Rissanen, J. | bioRxiv Science

bioRxiv2022

Lack of self-medication by fungus infected Lasius platythorax ants in a multitrophic experiment

Ants live in dense colonies with low genetic diversity between nestmates, creating favourable conditions for the spread of diseases. Due to the constant pressure by infectious disease, ants have evolved different mechanisms to cope with it. One way to combat pathogens is via consumption of biologically active compounds, which can inhibit the growth of the microbes or eliminate them. Although ants are known to respond to pathogen infections by altering their foraging to incorporate reactive oxygen species (ROS) into their diet to successfully combat disease under laboratory conditions, it is still unknown how ants would self-medicate in the wild. By designing a multitrophic environment, we try for the first time to identify potential sources of medicinal compounds for ants to use against fungal infections under natural conditions to better understand how the behaviour is expressed in nature. We investigated whether a bean plant suffering from an infestation of Megoura viciae aphids would be a source of ROS which the ant could use for self-medication, and whether the ant Lasius platythorax changes its foraging behaviour on bean plants if exposed to infectious spores of the entomopathogenic fungus Beauveria bassiana. We found no clear evidence that the ants significantly change their foraging behaviour on extrafloral nectaries in response to the pathogen or that the nectar produced by aphid stressed plants contains ROS. The aphids contained ROS in levels which ants could potentially use, however Megoura viciae is considered unpalatable to ants and we saw no evidence of predation on the aphids. Our results mean that the combination of species of the ant - plant - aphid interactions which we studied does result in a detectable self-medication response in ants. Therefore, we suggest that alternative species of aphids as well as other natural sources of ROS should be studied for self-medication.

animal behavior and cognition↗

bioRxiv2022

Pathogen prevalence modulates medication behavior in ant Formica fusca

Ants face unique challenges regarding pathogens, as the sociality which has allowed them to form large and complex colonies also raises the potential for transmission of disease within these colonies. To cope with the threat of pathogens, ants have developed a variety of behavioral and physiological strategies. One of these strategies is self-medication, in which animals use biologically active compounds to combat pathogens in a way which would be harmful in the absence of them. Formica fusca ants are to date the only species of ants proven to successfully self-medicate against an active infection caused by a fungal pathogen by supplementing their diet with food containing hydrogen peroxide. Here, we build on that research by investigating how the prevalence of disease in colonies of F. fusca affects the strength of the self-medication response. We exposed either half of the workers of each colony or all of them to a fungal pathogen and offered them different combinations of diets. We see that workers of F. fusca engage in self-medication behavior even if exposed to a low lethal dose of a pathogen, and that the strength of that response is affected by the prevalence of the disease in the colonies. We also saw that the infection status of the individual foragers did not significantly affect their decision to forage on either control food or medicinal food as uninfected workers were also foraging on hydrogen peroxide food, which opens up the possibility of kin medication in partially infected colonies. Our results further affirm the ability of ants to self-medicate against fungal pathogens, shed new light on plasticity of self-medication and raise new questions to be investigated on the role self-medication has in social immunity.

animal behavior and cognition↗