Males fight for dominance in order to gain access to the limited supply of female eggs. This is why male birds expend energy on brightly colored plumage, but female birds do not.
That said, if a male knows he's relatively weak, there is often no point in wasting energy attempting to out compete another dominate male. Evidently, this is the case with Australian red-backed fairy wrens. Male birds of this species will either display red plumage or not bother at all, depending on how healthy they are. And that can change from one mating season to another.
There is now some evidence that the same is true of men.
When Michael Petersen and Daniel Sznycer asked people to what extent they support political positions that increase the relative wealth of their economic classes, they, not surprisingly, found that poor people believe more money should be distributed to the poor, while rich people do not.
However, the interesting thing is that the extent to which men support policies that are in their self-interest is correlated with the size of their biceps. Just like strong male wrens are willing to display color and challenge other wrens for access to mates, strong men are more willing than weak men to fight for policies in their self-interest.
Of course, the study found that there was no correlation between female self-interest and the size of female biceps.
So, if you are a man and want to make money through politics, or you are bird and want to mate, lift weights.
- Webster, M. S., C. W. Varian, and J. Karubian. 2008. Plumage color and reproduction in the red-backed fairy-wren: Why be a dull breeder? Behavioral Ecology 19: 517-524.
- Rowe, M., J. P. Swaddle, S. Pruett-Jones, and M. S. Webster. 2010. Plumage coloration, ejaculate quality and reproductive phenotype in the red-backed fairy-wren. Animal Behavior 79: 1239-1246.
- Petersen, M. B*, Sznycer, D.*, Sell, A., Cosmides, L., & Tooby, J. (in press). The ancestral logic of politics: Upper body strength regulates men's assertion of self-interest over income redistribution. Psychological Science