Birds of prey have a macho image, but some male raptors look and act feminine, fooling everyone into thinking they are females.
These she-males often win out in the end. They avoid conflicts with other puzzled males, and yet they establish territories and enjoy successful mating with females.
"Permanent female mimicry," as the phenomenon is called, has already been confirmed in the shorebird ruff Philomachus pugnax. Now a paper in the latest Royal Society Biology Letters has determined that 40 percent of all sexually mature marsh harrier Circus aeruginosus males exhibit lifelong female behaviors and attributes.
Such female mimicry, also seen in fish, reptiles and insects, appears to emerge most often in species with males that fight a lot with each other.
"When intrasexual aggression is high, permanent female mimicry is arguably adaptive and could be seen as a 'permanent non-aggression pact' with other males," lead author Audrey Sternalski and her colleagues explained.
Sternalski, a researcher at the Instituto de Investigación en Recursos Cinegéticos in Spain, and colleagues François Mougeot and Vincent Bretagnolle tested out their theory. They presented plastic decoys representing each of the three types of birds -- males, she-males and females -- to each bird group during different stages of their breeding cycles.
Typical males were aggressive toward male decoys, but more tolerant of the she-males.
"By contrast," the researchers wrote, "female-like males tolerated male decoys (both she-male and male) and directed their aggression towards female decoys…Therefore, female-like males not only look like females, but also tended to behave like them when defending breeding resources."
Randolph Krohmer, an associate professor of biology at Saint Xavier University, recently led a study on she-male red-sided garter snakes, Thamnophis sirtalis parietalis. In this species, the female mimics don't look like females, but they release a pheromone that fools other snakes into thinking they are gals instead of guys.
"During courtship these snakes form mating balls that can consist of 50 males and one actual female," Krohmer told Discovery News. "It looks like a tumbleweed rolling around on the ground."
He said the snakes "taste" the sex pheromone by flicking their tongues. When males detect the chemical released by the she-males, they sometimes spend time courting them. While these males are busy being confused, the female mimics then make moves toward the actual females.
"The she-males diminish the costs of courtship at the same time as duping other males," said Morris Gosling, an emeritus professor of evolutionary biology at the University of Newcastle.
Krohmer at first wondered if the behavior somehow benefited the entire species, but he's been studying the snakes for decades. When he began his work, approximately 10 percent of the snake's population consisted of she-males. That has remained constant over the years.
"You'd expect the percentage to go up if she-males were advantageous to the species," he said.
Nevertheless, she-males persist and represent "a third category of sexual behavior," which is neither fully male nor female, according to Krohmer.