Image: Male marsh harriers respond to decoys
© Audrey Sternalski
The researchers observed how male marsh harriers responded to three types of decoys, typical males, females and transvestites.
By LiveScience contributor
updated 11/8/2011 8:06:00 PM ET 2011-11-09T01:06:00

Birds of prey may be thought of as fierce foes, but scientists find that some males disguise themselves as peaceful females.

These males belong to a species of raptor known as the marsh harrier. Using plastic decoys, French researchers learned that the transvestites among these predators are less aggressive than other males.

Some animals will use the tactic known as sexual mimicry in the cutthroat battle to survive. For instance, young male birds often have female plumage that helps camouflage them; they will acquire more striking plumage only after reaching sexual maturity, to help them attract mates.

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However, permanent lifelong female mimicry, in which males look like females throughout life, is extraordinarily rare in birds. Until now, it had been studied in only one species, the ruff (Philomachus pugnax), a shorebird in which some males engage in female behavior to sneakily get sex.

Why dudes dress like ladies
The only other bird in which this practice has been found is the marsh harrier (Circus aeruginosus). In one exceptional population in midwestern France, 40 percent of the males of this bird of prey disguise themselves in female plumage.

To study the marsh harriers there, ornithologists including Vincent Bretagnolle, directorof the Center for Biological Studies of Chizé, France, created decoys painted to closely resemble the females, typical males, and female-like males. Females are mostly brown with ocher-brown eyes, while typical males are mostly gray with yellow-white eyes, and female-like males are mostly brown with yellow-white eyes. Males are also approximately 30 percent smaller and lighter than females.

The researchers then spent three months watching how both kinds of males responded to decoys placed in the wild near the nests of 36 breeding pairs of marsh harriers. Some funny situations arose during this field work.

"I observed a female-like male marsh harrier trying to copulate, for almost 10 minutes, with a female decoy," Audrey Sternalski, a behavioral ecologistat the Research Institute for Hunting Resources in Spain, told LiveScience. "Furthermore, as our study area is also a well-known site for naturalists and photographers, it has been really funny to observe some tourists or photographers believing that the decoy was a real marsh harrier and attempting to approach the decoy to photograph it." [In Photos: Birds of Prey]

The researchers found typical males were territorial creatures, often attacking decoys resembling other typical males. They were significantly less aggressive against decoys painted like a female or a female-like male, however. This suggests the transvestite males mimic females to avoid costly fights with typical males.

The researchers' experiments also revealed that female-like males were much less violent than their brethren, never attacking decoys resembling typical males. (Female marsh harriers rarely assaulted any decoy.) These transvestites might essentially have a "nonaggression" pact with other males, researchers said.

Oddly, on the rare occasions when female-like males were aggressive, they pointed that aggression at decoys resembling females or other female-like males. Past studies of birds of prey suggest that females normally attack other females to protect their homes — the female-like males could be copying this behavior, Sternalski said.

Getting close to the gals
Sternalski suggested transvestite marsh harriers might also use their feminine plumage to get intimately close to females, just as transvestite ruffs do. Although it is very hard to catch marsh harriers mating because they prefer to have sex in reed beds, future research could involve paternity tests of hatchlings to see if transvestite males did indeed get around.

Future studies also could endeavor to tease apart the relationships between plumage and behavior: Are transvestite males relatively peaceful because they are hormonally wired to be so, or do they learn such behavior by watching others?

"The main obstacle we foresee in future research resides in the species itself," Sternalski said. "The marsh harrier is a difficult species to work with because the species is quite sensitive to human disturbance, and adults are difficult to capture and manipulate."

Sternalski, Bretagnolle and colleague François Mougeot detailed their findings online Tuesday in the journal Biology Letters.

© 2012 LiveScience.com. All rights reserved.

Explainer: 10 peeks at sex in the wild

  • Image: scroll fragment
    Jerome Maison  /  AP file / Warner Independent Pictures

    Emperor penguins endure some the harshest conditions on the planet — the Antarctic winter — to satisfy their primal urge for sex. The annual ritual begins with a days-long, up to 75-mile slog to their inshore breeding grounds. Once there, an elaborate courtship of calls and poses reunites old mates and enables young lovers to form lasting bonds.

    Copulation itself occurs under the cover of the dark of the polar night. Then the true sign of their devotion begins: Males huddle together to incubate the eggs as the females waddle back out to sea to feast, fattening up to provide for their newly hatched young. Once she returns, males depart in the first of a tradeoff that may, eventually, allow for healthy offspring.

    Click on the "Next" label for nine more peeps at sex in the wild.

    - John Roach, msnbc.com contributor

  • Captive pandas require help to get it on

    Image: Pandas
    Wichai Taprieu  /  AP file

    In the wild, biologists say pandas get it on without too much of a hitch. But since the wild population in China's mountain forests hovers around just 1,600, there's a push to boost the panda population in zoos around the world. And that's where the problems lie. Many pandas lack interest in their arranged mates and some inexperienced males who give it the old college try fail to engage the proper body part. As an aid, some zoo keepers attempt to arouse the bears and teach them appropriate technique with specially made DVDs, a.k.a. panda porn. When that fails — and it often does — artificial insemination is considered a last resort. These tricks combined with a better understanding of what makes the panda libido tick are beginning to pay off. The captive population has boomed in recent years.

  • Male macaques groom for sex

    Image: Japanese Macaque monkeys
    Shuji Kajiyama  /  AP

    Male macaques groom females in exchange for sex, according to a study that examined the market underpinnings of monkey sex. According to the research, based on 20-months of observation in an Indonesian nature reserve, a female is three times more likely to mate with a male if he grooms her first. Supply and demand also comes into play: Males spend more time grooming when competition for female attention is greater. Scientists refer to this practice as a biological market.

  • Male antelopes play hard to get

    Image: Antelopes
    Jakob Bro-Jorgensen / Zoological Society of London

    For highly-desired male topi antelopes in Africa, the frenzied six-week-long mating season is exhausting. Any given female is receptive for about a day, thus she wants to mate as many times as possible, especially with the fittest males. This creates intense competition for high quality antelope sperm, allowing those that possess it to be picky. A study of the phenomenon found the choosy males deliberately select the least mated females and fend off aggressive females they've already mated with. The aggressive female in the center of this image is attacking the male on the left as another female eyes the scene.

  • Sumatran rhinos tussle before they tango

    Image: Newborn rhino
    Tom Uhlman  /  AP

    Mating for the typically solitary and territorial Sumatran rhinoceros is a drawn-out affair. Studies of the critically endangered species in captivity show that when a female becomes receptive to a male's approach, she'll exhibit increased urine spraying, tail raising and swinging, and vocalizations. Foreplay includes head and genital butting, which can be a bit too much when a female isn't quite ready to tango with a young and aggressive male. But when the tussle is successful, a male will mount, often riding his mate for up to an hour. The image here shows the results of a successful mating at the Cincinnati Zoo. Such captive breeding efforts are a silver lining for conservationists hoping to keep the Sumatran rhino alive — just 300 are thought to remain in the wild.

  • Burliest walrus bulls get the harem

    Image: Walrus
    Liz Labunski  /  AP

    With a cacophony of clicks, clacks, whistles, and bellows, male walruses swim around the chilly Arctic waters vying for the attention of ice-bound females. Males will also fight off other males that get too close, sending the loser packing. Once the fussing and fighting is done, female harems surround the burliest males in the water for an underwater romp. Though scientists know little about what actually happens under the cover of the waves, they do know walruses are endowed with the mammal world's largest penis bone, called a baculum, which extends up to 30 inches.

  • Dolphin mating is brief, but bountiful

    Image: drainage channel
    Noaa  /  NOAA

    For playful dolphins, mating seems just like another carefree and pleasurable way to pass away the day. Along with some typical male posturing for access to females, there's plenty of chasing, rubbing, nuzzling and stroking that constitutes as foreplay. The belly-to-belly copulation act itself lasts less than a minute, though is often repeated several times over the course of an hour.

  • Virgin female spiders risk all for a big mate

    Image: archaeological site in Masada

    For some male spiders, sex is the ultimate sacrifice: females eat them as part of the reproductive ritual. But among the East African blood-gorging jumping spider Evarcah culicivora, shown here, males possess the coital-infused cannibalistic urge. Nevertheless, female virgins opt to be deflowered by bigger males before settling in with a small guy for the long haul. Scientists suspect females gamble with their fate once in hopes of producing larger, fitter offspring, but decide not to double down after the flirtation with danger.

  • Sappy sex for beetles of all sizes

    Kensuke Okada

    As this picture shows, Japanese sap beetles come in big, medium, and small sizes. Generally, in the beetle world, mating success is only bestowed on males with the biggest bodies because they can beat out the competition with brute force. Big sap beetles successfully employ this strategy, but when they do, the medium guys take to the air with their extra long wings and survey for sites where big males are absent. But the little guys have the biggest testicles. This allows them to hang with the big guys and sneak sex behind their backs. Since the little beetles have such big testes, they produce more competitive sperm, upping their chances at siring offspring when they get a shot.

  • Cycads have 'hot' plant sex

    Irene Terry  /  Univ. of Utah

    Hot sex has ancient roots. The males in a group of plants called cycads, which have been around for at least 250 million years, get all hot and bothered as a means to compel pollen-covered insects deep in their cones to flee forth and find a female to pollinate, according to scientists. The plants begin the process by emitting a fragrance that lures little insects called thrips into their cones. After a few hours of the thrips feasting and rolling around in there, the cycads heat up as much as 25 degrees Fahrenheit, which turns their sweet smelling fragrance into a stench. The thrips flee, some landing on benign-smelling female cycads and thus completing the pollination cycle.

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