An indirect approach to study sperm precedence in a subsocial spider

Abstract

The subsocial spider Anelosimus cf. studiosus has a sex ratio biased toward females, but unexpectedly males perform ritualised fights to gain first access to females. Usually females will mate first with the winner male, but they can remate with loser males, remaining as satellite. We tried to discover why loser males wait to mate with a copulated female, instead of looking for virgin females. We considered the first male as a winner and the second as a loser, without allowing them to engage in fights. The objective of the present study was to determine sperm priority, by performing double copulations, with normals and males sterilised with gamma radiation (2000 rad). They were randomly assigned to one of the three experimental groups (IN, NI or NN). In the group IN, 13 females mated with irradiated males and 24–48 hr later with normal males. In the group NI, 12 females mated with normal males and 24–48 later with irradiated males, and in the control group NN, 12 females mated with 2 normal males. We opened the egg sac and counted the number of embryos and eggs. In the IN and NN groups we obtained 80% successful egg sacs and 75% in NI. We did not find differences in the total number of eggs laid per egg sac (eggs + embryos) or between the total number of embryos, among the three groups. The results suggest that first and second males would have similar percentages of paternity. Future studies will focus on morphology and mechanisms of female choice in this species.