Reasons to cheat. Female Seychelles warblers have affairs to improve the vigor of their offspring.

Infidelity Has Its Benefits

Genetic paternity tests have revealed that many apparently monogamous species indulge in infidelity, but scientists have puzzled over the benefits of these illicit liaisons. New research shows that, in the Seychelles warbler, the benefits are likely linked to genes associated with immune function.

The Seychelles warbler is a songbird that inhabits a few small islands in the Indian Ocean's Seychelles group. On Cousin Island, breeding territories are in short supply, and females can't be too fussy about whom they choose as mates. But many make up for this lack of choice by having affairs: 40% of Cousin's warbler chicks are not fathered by their mother's mate.

Reasoning that affairs might be a way for females to improve the genetic quality of their offspring, molecular ecologist David Richardson, of the University of East Anglia in Norwich, U.K., and colleagues investigated the relationship between female mate choice and the diversity of each individual's Major Histocompatibility Complex (MHC) genes. The MHC is a group of molecules which plays a major role in the body's immune response to pathogens. Studies in some species have shown that individuals with higher MHC diversity are better able to resist pathogens.

The team found that there was indeed a relationship between MHC diversity and infidelity: Female warblers cheated far more frequently on males with below-average MHC diversity than on males with higher MHC diversity. Richardson says that the females don't appear to "sniff out" MHC characteristics the way mice do, so it may be that MHC diversity is associated with other features, such as superior singing or fighting, which make males more attractive or allow them to outcompete others. The team reports its findings in this week's online edition of Proceedings of the Royal Society B.

The results are important, says Scott Edwards, an evolutionary biologist at Harvard University in Cambridge, Massachusetts, because several studies on other species "have found no explanation for the pattern of extra-pair matings," but few have investigated the effects of MHC on mate choice in birds.

Related sites
David Richardson's homepage at the UEA
Seychelles Warbler factsheet from BirdLife International
Structure and function of Major Histocompatibility Complex proteins
Abstract of study