Patch departure rules in Bumblebees: evidence of a decremental motivational mechanism

Behavioral Ecology and Sociobiology (Impact Factor: 3.05). 08/2007; 61:1707-1715. DOI: 10.1007/s00265-007-0402-6

ABSTRACT The patch living rules of a pollinator, the bumblebee Bombus terrestris L., are studied here in the framework of motivational models widely used for parasitoids: The rewarding events found during the foraging process are supposed to increase or decrease suddenly the tendency of the insect to stay in the current patch and therefore to adjust the patch residence time to the patch profitability. The foraging behaviour of these pollinators was observed in two environment types to determine their patch-leaving decisions. The rich environment was composed of male-fertile flowers, offering pollen and nectar, and the poor one of male-sterile flowers, offering little nectar and no pollen. The experimental design consisted of a patch system in which inflorescences were evenly arranged in two rows (1 m distance). Residence times of foragers inside inflorescences and rows were analysed by a Cox proportional hazards model, taking into account recent and past experience acquired during the foraging bout. Most of the results showed a decremental motivational mechanism, that is, a reduction in the residence time on the inflorescence or in the row related to exploitation of flowers within inflorescences and inflorescences within rows These results indicate that bumblebees tend to leave the patch using departure rules similar to those found in parasitoids. The results also provide information on the memory, learning and evaluating capabilities of bumblebees especially when rich and poor environments were compared. The patch-leaving mechanism suggested by this study is consistent with the central place foraging theory.

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    ABSTRACT: Foraging bumblebees focus mainly on one flower species (their 'major'), and occasionally visit other flowers ('minors') that are less rewarding. Minoring allows tracking of potential alternative resources that might become more profitable with time. This is expected to provide an adaptive benefit under unstable foraging conditions, but to reduce foraging efficiency when resources are steady. We tested how predictability of food sources affects minoring and foraging success, using Bombus terrestris workers that fed on sucrose solution in artificial flowers. Bees exposed to three colour-distinct flower types with fluctuating food rewards performed more minoring (visits to a fourth, nonrewarding flower type) than bees that encountered a stable reward schedule. This difference was mostly due to a higher frequency of flower type shifts in the fluctuating-reward treatment. Flowers of the highest-rewarding type were visited less frequently in the fluctuating-reward than in the stable-reward treatment. This suggests that the fluctuating-reward schedule limited the bees' ability to track the most profitable flower type. Intervals between successive visits were longer in the fluctuating-reward than in the stable-reward treatment, possibly because the fluctuating schedule required time-consuming neural processing. As expected, the number of minoring visits correlated negatively with the number of colour shifts and with foraging success in the stable-reward treatment. In the fluctuating-reward treatment, on the other hand, sucrose intake rates were independent of minoring and colour shift frequencies. We suggest that novelty seeking by foragers can evolve when food sources offer fluctuating rewards, which reduce foraging success but also the cost of exploring new resources.
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    ABSTRACT: Several optimisation models, like the marginal value theorem (MVT), have been proposed to predict the optimal time foraging animals should remain on patches of resources. These models do not clearly indicate, however, how animals can follow the corresponding predictions. Hence, several proximate patch-leaving decision rules have been proposed. Most if not all of these are based on the animals’ motivation to remain on the patches, but the real behaviours involved in such motivation actually still remain to be identified. Since animals are usually exploiting patches of resources by walking, we developed a model simulating the intra-patch movement decisions of time-limited animals exploiting resources distributed in delimited patches in environments with different resource abundances and distributions. The values of the model parameters were optimised in the different environments by means of a genetic algorithm. Results indicate that simple modifications of the walking pattern of the foraging animals when resources are discovered can lead to patch residence times that appear consistent with the predictions of the MVT. These results provide a more concrete understanding of the optimal patch-leaving decision rules animals should adopt in different environments.
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