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Abstract
Male Mastophora cornigera exit egg sacs as adults, which allowed us to determine spiderling sex ratios and patterns of maternal investment in this species. We collected 15 egg sacs produced by seven mothers, which yielded 1945 emergent spiderlings which were sexed, 1850 of which were weighed. Two emergent broods were significantly male and female biased and were unaffected by pre-emergence mortality. The weights of male and female spiderlings differed in eight broods, with males and females being heavier in four cases each. Five of these broods were derived from multiple egg sac sets produced by one mother, and in each case, the total mean male and female spiderling weights for all broods in a set were biased in the same direction as the biased brood(s) within that set. Mean emergent spiderling weight was independent of brood size and sex ratio for both males and females. Despite such independence, sex allocation in M. cornigera can favor sons, daughters, or both equally, and by numbers, by weight, or both at once. The proximate mechanisms and adaptive significance of such variability is unknown. We also review evidence for gender-biased allocations in arachnid offspring and suggested mechanisms for their applicability to M. cornigera.
Acknowledgments
We thank Dina Abdulla, Lisa Liceaga, Allison Quan, and Ranielle Rodriguez for laboratory assistance; Peter Bryant for sharing with us his knowledge concerning potential Mastophora sample sites; and Victor Carmona-Galindo for the use of his Mettler Toledo microbalance. We are also grateful to Suzanne Nevius for allowing us to collect an M. cornigera female and its egg sac from her backyard. The helpful comments of two anonymous reviewers are greatly appreciated.