Vladimir Anisimov of the Science for Life Extension Foundation here discusses some of the differences in aging between individuals, and what might be learned from primate studies: "it is well known [that] heterogeneity is crucial feature of the aging process. There are individual peculiarities in vulnerability and resistance to stresses and stress-related pathologies among different persons due to the heterogeneity of the ageing process. It is of great importance to elucidate the individual specificity of age-related changes of the [hypothalamic-pituitary-adrenal (HPA) axis] in the context of aging of the organism as a whole and the stress-dependent age pathologies. ... [The aim of recent research] was see in which way the differences in the HPA axis function in young (6-8 years) and old (20-27 years) female rhesus monkeys depends on various behavioral types, under basal conditions, as well as under conditions of acute psycho-emotional stress. It has been found that the monkeys with depression-like behavior demonstrate age-related changes in the HPA axis function. ... taken together these results suggest that age-related dysfunctions of the HPA axis are individual features associated with peculiarities of adaptive behavior of animals. This approach seems very fruitful for future studies. Indeed, nonhuman primates and humans are [similar] in physiology of HPA: both species have cortisol as a main glucocorticoid hormone, similar circadian rhythm of HPA activity, age-related decline in DNEA sulfate secretion. Furthermore, unlike rodents, nonhuman primates feature the psycho-emotional reactions and adaptive behavior more similar to those in humans."