The specialization in groups of ants tending aphid colonies (Hymenoptera: Formicidae; Homoptera: Aphididae)

Data are reported on a comparative analysis of the behavi- our and work organization of different ant species tending aphids in multi-species communities. The teams of ants tending separate aphid colonies (milkers) are shown to in- clude both passive and active foragers. The functions of the former are usually restricted to collecting and porting honeydew. Active foragers are multifunctional. They can guard trophobionts, search for new aphid colo- nies and even coordinate the group activity to some extent in addition to the common functions (collecting and porting honeydew). The comparative analysis of the schem- es of interaction of different ant species with aphids has re- vealed that the depth of functional differentiation in work- ing groups of milkers is caused by the requirements of an ant colony for carbohydrate food. One of the main factors causing deep in the groups appears to be the ant colony numbers. Different species of ants have been found to use definite schemes of interaction with aphids; ranging from the individual foraging of unspecialized mil- kers to in working groups. The most complicated interaction was observed for ants with numerous colonies (Formica s. str.) dominating in multi- species communities (REZNIKOVA & NOVGORODOVA 1998, NOVGORODOVA 2004). It is characterized by the clear divi- sion of two main functions (protection of the aphid colony and honeydew collecting) in constant teams of ants tend- ing separate aphid colonies. The largest number of profes- sional groups (shepherds, guards, scouts and trans- porters) was noted only for red wood ants (Formica rufa LINNAEUS, 1761 group), whereas merely two groups (shep- herds and multifunctional guards) were discerned in the teams of Formica pratensis RETZIUS, 1783 ants living in smaller colonies (NOVGORODOVA 2005). Other members of multi-species ant communities demonstrate simpler schem- es based on a partial division of labour (Camponotus saxa- tilis RUZSKY, 1895) or individual foraging by unspecial- ized milkers (Formica fusca LINNAEUS, 1758, F. cunicula- ria LATREILLE, 1798 and Lasius niger (LINNAEUS, 1758)). Another factor regulating the depth of professional speci- alization appears to be the available resources (the num- ber and productivity of the trophobionts). On the one hand, insufficient carbohydrate food resources were found to lead to reorganization of the milkers' work and the ap- pearance of constant protection of the aphid colonies by F. fusca, F. cunicularia, and L. niger. This was observed in nature and was demonstrated in experiments involving a decrease in the available aphid colonies from 10 to 1. The number of ants (F. cf. cunicularia) attending the last re- maining aphid colony increased as did their aggressiveness. The ants started to protect the aphids to some extent. On the other hand, decrease in the aphids' productivity, on the underground parts of the plants in autumn in particular, proved to lead to a simplification of the organization of Formica s. str. milkers' work. The number of ants collect- ing honeydew in the separate aphid colonies decreased. Only non-aggressive shepherds (with / without transpor- ters) still took care of the aphids. There were no guards, so the effectiveness of protection was lower. On the whole, the specialization in the working groups seems to be facul- tative. A tendency for the functional differentiation in the ant teams to deepen is observed as the ant colony num- bers increase and also as a consequence of insufficient food resource at both intra- and interspecies levels. The study was elaborated within a joint research pro- ject of the Hungarian Academy of Sciences and the Rus- sian Academy of Sciences and was funded by the RFBR (No. 06-04-48288) and the President of the RF (Scientific