Evolutionary aspects of foraging efficiency and niche shift in two sympatric seed-harvesting ants (Messor) (Hymenoptera Formicidae)

Abstract

Messor capitatus and M. structor often coexist in narrow sympatry in central Italy and show very similar food preferences. These two species differ markedly from each other in their recruitment mechanism. By measuring the foragers' efficiency in transporting seeds into the nest, it appears that both species show a remarkable tendency to «re-create» the seed distribution most suited to its own recruiting mechanism. Such strategies, of course, imply a much greater number of foragers and longer handling times. The resulting apparent waste of energy can be justified by the fact that the gain obtained from a single seed is much greater than the energy necessary to search for and carrying it under any conceivable circumstances (NIELSEN & BARONI URBANI 1990). Nevertheless, this way of «improving» the natural seed distribution may be difficult to explain by mutual competition alone. The same selection pressure acting on both species may have successively shaped their behaviour as follows: secondary loss of trophallaxis → inadequate food distribution within the nest → need to involve a higher number of foragers for seed transport in order to ensure better intranidal distribution.

Nearctic seed harvesting ants of the genus Pogonomyrmex also seem to lack trophallactic behaviour, but it is argued that both genera are likely to have acquired this character by convergence as a secondary adaptation to solid food ingestion, rather than by common descent.

KEY WORDS:

• ants
• Formicidae
• behaviour
• evolution
• granivory
• foraging strategies
• adaptations
• trophallaxis