Abstract
Group living is favorable to pathogen spread due to the increased risk of disease transmission among individuals. Similar to individual immune defenses, social immunity, that is antiparasite defenses mounted for the benefit of individuals other than the actor, is predicted to be altered in social groups. The eusocial honey bee (Apis mellifera) secretes glucose oxidase (GOX), an antiseptic enzyme, throughout its colony, thereby providing immune protection to other individuals in the hive. We conducted a laboratory experiment to investigate the effects of group density on social immunity, specifically GOX activity, body mass and feeding behavior in caged honey bees. Individual honeybees caged in a low group density displayed increased GOX activity relative to those kept at a high group density. In addition, we provided evidence for a trade‐off between GOX activity and body mass: Individuals caged in the low group density had a lower body mass, despite consuming more food overall. Our results provide the first experimental evidence that group density affects a social immune response in a eusocial insect. Moreover, we showed that the previously reported trade‐off between immunity and body mass extends to social immunity. GOX production appears to be costly for individuals, and potentially the colony, given that low body mass is correlated with small foraging ranges in bees. At high group densities, individuals can invest less in social immunity than at low densities, while presumably gaining shared protection from infection. Thus, there is evidence that trade‐offs at the individual level (GOX vs. body mass) can affect colony‐level fitness.
Highlights
Group living is favorable to pathogen spread due to the increased risk of disease transmission among individuals (Schmid-Hempel, 1995)
Group density affected the activity of glucose oxidase (GOX), which was higher in bees kept in the low group density compared to the high group density (ANOVA: F1,27 = 5.09, p = .03, Figure 1a)
We found evidence for a group density mediated trade-off between GOX activity and body mass, supporting previous findings that immunity and body mass are under resource allocation and traded-off in insects (Bascuñán-García et al, 2010; Cotter et al, 2010; Krams et al, 2015; Rantala & Roff, 2005; Van Ooik et al, 2007)
Summary
Group living is favorable to pathogen spread due to the increased risk of disease transmission among individuals (Schmid-Hempel, 1995). Animals can alter their immune responses when crowded, a common phenomenon in insects (Wilson & Cotter, 2008) Insects can upregulate their immune system and increase disease resistance at an individual level when in high group densities (density-dependent prophylaxis), (Bascuñán-García, Lara, & Córdoba-Aguilar, 2010; Cotter, Simpson, Raubenheimer, & Wilson, 2010; Krams et al, 2015; Rantala & Roff, 2005; Van Ooik, Rantala, & Saloniemi, 2007).
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