Life in the dark : vision and navigation in a nocturnal bull ant

Abstract

While there are benefits associated with adopting a nocturnal lifestyle, there are also a number of challenges. For central place foragers, that are required to navigate to and from the nest, the challenge is the low light environment at night. As vision plays an essential role in navigation, central place foragers must be able to extract reliable navigational cues despite the extremely low ambient light levels. By studying the night-active bull ant Myrmecia pyriformis in its natural habitat, I show that the adaptations which allow for the transition from a diurnal to a nocturnal lifestyle do not simply involve any one behavioural or physiological characteristic, but are instead shared by every aspect of the ants' biology. The majority of activity to and from the nest is restricted to the dusk and dawn twilight, allowing foragers to navigate to and from the nest in sufficient light conditions. To ensure this occurs, foragers measure ambient light conditions and only begin activity when there is sufficient light. As foragers leave the nest, they are immediately oriented towards nearby Eucalyptus trees. By tracking individuals over many nights, I provide the first evidence that nocturnal ants form idiosyncratic routes. To do so, individuals rely on compass information provided by polarised skylight and the landmark panorama. Landmark guidance appears to be the primary mechanism of navigation, with foragers using landmark information from a variety of sources. As light levels fall, navigation suffers as landmark cues become harder to use; foragers become less directed, take longer to move between the nest and trees, walk more slowly, and stop for longer periods of time. Finally, when compared to day-active bull ants, the visual system of M. pyriformis is adapted to its night-active lifestyle via optical and physiological modifications which work to increase visual reliability in low light. As the majority of information on visual system adaptations to low light levels comes from work on nocturnal flying bees, studying the visual system of a nocturnal walking insect thus provides an interesting comparison which may be able to shed light on some of the fundamental questions relating to the costs and benefits of temporal niche partitioning. Show more