Measuring stress in captive animals

Stress is rarely thought to be a good thing, but is it true that all stress is necessarily bad? In the animal kingdom, such disparate events as mating, fighting, or hunting can trigger an animal's stress response.

Stress is a normal part of life, but what isn't so obvious, especially for zoologic institutions, is determining whether a captive animal is experiencing the good kind of stress that helps keep it alive or the harmful kind.

In her Oct. 11 AVMA Animal Welfare Forum presentation "Stress and distress: evaluating their impact for the well-being of zoo animals," Nadja Wielebnowski, PhD, a behavioral endocrinologist at Chicago's Brookfield Zoo, discussed some of the studies under way to measure stress in captive animals.

"Not all stress is negative, and some stressful situations, like exploring a new element in the environment or mating, usually are seen as beneficial and can help reduce animal boredom in captivity," Dr Wielebnowski said.

"In zoos, we are most concerned with chronic stress, when animals are repeatedly exposed to negative stressors and are not able to respond appropriately. Prolonged negative stress can become physically harmful."

Researchers at Brookfield Zoo are currently studying stress in 18 species using techniques that may be expanded and applied to more species. The procedure includesdaily collecting, testing, and validating of fecal samples for used hormone analysis along with extensive monitoring of behavioral and physiologic changes.

Brookfield's clouded leopard collection is part of a national study started in 1999 involving 12 zoos and 74 large cats. Difficult to manage in captivity, clouded leopards are often seen pulling out their fur, pacing excessively, hiding for long periods, and acting aggressively toward exhibit mates.

Research has shown that higher-than-average concentrations corticosteroids in the feces—an indicator of stress—are positively correlated with the occurrence of self-injuring behaviors, as well as the frequency of pacing and hiding.

In addition, when husbandry factors were analyzed, it was found that enclosure height, keeper time, being able to see potential predators, and public display were all associated with hormonal changes. The higher the enclosure, for example, the lower the hormone concentrations.

The Clouded Leopard Species Survival Plan Committee of the American Zoo and Aquarium Association has already recommended increasing enclosure height to a minimum of 10 to 12 feet for this big cat species.

"Zoos have made great progress in animal welfare over the past several decades," Dr. Wielebnowski said, "but there is still work to be done. We need to identify more accurate and reliable scientific measurements to increase our understanding of what well-being means from the individual animal's point of view. This includes species as different as the charismatic polar bear to the hissing cockroach."