Experimental studies on male-female mortality differences in nonhuman species are important because they provide insights into both the nature of age-specific gender differences and the concept of selective survival - whether one subgroup in a population (e.g., males) is consistently more frail than another subgroup (e.g., females). We found that it was not possible to classify either sex as more robust or longer lived since relative longevity was conditional on age (young or old), cage conditions (solitary confinement or grouped cages), and treatment (starvation, irradiation, or density). Implications of these findings are discussed including selective survival, demographic selection, a framework for male-female mortality differentials, and an evolutionary perspective on gender differences in longevity. Inglés