A study into the fertility of a population of wild Soay sheep on an island off Scotland suggests that ewes with highly developed immune systems that can fight disease and parasites well aren’t necessarily the best breeders under tough conditions.The researchers, whose work is reported in the Science journal, say it has to do with energy budgeting.
There is a trade-off between the energy an animal uses to ward off disease and energy needed to breed and raise offspring. Energy put into producing antibodies reduces energy put into producing a foetus. Ewes that are well-adapted to the island’s icy winters and high parasite loads tend to live longer but produce fewer lambs than those with weaker immune systems that die younger.
The study found that short-lived ewes had lower concentrations of antibodies than the longer-lived ones – the reason why their lives were so short. But those that died young bred almost every year, often having twins. Sheep that lived longer did not reproduce every year.
Source: Science, 29 October 2010.