Determining whether an animal has a good quality of life involves weighing their positive experiences against their negative ones. For animals used in biomedical research, these judgments are critical for categorizing how severe a procedure is, refining research methods, and deciding when a study should end. While researchers often evaluate one stressful event at a time, the combined impact of many stressors over an animal’s entire life ultimately determines their overall well-being.

This study investigated whether abnormal repetitive behaviors, such as pacing, rocking, or hair-plucking, could act as a reliable indicator of this combined lifetime stress. These behaviors are common in captive animals and can reflect chronic deprivation or past trauma. By cataloging various negative events, the researchers hoped to find a relationship between an animal’s history and their current behavior.

The researchers studied 240 rhesus macaques living at two research centers in the United States. The team identified 12 different types of stressful life events. These included two current housing conditions and 10 past experiences extracted from the animals’ records.

Current stressors included being housed alone or in an adverse area of the room, such as near a door. Past stressors included:

• Maternal loss in infancy
• Being born and raised indoors
• Living indoors
• Spending time alone
• Being relocated
• Having an aggressive cagemate
• Losing a cagemate
• Being used in research projects
• Health events, including research procedures
• Spending time in hospital

The macaques were monitored via cameras to record how often they performed repetitive behaviors. The researchers then assigned each monkey three negative experience scores — past, current, and lifetime — to see if higher scores predicted more frequent abnormal repetitive behaviors. The team was particularly interested in determining whether there was a dose-response relationship, where an increase in total hardship leads to a predictable increase in repetitive behavior.

The results showed that almost 60% of the macaques performed some form of repetitive behavior. Crucially, monkeys with higher lifetime negative experience scores were significantly more likely to display these behaviors.

The study found that the relationship between stress and behavior varied by facility. At one center, both current and past hardships predicted the monkeys’ behavior. At the other center, however, current housing conditions had little impact, but past experiences were still highly predictive — suggesting that some behaviors may be lasting signs of trauma or “scars of the past.”

The type of behavior also mattered. Hair-plucking appeared to be more closely tied to current stress, while motor behaviors like pacing and rocking were more reflective of an animal’s past. This mirrors findings in human psychology, where the number of prior negative life events can predict the severity of conditions like obsessive-compulsive disorder.

The study has certain limitations. Some repetitive behaviors, such as tooth-grinding, are difficult to see on camera and weren’t included. Additionally, the researchers focused only on negative events and didn’t account for positive experiences, like prolonged maternal contact or large outdoor enclosures, which might help protect animals from stress. They also assigned equal maximum weights to each of the 12 stressors, though some events may be more traumatic than others.

It’s also worth noting that roughly 40% of the macaques showed no repetitive behaviors at all, despite many having above-average lifetime stress scores. It’s unclear whether these individuals were somehow more resilient or responded to cumulative stress through other means, such as inactivity.

This research provides several useful insights for advocates working to improve the lives of animals used in research. The findings suggest that repetitive behaviors may serve as a useful indicator of cumulative welfare, accounting for the impact of both the number and severity of an animal’s negative experiences. This is more practical than invasive medical tests like measuring reductions in brain region volume.

The data also supports arguments to limit the re-use of animals in research. Involving animals in more than one experiment may be seen as a way to reduce overall animal use, an objective of the 3Rs principles, but this study adds nuance to that approach. If every experiment adds to a cumulative burden that monkeys can’t fully recover from, then limiting the total number of studies an individual animal participates in is vital for their quality of life.

Because these repetitive behaviors may reflect a cumulative burden of stress, the findings provide a scientific foundation for advocates and policymakers pushing for a shift away from animal use altogether. As it becomes clearer that laboratory environments can cause irreversible behavioral changes, the case for replacing these models with non-animal alternatives grows stronger to ensure that no animal is forced to endure a lifetime of hardship.

This summary was drafted by a large language model (LLM) and closely edited by our Research Library Manager for clarity and accuracy. As per our AI policy, Faunalytics only uses LLMs to summarize very long reports (50+ pages) that are not appropriate to assign to volunteers, as well as studies that contain graphic descriptions of animal cruelty or animal industries. We remain committed to bringing you reliable data, which is why any AI-generated work will always be reviewed by a human.