Sci. Aging Knowl. Environ., 13 October 2004
Vol. 2004, Issue 41, p. pe38
[DOI: 10.1126/sageke.2004.41.pe38]


Nuns and Monkeys: Investigating the Behavior of our Oldest Old

Judith A. Corr

The author is in the Department of Anthropology, Grand Valley State University, Allendale, MI 49401, USA. E-mail: corrj{at}

Key Words: primate • macaque • rhesus monkeys • behavior • personality • Alzheimer's disease


From every angle, aging is an important topic. Not one of us will escape the ravages of aging, but increased understanding of the processes of senescence and their behavioral consequences can help lessen its impact. All sexually reproducing organisms "senesce," that is, they experience physiological decline with increasing age. But not all organisms "age gracefully" or enjoy a good quality of life in later years. The goal of any research involved with the mysteries and mechanisms of aging must ultimately be to improve the health and quality of life in our oldest old.

How We Know What We Know

Traditionally, nonhuman primates have been considered the best model for the investigation of biological aging because of their close phylogenetic relationship to humans. In particular, aged rhesus monkeys (Macaca mulatta) are valued for their close physiological similarity to aged humans (1). Recently, however, several important longitudinal aging-related studies employing humans as study subjects have been reported in both the scientific and popular press: the MacArthur Foundation Study of Successful Aging (2), the Nun Study (3), and, most recently, the Harvard Study of Adult Development (4).

All three of these studies have stimulated a great deal of public interest in aging, and published accounts of them are available at your local bookstore (in large print) (see "Sister Knows Best"). In particular, the Nun Study was widely discussed in the print and broadcast media, earning one 91-year sister the cover of Time magazine (5). Snowdon's ongoing longitudinal study looks at 678 Catholic nuns aged 75 to 106, employing archival records from their early and middle years as well as interview and medical data taken by researchers from age 75 to death. Combining an individual life history approach with after-death examination of the sisters' brain tissue, Snowdon is, for the first time, able to say something specific about links between behavior, personality, and the risk of Alzheimer's disease and how the brutal consequences of this disease might be avoided.

The Old Monkey Model

Although use of a model is never as good as observing the "real thing," nonhuman primates continue to make a critical contribution in the understanding of both physiological and cognitive changes that occur with "normal" primate aging (6). Primate gerontology is an emerging field that employs an array of research methods to explore the management of normal aging processes and age-related disorders. Nonhuman primates experience the same "normal" cognitive and physiological changes with increasing age as humans, and their clinical presentations are similar as well (7). Both human and nonhuman primates age "normally." Like us, old monkeys and apes have hair that thins and turns gray, skin that wrinkles, and muscles that decrease in mass; they also experience age-related mental declines and loss of bodily functions (8). Like us, they have their forgetful "senior moments" and experience decreases in the ability to see, smell, taste, touch, and hear. This is why the use of nonhuman animal models to investigate specific human problems, such as aging, is well established.

Studying aging in wild free-ranging primate populations is difficult; unless a group has been monitored for generations, the histories and ages of wild individuals are not known. Further, because of environmental pressures, including predation, there is a basic lack of aged animals in the wild (see Holmes Perspective). For this reason, semi free-ranging colonies of nonhuman primates that are maintained specifically for observational research make an adequate alternative. Such a colony has been maintained since 1939 on Cayo Santiago, an island 1 km off the southeast coast of Puerto Rico. The Cayo colony of rhesus macaques (Macaca mulatta) is maintained at approximately 1000 animals, who are monitored, maintained, and censused daily. An aged female and an aged male macaque from the Cayo colony are shown in Fig. 1 and Fig. 2.

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Fig. 1. Aged macaque female. This female (designated 577) was 25 years old when the photo was taken (1999), and the oldest animal in the colony on Cayo Santiago at that time.


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Fig. 2. Aged macaque male. This monkey (designated A92) was 19 years old when the photo was taken (1999).

Now, however, the emerging phenomenon of growing numbers of nonhuman primates in captive populations with known life histories provides an enormous research potential spanning the primate spectrum from marmosets to the great apes (9). Primate caretakers are now responsible for an increasingly aged captive primate population found in zoos, research centers, and free-ranging colonies, as well as in several recently formed retirement/sanctuary centers for aged monkeys and apes (particularly chimpanzees) who come from the entertainment industry, the pet industry, and research settings (primates in this last category represent those who are now "surplus to research needs") (10). Using this resource, scientists can undertake cross-disciplinary research to inform the lives of both human and nonhuman primates. The Nun Study, for instance, can help us to appreciate the critical role of enrichment and psychosocial stimulation in the successful aged individual, an insight that can be directly applied in facilities that are now housing large numbers of "retired" primates. In turn, increased understanding of the role of individual personality in the adjustments of these "retired" primates to new challenges can inform the role of individual personality in the social and cultural adjustments of elderly humans.

Once again, all primates, human and nonhuman, are universally subject to the processes of senescence (11). As the earth's human and captive nonhuman primate population pyramids continue to swell at their heights, reflecting a growing proportion of aged individuals, it becomes increasingly important to investigate how the aged can continue to live active and fulfilled lives, thereby increasing both their quality of life and their independence. Currently, nonhuman primate gerontology is contributing to this important task in many physiological areas including (i) the use of drugs that enhance the function of the neurotransmitter gamma-amniobutyric acid to improve and maintain higher cortical function (12) (see "Old Neurons Revisit Their Youth"); (ii) the role of calorie restriction in increasing longevity (13) (see Masoro Subfield History); (iii) Alzheimer's disease (14); and (iv) age-related hormonal changes (15). All such studies aim to lengthen and enrich the lives of our oldest old.

What About Behavior?

Unlike the processes of biological aging, or senescence, changes in behavior with increasing age are not well understood for two major reasons. First, biological senescence and nonbiological (or behavioral) aging interact and affect each other. Senescence might, for instance, result in personality and behavioral changes in the aging individual (16). Is a withdrawn elder withdrawn because of behavior and personality, or because of sight or hearing impairments? Second, whereas senescence is a universal phenomenon, responses to becoming older vary widely in humans by culture, population, and the individual. This is also true of nonhuman primates whose aging style also varies by social structure, sex roles, and individual personality (17). Obviously, these factors complicate research inquiries and challenge the observer to separate senescence and its effects from individual behavior. I argue that they shouldn't be completely separated lest we miss seeing how one informs the other.

Beginning in the 1960s, gerontologists studying human aging formalized a number of models of behavioral aging to explain an observed decrease in social interaction among the elderly (18). Several theories were formulated based on data from the 1954 Kansas City Study of Adult Life, and variously fell in and out of favor. Most of these theories have been tested in nonhuman primates, most notably the theory of social disengagement, which argues that a gradual, voluntary withdrawal or disengagement occurs between the aged person and the society that ultimately culminates in death. The results from these studies are contradictory--which is, I would argue, a testament to the important role of varying primate societal structures. In tests of these gerontological theories in humans, the results are also varying, and for the same reason: Human societies and the embedded social roles of the aged also vary.

I have argued elsewhere (19) that theory development is vital to the maturity of any emerging discipline, but neither human or nonhuman primate gerontology, in my view, has developed such a theoretical framework. Most recently, the evolutionary theory of aging has been through a makeover of the "grandmother hypothesis" and suggests that natural selection does favor longevity in those family lines whose postreproductive elders are present to nurture younger generations (20). In other words, my sons are ensuring the survivability and success of their children by consulting me, the children's grandmother. Not a new idea, but an excellent opportunity to remind us that behavior matters.

Use It or Lose It!

"Owning an old brain is like owning an old car, careful driving and maintenance are everything." (4, pg. 213)

Although we share the substrate of primate biology with other primate species, humans uniquely embroider culture and cultural variation onto biology. And, as we gain insights from longitudinal studies of human aging, it becomes increasingly clear that culture, individual behavior, and personalities play a larger role in healthy longevity than was once thought (21). The Nun Study suggests this when it reports that positive emotions (as scored from archival documents) were strongly associated with longevity (22). Not only does behavior interact with the biological reality of aging, it is also reported that behavior may have the power to overrule it. For instance, Sister Mary, often described as an "energized" 101-year-old, had excellent cognitive and physical function prior to death, yet postmortem neuropathologic evaluation showed that classic Alzheimer's lesions were prevalent in her brain (23) (see "Detangling Alzheimer's Disease"). In the case of Sister Mary, her positive, upbeat personality and active lifestyle may have been preventive of Alzheimer's symptoms. I have observed many aged female macaques on both Cayo Santiago and Morgan Island (South Carolina) who remind me of this engaged, energetic nun...they go, and go, and go, until the end. Interestingly, both macaque females and the Nun Study sisters live with a certain group of females throughout their lives--the macaques within a "matriline," (that is, groups of mostly related females) and the Nun Study sisters in groups of females who, although generally not related, have close, interactive, and lifelong relationships. Based on the Nun Study, it would be informative, I believe, to look specifically at the relationship between individual personalities and longevity in aged macaque females.

Clearly, we need to take this "path wide open" toward a better understanding of the psychosocial influences on aging. The mind-body connection is a powerful one, and we will more accurately connect the dots in all primate life histories if we take advantage of the opportunity that is before us: an increasing aged primate population in both human and nonhuman primates (7). Ultimately, successful aging is more than simply surviving, as the Nun Study suggests; it is remaining active and engaged in society--behavior that, when you think about it, increases the quality of life at any age.

For me, I continue to take my silver vitamins, glucosamine-chrondroitin, and calcium. I try to cut down on fat, increase levels of the antioxidant lycopene in my diet, and make other beneficial choices. But along with that, I also follow my almost 90-year-old mother's, learn new things, talk to someone younger every day, and do difficult crossword puzzles.

October 13, 2004
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Citation: J. A. Corr, Nuns and Monkeys: Investigating the Behavior of our Oldest Old. Sci. Aging Knowl. Environ. 2004 (41), pe38 (2004).

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