The normal behavioral patterns of a species can be important to model choice, whether or not the researcher is interested in the animal's behavior per se. The normal social organization of a species affects such variables as how animals must be housed or fed and under what circumstances they will reproduce successfully. Social and individual behavioral characteristics may influence research variables in both obvious and subtle ways. An example of an obvious behavioral constraint is that, with many species, placing more than one adult male in a social group under the relatively confined conditions of captivity will result in serious aggression that may disrupt research. A more subtle, in this case positive, effect of social variables is the finding that female Macaques with irregular menstrual cycles can be induced to cycle normally, and thereby improve their breeding performance, when housed with conspecific females with regular cycles (Wallis et al., 1986). Even more subtle effects have been observed, e.g., as a result of behavioral sex differences in mice, females, not males, show enhanced immune responses when housed in groups (Riley, 1981). Such behavioral effects may introduce uncontrolled variables into experiments if they are not anticipated by the researcher.