Female Gouldian finches don't always stand by their mate. Given the opportunity, they will indulge in a promiscuous tryst with another male. But this infidelity is not merely cold-hearted cheating. It's an evolutionary ploy that enables the female finches to bolster their offspring's odds of survival.
The benefits of promiscuity in monogamous animals such as the Gouldian finch are straightforward for males but less clear for females. Promiscuity offers male finches a way to increase the number of offspring they father. If a brief romantic encounter enables a male to have more offspring than its mate could provide, then the act is an evolutionary success. But with females, the benefits of promiscuity are more complicated. There are only so many eggs a female can lay in one breeding season and having an affair doesn't increase the number of offspring that will come from those eggs. So why would a female finch take on a lover?
To answer that question we must first take a closer look at what's going on in the Gouldian finch population.
Gouldian finches are polymorphic. What that means is the individuals in the Gouldian finch population exhibit two different forms or "morphs". One morph has a red-feathered face (this is called the "red morph") and the other has a black-feathered face (this is called the "black morph").
The differences between the red and black morphs run deeper than the color of their facial feathers. Their genetic makeup differs as well-so much so, that if a mismatched pair of birds (a black and a red morph) produces offspring, their young suffer a 60 percent higher mortality rate than offspring produced by parents that are the same morph. This genetic incompatibility between the morphs means that females who mate with males of the same morph secure better survival odds for their offspring.
Yet in the wild, despite the genetic drawbacks of mismatched morphs, finches often do form monogamous pair bonds with partners of the other morph. Scientists estimate that nearly one third of all wild Gouldian finch mating pairs are mismatched. This high rate of incompatibility takes a toll on their offspring and makes infidelity a potentially beneficial option.
So if a female mates with a male that is more compatible than her mate, she's ensuring that at least some of her offspring will benefit from higher odds of surviving. Whereas promiscuous males can produce more offspring and bolster their fitness by sheer numbers, promiscuous females secure better evolutionary success by producing not more offspring but genetically fitter offspring.
This research was conducted by Sarah Pryke, Lee Rollins, and Simon Griffith from the Macquarie University in Sydney Australia and was published in the journal Science.
Gouldian finches are also known as rainbow finches, Lady Gouldian finches, or Gould's finches. They are endemic to Australia, where they inhabit the tropical savannah woodlands of the Cape York Peninsula, northwest Queensland, the Northern Territory, and parts of Western Australia. The species is classified as near threatened by the IUCN. Gouldian finches face threats from habitat destruction due to over-grazing and fire management.
Pryke, S., Rollins, L., & Griffith, S. (2010). Females Use Multiple Mating and Genetically Loaded Sperm Competition to Target Compatible Genes Science, 329(5994), 964-967 DOI: 10.1126/science.1192407
BirdLife International 2008. Erythrura gouldiae. In: IUCN 2010. IUCN Red List of Threatened Species. Version 2010.3.