Physical worker castes in colonies of an acacia-ant (Crematogaster nigriceps) correlated with an intra-colonial division of defensive behaviour

Crematogaster nigriceps (Emery) is one of the acacia-ants inhabiting the swollen thorns of Acacia drepanolobium trees in East Africa. The ants show two distinctive behaviours in response to being disturbed. Some ants ("Defenders") readily leave the swollen thorns to defend the tree, whilst others ("Domestics") remain inside the swollen thorns and will not move unless provoked into doing so. Analysis of head length and head width measurements of the ants shows conclusively that the two behavioural castes are also physical castes. Defenders have significantly larger heads (Head length 0.97 mm +/- 0.006, Head width 0.98 mm +/- 0.006) than the Domestics (Head length 0.78 mm +/- 0.008, Head width 0.81 mm +/- 0.007). It is concluded that there is an intra-colonial division of defensive behaviour within the worker ants of C. nigriceps, mirrored by a physical sub-division of the worker caste.