Sex investment ratios and natural history observations in a population of Trachymyrmex holmgreni (Formicidae) in southern Brazil

Nest architecture studies provide important information about the natural history of ants, such as number of workers in a colony and presence/absence of winged forms, larvae, pupae, and food resources. These aspects of the population biology may help answer questions related to sex ratio, kin selection, and parent-offspring conflict and to the impact of environmental conditions on the colony. Here, we describe the following aspects of the natural history of the fungus-farming ant Trachymyrmex holmgreni: (a) sex investment ratios; (b) intranidal population; and (c) external and internal nest architecture. In April 2007, we excavated ten nests of T. holmgreni in Itapeva beach in southern Brazil. In four of the ten nests, we measured height, width, and depth of all nest chambers found and counted and weighed all ant individuals in the laboratory. We found gynes and males in all four colonies, and larvae and pupae in three of them. The numerical sex ratio and the estimated sex investment in the colonies examined here suggest a higher expenditure of energy for the production of females. Our study provides new data for understanding the biology of T. holmgreni, which is a member of the Trachymyrmex iheringi group, a rather diverse group of fungus-farming ants that are closely related to leaf-cutting ants.