the sperm of different males sometimes compete for increased reproductive success. Species of crickets, specifically Gryllus bimaculatus, are known to exhibit polyandrous sexual selection. Males will invest more in ejaculation when competitors are in the immediate environment of the female. Evidence exists that illustrates the ability of genetically similar spermatozoa to cooperate so as to ensure the survival of their counterparts thereby ensuring the implementation of their genotypes towards fertilization. Cooperation confers a competitive advantage by several means, some of these include incapacitation of other competing sperm and aggregation of genetically similar spermatozoa into structures that promote effective navigation of the female reproductive tract and hence improve fertilization ability. Such characteristics lead to morphological adaptations that suit the purposes of cooperative methods during competition. For example, spermatozoa possessed by the wood mouse (Apodemus sylvaticus) possess an apical hook which is used to attach to other spermatozoa to form mobile trains that enhance motility through the female reproductive tract. Spermatozoa that fail to incorporate themselves into mobile trains are less likely to engage in fertilization. Other evidence suggests no link between sperm competition and sperm hook morphology. Selection to produce more sperm can also select for the evolution of larger testes. Relationships across species between the frequency of multiple mating by females and male testis size are well documented across many groups of animals. For example, among primates, female gorillas are relatively monogamous, so gorillas have smaller testes than humans, which in turn have smaller testes than the highly promiscuous bonobos. Male chimpanzees that live in a structured multi-male, multi-female community, have large testicles to produce more sperm, therefore giving him better odds to fertilize the female. Whereas the community of gorillas consist of one alpha male and two or three females, when the female gorillas are ready to mate, normally only the alpha male is their partner. Regarding sexual dimorphism among primates, humans falls into an intermediate group with moderate sex differences in body size but relatively large testes. This is a typical pattern of primates where several males and females live together in a group and the male faces an intermediate number of challenges from other males compared to exclusive polygyny and monogamy but frequent sperm competition. Other means of sperm competition could include improving the sperm itself or its packaging materials (spermatophore). The male black-winged damselfly provides a striking example of an adaptation to sperm competition. Female black-winged