Response thresholds to recruitment signals and the regulation of foraging intensity in the ant Myrmica sabuleti (Hymenoptera, Formicidae)

The optimal foraging theory predicts that colonies of social insects must b e able to adjust the intensity of their foraging behaviour asa function of the quality of the food discovered. Here. the mechanisms allowing the regul ation of recruitment as a function of food concentration in the ant Myrmica sabuleti were analyzed. Although the total number of foragers ei-gaged in food collection during recruitments increased with increasing concentration of sucrose solutions (0.1 vs. 1 M), neither the proportion of recruiting s couts nor the invitation behaviour performed by the scouts in the nest can explain this relationship. Foragers trail more when coming back from a 1 M than from a 0.1 M sucrose solution. However, this alone cannot explain the collective patterns observed since the mean numbers of workers leaving the nest after the entry of a scout coming back from either 0.1 or 1 M sources were not significantly different. We suggest-that a spatial distribution of the foragers in the nest as a function of their motivational state could b e part of the regulation process. The ants located near the nest entrance w ould respond to both low and high trail pheromone signals, but those locate d deeper in the nest would respond only to high level signals, resulting in higher recruitment rate towards richer sources. (C) 2000 Elsevier Science B.V. All rights reserved.