The relationship of nest rock dimensions to reproductive success and nest permanence in a high-mountain ant, Proformica longiseta (Hymenoptera: Formicidae)

Abstract

The relationship between the dimensions of nesting rocks and nest abandonment or permanence has been studied in the high-mountain ant Proformica longiseta Colllngwood, analyzing the advantages or disadvantages of different types of rocks and whether rock selection occurs in this species. The frequency distributions of available and occupied rocks showed statistical differences for thickness, surface and volume. Thirtynine percent of the ant nests were abandoned before the end of the active season, 16% overwintered once without a change of rock, and 45% overwintered at least two years under the same rock. Rocks protect against extreme temperatures as well as serve as a supplementary source of heat, maintaining optimal conditions for brood development. Rocks thinner than 3.5cm harbored broods for shorter daily periods than did rocks thicker than 3.5cm. The statistical analysis of the mean rock dimensions according to the time of permanence of the nests, showed differences for thickness, surface and volume. The sizes with longer permanence were those with between 2.5 and 4.5cm of thickness and a surface area between 100 and 250cm2. This kind of rock also proved conducive to the production of sesuals. Thus, for the fitness of the colony, there are suitable and unsuitable rocks. The occupation of the optimal rocks resulted from random discovery by ants, without the availability offavorable nest-sites (rocks) being a limiting factor. When relocation was needed because the rock proved unsuitable, workers relocated the nest under another rock without recognizing or learning to recognize suitable or unsuitable properties. Therefore, there is no prior selection of the rock. The thermal properties of this rock leads to abandonment (if >bad) or selection (if >good). Thus this process constitutes >a posteriori> rock selection.