Stress tested, but sparrows stay tough

You might think of stress as a bad thing, but it is extremely important to survival. Without stress, the body doesn't know if it's in danger. When you are in danger, your body responds by sending more energy molecules and oxygen to your muscles that help you fight or get away. All animals respond to stress, which is in part controlled by molecules called hormones that tell the body what to do. However, when danger is always present or the stress is intense, this may have negative consequences, such as making an animal sick or even damaging their DNA. To investigate this, Ursula Beattie of Tufts University, USA, and her colleagues from across the USA decided to look at how house sparrows (Passer domesticus) respond to stress, and whether being more stressed causes changes in the levels of stress hormones and damages DNA.The scientists collected 40 adult and 17 juvenile sparrows in eastern Massachusetts before letting the adult sparrows relax for 3 weeks. The researchers then stressed some of the adult sparrows for 2 weeks by exposing them to new ‘stressful’ experiences, such as playing a radio or flashing disco lights. Afterwards, the team let the birds relax for 2 weeks, then repeated the stressful process five more times over 6 months. The scientists also decreased the amount of stress for one group of sparrows with each round of stressful activities while increasing the amount of stressful time for another group. This way, some birds experienced more stress than others. After each round of stressful experiences, and again after six stress-free weeks, the researchers collected blood from the sparrows to look for the changes in the birds’ stress hormone – corticosterone – and to look for damage to their DNA. The team was surprised to find that the sparrows showed no differences in blood corticosterone or DNA damage regardless of how much stress they experienced. However, 6 weeks after the last round of new experiences, adult sparrows that experienced the most stress activities had higher corticosterone levels. Beattie suggests that this might be a way for the adults to prepare themselves for another round of stressful experiences by already having more stress hormone, but without causing any DNA damage.The scientists then wanted to investigate whether juvenile sparrows responded to stress in a similar way as adults. The team stressed half of the juveniles similar to the way they had stressed the adults with two main differences. First, the team only gave the juveniles 2 days to relax, so the stress of being caught was used as the first stress the sparrows experienced. Second, the juveniles only experienced the stressful lights and sounds four times over 18 days. Similar to the adults, the stressed juvenile sparrows had no changes in corticosterone levels or DNA damage compared with their relaxed counterparts. Also, both the adults and juveniles always had higher corticosterone levels after the stressful experiences, no matter how many previous rounds of stressful experiences they had. The researchers believe this shows the sparrows never got used to the new lights and sounds, and that stressing the sparrows for longer than 6 months, so they have more time to get used to the stress, might be useful in future experiments.Beattie comments that the results from these experiments show that different facets of the sparrows’ biology react in different ways to long-term stress. Therefore, Beattie suggests that looking at just one piece of the birds’ biology, such as corticosterone levels, is not enough to figure out whether a sparrow is stressed over a long period of time. This may be the same for other animals too, which might experience long-term stress without associated negative impacts such as DNA damage.

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