For zebra finch males, having a partner can be more important than having a mate.
Zebra finches form life-long relationships, usually with members of the opposite sex. But a study published in the journal Behavioral Ecology and Sociobiology found that male birds raised together form bonds with each other that are just as strong.
Sixteen male finches were raised to adulthood together. Upon reaching maturity, the males coupled up and showed signs of affection, like singing, preening and nuzzling beaks. When females were then introduced to the finches' cage, five of the eight male-male pairs stayed together and ignored the females.
"Relationships in animals can be more complicated than just a male and a female who meet and reproduce, even in birds," said lead researcher Julie Elie of the University of California Berkeley in an interview with the BBC.
The researchers did not report any sexual activity between the paired males, only that they formed cooperative bonds. This suggested to the researchers that the survival advantages of having a pair bond can be more complex than simply having a reproductive partner.
"A pair-bond in socially monogamous species represents a cooperative partnership that may give advantages for survival," said Elie. "Finding a social partner, whatever its sex, could be a priority."
Although some have reported this as evidence of homosexuality in the animal kingdom, the lack of any observed sexual behavior means there is an important piece missing from that argument. Just because males are affectionate to each other and live together doesn't mean their "bro-mance" is homosexual.
Same sex pair bonds have been observed in birds before. Elie noted the case of female albatross, who will bond with another female, then mate with a male from another pair bond in order to raise a chick with her female partner.
"Female partners copulate with a paired male then rear the young together," Elie said.
In the zebra finch study, females were not raised together, so it is unknown if female finches will form pair bonds as well.
The famous relationship of two male penguins, Roy and Silo, at Manhattan's Central Park Zoo is another example of same-sex pair bonding in birds. The two male chinstrap penguins formed such a strong bond that they were even able to incubate and hatch a fertilized egg that a keeper placed in their nest.
In situations like these, humans are quick to put their own sexual definitions on animals. But it is important to remember that our own sexual definitions of hetero- vs. homosexual are set by our cultures, and the labels we put on each other and on animals are based in our culture as well. Cultural sterotypes in some human societies about male-male affection may make people inclined to label affection between male animals as evidence of homosexuality.
Putting our labels on animal behaviors runs the risk of anthropomorphic interpretation, or seeing human characteristics in animal behaviors.