Foraging of a hypogaeic army ant: a long neglected majority

Abstract

Army ants have been studied thoroughly for more than a century. The conduction of column and swarm mass raids, featured by epigaeicly active species, is believed to be a central characteristic of army ant behavior. Most army ant species, however, lead a hypogaeic life. Due to the difficulties to observe them, nothing is known about their hypogaeic behavior in the field. Using palm oil baits, trail excavations, and laboratory observations, the hypogaeic foraging of Dorylus (Dichthadia) laevigatus was observed in Malaysia. D. laevigatus was found to construct stable hypogaeic trunk trail systems providing quick and easy access to all parts of its foraging area. Small column raids were conducted throughout the ground stratum and above the ground surface. These raids were caste specific, with the smallest workers predominantly following existing cracks and tunnels in the soil. In case of food location, larger workers were recruited from nearby trunk trails. Exploratory trails leading to prey had to be widened before larger workers could gain access and help to process the food. Bulky food sources such as baits or termite mounds could be exploited over several weeks to months. Besides raiding in columns, D. laevigatus came occasionally to the ground surface at night to conduct swarm raids. This combination of swarm and column raids with the use of trunk trails has never been demonstrated for a classical army ant species. The omnipresence of D. laevigatus within its foraging area stands in sharp contrast to epigaeicly active species, characterized by a very localized and temporary presence at foraging sites. D. laevigatus stayed in the same foraging area for several weeks to months. Having a broad diet and the ability to exploit bulky food sources over long periods of time, D. laevigatus seems to follow a sustainable use of the soil fauna. Summing up these particularities demonstrates a remarkable divergence of the hypogaeic foraging of D. laevigatus from that of epigaeicly foraging army ant species.

Temporal organization of bi-directional traffic in the antLasius niger(L.)

“…In L. niger, the subterranean galleries have a small diameter (down to 2·mm, allowing the passage of a maximum of two individuals) and their length ranges from 4 to 12·cm (Rasse, 1999), which includes the length of the bridge we used in our experiments. One can thus imagine that, up to a critical flow volume for which clogging may occur, the temporal organization we observed will allow maintenance of the same flow of individuals through a gallery without engaging in the costly work of enlarging it (Berghoff et al, 2002).…”

Nesting habits and colony composition of the hypogaeic army ant Dorylus (Dichthadia) laevigatus Fr. Smith

“…The 15 year old plantation contained 146 palm trees distributed in 14 rows over an area of 1.1 ha (see also Berghoff et al, 2002). The soil consisted of three distinct zones: 1) an uppermost black humus zone of about 5 cm depth, followed by 2) a yellow clay zone about 40 cm deep, containing a network of palm roots in 20-35 cm depth, and 3) a grayish-white zone with red iron streaks reaching the ground water level at a maximum depth of 80 cm.…”

“…Distribution of D. laevigatus and subterranean movements were monitored via sieve buckets (height: 9 cm, diameter: 30 cm) baited with palm oil (see Berghoff et al, 2002 for a detailed description of the method). The ants recruited in large numbers to the sieve buckets, where they fed on the oil.…”

“…Schneirla, 1971;Gotwald, 1995). On the other hand, for the majority of species, which forage hypogaeicly, we are just beginning to look into these traits (Berghoff et al, 2002). Likewise, colony attributes such as nest form, the way of its construction, and its internal conditions were studied mainly for army ant species which form epigaeic bivouacs (e.g.…”

“…Although D. laevigatus is scientifically known for almost 150 years and is locally very abundant (Weissflog et al, 2000;Berghoff et al, 2002), nothing is known about its colony composition or nesting habits. By attracting D. laevigatus to palm oil baits we were able to monitor a part of its hypogaeic movements and thus to locate the nest site.…”