Monogamous relationships, like the ones humans have, tend to be quite rare in nature. Most of the time, females will select several males to mate with throughout their life. Birds; however, are an exception as about 90% form monogamous partnerships and mate with 1 individual for life.
The zebra finch; however, takes this to another level in terms of human like relationships. In almost all bird species, females will choose the best looking male, which could mean many things including brightest plumage or the biggest tail, but not in this case.
A recent study has shown that zebra finches choose their mates based on compatibility with one another instead of choosing the ‘fittest’. While a female may not choose the ‘best’ mate, choosing a compatible mate has many advantages including the ability to coordinate tasks such as egg sitting or feeding young.
To study the mate choice in the zebra finches, the researchers put 20 males and 20 females together in a cage and gave them time to choose mates. They then split 10 of the pairs apart and left 10 together and allowed them to mate to see which pairs were more successful in mating.
What the team found was that pairs that were allowed to mate with their choice were much more successful in producing offspring, 37% more successful to be exact. Nests of the forced mates had a much higher proportion of infertile and missing eggs. Eggs are very ‘expensive’ for females to make in terms of energy output and the high frequency or eggs that had no change of hatching is a big problem.
These results show a significant difference in mating success that shows how big a gain in fitness zebra finches get from choosing a mate based on compatibility. Zebra finches that make poor mate choices in the wild would pass on few offspring and would quickly be eliminated from the population. In this case, choosing the fittest mate would not be of advantage.
It seems that when incompatible finches are paired, they don’t take good care of their young. This was especially apparent with males when it came to feeding the young and defending the nest. In several cases, females even tended to mate less with males they were forced to, reducing the number of offspring they left behind.
The team also wanted to investigate whether females chose males based on behavioral or genetic compatibility. The frequency of embryo mortality (chicks that died before hatching) was approximately equal in both chosen and forced pairings. This suggests that females didn’t choose based on genetic compatibility because embryo mortality is based on genetics and not behavior.
The idea of mate choice based on behavioral compatibility has not been studied in great detail yet and seems to be rare in nature but this study shows how effective a strategy it can be in the right circumstances. It offers a huge increase in fitness in zebra finches.