You may judge people who are clearly shallow individuals, but if we look at the wider animal kingdom, it’s easy to see humans are not unique in their propensity to hold looks in high regard. Take birds, for example – males with the biggest or boldest plumage tend to be the winners when it comes to the ladies.
That being said, when it comes to choosing a mate, is there a consensus on who is the best-looking beau or belle? Not always, evidently, as some species seem to display distinct differences in preference between individuals, suggesting some may base mate choice on some form of compatibility (no, not zodiac signs). Now, an intriguing new study is offering us some insight into such mate choice, and its consequences on offspring.
For the investigation, researchers from the Max Planck Institute for Ornithology chose to investigate zebra finches which, like humans, partner up for life and share the parenting duties. Females also don’t seem to be in agreement on which male is the most attractive, meaning mate choices are generally specific to the individual.
They wanted to find out whether preferences might serve to match up compatible mixtures of genes, i.e. genetic compatibility, or behavioral traits that could, for example, lead to better task coordination, which could be particularly vital in pairs that split childcare. Either way, the ultimate goal should be to create viable, healthy offspring that can keep the genes going, so the researchers also examined the effects of mate choice on offspring fitness.
To do this, they gathered 160 bachelor zebra finches that had recently been obtained from the wild and allowed each bird to pick a partner from a selection of 20 individuals of the opposite sex. Once the birds had paired up, half of the females were allowed to stick with their choice, while the other half were forced to pair with a male that had been picked by another female. Pairs were then put in separate cages for a few months so that they could bond, and then let loose in communal cages – three pairs from each group per aviary – for a period of five months so that they could breed.
They then repeated the whole process with the same birds, but this time two-thirds of the birds from the first breeding period were separated, and the rest were allowed to stay together. Behavior was monitored throughout the study, as was reproductive success, which was measured in terms of dead embryos, dead chicks and surviving offspring.
As described in PLOS Biology, they found that individuals from chosen pairs were 37% fitter on average than those from non-chosen pairs, which was calculated by looking at the number of offspring produced that reached independence compared with other birds in the same aviary. Males from non-chosen pairs were also more likely to stray than those from chosen pairs.
By comparing the numbers of dead embryos and offspring, they were also able to test the genetic and behavioral compatibility hypotheses, respectively. While the embryo mortality rates were similar between the two treatment groups, significantly fewer offspring died in the chosen pairs, suggesting they were better at raising offspring. This therefore lends weight to the behavioral compatibility hypothesis, the researchers conclude, which could also support what we see in humans. For example, some research has suggested that individuals are more committed when part of a love-based, rather than arranged, marriage.