Foraging ecology of the Namibian desert ant Ocymyrmex robustior in the Kuiseb River

Abstract

Abstract by author:

In the first part, this study addresses the temporal aspect of foraging. It focuses on daily activity patterns relating to microclimatic conditions of a few 0. robustior colonies during the winter and spring months. Summer data have been obtained in 0. robustior - as well as summer and winter data in 0. velox - by Wehner & Wehner (unpublished). Special attention was paid to abiotic environmental factors such as soil and air temperatures, which previously turned out to be most responsible for daily variations in activity of ants, followed by light intensity (Femandez-Escudero & Tinaut, 1998). Cerda (2001) concluded that in ant communities of open Mediterranean habitats the temporal activity patterns are basically conditioned by environmental factors rather than by interspecific competition

First, the present study documents the amount of intra- and internest differences that occured in activity and unravels the influencing external factors. The second part of the present work documents the structure of the Kuiseb River population based on colony density and colonies' foraging ranges. It has been claimed that territorial aggression is possibly the most important mode of population regulation in ants among other factors, including climate, predation, food, and availability of nest sites (HöIdobler & Wilson, 1990). However, offensive interactions, such as a controlled attack of a foreign colony, do not occur in 0. robustior colonies. Marsh (1985b) hypothesised that the foraging areas of 0. robustior colonies overlap. This was based on the occurrence of ants from different colonies at the same bait point. Now the question raises how the spatial resource for nesting sites and feeding is divided among the colonies and whether the proposed hypothesis is true or if a colony can defend an annexed foraging terrain against intruders of other colonies. In this context we also enquire about the monodomy/polydomy structure of the O. robustior colonies. Further investigations concern the spatial foraging strategies applied by individual ants. In particular, the following questions were posed: (1) How does the individual search characteristic of a forager look like and (2) is there individual persistence in foraging sites? This tendency to stick to a particular foraging direction (sector) is not only unique to trunk trail foragers (Hölldobler & Wilson, 1990) but also occurs in individual foragers of Cataglyphis (Wehner et al., 1983; Wehner, 1987, 2003). According to the observation in Cataglyphis, this peculiar sector fidelity of individuals is rather rapidly acquired during a forager's lifetime and improves with increasing number of previous successful foraging runs

Hence, much consideration has been given to record maps of foraging paths of individual ants. This provides information about foraging characteristics such as the path-structure, the length, the duration as well as the spatial distribution of foraging runs. In addition, the issue is addressed whether there is any behavioural plasticity concerning foraging ranges that can be tested with food baits

Finally, I compare to what extent the space use patterns and individual modes of foraging of the myrmicine species Ocymyrmex robustior agree with those of other ecologically equivalent thermophilic desert ant species such as members of the North African formicine genus Cataglyphis. The present account should, last but not least, add an additional piece to the large puzzle representing the similar adaptations of these convergent ant genera inhabiting the two of the most extreme deserts of the world