When it comes to courtship, some male fish species are willing to do anything - even pretending to be female - to get the girls.

New Zealand scientists have shed new light on the strange phenomenon of so-called "sneaker males" who disguise themselves as females to avoid aggression from larger males, and steal mating opportunities.

A just-published study, led by Otago University's Dr Erica Todd, shows these deceptive dudes can achieve their extraordinary feat of subterfuge by turning specific genes in their brains and gonads on or off.

Todd studied bluehead wrasse, which she said had a social organisation that rivals the most outrageous soap opera.

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"There are two types of males — large, aggressive blue-headed males that openly court females, and smaller 'sneaker' males that look, and act, like females to sneak in matings."

Working with Professor Neil Gemmell and other researchers from the University of Otago and North Carolina State University, Todd used high-throughput RNA-sequencing to uncover which genes are active in sneaker male brains, compared with territorial males and females.

She and her colleagues were surprised to discover that sneaker males had brain gene expression patterns near-identical to females, but very different to territorial males.

The study also revealed how sneaker males make themselves look like females to sneak past other males and avoid confrontation.

The blue-headed wrasse. Photo / Tibor Marcinek/CC
The blue-headed wrasse. Photo / Tibor Marcinek/CC

"Males of many species use bright colours and other ornamentation to attract mates and compete with rivals, which are often regulated by male sex hormones produced in the testes," Todd said.

"In sneaker male testes, we found that many of the genes critical for male sex hormone production were turned off — making them look female."

Although they look and act like females, the sneaker males were reproductively potent — their testes are three to four times larger and produce 60 per cent more sperm than territorial males.

Todd discovered that the larger testes of sneaker males had higher expression of genes involved in cell proliferation and sperm quality control.

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The study also gave insights into how different males cope with their divergent lifestyles.

"Sneaker males express genes for neuroplasticity that may help them elude territorial males and steal mating opportunities with females," she said.

"Territorial males express genes associated with stress and protection against cellular damage, suggesting that life is tough at the top of the social hierarchy."

But that's not all that's going on in the bluehead wrasse: astonishingly, females can change sex, and sneaker males can change roles, to become territorial males when they grow large enough.

Todd and her colleagues are now investigating the genes and environmental signals that trigger sex change in bluehead wrasse and the New Zealand spotty.

They are funded by the Royal Society of New Zealand Marsden Fund, the University of Otago and the US National Science Foundation.