Stress is our coping response. Whether emotional or physical, stress is how organisms react to upheaval in their lives. And in many cases, that response requires tradeoffs. An animal will make it through now, but may come out with fewer fat stores or a shorter life span. But a new study shows that under certain conditions, developmental stress in male zebra finches might have a positive effect, in the form of more offspring to carry on his genes.
Ondi Crino, a biologist now at Macquarie University in Sydney, examined how stress during development might affect reproductive success in male zebra finches. She purchased 10 male and 10 female zebra finches from pet shops near the University of Montana. The birds were allowed to pair off and nest. When the first batch of chicks was 12 days old, Crino fed half of the male offspring peanut oil, and half peanut oil with the hormone corticosterone mixed in.
Both humans and finches produce stress-related hormones. Humans produce cortisol, while finches produce corticosterone. These two hormones increase during times of stress and cause many of the negative effects we associate with worry and pressure. So administering corticosterone is one method of “stressing” an animal without changing anything else in its environment. The dose was in the range of what a young bird might experience in the midst of a natural upheaval such as a cold snap or famine.
After 16 days of the peanut oil supplement, the young male birds receiving corticosterone were smaller than their relaxed counterparts. They also had a larger spike in their own corticosterone levels when they were stressed. But over time, the chicks that received corticosterone appeared to grow out of their stressful upbringing. By adulthood they were the same size as controls, and they did not show frazzled feathers or pale colors that might indicate a rough chickhood.
Then, Crino released five stressed males and five control males into a cage with 10 females to see how developmental stress affected their mating abilities. Crino had hypothesized that stress during development would hurt the birds’ reproductive success. She was wrong. Not only did the stressed males mate as well as control males, they also produced more offspring than their unstressed brethren.
This impressive paternal achievement appears to be the result of a different parenting strategy between stressed and relaxed males. Male finches that received corticosterone during development fed their offspring more, resulting in baby birds with a higher weight for their size.
The stressed fathers also didn’t waste resources raising chicks that weren’t their own. And their mates didn’t mate as much with other males. The results suggest that fathers that received corticosterone during development might spend more time with their mates, resulting in more of their own offspring rather than those from another male. A stressed childhood resulted in a more devoted dad, Crino and her colleagues report October 8 in the Proceedings of the Royal Society B.
The results show that a bit of a tough upbringing might, under certain conditions, produce fitter finches. “We are currently undergoing a paradigm shift in how we think about developmental stress,” Crino explains. “Scientists are starting to think of developmental stress as a cue that induces phenotypic changes that help animals cope in stressful environments.” Sometimes, that coping might produce a benefit by passing on a bird’s genes to the next generation.
“Few studies have actually looked at the [reproductive] fitness costs of growing up in a harsh environment, and this study goes a long way to redress this imbalance,” says Karen Spencer, a behavioral neuroendocrinologist at the University of St. Andrews in Fife, Scotland. “This is one of the most convincing pieces of evidence in favor of the idea that experiencing stress during development may not be all bad.”
It is not certain how this extra parenting effect is achieved. Creagh Breuner, a behavioral endocrinologist at the University of Montana and an author on the study, says in the families of stressed fathers, both the male and the female finches fed their chicks more food as they were growing. It’s not clear how the stressed male is recruiting the female to the cause.
The underlying mechanism allowing the stressed dads to switch their parenting strategy is still unknown. Breuner hypothesizes that the corticosterone treatment might have made the adult finches less aggressive. They might then spend less time fighting other males, and more time guarding their mate and spending time with the kids. Stress could make the males more lover than fighter.
But it’s not necessarily all positive. The stressed finches could be “investing more heavily in their current reproductive bout,” says Crino, putting more parenting effort in the short term in anticipation of a shorter life span. Other studies have shown that developmental stress shortens finch life span. They might have won the current reproductive battle, but if they live shorter lives and therefore have fewer chances to reproduce, they may still lose the war.