Characterizing the phenotypic changes associated with aging in a short-lived primate is necessary in order to develop better translational models for human health, aging, and disease research. A population of conventionally housed marmoset monkeys was assessed to determine if phenotypes of body composition, hematology, and morphometrical measures were associated with age or risk of death. We found that the cause of mortality in older marmosets was more likely to be due to cardiac and chronic kidney disease than in younger marmosets. Older marmosets have decreased fat mass, morphometric measures, and serum albumin. Older marmosets are more likely to show a modified posture while at rest and this modified posture was significantly associated with an increased risk of imminent death. These assessments provide an initial definition of aged health in marmosets and a base for future translational aging research with this species. 1. Introduction The relationships between health, aging, tissue function, and disease in primates and humans is often not well-modeled by rodent studies [1]. Nonhuman primates are our closest evolutionary relatives and as such are more similar to humans in terms of anatomy, embryology, fetal development, immunology, biochemistry, gene interactions, sensory apparatus, and overall physiological and psychological function than any other animal group. As a consequence, research with nonhuman primates is particularly relevant for the understanding of human health, disease, and therapeutics. The characterization of aging in a short-lived primate will open new possibilities for the assessment of health in the context of aging. Marmosets are small new world primates that offer a valuable resource as an animal model to examine adult disease risk, aging, and functional decline because they have the shortest average lifespan and fastest reproduction of any anthropoid primate [2]. Additionally, the long-standing use of marmosets as a model for family interactions, hormonal development, reproductive output, and medical research has resulted in a large base of average values for growth, body weight, and hematological measures. Marmosets are sexually monomorphic, and adults weigh an average of 300–500 grams in captivity [2]. They typically produce litters consisting of fraternal twins, with a gestational length of 143 days. Marmosets reach sexual maturity at approximately eighteen months, and the average lifespan in captivity for Callithrix jacchus is 4 to 6 years [2–4]. Marmosets are often considered “aged” at 8 years of age at which point studies
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