Mating is one of those things that seems like it should be innate, particularly for an animal like the cowbird-a parasite that lays its eggs in other birds' nests and provides no parental care to its young. But, in yet another example of the complex interaction of nature and nurture, new research finds that young male cowbirds learn mating skills through direct social interactions with adult males. Even more interesting is the discovery that only the briefest interaction, occurring when the adults aren't demonstrating any real mating behavior, is enough to influence a young bird's mating style for life.
The findings, published in the May issue of the Journal of Comparative Psychology (Vol. 121, No. 2), excites researchers because it suggests that cowbirds have an innate ability to pick up the information they need from the environment, with little exposure to it.
This research, conducted by the University of Pennsylvania comparative psychologist David J. White, PhD, and colleagues at Indiana University, "gives us a picture of development that allows for a great deal of preprogramming, but also a strong influence ofexperience," says Robert Seyfarth, PhD, who studies primate behavior at the University of Pennsylvania.
Indeed, White sees his research as a first step in teasing apart the intricate weave of nature and nurture by determining the specific mechanisms by which animals learn social behavior.
"The key finding here is this idea of the importance of the early learning environment," says White. "This work shows that early learning environment biases what's available to learn. Translate that to humans and you can ask, what's the most likely thing someone is going to pick up in this environment?"
Passing down behavior
White began his fascination with cowbirds as a postdoctoral student in the laboratory of Indiana University comparative psychologists Meredith West, PhD, and Andrew King, PhD.
In an earlier study, they found that if they housed a group of juvenile male cowbirds with adult males from Indiana, the younger birds would develop a typical Indiana cowbird mating style-monogamous, aggressively competitive, and prone to engage in "counter singing" in which males sing to other males to establish dominance. The researchers also found that if the young birds had no adult male contact, they developed a more egalitarian mating style with little counter singing and promiscuous mating.
In this latest study, White and his colleagues explored whether these two distinct groups of males could transmit their mating styles to a new generation of adolescent birds. This time, for several months before mating season, they housed juvenile males with either the competitive maters or the more egalitarian maters. As expected, the juveniles developed the same mating style as the adult males they lived with. The differences were even more pronounced after the researchers put the young birds together in the same aviary.
The nice thing about this study, says Seyfarth, is that White went beyond measuring just birdsong, which has long been used to show the importance of environment for normal mating development.
"He's measuring a much broader kind of behavior, including wing gestures and how often a bird flies off when another bird is singing," says Seyfarth. "Song is not the only end product, making this a much more powerful finding."
Powerful, maybe, but the findings didn't tell the researchers how the juveniles learned from their mentors. Most studies of behavioral transmission assume that animals learn by direct imitation. White thought there might be something more subtle going on.
To find out, he and his colleagues studied two more groups of juvenile males. They housed one group with typical Indiana adult males and the other group in an adjacent aviary where the juveniles could see the adult birds, but had no direct social contact. The adult males only stayed for three weeks in the fall-a time when adult males display little to no mating behavior.
"We basically controlled the dosage of adult male exposure," says White. "That way we could see what they're learning and when."
They didn't expect the young birds to learn anything about mating. "In the fall, adults are basically doing nothing," explains White. "They're molting so they can't fly well, and they're not singing. There's very little that juveniles could learn from adult males at that time, we thought. Lo and behold, we were wrong."
In fact, the juvenile males housed with adult males developed the same competitive mating style of their mentors. Those housed with juveniles only developed the more egalitarian and promiscuous mating style seen in the first study.
"They got this pronounced effect but with just three weeks of contact with adult birds," says University of Tennessee comparative psychologist Todd Freeberg, PhD. "Most researchers would be looking for the big obvious behaviors, but this shows that it can be pretty subtle things going on at times you wouldn't expect that lead to a behavioral cascade."
White and his colleagues believe this "behavioral cascade" explains how the juvenile males end up with the same mating style as the adults. It all starts with singing: While adult males don't sing much in the fall, juveniles practice their songs by singing to anyone who will listen.
But males with a competitive mating style see singing as a sign of aggression-a way of competing. So if a juvenile male sits next to a competitive adult male and starts singing, the older bird will stand his ground. In this way, White believes, juvenile males learn not to fly away when another male comes over to sing, encouraging competitive counter singing. In contrast, juveniles who aren't around adult males tend to fly away from a competition.
"An individual that doesn't stick around when others are interacting with it might miss out on social interaction and key social experiences that may be important for normal development," says Freeberg. "And that could be just enough to trigger this differential experience."
White believes his work has implications for how researchers think about cultural transmission.
"This type of approach is the way to go in trying to understand the importance of social life in shaping individual organisms," says White. "By raising these birds in a different culture, we can see how the early learning environment biases what's available for them to learn."
The finding, says White, shows how studying animals' early-life experience sheds light on their behavior as adults.
Without that, "you might get false impression of where evolution is having its impact," says White. "Environment is shaping the ability to learn socially throughout the animal's lifetime."
Beth Azar is a science writer in Portland, Ore.