C) and below (≤ 20°C) the lower critical limit of the thermoneutral zone of manatees. We found that with increased distance, manatees decreased foraging duration in cold water temperature and increased foraging duration in warmer temperatures. We also found that manatees returned to closer thermal refuges more often. Our results suggest that the spatial relationship of thermal and forage habitats can impact behavioral decisions regarding foraging. Addressing foraging behavior questions while considering thermoregulatory behavior implicates the importance of understanding changing environments on animal behavior, particularly in the face of current global change.

Not just a matter of taste: palatability of bait markers is influenced by the need to search for alternative food

Context Bait palatability is a key issue influencing the uptake of toxic baits or non-toxic bait markers. Animals often reject baits with high concentrations of the active compound (whether it is a toxin, vaccine or marker) because of poor palatability, thus reducing the efficacy of baiting. Foraging theory predicts that palatability will be affected not only by the taste of active ingredients in bait but also by an animal’s ability to access alternative foods. Yet few studies of bait palatability are measured in the context of an animal’s need to search and forage for other food types. Aims The present study examined whether the palatability of Rhodamine B (RB) baits for black rats (Rattus rattus) was affected when foraging constraints were placed on access to alternative food compared with when alternative food was freely accessible. Rhodamine B is a bait marker and was used as a surrogate for other active ingredients likely to be used in pest control management. Methods Each day, RB bait at one of four concentrations was provided to an individual rat along with an alternative food that was either freely available (spatially clumped with foraging constraints absent) or hidden within a matrix of tubes (spatially scattered, thus with foraging constraints present). Key results Black rats exhibited a gradient in how palatable they found RB and preferred baits that contained the lowest concentrations of RB. Importantly, RB baits were more palatable when access to alternative food was made more difficult by applying a foraging constraint. In particular, a 0.2% RB concentration appeared to represent a threshold in palatability where intake at or above this concentration was significantly affected by a rat’s ability to freely access alternative foods. The ingestion of RB dye (mg kg–1) was highest in rats that consumed the highest concentrations, even though food intake was reduced. Conclusions The consumption of baits at high RB concentrations was greatly affected by the ease of access to other foods. We suggest the willingness of the animal to consume the bait can be influenced by the effort needed to find alternative foods. Implications A higher incidence of marking in the whiskers or hair of target individuals in the field will only be achieved with the use of the most palatable concentrations of RB and environments providing low alternative food access and abundance. A trade-off between reliable marking and palatability of RB at varying concentrations must be achieved if actual bait uptake in the field is to be more accurately represented. A re-evaluation of palatability experiments may be required as access to alternative foods can have profound impacts on bait uptake.

Field evaluation of a bioenergetics-based foraging model for kokanee (Oncorhynchus nerka)

We used a bioenergetics-based foraging model to determine if bioenergetic and foraging constraints could explain kokanee (Oncorhynchus nerka) diel vertical migration in Blue Mesa Reservoir, Colorado. We compared model predictions of daily growth and migration strategies with observed growth and diel vertical distributions on three dates during the summer. Results suggest that bioenergetic and foraging constraints were not sufficient to explain diel vertical migration early in the summer, when thermal stratification was weak. However, these constraints could explain observed patterns later in the summer, when optimal thermal habitat for kokanee was spatially segregated from food-rich surface waters. The onset of a strong thermocline, and its exclusion of piscivorous lake trout (Salvelinus namaycush) from surface waters, appeared to determine the relative importance of predation risk for kokanee diel vertical migration patterns. Our observations and modeling results suggest that the relative importance of various factors driving diel vertical migration changes seasonally. Furthermore, the relative importance of each factor likely varies from system to system and may have caused the variety of single-factor hypotheses proposed to explain kokanee diel vertical migration. The model provides a framework for studying diel vertical migration across systems of differing thermal regimes, productivity, and predation pressures.

Comparison of food habits and of nutrients in the stomach contents of summer- and winter-trapped voles (Microtus pennsylvanicus)

Meadow voles (Microtus pennsylvanicus) have low fat reserves in winter. Since plant parts with high nutritive values are less abundant at this period, we hypothesize that voles must select different items or face a lower quality diet than during summer. Food habits determined from epidermal plant fragments of fecal matter showed that summer- and winter-trapped voles were using the same plant species though in somewhat different amounts. Nutritive constituents evaluated from stomach contents varied significantly by season. Stomach contents of winter-trapped voles had lower levels of protein and total phenolics and higher levels of total nonstructural carbohydrates. Forage quality ratios involving protein/total phenolics did not vary between seasons. These results suggest that overwintering voles of this study, trapped during low density, did not face obvious nutritive constraints. However, voles living under more crowded conditions could still face foraging constraints.