Infants can be taxing, or so I'm told. Being a trusting fellow, I used to just take my brother at his word, but now I have definitive proof, and it comes in the form of poop. How does one go about measuring stress, and where does the poop come in?

Most animals respond to stress by releasing steroids to liberate extra energy. In some animals, this steroid release follows a seasonal cycle; in others, it is released in response the environmental stresses. In mammals, for example, steroid release is often related to behaviors such as aggression. It can also be related to social contexts, such as the number of males present in a group, or simple factors such as the individual's age.

One challenge of monitoring animals for stress is that one must avoid causing additional stress in the process. Otherwise, one might fall prey to some macroscopic form of the Heisenberg uncertainty principle. As a result, one shouldn't go around capturing animals to measure their steroid levels. Fortunately a wonderful thing happens naturally every morning: some animals find a quiet place to leave their own little fecal samples.

Perhaps not surprisingly, it turns out that the steroid content of their feces is related to their internal steroid levels. Thus, a researcher has only to come along afterwards with a pooper-scooper and can collect all of the samples he or she desires, all without stressing out the subjects. That is precisely what a group based in North Carolina and Georgia did.

In a study published in the Proceedings of the Royal Society, the researchers reported the results from 867 fecal samples that were collected, oven-dried, and analyzed in a lab for the steroid glucocorticoid. These samples came from the primate Sifaka (a type of lemur). In addition to these samples, the researchers kept records of the behavior patterns of the Sifaka, allowing them to correlate behavior and stress.

Among other things, they analyzed the glucocorticoid levels for relationships to age, social group stability, alpha male status, and the presence or absence of infants. Surprisingly, the group was unable to find any statistically significant difference in glucocorticoid levels for any factor except the presence of infants.

They hypothesize that infants will cause increases in steroid levels to allow the males to protect the infants against invaders. During their field campaign, they witnessed 5 such invasions and, in all cases in which the invasion was successful and there were initially infants present, the infants disappeared, most likely as a result of infanticide.

OK, so maybe Sifaki poop isn't the definitive proof that infants cause stress in humans, but I would be willing to wager that if you measured my fecal glucocorticoid levels, they would be elevated if and/or when I have a little one of my own.

Proceedings of the Royal Society, 2009. DOI: 10.1098/rspb.2008.1912

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