Seasonality and colony-size effects on the life-history characteristics of Rhytidoponera metallica in temperate south-eastern Australia

Abstract

In social insects, colony growth is assumed to follow a logistic growth curve, with small, newly founded colonies experiencing exponential growth (ergonomic stage) that slows down with increasing colony size and approaches an asymptote when the colony produces sexuals (reproductive stage). Environmental factors may also influence colony growth, particularly in temperate climates where colder temperatures in winter slow development. However, growth patterns are rarely studied in detail in social insects. In this study, I investigated colony size and seasonality effects on life-history parameters of the ponerine ant Rhytidoponera metallica. I followed the growth of 10 laboratory colonies monthly over two years in conjunction with monthly excavations of 5 field colonies. Colony composition was highly seasonal in both laboratory and field colonies, with pupae and larvae produced only during the warmer months. Males, however, were present in colonies throughout most of the year. An expected logistic growth pattern was found in the majority (4 of 6) of laboratory colonies that had positive growth, one colony followed a Gompertz growth pattern and another a power curve. Two laboratory colonies decreased in size and two colonies didn't change in size. The slowing of growth observed with increasing colony size in the majority of laboratory colonies was related to a decrease in per capita brood production with increasing colony size. Colony size also related to the presence of males: field colonies containing males were significantly larger than field colonies where males were absent. By using a combination of laboratory and field colonies, I was able to obtain information on seasonality of brood and male production, in addition to important demographic data on mortality and natality rates that is difficult to obtain in social insects using only field excavations.