During this last frenetic quarter of the 20th century, stress has become a byword for everyday life. It is the project deadline at work, the in-laws coming to visit, the search for new daycare and the checkbook that refuses to be balanced. It is cell phones, fax machines and e-mail; headaches, ulcers and heart attacks. But surprisingly, it is not limited to humans."Just because you can't put a cow down on a psychiatrist's couch and ask it about its relationship with its mother doesn't mean that the animal is not stressed," says Gary Moberg, a professor of animal science and head of the Stress Research Unit. In fact, notes Moberg, every living organism, from the most developed primate down to the lowliest oyster, experiences stress. And all creatures respond to stressors in remarkably similar fashion. Curious about the nature of such universal stress responses and the implications for the well-being of both humans and animals, a group of UC Davis researchers is trying to better define the basic biology of stress.
stress experts wince at the question. That, perhaps, is the biggest mystery of their science, they say.
The term stress was borrowed from the engineering concept of a physical strain or tension that threatens to upset a state of equilibrium or homeostasis. "Stress is any perceived threat to the delicate balance that the body tries to maintain," says Moberg. "The body is designed to deal with stress by making mild adjustments in response to that threat. You get into problems only if the human or animal continues to experience stress for so long that the biological adjustment is too great." Moberg, a physiologist, has spent the past 30 years studying and deÞning how animals, ranging from fish to sheep to monkeys, respond to stressors in their environments. For both humans and animals, the cause of stress may be as simple as a change in temperature or as complicated as the loss of a critical relationship. As Moberg and others in his field scrutinized the response of various organisms to stressful situations, they discovered an exquisite chain reaction of events occurring as the individuals sought to biologically right themselves. The first line of defense to a stressor is often a change in behavior. For example, if it's cold outside, we humans quickly pull on a sweatshirt. If such behavioral changes don't alleviate the stress, the body shifts into second gear, triggering changes first in the autonomic nervous system, then the endocrine system and the immune system. Once the immune system has been tinkered with, the person or animal is in a "pre-pathological" state and disease is likely to follow, Moberg says. "These are all ways the body tries to defend itself," he explains. "As the biological costs of responding to a stressor build up, something has to be given up--something like reproduction, growth or normal cardiovascular function." And while all creatures follow this same basic pattern of stress response, the responses are tremendously varied not only between species but even between individuals of the same species. "Why does one person respond to extreme stress by getting a heart attack and another by developing cancer?" Moberg asks, noting that this variance is one aspect of the field that has long intrigued him. "Research suggests it is a combination of early life experiences, genetics and/or particular life stages," he says. Moberg's early research focused on stress responses in monkeys, looking at how early life experiences affected their individual responses to stress. In the early 1980s he broadened his work to address animal welfare issues. In one study he is monitoring how environmental stresses make sturgeon, those mammoth fish valued for both their caviar and meat, vulnerable to disease. Fish may be able to deal with chemical pollutants in the water, but then they may get hit by mine-tailing pollution and the warming of the river water. "Any one of these alone might not be a biological threat, but combined they are, and the fish may stop growing and reproducing," he says. Moberg has been asked to join an international research group examining a method of fishing in which fishing boats locate schools of tuna by tracking the dolphins that swim over the tuna. "The boats encircle the dolphins, drop their nets and then release the dolphins while netting the tuna," Moberg explained. "We want to know if this whole process represents a serious stress to the dolphins." He also is beginning a study on transgenic mice--laboratory animals whose genetic makeup has been altered for research purposes. It has been suggested that changing the genes of a species may induce stress in individual animals. "Normally, as animals evolve one characteristic, they have the opportunity also to develop biological mechanisms to compensate for that change," he says. "Transgenic animals don't have that chance to adapt. "I don't think all foreign genes would be a problem, but some genes--particularly those related to energy use, such as growth hormone genes--might be," he says.
For decades the stress response system has been defined as that network of mechanisms that prepares the animal for "fight or flight." Researchers are now discovering, however, that some of the biggest stressors occur not in the form of predators on the plains of Africa but in the passive environments of laboratories, zoos and farms. The stresses of confinement often manifest themselves in plain and simple boredom, just now being understood as a serious form of suffering in captive animals.
"Many animals spend a large percentage of their day in search of food," says Professor Joy Mench, an authority on animal welfare issues and director of the Center for Animal Welfare at UC Davis. "Then we place them in captivity and feed them concentrated high-energy foods that take them just a few minutes to eat." The animals are left with nowhere to go and nothing to do. It is not surprising that these animals begin to demonstrate what are known as abnormal and "stereotypic" behaviors--actions similar to those seen in people who suffer from obsessive compulsive disorder and other psychiatric disorders, Mench says. Rabbits relentlessly suck on their water bottles, wolves in zoos pace back and forth, pigs gnaw on the bars of their pens, parrots pluck out their own feathers, and monkeys bite themselves. They may even turn on other animals. Mice, for example, are known to remove most of the hair of their cagemates, leaving them with only a Mohawk-style strip of fur on their heads, Mench says. Having accepted the fact that boredom is a form of chronic stress for captive animals, how can such suffering be alleviated? Mench is convinced that it is a problem that both science and society are obligated to address. "Society will have to make the decisions, but as scientists we have to come up with reasonable solutions," she says. For Mench and her graduate students that means taking a close look at the cages and enclosures used to house laboratory and farm animals, then devising more interesting captive habitats. They are working with a number of species of animals, including quail, chickens, parrots, burrowing owls and rabbits. Cheryl Meehan, a second-year Ph.D. candidate working with Mench, is studying how cages housing orange-winged Amazon parrots can be "enriched" with toys, perches and climbing apparatuses to provide a more stimulating and thus less stressful environment for the bright-green tropical birds. To some of the cages she adds ladders, swinging perches, ropes and foraging toys that require the parrots to work for their food. She fashioned one such hanging toy out of two doll-sized straw hats, sewn together to encase a cloth bag of in-shell peanuts. To get to the peanuts, the parrots must tear open the straw hats and peck through the bag and peanut shells. Meehan is videotaping the parrots' activities, expecting to capture such telltale signs of stress as feather-plucking and repetitive pacing among the parrots in the non-enriched cages. She's also comparing behavioral development of the birds in the enriched cages with that of the birds in standard cages. And she'll monitor levels of the hormone "cortisol" in the blood, the most commonly used physiological measure of stress.
Photos by Neil Michel/Axiom |
