Micro-Environmental Factors Affecting Diel Patterns of Foraging in the Leaf-Cutting Ant Atta cephalotes (L.) (Formicidae: Attini)

Abstract

Long-term, regular and widespread changes in the physical environment of leaf-cutting ants do not seem directly responsible for their characteristic rhythmic foraging behaviour (Lewis, Pollard & Dibley 1974). However, there are suggestions that the micro-weather near each nest, or even peculiar to each trail or nest-entrance may affect their foraging rhythms. For example, in Trinidad many farmers and gardeners claim that nocturnal foraging is encouraged by warm nights, by cool or very hot dry days, and by full clear moonlight; conversely, diurnal foraging is supposedly encouraged by cool nights, warm days and moonless or dull nights. In other Caribbean and Central American territories scientific, as well as lay opinion, seems to be confused. Belt (1874) suggested that Atta in Nicaragua became nocturnal during hot weather, and Cherrett (1968) found A. cephalotes (L.) in Guyana least active in the warmest (32-35? C) time of day. In the U.S.A., Moser (1967) thought that trail temperatures determined whether A. texana (Buckley) could forage, but that the range of temperatures over which foraging occurred (1 1-29? C) was so wide that other undetected factors might also be important. By contrast, Hodgson (1955) claimed that foraging by A. cephalotes isthmicola (Weber) in Panama started when morning light fell on the nest entrances, although an inherent rhythm caused the ants to move towards the entrances while light was still very dim. The only way to test these explanations was to make detailed measurements of the components of the micro-weather experienced by ants in their nests, at the nest entrances, and on trails, to compare with the numbers of foragers emerging to collect vegetation. This paper, therefore, describes a series of analyses of the separate and combined effects of components of micro-weather on foraging behaviour. Before each foraging period is well established, unladen ants mill around the nest entrances and then 'explore' the first few metres of the trail as though 'sensing' the environment. Many of these ants re-enter the nest, then the foragers begin to stream outwards to a cutting site. Once this exodus has started, most of the foraging population seems to have been recruited within 2-3 h (Lewis et al. 1974). Therefore, the search for physical stimuli to foraging was concentrated on the conditions experienced over the short period just before and after exodus started each day or night.