A stressful early life can have long-lasting consequences, not just for you but for your partner. Pat Monaghan from the University of Glasgow found that zebra finches are more sensitive to stress as adults if they had unusually high levels of stress hormones as chicks. To no one’s surprise, they also died at a younger age. But it came as more of a shock that these tightly wound finches passed the consequences of their early hardships to their partners. They too died earlier even if their early days had been stress-free.
A wide variety of animals react to stressful conditions by become physiologically more jumpy. Their bodies flood with stress hormones at slight provocations, and they take longer to return to normal. In the short term, these changes help animals to survive through difficult times. But they can also be harmful in the long term. Not only can they shorten an individual’s life, but they can turn it into an undesirable partner.
Many studies have found that individuals across many different species are less likely to secure mates when they grow up if they have stressful upbringings. This might be because they are less physically attractive but, based on his study, Monaghan thinks that it could also be that such individuals pose a health hazard to their mates.
Monaghan raised almost 200 zebra finches in captivity. When they were just 12 days out of their eggs, she fed 70 of them with food laced with CORT, a stress hormone. It was an artificial dose of stress, which raised the birds’ CORT levels to those they would normally get during hard conditions. Monaghan kept this up twice a day for the next 16 days, before leaving the chicks to grow up undisturbed.
Three years later, around 30 percent of the CORT-fed finches were dead, around twice as many as their peers whose chick-hood had been undisturbed. The extra dose of stress hormone, delivered over just 16 days at the very start of their lives, had a significant effect on their odds of survival. It’s not clear what killed them but it was clearly something subtle. They weighed the same as their peers, and their bodies showed no signs of fighting or infections. Monaghan is now studying their frozen bodies to find out.
However, it’s clear that their partners also suffered. When the birds were two years old, Monaghan placed her finches in arranged partnerships, pairing them with other birds in small cages three times over the year, to allow them to mate.
Monaghan found that the finches were more likely to die early if they were paired with a bird that ate CORT-laced food as a chick. If they had been treated with CORT themselves, around 20 percent died within three years if they were paired with a normal finch and around 40 percent died if they were paired with a CORT one. For normal birds, just 5 percent died if they had a normal partner, and 20 percent died if they had a CORT one.
Monaghan thinks that the partners of CORT birds don’t get the same benefits that they would normally expect from their partners. Zebra finches form strong social bonds when they pair up, and like many other back-boned animals, these bonds are accompanied by higher levels of a hormone called oxytocin. This buffers them against stress, making them less sensitive to difficult environments and bolstering their health. But if a partner is sensitive to stress themselves, they’re less able to provide this “social buffering”. So it’s not that the CORT birds were actively harming their partners; they just weren’t helping them.
The fact that Monaghan’s birds had no choice over their partners could have made this problem even worse. Perhaps the birds sensed that they had been stuck with a dud and, as a result, became stressed themselves.
Reference: Monaghan, Heidinger, D’Alba, Evans & Spencer. 2011. For better or worse: reduced adult lifespan following early-life stress is transmitted to breeding partners. http://dx.doi.org/10.1098/rspb.2011.1291
Image by Keith Gertstung