This post initially appeared on Science Blogs

Most biological systems have trade-offs, so this really shouldn't come as a surprise. A recent paper published in Science shows that in sheep, some females have a stronger immune system and tend to live longer, but also tend to reproduce less often. This seemed to translate to roughly equal reproductive fitness over the course of their lives. It's a cool study, clearly involving a lot of work (they took samples over the course of a decade). The New York Times comes soooo close to having a great science article - they just forgot one thing.

They found that the average life span of the 410 ewes surveyed was 6 years. But there was a wide variation, with some living 15 years, and many others dying at age 3 or 4. The short-lived ewes had lower concentrations of antibodies than the longer-lived ones, which suggested why their lives were so short. But why was natural selection not weeding them out?

Dr. Graham said the researchers found this to be a puzzle: "What are all these sheep doing with low antibody concentrations?"

The answer, she said, was that the short-lived ewes were more likely to produce lambs. Those that died young reproduced almost every year, often having twins. Sheep that lived longer did not reproduce every year.

The researchers make the point that it could be an energy expenditure issue, and that's certainly plausible. The immune system requires a great deal of energy to fight infections, and babies require a great deal of energy. Resources are limited, yada yada yada... All of this is true. There's no wild misinterpretation of the data, no far reaching conclusions (I can just imagine the headline: "Having trouble conceiving? New study suggests taking immunosuppressives will increase your chances of having a baby" - ugh), and they even give you an interesting bit of history about why these sheep were good experimental subjects - something you couldn't have gotten just reading the paper.

The only major qualm I have with the report is the credulousness that starts with the title. There's absolutely no data to suggest that the Ewes with weak immune systems have more offspring because they have weaker immune systems. The two are correlated, but no causal relation was demonstrated. Not that I blame the reporter - the fact that the explanation for a causal link makes such intuitive sense makes it easier to fall prey the fallacy