New research in hamsters now suggests that without companionship, wounds on the animals don't heal as fast.
Researchers looked at the effect social contact had on wound healing in stressed hamsters. Results showed that skin wounds healed nearly twice as fast in the hamsters paired with a sibling. These animals also produced less of the stress hormone cortisol than unpaired hamsters.
"Stress delays wound healing in humans and other animals, and social contact helps counteract this delay," said Courtney DeVries, the study's lead author and an assistant professor of psychology and neuroscience at Ohio State University. "Our goal is to understand the physiological mechanisms by which social support improves health."
She and her colleagues also treated a group of socially isolated hamsters with oxytocin, a hormone released during social contact and associated with social bonding in monogamous animals. Oxytocin treatment seemed to ameliorate the effects stress had on wound healing, as the treated animals healed about 25 percent faster than the untreated lone animals.
The study appears in a recent issue of the journal Psychoneuroendocrinology.
The researchers conducted a series of experiments to learn how social interaction affects health, and to better understand the mechanisms by which it does so. Female Siberian hamsters were housed with a sibling or isolated during the three-week study. All animals received minor skin wounds about the size of a sunflower seed on the backs of their necks. The researchers photographed and measured the wounds each day.
Some of the hamsters were confined to small Plexiglas tubes for two hours a day for up to 14 consecutive days. Other studies have shown that such confinement causes stress and also delays wound healing. While the animals couldn't turn around in the tubes, they could move back and forth and stand up or lie down.
Hamsters were separated into four groups for one of the experiments: socially isolated, non-stressed; socially isolated, stressed; paired, non-stressed; and paired, stressed. As soon as a single day after injury, the wounds on the socially isolated, stressed animals remained about 25 percent larger than the wounds of the other three groups, and stayed this way for about a week.
In a second experiment, the researchers compared levels of cortisol – the stress hormone – of paired animals to those in isolated animals. Blood samples were collected immediately after the hamsters were stressed, and again 45 minutes later. Right after stress, the cortisol levels of the isolated animals were one-and-a-half times greater than those of the paired animals.
"We expected cortisol concentration to increase substantially in stressed and lone animals," DeVries said. But cortisol levels didn't increase in the paired, stressed hamsters.
In a third experiment, the researchers found that surgically removing the adrenal glands – the main source of cortisol – from some of the isolated hamsters was just as beneficial to wound healing as having a companion in the cage. Wounds on the stressed and non-stressed animals healed at the same rate, suggesting that cortisol is a key player in delaying wound healing during stressful conditions.
Finally, the researchers experimented with the effects of treating two groups of socially isolated hamsters with either oxytocin or a substance that blocks oxytocin production. Hamsters in the oxytocin treatment group were given oxytocin injections daily for five days prior to wounding and restraint stress, while control animals were given a saline placebo. The treated animals' wounds healed at a faster rate than did wounds on the control animals.
Treatment with an oxytocin antagonist delayed healing even in paired animals.
"This suggests that oxytocin may be the main buffer against delays in wound healing, since the presence of a sibling also increased oxytocin secretion," DeVries said.
Wounds on all of the hamsters – stressed and non-stressed alike – healed in less than two weeks.
"The problem lies in how long it takes a wound to heal," DeVries said. "This can spell trouble for diabetics and people who are otherwise immunocompromised, such as cancer or AIDS patients. But having a companion may help wounds heal faster during stressful times."
DeVries conducted the study with Erica Glasper, Courtney Detillion and Tara Craft, all doctoral students in psychology at Ohio State, and Brian Prendergast, an assistant professor of psychology at the University of Chicago.