competition through a process known as "sperm choice". Proteins present in the female reproductive tract or on the surface of the ovum may influence which sperm succeeds in fertilizing the egg. During sperm choice, females are able to discriminate and differentially use the sperm from different males. One instance where this is known to occur is inbreeding; females will preferentially use the sperm from a more distantly related male than a close relative. Post-copulatory inbreeding avoidance Inbreeding ordinarily has negative fitness consequences (inbreeding depression), and as a result species have evolved mechanisms to avoid inbreeding. Inbreeding depression is considered to be due largely to the expression of homozygous deleterious recessive mutations. Outcrossing between unrelated individuals ordinarily leads to the masking of deleterious recessive mutations in progeny. Numerous inbreeding avoidance mechanisms operating prior to mating have been described. However, inbreeding avoidance mechanisms that operate subsequent to copulation are less well known. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between sperm of rival males for achieving fertilization. In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed. In vitro fertilization experiments in the mouse, provided evidence of sperm selection at the gametic level. When sperm of sibling and non-sibling males were mixed, a fertilization bias towards the sperm of the non-sibling males was observed. The results were interpreted as egg-driven sperm selection against related sperm. Female fruit flies (Drosophila melanogaster) were mated with males of four different degrees of genetic relatedness in competition experiments. Sperm competitive ability was negatively correlated with relatedness. Female crickets (Teleogryllus oceanicus) appear to use post-copulatory mechanisms to avoid producing inbred offspring. When mated to both a sibling and an unrelated male, females bias paternity towards the unrelated male. Empirical support It has been found that because of female choice (see sexual selection), morphology of sperm in many species occurs in many variations to accommodate or combat (see sexual conflict) the morphology and physiology of the female reproductive tract. However, it is difficult to understand the interplay between female and male reproductive shape and structure that occurs within the female reproductive tract after mating that allows for the competition of sperm. Polyandrous females mate with many male partners. Females of many species of arthropod, mollusk and other phyla have a specialized sperm-storage organ called the spermatheca in which