Cheaters never prosper. Our parents and teachers drilled that adage into our heads when we were young. Of course, the saying presupposes ethical, desirable behavior but we all know that it is not true. Bending the rules, deception, and even more insidious behaviors often result in getting ahead in society, in sports, in most walks of life. Basically, humans are selfish, looking to gain an advantage. If cheating is a way to get a leg up, then cheating it is.
Birds do their share of cheating as well. Today we will explore cheating in the reproductive life of birds.
We know that over 90% of bird species are monogamous. A male and a female establish a pair bond and attempt to raise of brood of babies. In some species like loons and eagles, the pair bond is life-long. Most songbirds have shorter-term relationships, often lasting only a year.
As humans, we tend to think of monogamous birds in the same way we think about human couples. How nice that two individuals are in love and will raise a happy family of youngsters.
That image began to crumble in the 1980’s when Dr. Patty Gowaty used DNA fingerprinting techniques to show that some of the Eastern Bluebird nestlings in South Carolina had a different father than the male pair-bonded with the mother.
Subsequent researchers have studied other species to see if females were being unfaithful to their mates. At least 100 species of songbirds have been tested for extra-pair paternity and over 75% of those species show evidence of cheating.
Does cheating on one’s mate offer an advantage? We believe that it does. We can start with the observation of the Greek philosopher Heraclitus that “No man every steps in the same river twice”. The environment is always changing.
Let’s consider a pair of birds. Every offspring will have half of the genes of the mother and half of the father. The offspring will resemble mom in certain features and dad in others. Each sibling will be a little different. Some of those offspring will be better adapted for the environmental challenges of the day. They may have the perfect bill size, the ideal plumage, the optimal size.
But if a female takes a mate outside the pair-bond, her offspring will show more variation because the cheating male brings a different set of genes to the offspring of the cheating female. The young therefore show more variability and increase the chance that one of the them will be better equipped for a changed environment in the future.
So, taking multiple partners generates more variability in one’s offspring. Increased variation is a way to hedge one’s bets against future changes in the environment.
Let’s revisit our notion of a cooperative, amicable monogamous relationship. We realize now that a monogamous relationship is a battle of the sexes, resulting in a grudging compromise.
A male wants to have a faithful female but to contribute as little as possible to the feeding of the young and to seek as many opportunities to cheat with other females as possible. By fathering many young, he has many chances to get his genes into the next generation. He cannot be a total dead-beat dad as the female cannot raise her young on her own.
The female on the other wants her mate to be entirely faithful to her but dalliances with other males are an advantage to her. She may seek out matings with other males or may be assaulted by a marauding male when her mate is off the territory. She wants her mate to provide lots of help in raising their young.
We therefore have this tension between the sexes. Both males and females want their mates to be faithful but want to have the freedom to cheat on their mates. Cheating pays off in so-called monogamous birds.