FACULTATIVE AND OBLIGATE SLAVERY IN FORMICINE ANTS - FREQUENCY OF SLAVERY, AND PROPORTION AND SIZE OF SLAVES

Slave-making ants raid nests of other ant species, capture the develop ing offspring and rear them to slave workers. Here we compare slave-ma king of three formicine slave-making ants: the facultative Formica sub nuda, the obligate Polyergus breviceps, and F. subintegra which previo usly has been considered facultative but appears to be an obligate sla ve-making ant If F. subintegra is an obligate slavemaker, slave-making off. subintegra should differ from that off. subnuda but closely rese mble slave-making of P. breviceps in the following aspects: (1) Obliga te slavemakers are rarer than facultative slavemakers. (2) Slaveless c olonies of facultative slavemakers are found, but obligate slavemakers always have slaves. (3) Because obligate slavemakers depend on their slaves, they should have a higher proportion of slaves than facultativ e slavemakers. (dr) Owing to special adaptations obligate slavemakers are able to raid bigger colonies, and hence have bigger slaves than fa cultative slavemakers. (5) Dufour's gland of F. subintegra should be l arger than that of F. subnuda. Per 100 free F. podzolica colonies, the number of P. breviceps and F. subintegra colonies with F. podzolica s laves were 1.3% and 3.9%, respectively, and the number of F. subnuda c olonies with F. podzolica 3.7%, and without F. podzolica 7.5%. The pro portion of slaves, when present, varied between 1-30% in the colonies off. subnuda, and between 70-90% in the colonies of the other species. The slaves off. subnuda were significantly smaller than those off. su bintegra and P. breviceps. The length of F. subnuda's Dufour's gland w as one third of the length off. subintegra's gland. The results show t hat slave-making off. subintegra parallels that of P. breviceps, and c ontrary to the earlier notion, F. subintegra is an obligate slave-maki ng ant. We suggest that F. subnuda and F. subintegra represent extreme modes of slave-making behaviour in the Formica sanpuinea group.