Strong immunity comes at cost to fertility, sheep study shows

Strong immunity may play a key role in determining long life – but at the expense of reduced fertility, a study suggests.
 
Scientists studying wild sheep found that those which are less susceptible to infection tend to live longer than their less hardy counterparts – but produce offspring less frequently. Those with weaker immune responses reproduce more often, but die sooner.
Over a lifetime however, both sheep with strong and weak immunity tend to produce the same number of offspring on average. Scientists say this overall balance may help explain why immunity can vary between individuals, including humans – for example, it may explain why some people get sicker than others when exposed to the same infection, or why vaccines seem to protect some people better than others.
 
The 11-year study, carried out by the University of Edinburgh and Princeton University, focused on wild Soay sheep on the remote island of Hirta, St. Kilda. Researchers tested the sheep’s blood for antibodies, and monitored which sheep produced lambs each spring.
 
Those sheep whose blood contained the most antibodies were more likely to survive harsh winters, but produced fewer offspring each spring. Sheep with fewer antibodies had more lambs each year but in the end left just as many descendants as sheep that lived longer.
 
Dr Andrea Graham, of the University of Edinburgh’s School of Biological Sciences, who took part in the study, said: “We have long suspected that there is a trade-off between life-prolonging immunity and reproductive ability. This trade-off – which is rarely demonstrated – could explain why there is so much variation in the strength of immune responses and even in predisposition to infection or autoimmunity.”