Sheep Study Finds an Upside to a Weak Immune System

A weak immune system makes an animal vulnerable to disease and parasites, but strangely, natural selection does not always weed out creatures that have what appear to be severe disadvantages, according to a new study.

That may be because the weaknesses confer some advantages. The study, in the journal Science, reports that while a strong immune system helps female sheep live longer, it is also associated with reduced reproductive success.
Andrea Graham, an ecologist at Princeton University, and her colleagues studied Soay sheep, an ancient breed found on the Scottish archipelago of St. Kilda.
The last inhabitants of St. Kilda moved away about 80 years ago because of the harsh winters and difficult living conditions. But they left behind their sheep, unmanaged and roaming freely. No longer selected by breeders, the sheep are now subject to natural selection, forming a contained, relatively accessible study population.
And studied they are. Every August for 11 years, Dr. Graham and colleagues sampled sheep feces to count parasites and drew blood from the sheep to measure antibody productivity.
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 without fail, often having twins. Sheep that lived longer did not reproduce every year.
In the end, both groups produced about the same number of offspring, Dr. Graham said.
The researchers were not sure why this phenomenon occurred.
“It could just be an energy budget issue,” Dr. Graham said. “Energy put into producing antibodies could be reducing energy put into producing a fetus.”