Inter-generic and inter-habitat variation in the demand for sodium by Neotropical ants

Abstract

Sodium is a potentially limiting element for ant populations. Here we examined the extent to which the demand for sodium varies among different ant genera and habitats. Sucrose and NaCl baits were placed on the ground and in trees located within three types of savanna and one type of forest. Tree-dwelling ants were found ca. two times more often on NaCl than on sugar baits, whereas on ground the frequency of ants on NaCl baits was about 20 % lower than on sugar baits. This difference seems to be explained by compositional differences between the tree- and ground-dwelling faunas. Camponotus and Cephalotes were much more frequent in trees than on ground, and both showed a preference for NaCl over sucrose baits. In contrast, the most frequent ants on the ground were Pheidole, and these were mainly attracted to sucrose baits. Genera-specific patterns of bait use did not differ significantly among habitats in spite of potential differences in the availability of sucrose (in honeydew and plant exudates) and sodium (in animal matter) for ants among these same habitats. In all habitats, genera that are predominantly carnivorous were comparatively less attracted to NaCl baits than those that rely more on plant matter. These findings lend additional support to the idea that sodium limitation is associated with the ant´s trophic position. At the community level, thus, the attractiveness of sodium sources to ants will depend both on rates of external sodium inputs as well on community composition.