Cryptic choice hypothesis
Cryptic choice hypothesis assumes that the processes occurring in the female reproductive organs are a continuation of sexual selection (Birkhead 1995). Through manipulation with the ejaculate, the female is capable of subsequently determining which sperm of the males with whom she recently copulated will finally fertilize the egg cells. Some authors have even suggested that, in this “decision-making”, the females are capable of “taking into consideration” the genotype of the individual sperm and their own genotype so that, in the final analysis, they allow the egg to be fertilized, for example, only by those sperm that ensure heterozygosity of the progeny for genes important for resistance, for example genes for MHC-antigens (Wedekind 1994; Lopez-Leon, Cabrero, & Camacho 1996; Stockley 1999) (Fig. XIV.3). Similar phenomena have also been observed for plants. Here, selection apparently occurs through affecting the rate of growth of the individual pollen tubes (Matthys-Rochon, Gaude, & Dumas 1987; Charlesworth, Schemske, & Sork 1987; Wendel, Edwards, & Stuber 1987).s
Unambiguous interpretation of the results of experiments studying cryptic choice is somewhat complicated by the ability of females to affect the viability of progeny through uneven investment into the individual embryos. It has, for example, been observed that the females of zebra finch (Taeniopygia guttata) that were able to copulate with more attractive males produce eggs with higher testosterone concentrations (Vogel 1999). In the traditional arrangement of experiments, i.e. without the use of artificial insemination, it is thus frequently not possible to decide whether the uneven success of the individual males is a result of cryptic choice occurring at the level of competition between sperm or classical sexual selection occurring at the level of the adult individuals. Results to date using artificial insemination indicate that it is possible that the cryptic choice mechanism also allows females to select males exhibiting quite specific phenotype traits, including the degree of expression of classical secondary sexual traits or body size (Evans et al. 2003) (Fig. XIV.4).