DENSITY-DEPENDENT MATING SUCCESS AND COLOR POLYMORPHISM IN FEMALES OF THE DAMSELFLY ISCHNURA-GRAELLSII (ODONATA, COENAGRIONIDAE)

1. Female-limited colour polymorphism is very common in odonates. One of the forms has male-like colouring (androchromotypics), while the other(s) is cryptic (gynochromotypics). Three hypotheses have been proposed to explain the maintenance of this polymorphism: (1) higher reproductive isolation of androchromotypics, balanced by higher predation on this form; (2) androchromotypics avoid unnecessary long matings, but suffer greater predation; and (3) androchromotypics avoid male harassment at high density, but have lower mating success at low density. 2. To test these hypotheses I measured survivorship and mating success of andro- and gynochromotypic females of Ischnura graellsii in two natural populations with different densities in Galicia (NW Spain). 3. Contrary to the predictions of hypotheses 1 and 2, mean longevity and daily survival rate was the same for andro- and gynochromotypics. 4. Mating success was the same for andro- and gynochromotypics at high density but the proportion of unmated females was greater in andro- than gynochromotypics at low density. Furthermore, androchromotypics mated with longer inter-copula intervals at high density, and mating duration was also density-dependent. These results suggest that hypothesis 3 is the most appropriate to explain the maintenance of the polymorphism. 5. Androchromotypics were larger than gynochromotypics in the high density population. As body size is related to larval nourishment, this suggests an effect of larval competition on the maintenance of polymorphism. 6. It is proposed that both the mating behaviour of this species (long copulations) and the existence of changes in population density during the season are the main factors that maintain the polymorphism in I. graellsii.