Newswise — Some people say that we become more like our partner as time goes on. Surprisingly, the same seems to be true in the animal world—studies on compatibility between mates show that individual animals are more successful when they behave in a similar fashion to their mate. By studying birds, Dr. Jenny Ouyang of the Netherlands Institute of Ecology is showing that similarity between mates can go beyond behavior, extending to their hormones.

Ouyang and her colleagues study similarity in animals using great tits, a small, European songbird, measuring levels of the hormone corticosterone in breeding pairs of males and females. They found that pairs in which the male and female had similar corticosterone levels made better couples—they were more likely to stay together. These pairs become even more similar in corticosterone levels over time. “For at least three years,” Ouyang says, “the pairs that stay together increase their similarities year after year after year.”

Much like previous studies on behavioral similarity, pairs that had similar corticosterone were more successful. As is the case with animals, success is often based on the number of offspring produced. Great tit pairs that increased corticosterone similarity within a year raised more nestlings that successfully left the nest.

Those unlucky pairs who did not have similar corticosterone were more likely to “divorce,” leaving the birds without a mate. For birds, like humans, the cost of divorce is high—birds might miss out on the opportunity to breed and produce offspring that year. But all is not lost for divorcee birds. The researchers found that divorced birds paired up with a mate with whom they were more successful in the subsequent year.

Great tits normally build nests in holes in tree trunks, but they will readily accept any kind of suitable cavity, including wooden nest boxes. At their study site in the forests of southern Germany, Ouyang and her colleagues have hundreds of nest boxes set up to attract the birds. Ouyang catches the adult birds during the spring and summer, and takes a small blood sample to measure corticosterone.

Corticosterone is related to the stress response and increases when birds get stressed out. In fact the mere act of being caught by a human is stressful for a bird, so researchers must be as quick as possible when catching birds and taking blood samples. When Ouyang traps the great tits, she hides in a camouflaged blind and watches for the male or female to enter the nest box. When a bird goes into the nest box, Ouyang springs into action—she remotely triggers a device to trap the bird inside the box, runs to the box, removes the bird, and takes the blood sample, all in under a speedy three minutes.

Ouyang also tracked which birds paired off and raised nestlings, leading to her discovery that stable pairs had similar corticosterone levels and more offspring.

The reason why pairs became more similar is still an open question. One idea is that, since corticosterone is related to how often the adults feed the nestlings, male and female great tits might become more coordinated in their parental efforts as time goes on, resulting in similar corticosterone levels. Because Ouyang’s study looked at correlations, rather than causes of corticosterone similarity, her work serves as a starting point for future studies to determine exactly how males and females become more similar over time.

Ouyang presented this study at the 2015 annual meeting of the Society for Integrative and Comparative Biology in West Palm Beach, Florida. The findings of the study have also been published in the December 2014 issue of the scientific journal Animal Behaviour.

Meeting Link: 2015 annual meeting of the Society for Integrative and Comparative Biology