Abnormal behavior cannot be explained solely by the current environment — How lifelong stress research is changing welfare assessments

The "abnormal behavior" of monkeys does not occur because they are currently suffering. It may be that a lifetime of accumulated stress remains as physical movements.
Hearing this, some might think it resembles the psychological scars of humans. A new study introduced by Phys.org on March 27, 2026, suggests that the repetitive behaviors of rhesus macaques housed in experimental facilities—such as pacing back and forth in the same spot or hair-plucking—may be linked not only to their current environment but also to "cumulative negative experiences" from past to present. The paper, published in Biology Letters, aims to reinterpret these behaviors as signs reflecting the lifelong burden carried by the animals.

In the study, the life histories of 240 rhesus macaques housed in two research facilities in the United States were examined. Researchers organized 12 types of negative events and treatments, including current stressors like isolated housing and past experiences like early weaning and medical procedures, and assigned each monkey a "lifetime negative experience score." They then conducted video observations to examine how frequently abnormal repetitive behaviors occurred. The core of this research is not merely evaluating the current state of the cage as being too small, but rather considering the history each individual has gone through.

The results were clear. Individuals with higher negative experience scores tended to exhibit more abnormal repetitive behaviors, and the increase followed what the paper describes as a "dose-response pattern," meaning the more the burden accumulates, the more pronounced the behavior becomes. However, not all behaviors carry the same meaning. Hair-plucking was more strongly associated with current stress, while behaviors like pacing and rocking were more strongly linked to past negative experiences. Furthermore, a comparison between the two facilities showed that in one, both current and past factors had an impact, while in the other, even with some improvements in the current environment, the influence of past experiences remained strong. This suggests that while environmental improvements are important, they do not necessarily erase the scars of history.

What makes this study significant is that it challenges how we have viewed the welfare of experimental animals. Abnormal behaviors have often been discussed as indicators of "current stress" or "current housing conditions." However, the new findings suggest that the behaviors observed are not only signs of "current discomfort" but could also be "traces of accumulated experiences." The research team suggests that these behaviors could potentially be used as non-invasive indicators of animal welfare, leading to discussions about setting limits on the repeated use of a single animal in multiple studies and possibly retiring individuals with high levels of abnormal behavior from research.

The important point here is that the study does not simplistically equate "abnormal behavior = mere evidence of distress." The fact that there were differences between facilities itself indicates that animal behavior is shaped by a combination of the current environment, past experiences, individual differences, and their interactions. Therefore, the value of this study lies not in crudely anthropomorphizing the inner lives of animals but in highlighting the risk of interpreting visible behavior based solely on a single moment in time. A monkey moving restlessly in front of you may not just be "lacking stimulation now" but could still be "carrying past burdens." This suggests that welfare evaluations should be conducted with a "map including history" rather than just a "current location."

This perspective is not limited to experimental monkeys. According to the introduction by Phys.org, the research team hopes that similar studies will extend to other captive animals, such as farm animals, zoo animals, working animals, and pets. Discussions on animal welfare tend to focus on current conditions like "how clean it is now," "whether there is enough food," and "whether space is secured." While these are important, if the accumulation of lifetime experiences casts a long shadow on behavior and condition, welfare becomes a concept with a much longer time axis. It cannot be dismissed with "it's okay because it looks comfortable today." Only by including past treatment can the quality of life of the individual truly be seen.

So, how is this topic being received on social media? As of March 29, 2026, within the range that can be confirmed from the public index, it is in the initial stage of quietly spreading from the sharing of scientific news to ethical points, rather than an explosive controversy or a huge swirl of discussion. Phys.org's article had been shared 152 times on the site within about two days of publication, and official posts were also made on X, Facebook, and LinkedIn. On LinkedIn, Phys.org's post was shared with a summary viewing the study as a "non-invasive indicator of animal welfare," receiving a few reactions. On Facebook, there were also a few comments and shares, while at least one repost on Reddit was moderated and deleted not due to misinformation about the content but due to issues with community fit or reaction scores. While the visible reactions in the public domain are not yet large, what stands out is not the emotional reaction of "poor thing," but rather the ethical and institutional perspective of "shouldn't we consider lifetime burdens when evaluating research animals, not just their current state?"