Ruff faeders pose as females to dupe rivals and get ahead in the mating game. Image: Lesley Evans Ogden

The animal kingdom is rife with imitation and trickery, but it's a rare bird indeed that can hide its true sex and profit from its deception. Such is the occupation of the male faeder, an uncommon type of male ruff (Philomachus pugnax), a shorebird species that breeds across northern Eurasia, spending the non-breeding season in sunny climes like Africa.

Ruffs get their name from the more typical adult male appearance, notably their attractive feathery neck ruffle that females of the species apparently find irresistible. Ruffs’ elaborate plumage is a classic outcome of what Darwin called sexual selection – with show-off males and drab females. Ruffs have long intrigued scientists because of their two alternate male plumage types. During the breeding season, showy, darker-coloured 'independent' males attempt to court multiple females while parading and strutting their ruffles around a small breeding area known as a lek (it goes something like this). Lighter-coloured subdominant 'satellite' males keep a lower profile but may take the opportunity to mate with females when independent male showoffs are temporarily distracted, such as when they're attacking neighbouring rivals.

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Come breeding season, darker 'independent' males flaunt their elaborate plumage to attract females, while the paler subdominant males prefer a low-key strategy. Image: Lesley Evans Ogden

In the Netherlands, especially in the northern region of Friesland, artisanal trapping of shorebirds for food was a centuries-old source of livelihood in rural communities. These trappers, known as wilsterflappers, traditionally used wind-driven nets, decoys and impeccable timing to capture flocks midair. When commercial shorebird hunting was outlawed in the late 1970s, Dutch researchers helped to redirect some of the unique skills of wilsterflappers to aid shorebird science.

And so it was that while conducting research on ruffs, the University of Groningen’s Dr Theunis Piersma was working alongside Joop Jukema, a Friesen potato farmer and wilsterflapper. Jukema became puzzled by the capture of a particularly weird bird. In this species, males are much bigger than females, and this weird bird was intermediate in size. Though it had female plumage, it was too big to be a female, thought Jukema. He suggested as much to Piersma, who was initially unconvinced, but later discovered that this was not just a cryptic male, but also one with ... extreme balls. The testes of these rare males (disguised as females), it was later discovered, have a volume 2.5 times larger than those of other males!

Whereas it's not uncommon in many birds for juvenile males to temporarily resemble females, this male ruff, it turned out, was a permanent female mimic. Subsequent work has revealed that these girlie guys make up about 1% of males in ruff populations. Jukema gave these males the name 'faeder', a word that means 'father' in both Old English and Frisian languages.

Self-proclaimed 'ruff farmer' Dr David Lank has bred and studied a captive ruff population for more than three decades. Image: Lesley Evans Ogden

The mysteries of how and why this unusual sexual mimicry system is maintained are still being worked out. At Simon Fraser University, Dr David Lank has maintained, bred and studied a captive ruff population founded with wild-caught birds for more than three decades. So long, he jokes, that he’s now a 'ruff farmer'. Flying two faeders to Canada from The Netherlands as hand luggage in a gym bag and breeding them into his captive ruff population has allowed investigation of how genes for this rare male type are inherited. Lank and collaborators discovered that the faeder type is carried by an autosomal-dominant allele, meaning females as well as males can inherit it. If inherited by females, Lank explains, it results in 'mini females' much smaller than the norm.

Male faeders appear to have several behavioural tricks in their repertoire. “They hang out in leks when things are happening, and try to sandwich in on top of females – to jump females when they crouch,” says Lank. (In ruffs, females crouch to indicate willingness to be mounted by males, who then jump on top to do the deed). Faeders also appear to decoy ornamented males away from mating with real females, explains Lank, who has observed these behaviours in his captive ruffs.

Studying faeder behaviour in the wild for a bird that is 1 in 100 is necessarily challenging. And one theory is that in the wild, this third male type, the female mimic, relies on a type of deception that only works because it is rare, existing at a prevalence of about 1% in the population, compared with 85% territorial and 14% satellite males, as research by Lank and others has revealed.

Humans might be duped, but are ruffs fooled by such sexual deception, too? That's still unclear. A recent study showed that faeders were just as often 'on top' in homosexual mountings as were 'true' males, suggesting that some males, at least, know what faeders are up to. “They may appear to be 'female mimics' to us, but not necessarily to the ruffs themselves,” says the University of Groningen’s Dr Yvonne Verkuil. She has led recent research revealing that though faeders resemble females, when not breeding, they’re more likely to hang out with males.

Ruffs are no longer the only known bird to exhibit this permanent female mimicry – it’s also been discovered recently in the marsh harrier. Nevertheless, no matter how you look at it, the faeder is a rare and unusual bird, and cross-dressing, it appears, is just one more of the clever tricks up Mother Nature’s sleeve. 

For other sneaky cross-dressing strategies from the animal kingdom, check out this episode of Earth Touch's Wild Sex series. 

Top header image: Ian White, Flickr