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Birds may boost chick survival through infidelity

July 28, 2010
Courtesy of the University of East Anglia
and World Science staff

Why is mate in­fi­del­ity so common among animals? A new study in­di­cates fe­male birds may in­crease their chicks' sur­viv­al through their un­faith­ful ways, sug­gest­ing one rea­son why na­ture may have kept this un­sa­vory prac­tice in her tool­kit.

Al­though in many an­i­mals fe­males may pair up with a spe­cif­ic “so­cial” mate who helps raise the pairs' off­spring, DNA fin­ger­print­ing stud­ies across a wide range of an­i­mals re­veals that off­spring are of­ten sired by males oth­er than the pair ma­le.

Sci­en­tists have won­dered why fe­males en­gage in such in­fi­del­ity – what is the ben­e­fit of be­ing fer­ti­lised by these oth­er males that don’t help raise the off­spring.

Sey­chelles war­bler birds are one spe­cies whose fe­males pair with the same male for life, yet of­ten pre­fer to be fer­ti­lised by oth­er ma­les. This seems to in­crease the ge­net­ic qual­ity of their off­spring, ac­cord­ing to au­thors of the stu­dy.

These fer­tilisa­t­ions, they found, can re­sult in a high­er di­vers­ity of cer­tain immune-system genes that de­tect dis­ease in off­spring. The re­sult is long­er-lived off­spring. The stu­dy, led by Da­vid Rich­ard­son of the Uni­vers­ity of East An­glia, U.K, is pub­lished in the cur­rent is­sue of the re­search jour­nal Mo­lec­u­lar Ecol­o­gy.

The investigation is one of a num­ber of new stu­dies that have re­vealed pos­sible roles for in­fi­del­ity in the ani­mal king­dom. Research pub­lished ear­lier this year found that some spe­cies of fruit flies might not even be able to sur­vive with­out fe­male pro­mis­cuity.  

In the new study, more than 97 per cent of war­blers on the ti­ny is­land of Cous­in in the Sey­chelles were tracked over a decade, their blood sam­pled, and their breed­ing at­tempts fol­lowed. The re­search­ers mon­i­tored the fates of 160 birds born on the is­land be­tween 1997 and 1999.

They found that fe­males paired to males with a low di­vers­ity of dis­ease-de­tecting genes, called ma­jor his­to­com­pat­ibil­ity com­plex or MHC, in­crease the 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 per cent of off­spring.

Off­spring born of the in­fi­del­ity have high­er ge­net­ic di­vers­ity in these dis­ease-de­tecting genes than they would have had if sired by the cuck­olded pair ma­le, the re­search found. The sci­en­tists al­so this high­er di­vers­ity to be as­so­ci­at­ed with long­er ju­ve­nile sur­viv­al, in­creas­ing life­span up to more than two­fold.

"On av­er­age ex­tra-and within-pair off­spring sur­vived equally well,” Rich­ard­son ex­plained. But “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.”

How­ev­er, "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,” he added, “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."


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Why does mate infidelity occur so frequently throughout the animal kingdom? A newly released 10-year study indicates female birds may increase their offspring's survival through their unfaithful ways. Although in many animals females may pair up with a specific “social” mate who helps raise the pairs' offspring, DNA fingerprinting studies across a wide range of animals reveals that offspring are often sired by males other than the pair male. Scientists have wondered why females engage in such infidelity – what is the benefit of being fertilised by these other males that don’t help raise the offspring. Seychelles warbler birds are one species whose females pair with the same male for life, yet often prefer to be fertilised by other males. This seems to increase the genetic quality of their offspring, according to authors of the study. These fertilisations, they found, can result in a higher diversity of certain immune-system genes that detect disease in offspring. The result is longer-lived offspring. The study, led by David Richardson of the University of East Anglia, U.K, is published in the current issue of the research journal Molecular Ecology. Since 1997 more than 97 per cent of warblers on the tiny island of Cousin in the Seychelles were tracked, their 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, called major histocompatibility complex or MHC, increase the 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 per cent of offspring. Offspring born of the infidelity have higher genetic diversity in these disease-detecting genes than they would have had if sired by the cuckolded pair male, the research found. The scientists also this higher diversity to be associated with longer juvenile survival, increasing lifespan up to more than twofold. "On average extra-and within-pair offspring survived equally well,” Richardson explained. But “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.” However, "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,” he added, “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."