Social immunity

Given the lifestyle of social insects they are particularly vulnerable to the threat posed by parasites. Colonies are made up of many highly related individuals that live in close proximity in a comparatively stable environment. The larger the colony the greater the probability that it is exposed to parasites given increased numbers of foragers (Naug and Camazine, 2002). Once exposure takes place, the high density of related individuals is likely to facilitate the spread of infection (Gadagkar, 1992; Schmid-Hempel, 1998; Pie et al., 2004). As discussed earlier, diversity and related-ness of individuals within a group can influence parasitism in a number of ways (Table 14.2). Fitting with predictions concerning increased sociality and parasitism, it has been shown that in bees antibacterial activity correlates positively with the level of sociality (Stow et al., 2007).

Outside of individual-based immune defence, social insects have additional possibilities to protect themselves through their social living and co-operation (Cremer et al, 2007). Indeed, in Acromyrmex leaf-cutting ants these social immune traits are effective to such a degree that life in a group gives a net benefit on exposure to parasites, despite the increased risks (Hughes et al., 2002). Following the sequencing of the first social-insect genome, that of Apis mellifera, it was even suggested that the apparent paucity of immune genes, relative to the numbers found in other insects, might have been due to the presence of these social defences (Evans et al., 2006). Colony structure and division of labour between the social individuals may act as a defence (Schmid-Hempel and Schmid-Hempel, 1993). More active defences may also be employed, for example increased allo-grooming to remove fungal spores from nest mates (Rosengaus et al., 1998) and alarm signals leading to avoidance of pathogen-exposed individuals (Rosengaus et al., 1999a). These social immune traits on the colony level may also mimic individual responses to infection. Fever, the raising of the body's temperature in an attempt to counter a perceived parasite or pathogen threat, is a well-understood response against infection in individual organisms. In honey bee colonies exposed to the fungal pathogen that causes chalkbrood disease in larvae, Ascosphaera apis, nursing adult workers were found to raise the nest temperature (Starks et al., 2000). Against a heat-sensitive pathogen such as A. apis, this colony-level response can been seen as analogous to fever within an individual.

Additionally, social immune priming may occur. Immunity of a naive individual within a social colony is induced not only on contact with the infectious agent directly, but also on contact with infected nestmates. Social immune priming of this kind has been demonstrated for fungal resistance in termites (Traniello et al., 2002) and for immune traits in bumblebees (Moret and Schmid-Hempel, 2001).

All the aforementioned defences of social insects are collectively referred to as social immunity (Cremer et al., 2007). While it is yet to be investigated, it is reasonable to expect that they may also show some level of specificity.

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