Many of the methods and constructs used to measure human well-being are not directly applicable to nonhuman primates. However, psychological well-being is a topic that has become of great concern for the ethical care of captive nonhuman primates. In captive populations, animals are said to have psychological well-being when they show few physiological and behavioral signs of stress
(Moberg, 1985). Recently, this also has been found to be an indicator, or by-product, of psychological well-being in humans in that higher levels of psychological well-being have also been associated with lower levels of cortisol release (Lindfors and Lundberg, 2002). Environmental enrichment is often used to alleviate stress and promote psychological well-being in captive nonhuman primates. This is particularly important as stress has been clearly associated with a variety of undesirable physiological effects, mostly as the result of compromised immune system functioning (Kelly, 1985; Shapiro et al., 1998).
Typically, psychological well-being in nonhuman primates is measured through behavioral observations and/or hormonal analyses. However, at least one recent study has shown that well-being or "happiness" can be reliably estimated by human raters who are familiar with the individual (King and Landau, 2003). In these studies, zookeepers or researchers with extensive experience working with or observing individual chimpanzees were asked to rank them on overall mood, quality of social relationships, extent to which they were successful in achieving goals, and how much they would like to "be" a particular chimpanzee. Results showed that reliability across raters was as consistent as have been found in human studies of subjective well-being. Furthermore, this construct appears to be heritable and generically correlated with dominance (Weiss, King, and Enns, 2002). There was a strong correlation between personality and subjective well-being in chimpanzees, and many aspects of well-being in chimpanzees are surprisingly similar to those observed in humans (King and Landau, 2003).
There are, however, methodological issues associated with personality testing in nonhuman primates (Itoh, 2002). Personality in animals is often measured using subjective assessment methods such as questionnaires, which may have problems with construct validity. Most principal component analyses have not been tested for reliability across research groups. As deWaal (2002) has cautioned, subjective human ratings of animal personality traits may reflect anthropomorphic projections of human traits onto animal behavior. For a complete understanding of both the physiological and behavioral changes associated with aging, objective tests in which behaviors are systematically recorded coupled with subjective assessments from animal care providers may be the most useful methodological approach.
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