conflict-centric differential allocation hypothesis, which states that there is a tradeoff between investment and attractiveness. Genetic monogamy Genetic monogamy refers to a mating system in which fidelity of the bonding pair is exhibited. Though individual pairs may be genetically monogamous, no one species has been identified as fully genetically monogamous. In some species, genetic monogamy has been enforced. Female voles have shown no difference in fecundity with genetic monogamy, but it may be enforced by males in some instances. Mate guarding is a typical tactic in monogamous species. It is present in many animal species and can sometimes be expressed in lieu of parental care by males. This may be for many reasons, including paternity assurance. Evolution of monogamy in animals While the evolution of monogamy in animals cannot be broadly ascertained, there are several theories as to how monogamy may have evolved. Anisogamy Anisogamy is a form of sexual reproduction which involves the fusion of two unequally-sized gametes. In many animals, there are two sexes: the male, in which the gamete is small, motile, usually plentiful, and less energetically expensive, and the female, in which the gamete is larger, more energetically expensive, made at a lower rate, and largely immobile. Anisogamy is thought to have evolved from isogamy, the fusion of similar gametes, multiple times in many different species. The introduction of anisogamy has caused males and females to tend to have different optimal mating strategies. This is because males may increase their fitness by mating with many females, whereas females are limited by their own fecundity. Females are therefore typically more likely to be selective in choosing mates. Monogamy is suggested to limit fitness differences, as males and females will mate in pairs. This would seem to be non-beneficial to males, but may not be in all cases. Several behaviors and ecological concerns may have led to the evolution of monogamy as a relevant mating strategy. Partner and resource availability, enforcement, mate assistance, and territory defense may be some of the most prevalent factors affecting animal behavior. Facultative monogamy First introduced by Kleiman, facultative monogamy occurs when females are widely dispersed. This can either occur because females in a species tend to be solitary or because the distribution of resources available cause females to thrive when separated into distinct territories. In these instances, there is less of a chance for a given male to find multiple females to