The effect of nectar concentration on foraging patterns of the lesser long-nosed bat (Leptonycteris yerbabuenae)

Background/Question/Methods

Many studies have documented the wide variation of nectar among species in volume, solute concentration, and composition, and this variation is thought to be an adaptation to particular pollinators. The fact that nectar is a resource produced in relatively small but replenishable quantities poses a unique challenge to animals that depend on this resource. The ability of pollinators to choose optimal nectar for their physiological needs and to meet their energy requirements is fundamental to understanding flower-pollinator relationships and the evolution of nectar attributes. Previous studies with nectarivorous bats have shown that these animals are capable of evaluating time intervals between successive visits to optimize their nectar reward. Nevertheless, how changes in concentration may affect bats’ foraging decisions and time intervals between successive visits have not been evaluated.  The current study was designed to evaluate how foraging patterns (i.e. intervals between successive visits and energy intake) change as a consequence of different stimuli (nectar concentration).  Thirty-two adult male Leptonycteris yerbabuenae were captured in the tropical dry forest in Chamela, Jalisco, Mexico, and maintained in captivity for foraging experiments. Individual bats were placed in 1.5m² cages and offered two artificial nectar solutions representing one of three treatments. Each treatment offered a different combination of concentrations resulting in a different stimulus. The law of Weber-Fechner was used to calculate the stimulus S=(C_o+C_i)/(C_o+C_i)²  were C_o  is the greater concentration and C_i is the smaller concentration. Eight bats were used for each of the three treatments: high W-F stimuli of 1.2 (8 and 32 % concentration); low W-F stimuli of 0.7 (13 and 27%); and control with stimuli of 0 (20% and 20%).

Results/Conclusions Bat visit sequences were recorded using infrared photogates and dataloggers. For both low and high stimuli treatments, visit intervals were significantly shorter at high concentration sources compared to low concentration sources, indicating that bats actually plan temporal visit intervals differently depending on concentration. However, when time intervals are converted into energy obtained at each visit, bats’ foraging success is expected to be significantly higher in the low stimuli treatment. These results suggest that when bats are exposed to large differences in concentration, as found in the high stimuli treatment, they appear to be less efficient in appraising the tradeoff between time-lapse and energy accumulation rate. This could be explained by a "taste saturation affect” in which individuals' ability to assess nectar energy content is altered.