strong immunity=low fertility

The Scientist | A study in wild sheep may help explain why natural selection has not eradicated weak or self-destructive immune systems. A fluctuating trade-off between reproduction and survival in a feral population of Soay sheep may resolve the age-old question of why natural selection has failed to eradicate genes for both infection-prone and self-assailing immune systems.

The potential answer, published in Science this week (29th October), comes from the nascent field of ecoimmunology, which examines how different levels of antibodies in the blood of wild animals can influence their ability to survive and produce young.

Specifically, the authors found that, among a population of isolated, wild sheep, individuals with higher levels of antibodies associated with autoimmunity in other species were more likely to survive harsh weather conditions, but also reproduced less. Consequently, the benefits of high immunity, such as quick and efficient riddance of infection, may come with a cost -- less energy for reproduction.

"This paper reveals that more [antibodies] might not always be better, and that to understand the evolution of immune systems, it will be critical to study them in free-living, outbred organisms," Lynn Martin, an ecoimmunologist at the University of South Florida, who was not affiliated with the study, said in an email to The Scientist.