SLAVE ADDITION INCREASES SEXUAL PRODUCTION OF THE FACULTATIVE SLAVE-MAKING ANT FORMICA-SUBNUDA

Formica subnuda is a facultative slave-making ant, and colonies withou t slaves are often found. We studied the effect of slave workers on se xual production of F. subnuda by experimentally increasing the proport ion of slaves. We added c. 4000 worker pupae of the ant F. podzolica t o 15 F. subnuda colonies and kept 15 colonies as controls. The followi ng year we excavated all colonies, counted the proportion of slaves, t he total number of workers (colony size) and the number of sexual offs pring. The proportion of slaves was significantly higher in the slave- added colonies than in the control colonies. The total production of s exual offspring increased 57% in the treatment colonies in comparison to the controls. When colony size was adjusted to the number of sexual offspring, the treatment colonies produced significantly more sexual offspring than the controls. Slave addition did not alter sex ratios. We suggest that two alternative mechanisms, not mutually exclusive, ca used the increase of sexual production in F. subnuda colonies: (1) Mos t of the added pupae were consumed and stored as fat body in workers a t the end of the experimental year; the following spring the excess fa t was metabolized and fed to the developing sexual larvae, or (2) a pr oportion of the added pupae hatched to become slaves; the following sp ring these slaves foraged actively for protein-rich food for the devel oping sexual offspring.