Shape-shifting and rolling in an arboreal ant from southern China (Cataulacus granulatus)

Many small terrestrial arthropods defend themselves by transforming their bodies into spherical shapes. While some, like pill millipedes, have evolved specialised morphology for this purpose, others, although lacking obvious morphological adaptations, can still form rough spheres for defence. Here, we present our observation of such a curling behaviour in the small arboreal ant Cataulacus granulatus. When disturbed or pinched, a C. granulatus worker can curl its body segments and retract its legs, forming a rough sphere with a maximum dimension of 2.1–2.5 mm along the main anatomical axes. After being released onto a wood board with a 30° slope, the curled ants rolled for an average of ~11 cm. After 0.1–1.4 s of rolling, the ants performed a braking behaviour by uncurling their body and legs, an action which effectively reduced the rolling speed. The rolling movement was eventually stopped as the ant clung onto the substrate surface with one tarsus. Our results suggest the defensive curling in C. granulatus may allow for passive rolling along solid slopes, which may facilitate escaping and concealment under natural conditions. In addition, the braking behaviour helps to prevent prolonged rolling and thus conserve the travelled distance due to passive displacement. These behaviours are potentially adaptive for escaping in a variety of habitats from forest floor to tree canopies.