Heavy rainfall induced colony fission and nest relocation in nocturnal bull ants (Myrmecia midas)

In recent years the bull ant Myrmecia midas Clark, 1951, has attracted attention for its impressive visual navigation during its nocturnal activity. Still, a basic understanding of this species’ ecology is lacking. Nest relocation and colony fissions are rarely observed and largely undocumented in M. midas colonies. In the current study, we quantified the nest-relocation and colony-splitting behaviour of suburban populations of M. midas in response to flooding in its habitat, caused by record rainfalls in eastern Australia in 2022. The flooding caused the destruction of nest chambers and disrupted colony activities. We documented nine relocations of the fifty observed colonies with another four split into 12 different colonies. Most relocations occurred a few days after one period of especially heavy rains. We collected several characteristics of these relocations: the distance of a nest from the nearest tree, the elevation of the nest above ground level and the diameter of the nest entrance, with all of these metrics predicting whether a colony moved after heavy rains. Moreover, we conducted excavations of some abandoned colonies, uncovering evidence of an internal collapse of nest chambers due to heavy flooding, which likely induced the nest relocation and colony fission in M. midas. In normal weather, we have not observed any colony relocating, suggesting that moving may bear a fitness cost.