BOTH EVALUATION OF RESOURCE QUALITY AND SPEED OF RECRUITED LEAF-CUTTING ANTS (ACROMYRMEX-LUNDI) DEPEND ON THEIR MOTIVATIONAL STATE

During recruitment, running velocity of both outbound and laden worker s of the leaf-cutting ant Acromyrmex lundi depended on the information about resource quality they received from the first successful recrui ter. In independent assays, single scout ants were allowed to collect sugar solutions of different concentrations and to recruit nestmates. Recruited workers were presented with standardized paper discs rather than the sugar solution given to the original recruiting ant. Out-boun d recruited workers were observed to run faster the more concentrated the solution found by the recruiter. Speed of disc-laden workers also depended on the concentration of the solution found by the recruiter, i.e. on the information about food quality they received, since they h ad no actual contact with the sugar solution. Disc-laden workers ran, as intuitively expected, slower than outbound workers. The reduction i n speed, however, could not be attributed to the effects of the load i tself, because workers collecting discs of the same weight, but with a dded sugar, ran as rapidly as out-bound, unladen workers. Workers coll ecting standardized sugared discs reinforced the chemical trail on the ir way to the nest. The percentage of trail-layers was higher when wor kers were recruited to 10% than to 1% sugar solution, even though they collected the same kind of discs at the source. Their evaluation of r esource quality, therefore, depended on their motivational state, whic h was modulated by the information they received during recruitment. U sing previously published data on energetics of locomotion in leaf-cut ting ants, travel costs of A. lundi workers recruited to sugar solutio ns of different concentration could be estimated. For workers recruite d to the more concentrated solution, both speed and oxygen consumption rate increased by a roughly similar factor. Therefore, although worke rs ran faster to the high-quality resource, their actual energy invest ment per trip remained similar to that made by workers recruited to th e low-quality resource. It is suggested that the more motivated worker s reduced travel time without increasing energy costs during the trip. The adaptive value of these responses seems to be related to a rapid transmission of information about a newly discovered food source.