Ant Societies Buffer Individual-Level Effects of Parasite Infections

Abstract

Parasites decrease host fitness and can induce changes in host behavior, morphology, and physiology. When parasites exploit social insects, they influence not only infected individuals but also the society as a whole. Workers of the ant Temnothorax nylanderi are an intermediate host for the cestode Anomotaenia brevis. We studied a heavily parasitized population and found that, although parasite infection had strong and diverse consequences for individual workers, colony fitness remained unchanged. At the individual level, we uncovered differences among the three worker types, infected and healthy workers from parasitized colonies and healthy workers from nonparasitized colonies. Infected workers were smaller than healthy ones and had smaller heads as parasite load increased. Behavioral changes extended to all workers from parasitized colonies; such workers were less active than workers in nonparasitized colonies but engaged in more grooming. Healthy workers from parasitized colonies showed behavioral patterns intermediate to those of infected workers and healthy workers from nonparasitized colonies. Despite the lower activity level found in ants from parasitized colonies, an important fitness parameter, per-worker productivity, remained unaltered in parasitized colonies. However, the investment strategies of parasitized colonies changed as their sex ratio became male biased and as male body size increased. In short, ant colonies can buffer the drain of resources by the parasite despite strong effects on individual workers.

The parasite's long arm: a tapeworm parasite induces behavioural changes in uninfected group members of its social host

“…First, the reduced flight response of infected T. nylanderi workers in response to nest attacks would presumably increase the tapeworms' transmission into the definitive avian host, which preys upon ant brood or beetle larvae. Causes of the lower escape rate might be, in addition to the lower activity of infected workers [26], their smaller eye and body size, and shorter legs [25]. Second, the higher survival of infected ants would extend the parasite's time period for transmission, because predation events by woodpeckers might be rare.…”

“…It is puzzling, as workers-both from parasitized and unparasitized colonies-were more aggressive towards infected ants, be it non-nest-mates or nest-mates [26]. This suggests that workers discriminated infected from uninfected ants due to the quantitative changes in their chemical profile [25], although aggression did not further increase with parasite load.…”

“…A previous study showed that uninfected workers provide better care for infected than for uninfected nest-mates [26], which may come at the expense of their own survival. As infected workers are tolerated in their colony and well cared for [26], we finally investigated whether colony aggression, an important collective trait involved in nest-mate recognition and nest defence, is affected by the presence of infected workers. Specifically, we evaluated the aggressive responses of ant colonies that were either naturally parasitized (i.e.…”

Worker senescence and the sociobiology of aging in ants

“…Group size may improve social mechanisms of infection control. Temnothorax nylanderi colonies infected with a cestode parasite were larger than uninfected colonies; worker productivity did not differ with infection state, suggesting that larger colonies may buffer parasitism (Scharf et al 2012). In Acromyrmex echinatior , workers experimentally infected with the fungus Metarhizium had significantly lower survival in isolation than when maintained with nestmates (Hughes et al 2002).…”

Impact of plant cover on the cavity‐nesting ant Temnothorax crassispinus

“…When inhabiting poorer nest sites, Temnothorax ants produce a male‐biased sex ratio (Strätz & Heinze, ). It was also shown that the presence of parasites influences the sex ratio, and parasitised colonies produce a male‐biased sex ratio (Scharf et al ., ). In the present study, colonies more frequently inhabited nests located outside balsam patches, but, unexpectedly, when living inside patches of alien balsam, they produced a more female‐ – not male‐ – biased sex ratio.…”