[...] Here, we tested how dietary protein : carbohydrate (P : C) ratio affects social immunity in a tropical ant, Ectatomma ruidum, combatting the parasitic fungus Metarhizium anisopliae, a model pathogen for studying social immunity [19–22]. Ants have two prominent mechanisms for social immunity (for additional mechanisms, see [23,24]). Allogrooming is a time-intensive [25] activity where nest-mates clean each other's cuticle, reducing the number of fungal spores [20,26,27]. The metapleural gland, a structure unique to ants [28], secretes antibiotics that reduce spore viability [20,29–31]. Both allogrooming and metapleural gland production can thus lower infection rates of ant workers exposed to Metarhizium. As dietary P : C constrains many worker- and colony-level traits [32], including storage biochemistry [33], worker longevity [32] and possibly worker activity rates [34] (but see [35]), we predicted that lower dietary P : C ratios would increase nest-mate interaction frequency and, in turn, the benefits of social grouping after exposure to Metarhizium. Our results provide new insight into the social value of carbohydrate resource exploitation. [...]

[...] We show for the first time that diet macronutrient composition affects immunity gains that arise from social grouping. When challenged with a pathogenic fungus (M. anisopliae), E. ruidum ants in worker groups survived longer when reared on a high-carbohydrate (1P : 3C) diet than did those reared on a high-protein (3P : 1C) diet, while solitary ants had similarly low survival rates when reared on either diet. These results do not simply reflect a general response to a low-quality diet because benefits from the high-carbohydrate diet only emerged in worker groups; there was no significant effect of diet in solitary conditions. In addition, a carbohydrate-rich diet reduced worker mortality rates when whole colonies were challenged with Metarhizium. Together, these results provide a novel mechanism by which nutrient balance can affect social groups and suggest an evolutionary advantage of carbohydrate exploitation by social insects.

As implied by the adage ‘feed a cold, starve a fever’, bulk food availability is generally considered an important mediator of immunity [6]. Immune functions have high metabolic costs relative to other physiological systems [7], and many studies have used starvation or caloric restriction treatments to show how bulk energy availability impairs individual disease resistance [8–10]. However, traits underlying immunity can require different nutritional mixtures and, as a result, the extent and nature of immune responses can depend on the relative availability of dietary components [12,18], including P : C balance [16,17]. Our results provide the first evidence that the immune consequences of nutrient balance can extend to social groups and may emerge as a consequence of nest-mate interactions.

Antibiotics played a key role in this nutrition-related social immunity. Blocking metapleural glands, the main source of antibiotic secretions, eliminated both diet and social grouping effects on immunity (figure 2), and increased the likelihood that cadavers showed signs of Metarhizium infection. Metapleural gland function can account for 13–20% of basal metabolic rate [40], so carbohydrate scarcity in a high-protein diet could reduce metabolite allocation to metapleural glands or exact trade-offs on other metabolic functions. Note that Metarhizium resistance does not depend on the metapleural gland in some ant species [23,24], and thus constraints on metapleural secretions will not explain nutrition-related social immunity in all ants.

By contrast, our results do not support the hypothesis that increases in energetically expensive allogrooming result from a higher-carbohydrate diet. Even though carbohydrate-rich diets can lead to higher metabolic rates [41], and possibly higher activity rates in insects, we found no evidence for nutritional effects on grooming frequency. Note, however, that we only observed grooming behaviour for 50 min after treatment (as in [26]); it is possible that significant grooming frequency differences among treatments could emerge with a longer observation period.

Results from the whole-colony experiment (figure 4) also revealed the benefits of a high-carbohydrate diet to immune function. These results suggest that colonies can buffer isolated effects of high dietary protein or Metarhizium challenge, but the effect of these factors in combination can overwhelm colony resistance and lead to higher worker mortality rates. We also found that removing larvae from colonies eliminated diet effects on worker mortality rates when colonies were challenged with Metarhizium (figure 4). Larvae, which (unlike workers) can digest protein, can serve to dampen effects of nutritionally imbalanced diets for ants [32]. Our results suggest that these protein digestion benefits may improve worker immunity in colonies on high-carbohydrate diets. [...]

Homwer hat geschrieben:Dabei stellt sich mir die Frage ob Ameisen Ausscheidungen produzieren.
Das klingt total doof, aber ist ernst gemeint. Sammeln die das dann auf dem Müllhaufen? Stinkt ein Ytong nach jahrelanger nutzung?