Gouldian Finches are beautiful colourful birds and, although commonly described as a monogamous species, female Gouldian Finches are anything but faithful. As Sarah Pryke and her colleagues at Macquarie University in Australia observed, female Gouldian finches will cheat on their mate at the slightest opportunity. Why would they do that? It's easy to explain why males cheat; increased progeny leads to increased fitness. However, breeding is more costly for females and an understanding of the benefits of promiscuity is more elusive. A recent paper in Science by Pryke and her colleagues proposes that infidelity, together with post-copulatory sperm selection, aids a female in choosing the best candidate as the father of her chicks.
Of the three morphs of Gouldian finches – red heads, black heads and yellow heads – the red and black head morphs are genetically incompatible, and – as Pryke and Simon Griffith noted in 2009 – interbreeding between the two morphs results in a 60% increase in offspring mortality. However, compatible males are not always around, and as some red head males are heterozygous and carry the black head allele, it's not always that easy for a female Gouldian finch to tell which prospective partner is the most compatible choice. Pryke and her colleagues performed a series of experiments to determine how often female Gouldian Finches engage in cheating, whether the propensity to cheat depends on the female's compatibility with her partner and the interloper, and whether post-copulatory mechanisms play a role in selecting the most compatible father for a female's offspring.
The researchers used 40 Gouldian finch females bonded to either a compatible or an incompatible male. During the experiments the females were visually separated from their social mate by an opaque plastic divider that split the cage in two. Pryke and her colleagues then presented the female with a compatible or an incompatible male, giving her the opportunity to copulate with the new male. The visiting males performed courtship displays in 95% of the cases and the females were quite promiscuous in response; 77.5% of the females solicited and copulated with the new males, irrespective of their compatibility status with their mate or the interloping male. However, fertilization of the eggs was not so indiscriminate; all of the offspring were fathered by her long-term mate when he was compatible and the interloping male was incompatible, but compatible interlopers fathered more offspring when the long-term mate was incompatible. This happened despite the fact that females had only one chance to indulge in an extramarital affair, but multiple opportunities to copulate with their long-term partner.
For a long time, the adaptive reason for female promiscuity has been a mystery. This new study provides insight into the evolution of female infidelity in birds. When compatible males are unavailable, or when unable to determine whether their life partner is genetically compatible or not, Gouldian finch females are faced with a conundrum. And how do they circumvent these problems? Well...they cheat... and then, they select the most compatible sperm and fertilize their eggs with it to give their chicks the best father they can get.