Who suffers from stress?
Recently, I wrote about how grooming each other can reduce levels of stress hormones, for example, in baboons and birds. But I asked, "why should natural selection allow excessive levels of this stress hormone?"
This week's paper shows one way that natural selection can lead to harmful levels of stress hormones. The question, of course, is "harmful to whom?"
Writing in American Naturalist, Oliver Love and Tony Williams report that stressed mother birds pass stress hormones to their offspring. (Passing your stress on to others seems to be popular in humans also.) These hormones increase the risk of chicks dying, especially male chicks. But they may also increase the mother's lifetime reproductive success.
How might this work? Birds that work too hard feeding their chicks may be less likely to survive to breed in future years. If conditions were the same every year, natural selection could adjust brood size over generations until it got the right balance between current and future reproduction. But conditions are not the same every year, so birds need some way to adjust brood size, decreasing it in bad years. One solution would be to lay fewer eggs in bad years (analogous to birth control). Stress hormones in the eggs could be an alternate solution, if those hormones reduce nestling survival (analogous to infanticide).
To test this hypothesis, they simulated bad years by trimming feathers on half of the birds. Trimmed and untrimmed birds had access to the same food supply, but trimmed birds had to work harder to bring the same amount of food to their chicks. The trimmed vs. untrimmed treatments were further divided into hormone treatments: half of the eggs were injected with stress hormone.
If their mother was untrimmed (simulating a good year), most chicks survived. If the mother was trimmed and the eggs were injected with stress hormone, then many chicks died. If the mother was trimmed but eggs were not injected with hormone, results were intermediate.
So far, stress hormone in the eggs looks harmful, not only to the chicks but also to the mother's reproductive success. But those are only the results for the first breeding cycle. Feeding a nest-full of chicks is hard work, especially for a mother with trimmed feathers. Those mothers lost more weight than those in any of the other treatments. In particular, they started the second breeding cycle smaller than those in any of the other treatments. As a result, they had lower chick survival in their second brood and they themselves were less likely to survive to the next year. Trimmed mothers who had fewer chicks to feed in their first brood (due to hormone treatment of eggs) had greater chick survival in their second brood and were more likely to survive until the next year.
What about overall reproductive success? This was measured over two broods in one year and one the next. Mothers that were not trimmed had the most surviving chicks, of course. Within the untrimmed treatment, those without stress hormone added to their eggs did best. (Presumably, birds in good condition have low stress hormone levels and pass little stress hormone to their eggs.) But for birds whose ability to feed their first brood was reduced by trimming, overall reproductive success was higher if the first brood was smaller, as it was when hormone was added to their eggs.
As the authors put it, mothers with a full-size brood but reduced ability to feed them (due to clipping, analogous to a bad year) were "mismatched" the demands of their brood. They ran themselves ragged trying to feed to many chicks, which had long-term negative effects on lifetime reproductive success. Stress hormone in the eggs essentially sacrifices some chicks to enhance their mother's lifetime reproductive success.
Which chicks were sacrificed? Mostly males. The sex ratio at laying was close to 50:50, but hormone treatment caused a small decrease in the percent of male eggs that hatched. This trend in sex ratio continued, with more male chicks dying than female. In species where males compete for mates or territories, small males may not reproduce, whereas every female can reproduce. Therefore, a bird with only enough resources to produce small offspring should concentrate on females. This appears to be an important secondary effect of stress hormones in eggs. See a previous post on control of offspring sex ratio in mice.