Charnov and Finerty (1980) suggested that vole population density can be regulated by aggressive interactions that change with the relatedness of neighbors. As density increases, the relatedness of neighbors will decrease because of dispersal. Presumably, unrelated individuals will behave less amicably (more aggressively) toward one another. As density increases, dispersal will cause the relatedness of neighbors to decrease and therefore increase the levels of aggression in the population. Increased aggressive levels are postulated to cause the crash of numbers seen in vole population fluctuations. Inherent in this mechanism is the ability for voles to distinguish, and respond differentially between relatives and nonrelatives. As we will show, for this hypothesis to be correct, it must take into consideration cryptic relatedness, relatedness in which the members of the dyad cannot, in fact, determine that they are related. Familiarity, based on association prior to weaning, is the proximate cue that microtine rodents use to assess kinship (Blaustein et al. 1987a,b, Ferkin 1990). Familiar nestmates are recognized as close relatives, whereas unfamiliar nestmates are not identified as relatives (Schadler 1983, Gavish et al. 1984, Boyd and Blaustein 1985, Ferkin 1989, Ferkin and Rutka 1990). To date, there is no evidence to show that microtine rodents employ phenotype matching, recognition alleles, or the major histocompatibility complex to assess kinship (Yamazaki et al. 1980). Genetic relatedness, per se, plays no apparent role in kin recognition for microtine rodents (Bekoff 1981, Blaustein et al. 1987a,b, Ferkin 1990). Thus, the ability to assess genetic relatedness of unfamiliar relatives appears unlikely for microtine rodents. The persistence of kin recognition is dependent on repeated social contacts with close kin (Ferkin 1988, 1990). Although kin recognition and kin-directed behaviors, such as reduced agonism, sharing of nest sites, increased nepotism, and the avoidance of inbreeding, can operate without kin recognition if related individuals interact frequently (Hamilton 1964), repeated contacts with close relatives may not be a feature common to most microtine social organizations (Wolff 1985, Cockburn 1988). The opportunity to identify and be recognized as a close relative will be affected markedly by the type of mating system, population turnover rate, dispersal tendency, nesting patterns, and parental care displayed in a particular species. Therefore, the opportunity to identify and be recognized as a close relative will be affected markedly between microtine species. Microtine rodents that do not have protracted interactions with close relatives may not be able to recognize unfamiliar close relatives.