Mate guarding is a reproductive behaviour, primarily of males, in an attempt to increase the likelihood of success of their own sperm fertilizing the eggs of females when competition is involved. Mate guarding behaviour can be pre- or post-copulatory. Studies on the behaviour both based on theory and through experiments have been conducted on a wide range of species. Mate guarding also occurs in humans. 
In organisms where polygamy is dominant such as in most mammals, birds, and insects, there is significant male-male competition even prior to copulation. When organisms mate with multiple males and store sperm, maintaining their viability within storage structures, there is sperm competition and males evolve mechanisms to increase their odds of successful parenting. In some species such as the Apollo butterflies the males produce mating plugs that prevent the female from mating again. In some dragonflies, males will physically hold females after copulation to increase the chances that their own sperm fertilize the eggs laid. In some moths where males prefer virgin females to mate, males may mark females with chemical compounds after copulation that make them less attractive to males. Mate guarding by males comes with costs for them in terms of reduced time available for feeding and other activities. Males may also have to choose between strategies of mating with multiple females or in choosing fewer females and guarding them during the critical phases. Female strategies can include multiple mating to ensure that they obtain fitter sperm to pass on benefits to their offspring and so mate guarding can sometimes appear as a hindrance to them, however females may also receive other benefits such as protection from predation through the actions of the guarding males. 
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