Parker’s sneak-guard model revisited: why do reproductively parasitic males heavily invest in testes?
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Alternative reproductive tactics are widespread in males and may cause intraspecific differences in testes investment. Parker’s sneak-guard model predicts that sneaker males, who mate under sperm competition risk, invest in testes relatively more than bourgeois conspecifics that have lower risk. Given that sneakers are much smaller than bourgeois males, sneakers may increase testes investment to overcome their limited sperm productivity because of their small body sizes. In this study, we examined the mechanism that mediates differential testes investment across tactics in the Lake Tanganyika cichlid fish Lamprologus callipterus. In the Rumonge population of Burundi, bourgeois males are small compared with those in other populations and have a body size close to sneaky dwarf males. Therefore, if differences in relative testis investment depend on sperm competition, the rank order of relative testis investment should be dwarf males > bourgeois males in Rumonge = bourgeois males in the other populations. If differences in relative testis investment depend on body size, the rank order of relative testes investment should be dwarf males > bourgeois males in Rumonge > bourgeois males in the other populations. Comparisons of relative testis investment among the three male groups supported the role of sperm competition, as predicted by the sneak-guard model. Nevertheless, the effects of absolute body size on testes investment should be considered to understand the mechanisms underlying intraspecific variation in testes investment caused by alternative reproductive tactics.
KeywordsSperm competition Alternative reproductive tactics Body size Lamprologus callipterus
We are indebted to Masta Mukwaya Gashagaza, Nshombo Muderwa, Harris Philli, Danny Sinyinza and other colleagues of the Democratic Republic of Congo, Republic of Burundi and Republic of Zambia for their kind collaboration and support for the entire research. We are also very grateful to the Japanese Research Team for sampling and practical assistance, Yasuoki Takami (Kobe University), Michael Taborsky (University of Bern) and three anonymous reviewers for providing helpful comments on earlier drafts of the manuscript, and the editor-in-cheif Sven Thatje for his advice. The research presented here was conducted under permits for fish research in Lake Tanganyika from CRSN, the Burundi government, and the Zambian Ministry of Agriculture, Food and Fisheries, and complies with the laws of each country and the guidelines of the Animal Care and Use Committee of Osaka City University, Kyoto University, and the Japan Ethological Society. Funding was provided from a Grant-in-Aid for Research Fellowship from JSPS for Young Scientists and an Overseas Scientific Research grant (MEXT).
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