Consistent Cuticular Hydrocarbon Discrimination of Alkane Mixtures with Proportional Differences in the Ant Camponotus floridanus.

Discrimination of odorant mixtures is essential in communication and individual recognition in many animals. The number of compounds and the similarity in compound structure can, however, make discrimination challenging. An example is the use of surface hydrocarbons in ants and other social insects to differentiate between colony members and foreigners, or to identify the reproductive status of individuals. These cuticular hydrocarbon (CHC) mixtures consist of long-chain compounds, including n-alkanes, that show quantitative variability across individuals and high structural similarity. We explored the limits of discrimination in synthetic CHC mixtures in Camponotus floridanus worker ants. We used differential conditioning to determine whether workers can differentiate between heptacosane (C27) and hentriacontane (C31) alone, in binary mixtures, and in mixtures with pentacosane and nonacosane. Worker ants successfully discriminated all odor pairs in one direction with consistent responses for the mixtures across trials, regardless of whether the ratio of C27 to C31 was 2:1 or 1.33:1 or whether C27 and C31 were increased by 100% or decreased by 50% in the four-compound mixtures. Differentiation success depended on an increased C27/C31 ratio in the rewarded mixture, while workers did not differentiate when C27/C31 was high in the punished mixture. The asymmetry could be due to increased concentration, volatility, or an inherent preference for C27. The successful discrimination indicates n-alkanes could be used for nestmate discrimination and identification of reproductive individuals. The workers’ ability to discriminate among quantitative differences in structurally similar CHC mixtures demonstrates the highly accurate tuning of the ant olfactory system.