Sperm allocation, the Coolidge effect and female polyandry

Abstract

In their recent comprehensive review in TREE, Wedell et al. [1xSperm competition, male prudence and sperm-limited females. Wedell, N. et al. Trends Ecol. Evol. 2002; 17: 313–320Abstract | Full Text | Full Text PDF | Scopus (654)See all References[1] demonstrate convincingly that, consistent with theory, males have evolved a diverse suite of strategies to allocate sperm strategically according to the reproductive value that is associated with individual inseminations. As the authors point out, strategic sperm allocation by males has important repercussions for female fitness. When a male limits his sperm investment in a female to save sperm for novel partners, he might inseminate too few sperm to fertilize all of her eggs, thus limiting the reproductive success of individual females [1xSperm competition, male prudence and sperm-limited females. Wedell, N. et al. Trends Ecol. Evol. 2002; 17: 313–320Abstract | Full Text | Full Text PDF | Scopus (654)See all References[1], or forcing females to seek more copulations.Wedell et al. suggest that preferential sperm investment in novel females (i.e. females not previously inseminated) might be mediated by a behavioural process that is commonly known as the Coolidge effect [1xSperm competition, male prudence and sperm-limited females. Wedell, N. et al. Trends Ecol. Evol. 2002; 17: 313–320Abstract | Full Text | Full Text PDF | Scopus (654)See all References[1]. This is the decline in the propensity of a male to copulate with a particular female as she becomes sexually familiar (i.e. the more a male copulates with her) and the resuscitation of his sexual interest when the sexually familiar female is replaced by a novel one. The Coolidge effect might mediate differential sperm investment, enabling a male to reduce progressively the number of sperm that he invests in a female according to how many sperm he has already inseminated into her, therefore distributing his sperm more evenly and adaptively across multiple females.That sexual familiarity progressively reduces male sperm investment in a female has one crucial consequence for females in polygynous species: successive copulations with the same male result in diminishing numbers of sperm inseminated. Therefore, a female obtains more sperm by copulating once with n novel partners than she does by mating n times with the same male. By copulating few times with multiple, novel partners, females can thus minimize the number of inseminations that are necessary to fertilize their eggs. Copulations are often costly to females [2.xCost of mating in Drosophila melanogaster females is mediated by male accessory gland products. Chapman, T. et al. Nature. 1995; 373: 241–244Crossref | PubMedSee all References, 3.xLongevity in Caenorhabditis elegans reduced by mating but not gamete production. Gems, D. and Riddle, D.L. Nature. 1996; 379: 723–725Crossref | PubMed | Scopus (123)See all References, 4.xCorrelated effects of sperm competition and post-mating female mortality. Civetta, A. and Clark, A.G. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 13162–13165Crossref | PubMed | Scopus (107)See all References, 5.xTraumatic insemination and sexual conflict in the bed bug Cimex lecticularius. Stutt, A. and Siva-Jothy, M.T. Proc. Natl. Acad. Sci. U. S. A. 2001; 98: 5683–5687Crossref | PubMed | Scopus (184)See all References]. Nevertheless, females of many species actively seek copulations with multiple males, and although many hypotheses have been suggested, the evolution of female polyandry remains unresolved [6xPost-insemination sexual selection. Birkhead, T.R. and Pizzari, T. Nat. Rev. Genet. 2002; 3: 262–273Crossref | PubMed | Scopus (323)See all References[6]. Male differential sperm allocation mediated by the Coolidge effect puts the evolution of female polyandry in a novel perspective and might contribute to our understanding of this puzzling female trait.