Dopaminergic expression and the regulation of foraging in response to water stress

Collective behavior allows social groups to respond to changing conditions through local interactions without central control. We examined the neural mechanisms that support the collective behavior that regulates foraging activity in response to water stress in seed-eating ants, Pogonomyrmex barbatus. A forager decides to leave the nest on its next trip based on its rate of olfactory encounters with returning foragers and the desiccation it experienced on the last trip. Colonies differ in how foragers assess the risk of water loss: in some colonies, foragers are more risk-averse and less likely to leave the nest to forage in dry conditions, while other colonies are more risk-tolerant and do not reduce foraging activity in low humidity. Previous pharmacological experiments showed that foragers treated with dopamine were more likely to leave the nest to forage in dry conditions. Because risk-averse foragers were more sensitive to this treatment, we tested whether these foragers have fewer dopamine neurons by comparing the dopamine neuron expression patterns across functionally distinct brain regions in foragers from the two colony phenotypes. Foragers of risk-averse and risk-tolerant colonies did not differ in dopamine neuron distribution or volumes of functionally distinct brain regions. They showed similar dopamine neuron cluster locations as other insects. These results suggest that behavioral differences between risk-averse and risk-tolerant colonies may be due to differences in how dopamine is synthesized or released in response to dry conditions or to differences in the distribution of dopaminergic synapses rather than the number of dopamine neurons.