Jouni Laakso á Heikki SetaÈlaÈ Nest mounds of red wood ants (Formica aquilonia): hot spots for litter-dwelling earthworms Received: 21 November 1996 / Accepted: 3 April 1997 Abstract A previously undocumented association be- tween earthworms and red wood ants (Formica aquilonia Yarr.) was found during an investigation of the in¯uence of wood ants on the distribution and abundance of soil animals in boreal forest soil. Ant nest mounds and the surrounding soil of the ant territories were sampled. The ant nest mound surface (the uppermost 5-cm layer) harboured a much more abundant earthworm commu- nity than the surrounding soil; the biomass of the earthworms was about 7 times higher in the nests than in the soil. Dendrodrilus rubidus dominated the earthworm community in the nests, while in soils Dendrobaena octaedra was more abundant. Favorable temperature, moisture and pH (Ca content), together with abundant food supply (microbes and decomposing litter) are likely to make a nest mound a preferred habitat for earth- worms, provided that they are not preyed upon by the ants. We also conducted laboratory experiments to study antipredation mechanisms of earthworms against ants. The experiments showed that earthworms do not escape predation by avoiding contact with ants in their nests. The earthworm mucus repelled the ants, suggest- ing a chemical defence against predation. Earthworms probably prevent the nest mounds from becoming overgrown by moulds and fungi, indicating possible mutualistic relationships between the earthworms and the ants. Key words Dendrobaena octaedra á Dendrodrilus rubidus á Formica rufa nest mound á Predator avoidance á Mutualism Introduction The nest building activity of Formica rufa group (Hy- menoptera, Formicidae) ants generates clearly distin- guishable patches in forests. It is not only ants that populate the nest mounds; they live in more or less close association with an array of species of various phyla (HoÈlldobler and Wilson 1990). The presence of earth- worms (Oligochaeta) in inactive ant nests during winter has been reported in one study (GoÈûwald 1990), but there is no quantitative or qualitative data on the pres- ence of earthworms in active nest mounds. A search of a database (FORMIS 1995; World Wide Web: http:// www.public.iastate.edu/~entomology/Formis) with more than 20 000 references on ants resulted in no informa- tion on ant-earthworm associations. Kistner (1982) and HoÈlldobler and Wilson (1990) give extensive lists of ant- associated organisms throughout the world, but there is no reference to earthworms. This is surprising because our recent ®ndings show that earthworms are common in ant nests, where their biomass is much higher than that of any other faunal group, except the ants them- selves. Studies on ant-associated arthropod fauna have re- vealed several speci®c adaptations to avoid predation by ants. Similarly, earthworms ± as potential prey for the ants ± are likely to have evolved an eective predator avoidance mechanism. The protective adaptations of arthropods include protective structure, avoidance by agility, and imitation of ants by producing odours that induce co-operative or alarm behaviour, or by excreting toxic chemicals to deter the ants (HoÈ lldobler and Wilson 1990). In the apparent absence of a protective carapace or the ability to imitate the shape and behaviour of the ants, the most plausible mechanisms for the earthworms to survive in the nest mounds are physical avoidance due to their cryptic lifestyle, and excretion of co-operative or ant-repelling chemicals. We report data on the existence of earthworms, particularly of two litter dwelling species Dendrobaena Oecologia (1997) 111:565±569 Ó Springer-Verlag 1997 J. Laakso (&) á H. SetaÈlaÈ Department of Biological and Environmental Science, University of JyvaÈskylaÈ, P.O. Box 35, FIN-40351 JyvaÈskylaÈ, Finland Fax: +358 14 602321; e-mail: Jouni Laakso: jotla@jyu.®