The Plug of Nephilid Spiders – A Raffles Museum Fellowship Research

Mate plugging via genital mutilation in nephilid spiders: an evolutionary hypothesis
M. Kuntner, S. Kralj-Fiser, J. M. Schneider & D. Li
Journal of Zoology 277 (2009) 257–266

Abstract
Nephilid spiders are known for gigantic females and tiny males. Such extreme sexual dimorphism and male-biased sex ratios result in fierce male–male competition for mates. Intense sperm competition may be responsible for behaviors such as mate guarding, mate binding, opportunistic mating, genital mutilation, mating plugs and male castration (eunuchs). We studied the mating biology of two phylogenetically, behaviorally and morphologically distinct south-east Asian nephilid spider species (Herennia multipuncta, Nephila pilipes) in nature and in the laboratory. Specifically, we established the frequencies and effectiveness of plugging (a plug is part of the male copulatory organ), and tested for male and female copulatory organ reuse. Both in nature and in the laboratory, plug frequencies were higher in H. multipuncta (75–80% females plugged) compared with N. pilipes (45–47.4%), but the differences were not significant. Plugs were single and effective (no remating) in H. multipuncta but multiple and ineffective (remating possible) in N. pilipes. In Herennia, the males plugged when the female was aggressive and in Nephila plugging was more likely when mating with previously mated and larger females. Further differences in sexual biology are complete palpal removal and higher sexual aggressiveness in Herennia (sexual cannibalism recorded for the first time), and mate binding in Nephila. Thus, we propose the following evolutionary hypothesis: nephilid plugging was ancestrally successful and enabled males to monopolize females, but plugging became ineffective in the phylogenetically derived Nephila. If the evolution of nephilid sexual mechanisms is driven by sexual conflict, then the male mechanism to monopolize females prevailed in a part of the phylogeny, but the female resistance to evade monopolization ultimately won the arms race.

Raffles Museum Fellowship
RMBR has a short-term visiting fellow program which offers research stints of 2-6 weeks in Singapore. This program endeavours to bring in experts in relevant fields to work with RMBR staff and students to further the cause of Southeast Asian biodiversity research. Fellowships are given on a competitive basis, dependent on funds and research areas identified. A long-term fellowship program is also available (6-12 months).

Matjaž Kuntner is a Slovenian evolutionary biologist, arachnologist and nature photographer. He is a Research Associate (Zoology) at the Institute of Biology, Scientific Research Centre of the Slovenian Academy of Sciences and Arts. He is also the Research Collaborator at the National Museum of Natural History, Smithsonian Institution. His research interests are in Systematic and Evolutionary Biology, Arachnology, Taxonomy of orb weaving spiders, Evolution of mating systems in spiders, Spider behaviour including web architecture, Tropical and Temperate spider diversity and in Slovenian arachnid fauna. He worked on the mating behaviour and sexual mating frequencies and choice on several species of orb weaving spiders which includes Nephila antipodiana, Nephila pilipes, Herennia multipuncta and Nephilengys malabarensis, while here in Singapore from 20th April to 28th Jun 2007. Together with Prof. Li Daiqin (Associate Professor, Dept of Biological Sciences, NUS), they were funded by the RMBR fellowship on the above study.

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