When it comes to finding a mate in the animal kingdom, females tend to gravitate toward males who appear strong and healthy. But a new study in zebra finches reveals that the small, gray-striped birds prefer mates with similar interests, such as a penchant for exploring the world. The drive for this compatibility is so strong that when scientists forced the females to mate up with males not of their choosing, the birds were more likely to cheat and shirk their parental duties, leading to more deaths among their chicks.
The research “suggests that having a mate you’re behaviorally compatible with is very important from an evolutionary perspective for zebra finches,” says behavioral ecologist Sasha Dall of the University of Exeter, Cornwall, in the United Kingdom, who wasn’t involved in the study.
Scientists have long been puzzled by female zebra finches. As opposed to females of other bird species, which tend to prefer males with brighter plumage or longer tails (traits that suggest that the males have good, healthy genes), female zebra finches seem to choose mates for some mysterious reason known only to each lady bird. Sometimes one will go for the guy with the bright red beak, sometimes the male with the thrilling song, sometimes neither.
So what’s really driving this mate choice? In the past, scientists have speculated that a desire for compatibility might be the answer. But they have disagreed over whether the birds are on the lookout for males with the right genes (genetic compatibility between the partners might help lower the high rates of embryonic mortality seen in the species) or the right behaviors. (A male that is as exploratory and active as a female, for example, may make it easier to coordinate chick rearing in the often large and chaotic colonies where these birds breed).
To find out, behavioral ecologist and lead author Malika Ihle of the Max Planck Institute for Ornithology in Seewiesen, Germany, and colleagues split 160 zebra finches in the lab into four groups of 20 females and 20 males and let the courtship rituals commence. After the birds had chosen their mates, the researchers left half of the happy couples paired off. The rest they separated and randomly paired up. “The separated females must have assumed that their mates had died, and they gave in and bred with the guys that we assigned,” Ihle says.
But they didn’t seem happy about it. The females weren’t as into having sex with their assigned mates, and they were more likely to be promiscuous in the short term than their free-choice counterparts. What’s more, the nests of arranged pairs had nearly three times as many unfertilized eggs as the free-choice pairs. And forced males weren’t as good parents: They didn’t attend to new chicks as much as they should have. In all, more than a third fewer chicks from assigned pairs survived to leave the nest than those of the free-choice birds, the researchers report online today in PLOS Biology.
In a previous experiment, the scientists had learned—by switching eggs between nests and taking advantage of the fact that the birds don’t notice the difference—that embryo mortality reflected genetic incompatibility. In this study, the researchers compared the rates of embryo mortality between both free-choice and assigned pairs and found they did not differ. As a result and given the difference in chick mortality, behavioral compatibility is the most likely factor behind female mate choice, the team says.
The fact that birds that chose each other had so many more of their chicks survive is “eye-catching,” Dall says. “The effects of these types of mate decisions are not usually that big.” Although the birds might have been expected to throw themselves wholly into their arranged relationship for the benefit of their chicks, they didn’t. “It seems like they can’t,” he says. “The idea is that there isn’t much the birds can do when their behaviors don’t match up.”
The major question the work raises, Ihle says, is how exactly behavioral compatibility leads to better care. Teasing out the details of how the free-choice parents feed their chicks better, for example, is the next step, she says.
For now, she thinks the work may say something about how we fall in love. “To me, love is a peculiar attraction toward a specific individual that is not necessarily shared by other choosing individuals,” she says. “It seems that the chosen pairs, those 'love marriages,' invested more into reproduction, were more committed, more faithful, and more motivated to raise their family.” Such findings might point to an explanation of love’s evolutionary benefits, such as increased survival of young, she says.
Leaving the love question aside, it’s at least clear that the study suggests that females are choosing males based on how well the partners work together during parenting, says Michael Jennions, a behavioral ecologist at Australian National University in Canberra, who wasn’t involved in the study. Although it is too early to tell how widespread this type of choice is in birds and the fairly few other groups of animals where there is biparental care, “it might be more common than we think,” he says.