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Abstract
Social insects have evolved a suite of sophisticated defences against parasites. In addition to the individual physiological immune response, social insects also express ‘social immunity’ consisting of group-level defences and behaviours that include allogrooming. Here we investigate whether the social immune response of the leaf-cutting ant Acromyrmex echinatior reacts adaptively to the virulent fungal parasite, Metarhizium anisopliae. We ‘immunized’ mini-nests of the ants by exposing them twice to the parasite and then compared their social immune response with that of naive mini-nests that had not been experimentally exposed to the parasite. Ants allogroomed individuals exposed to the parasite, doing this both for those freshly treated with the parasite, which were infectious but not yet infected, and for those treated 2 days previously, which were already infected but no longer infectious. We found that ants exposed to the parasite received more allogrooming in immunized mini-nests than in naive mini-nests. This increased the survival of the freshly treated ants, but not those that were already infected. The results thus indicate that the social immune response of this leaf-cutting ant is adaptive, with the group exhibiting a greater and more effective response to a parasite that it has previously been exposed to.
Highlights
Parasites may be a significant threat for social insects because their colonies normally contain dense aggregations of highly related individuals (SchmidHempel 1998; Boomsma et al 2005)
The experiment showed that the social immune response depends on the nature of the threat (Metarhizium parasite or control) and whether the group has been previously immunized against it
Ants directed more allogrooming at ants exposed to the Metarhizium parasite than to those treated with the control solution
Summary
Parasites may be a significant threat for social insects because their colonies normally contain dense aggregations of highly related individuals (SchmidHempel 1998; Boomsma et al 2005). Just as in other animals, social insects have individual-level defences against parasites, such as the physiological immune response. These include allogrooming, a behaviour that is effective at removing parasites and increasing resistance (Rosengaus et al 1998; Hughes et al 2002; Yanagawa et al 2008). Such grouplevel defences can be regarded as a form of ‘social immunity’ (Cremer et al 2007; Cremer & Sixt 2009). The social immune response may even be transferable, with the resistance of naive individuals being increased by interacting with individuals that have been exposed to a parasite (Traniello et al 2002; Ugelvig & Cremer 2007)
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