MORTALITY RISK VS. FOOD QUALITY TRADE-OFFS IN A COMMON CURRENCY: ANT PATCH PREFERENCES'

Many studies have shown that predation risk affects foraging behavior, but quantitative predictions are rare because of the lack of a common currency for energy intake and mortality. This problem is soluble in ants. We gave 12 Lasius pallitarsis colonies the choice between foraging in two patches that differed both in food quality and in associated mortality risk. We independently measured the growth that colonies could achieve on the diets offered in those patches. With no risk at either patch, colonies always preferred the higher food quality patch. When mortality risk (a large Formica subnuda ant) was associated with the trail to the higher food quality patch, the use of that patch depended on the magnitude of the growth differential between feeding in risky or safe patches: the greater the benefit of feeding in the risky patch, the greater was its relative usage. Colonies valued risky patches equally to safe patches at the point where forager mortality rates were approximately offset by colony growth gain. This ability to reduce mortality risk while foraging may be a factor that favors insect sociality, in general.