Behavioral characteristics of an extremely old rhesus macaque in a zoo: Dementia-like symptoms and implications for quality of life of geriatric animals
Posted on: 9 June 2026
How dementia symptoms in a Guinness-record-aged female rhesus macaque influenced her overall behaviors
Selected by Stefan Friedrich Wirth
Categories: animal behavior and cognition, neuroscience, pathology, zoology
Introduction
The authors of this preprint investigated dementia-like behavioral abnormalities in an unusually old female rhesus macaque, Macaca mulatta (Papionini; Fig 1).The research was performed based on longtime video documentations and corresponding scientific analyses using statistical and qualitative approaches. Only little is known about the overall aging process in rhesus macaques. Studies on pathological behavioral changes due to aging have rarely been conducted in macaques, partly because these animals do not typically live very long lives, either in the wild or in zoos (see for example M. G. Baxter et al., 2023). The female animal studied here, referred to as „ISK“, was so far a notable exception. The individual ISK died in 2021 in the Kyoto Zoo (Japan) at the age of 43 years and 4 months.
![Fig.1) Rhesus macaque, Red Fort, Agra, India. Photo credit: By Yann (own work) [CC BY-SA 4.0-3.0-2.5-2.0-1.0], via Wikimedia Commons.](https://prelights.biologists.com/wp-content/uploads/2026/06/rhesus-macaque-300x200.jpeg)
Previous research on rhesus macaques has found that while associations between tau pathology and neurological symptoms can be observed, these are not characterized by general cognitive decline, but rather, as in the grey mouse lemur, negatively affect object discrimination learning or, as in cynomolgus macaques, lead to Parkinson’s-like symptoms.
In this preprint, the authors used various data sources to assess ISK’s activity budgets, behavioral rhythms, gait, quality of life (QoL) scores and anecdotal episodes, allin relation to senile plaques and phosphorylated tau deposition in her brain.
Main findings
- An autopsy performed after ISK’s death revealed senile plaques and deposits of phosphorylated tau protein in her brain; additionally small amounts of neurofibrillary tangles had accumulated in the hippocampus. While these findings are indicative of Alzheimer-like dementia in humans, their significance in macaques had not previously been established. The behavioral study noted signs of age-related pathology, though these did not reach the severity observed in human Alzheimer’s pathology.
- ISK was active both day and night, albeit with a stronger propensity for diurnal activity; ISK was however more active at night than its conspecifics (Fig. 2).
- ISK interacted with her conspecifics but was last observed grooming a conspecific in January 2021. She did continue to react to other individuals also later on. However, these interactions became progressively less frequent during the final year of her life.
- ISK exhibited progressive changes in her gait, characterized by an altered walking rhythm and a tendency to asymmetrically use of her body.
- ISK spent an increasingly large amount of time in the indoor enclosure. A detailed case observation revealed that when venturing into the outdoor enclosure, ISK displayed pronounced signs of disorientation and diminished obstacle recognition; this led her to first become entangled in dense undergrowth and subsequently fall into a ditch, from which she had to be rescued by a zookeeper. However no direct link could be established between Tau pathology and cognitive decline based on cognitive tests.
- Compared to younger conspecifics, ISK exhibited longer periods of sleep relative to her waking hours.
Why I am highlighting this preprint
Aging research is important not only for better understanding our own age-related frailties and improving the treatment of pathologies in humans, but also for zoo animals – primates in particular. Effective veterinary and husbandry care for all forms of age-related impairments (including cognitive ones) is possible only if we first understand what the aging process actually looks like in these animals. Yet, particularly for primates, there is still limited knowledge on this subject.
Given that the physiological state of the studied rhesus macaque, as revealed by a neuroanatomical autopsy showing tau protein deposits, bears striking similarities to Alzheimer’s disease in humans, and given that the corresponding external symptomatology in primates remains largely unknown, I found it of great interest that the preprint authors investigate in detail which degenerative processes can occur in rhesus macaques.
As vital as aging research in primates is, it can only be conducted if individuals who have reached a noteworthy age are actually available for study. The individual examined here fulfilled this criterion in such an exceptional manner that it even earned an entry in the Guinness Book of World Records.
In my view, the multifaceted methodological approach employed here, which combines anatomical analysis via autopsy with observations of the living animal (utilizing long-term video analysis) and anecdotal individual observations, could serve as a pioneering model for future aging research projects involving also other primate species both in the wild and kept in the zoo.
Questions to the authors
- How might the animal’s condition during her final year of life be distinguished from her former social acceptance within her younger conspecifics? Was ISK’s behavior so markedly abnormal that her conspecifics actively avoided her? Was she ostracized? Or did her social standing merely shift gradually, essentially as a consequence of her own choices to become more withdrawn? And did the reaction of her conspecifics to her decline, at least ostensibly, resemble the kind of empathy we extend to the elderly: a willingness to offer support rooted in the knowledge that they have long rendered faithful service to our society?
- Disorientation, such as that described in the anecdotal account wherein ISK first became entangled and subsequently fell while in the outdoor enclosure, may be attributed to neurodegenerative processes; however, it could also or additionally be a consequence of declining sensory acuity. What was the state of the animal’s vision, hearing or sense of touch? Could these sensory deficits have contributed, at least in part, to the observed behavioral anomalies?
- ISK set a longevity record for rhesus macaques. To what extent can this be explained by her life history? Is this longevity attributable to her upbringing in a zoo environment, and specifically to the improved understanding of optimal housing conditions for primates that has emerged in recent times? In this context, is it reasonable to anticipate that future generations of zoo-housed rhesus macaques will more frequently attain advanced ages, thereby allowing the insights gained from ISK’s case to enhance the care provided to future geriatric rhesus macaques?
- What is known about behavioral and cognitive signs of aging in wild rhesus macaques? Are there at least anecdotal reports regarding this?
Future perspectives
The results of this study may contribute to a better understanding of physiological degenerative signs of aging and their link to behavioral components in macaques and other non-human Old World monkeys. This could also yield evolutionary biological insights regarding the underlying mechanisms of age-related dementia in humans.
This study, as well as future follow-up studies on age-related degeneration in rhesus macaques, should be used to establish more animal-appropriate housing conditions for older animals, for instance, through the provision of navigational aids in outdoor enclosures or even the implementation of an active training program aimed at mitigating cognitive decline. Preventive measures could also be applied, for example, in the form of enhanced environmental enrichment within enclosures to exercise cognitive abilities already in younger animals, with the ultimate goal of preventing age-related impairments from arising in the first place.
Reference
Baxter, M. G., Roberts, M. T., Roberts, J. A., & Rapp, P. R. (2023). Neuropsychology of cognitive aging in rhesus monkeys. Neurobiology of Aging, 130, 40–49. doi:10.1016/j.neurobiolaging.2023.06.011
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