A 10-year study from the University of East Anglia seems to have an answer. It shows that Seychelles warblers (Timerl dezil) may increase their offspring’s survival through their infidelity.
 
Although in many animals females may pair up with a specific “social” mate who helps raise their offspring, DNA fingerprinting studies across a wide range of animals reveals that offspring may often be sired by other males.
 
What has perplexed scientists is why females engage in such infidelity. What is the benefit of being fertilised by these other males – males which do not contribute towards raising the offspring?

Despite being apparently monogamous and pairing with the same male for life, female Seychelles warblers often prefer to be fertilised by other males, and this appears to increase the genetic quality of their offspring.

The study has shown that these extra-pair fertilisations can result in a higher diversity of specific genes which detect disease and trigger an immune response in offspring. As a consequence, the offspring survive longer, probably as a result of having greater resistance to a wider range of diseases.

The research – entitled MHC-dependent survival in a wild population: evidence for hidden genetic benefits gained through extra-pair fertilisations – has been led by Dr David Richardson and is published in the Molecular Ecology journal.
 
It was conducted along with the University of Sheffield, the University of Groningen and Nature Seychelles and used the warbler population on Cousin island.

Since 1997 more than 97% of warblers on Cousin have been ringed, blood-sampled, and their breeding attempts followed. The researchers monitored the fate of 160 birds hatched on the island between 1997 and 1999, over 10 years.

They found that females paired to males with a low diversity of disease-detecting genes (known as major histocompatibility complex or MHC) elevate gene diversity of their offspring by gaining extra-pair fertilisations from males with higher diversity. This extra pair fertility was found to be common – accounting for 40% of offspring.
 
Importantly, the offspring born as a result of this female infidelity have higher genetic diversity in these disease-detecting genes than they would have had if sired by the cuckolded pair male.

The researchers then found a positive association between diversity of MHC genes and juvenile survival. A higher than median MHC diversity was found to increase lifespan more than twofold.

Dr Richardson said: “We first tested whether extra-pair offspring have a survival advantage compared to within-pair offspring. Then we tested whether there are genetic benefits to the patterns of the MHC-dependent extra-pair fertilisations observed in this species.”

“We did not find any evidence for genetic benefits of extra-pair fertilisations per se, as on average extra-and within-pair offspring survived equally well.

“However, by not being faithful to a pair male with low MHC diversity, females are ensuring that their offspring do not end up with below-average levels of MHC diversity and therefore lower survival.

“We have shown that the association between survival and MHC diversity levelled off with increasing diversity, so choosing males with above-average MHC diversity would not have resulted in any additional fitness benefits for the offspring.”

“One thing that remains unknown, however, is what mechanism drives the patterns of MHC-dependent extra-pair mate choice.

 Experiments are needed to determine whether females actively choose more diverse MHC males or whether other factors like male-male competition or sperm competition play a role.”