Death in the Distance: Mortality Risk as Information for Foraging Ants

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Abstract
Ant foragers are known to communicate food presence in distant patches, but do they also communicate information about distant mortality risk? Recruitment to a food source in a laboratory Lasius pallitarsis colony depended upon whether the initial returning foragers had encountered mortality risk (a larger Formica subnuda) and the quality of the food they found. When food quality was high, risk appeared to not affect recruitment; when quality was low, risk inhibited foraging. In a second experiment, L. pallitarsis colonies had access to food of several qualities through a divided trail, which forced foragers to take different routes to and from the food patch. Danger was either entirely absent, present only on the way to the patch, present only on the way from the patch, or along both routes. When food quality was poor colonies recruited to food when risk was absent, but risk anywhere significantly reduced the level of foraging. This included the situation where only returning foragers could encounter danger, which strongly suggests that their behaviour was a critical factor in whether nestmates would continue foraging. When food quality was moderate, only treatments with risk on the way to the food significantly inhibited foraging. When food quality was good, colonies continued to forage at a high rate, irrespective of presence or location or risk. In total, L. pallitarsis foragers appear to communicate and use information about both food and mortality risk in deciding whether to exploit patches. Ant foragers are known to communicate food presence in distant patches, but do they also communicate information about distant mortality risk? Recruitment to a food source in a laboratory Lasius pallitarsis colony depended upon whether the initial returning foragers had encountered mortality risk (a larger Formica subnuda) and the quality of the food they found. When food quality was high, risk appeared to not affect recruitment; when quality was low, risk inhibited foraging. In a second experiment, L. pallitarsis colonies had access to food of several qualities through a divided trail, which forced foragers to take different routes to and from the food patch. Danger was either entirely absent, present only on the way to the patch, present only on the way from the patch, or along both routes. When food quality was poor colonies recruited to food when risk was absent, but risk anywhere significantly reduced the level of foraging. This included the situation where only returning foragers could encounter danger, which strongly suggests that their behaviour was a critical factor in whether nestmates would continue foraging. When food quality was moderate, only treatments with risk on the way to the food significantly inhibited foraging. When food quality was good, colonies continued to forage at a high rate, irrespective of presence or location or risk. In total, L. pallitarsis foragers appear to communicate and use information about both food and mortality risk in deciding whether to exploit patches.