Ovarian indexes as indicators of reproductive investment and egg-laying activity in social insects: a comparison among methods

High reproductive skew among colony members is a fundamental feature of insect's societies, where one or few individuals monopolize reproductive output. However, many social insect species retain a high reproductive plasticity, with all colony members able of developing ovaries and laying eggs. Understanding the partition of reproduction inside these plastic societies is a key step in the comprehension of social evolution. The reproductive status of social insects is usually evaluated by looking at the degree of ovarian development, and a great variety of different methods has been used even in the same species. It is not known, however, to which extent the different methods give comparable results and represent good indexes of individual egg-laying activity. We used the model paper wasp species Polistes dominula to tackle these issues, by compiling the main methods of ovarian development assessment on the same set of individuals and quantifying their egg-laying activity. Our results showed that all examined methods give similar and highly correlated results, but none of them allows reliable estimates of the egg-laying activity. Ovarian indexes (whatever method is used) are good indicators of individual reproductive potential, but can only roughly reflect actual reproductive activity. We thus propose to be very cautious in extrapolating egg-laying activity levels from ovarian index estimations.