Stress has a lot of negative effects. It can cause anxiety and anger. It’s associated with everything from sleepless nights to drug dependence. But stress is not actually positive or negative. It is only how organisms react to a challenge.
A new study shows that adding stress hormones to the diet of developing zebra finches has an unexpectedly positive effect. When the birds reach independence, they end up with more finch friends than their unstressed counterparts. The results show a subtle effect of developmental stress and indicate that stress is more complicated than its negative reputation.
To study sociality in birds, Neeltje Boogert and Karen Spencer at the University of St. Andrews in Scotland turned to the highly social zebra finch (Taeniopygia guttata). “Finches do everything together,” says Boogert. “They eat together, fly to perches together and sleep together.”
Zebra finches have a particular advantage when studying sociality. Unlike rats or mice, zebra finches don’t spend a lot of time bullying others to assert their dominance. “We can stick finches in a big room and just let them spend time with each other,” Spencer says.
And zebra finches associate together in groups. Male and female mated pairs, for example, will stick together. The offspring tend to start out hanging around with mom and dad, eventually heading off to form their own independent friendships.
Boogert and Spencer wanted to see how developmental stressors might change how finches associated with each other. They took nests with 12-day old chicks and fed half of them peanut oil, and half of them peanut oil spiked with the hormone corticosterone. Similar to cortisol in humans, corticosterone is a major stress hormone in birds. So it can be used as a stand-in for behavioral stress without changing the birds’ environment, nutrition or parental care.
After 16 days on a diet of supplemental corticosterone or peanut oil placebo, all the birds were released into a large aviary where they could fly around, perch and hang out with whomever they pleased.
To keep track of the birds, Boogert attached tiny microchips to their legs, similar to the chips many pet owners get implanted into their dogs or cats to track down Fluffy if he roams too far. But in this case, Boogert placed barcode readers at the entrance to each feeding station. When the birds popped in for a snack, Boogert could tell exactly which finches stopped to feed, when they did and who they were with.
The technique is a vast improvement over previous studies. “Before you had to watch them a lot, to see who was sitting next to whom,” Boogert says. “It’s really time intensive, hours and hours of video, and it’s so tedious that people will start to make mistakes.” With the barcode readers, however, the researchers were able to collect a large amount of accurate data without having to stare at a single videotape.
The scientists hypothesized that finches exposed to corticosterone during development would be less social than their unstressed brethren. Previous studies had shown that great tits (Parus major) with higher corticosterone responses to stress formed fewer, but more stable, associations with other birds.
But it turns out that developmental stress in zebra finches produces a very different effect. Instead of more cautious and less social individuals, finches exposed to developmental stress were more social in the aviary, associating less with their parents and spending more time with many different birds compared to controls. Instead of forming a few, tight-knit associations, they formed many weaker ones. “They have more friends, but they are kind of flaky friends,” Boogert says. The authors published their findings October 29 in Biology Letters.
Early-life stress may yield benefits for these highly social birds. More groups to associate with means the finches might be more likely to find food sources in the wild. And hanging out with more finches means more exposure to potential mates. “It’s another piece in the puzzle that could explain how different development and behaviors influence reproductive success,” says Creagh Breuner, a behavioral endocrinologist at the University of Montana in Missoula.
But there are pros and cons to a good social life, notes Cédric Sueur, an animal behaviorist at the University of Strasbourg in France. The birds might have decreased risk from predators and a higher chance of finding food. But they might also have a higher risk of catching diseases and may have to compete for food with other finches. “How an individual associates with other group members is a way to balance between these advantages and inconveniences,” he says.
Inconveniences aside, Boogert hypothesizes that the gregarious social flair of stressed finches might be part of a live fast, die young lifestyle. “There is some evidence suggesting that if birds are stressed during development they don’t live as long,” she explains. “You need to find a mate quickly and get some chicks.” But, she notes, this hypothesis remains to be tested.