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Researchers at Hamilton's McMaster University found that some small, Casanova-like fish are infiltrating the nests of other unsuspecting males, leaving their sperm on the eggs inside before slipping out to wreck another home.

Julie Marentette, lead researcher and PhD student in psychology, neuroscience and behaviour, said that this type of cuckolding happens in many species of fish where the fathers often raise the young.

Using data collected from 1,300 Lake Ontario round gobies between 2006 and 2007, researchers found that the smaller, light-coloured goby bachelors actually look more like the females, while the conventional dads are larger, black and have a round, swollen head.

Scientists aren't sure if the rogues get into the nest by flirting with the fathers who guard the roost, or by sneaking in, fertilizing the eggs and then turning tail in search of another nest to infiltrate.

Marentette said that with up to 10 females per nest, the sneaky bachelor has the opportunity to really spread his seed.

And spread it does. Marentette said the roaming single males, who have developed bigger testes, can blanket the eggs with a larger volume of more-potent ejaculate than the stay-at-home-dads.

However, the bachelors need to swim carefully: the stay-at-home dads, who spend more time courting females and defending their territory, are bigger and have higher levels of fish testosterone.

Marentette said two-male fertilization only happens after a species has become established in an area. As a population migrates to a new area, like the gobies did in the Great Lakes in 1990, most males stay parental because nesting sites are readily available.

However, as the population grows, bachelors emerge to seize the opportunity to sow their wild oats.

While the gobies have yet to be subjected to an aquatic paternity test, Marentette said that may be part of future research to find out which type ends up fathering more fry.

She hopes the research will help predict what kind of effect the invasive gobies will have on the Great Lakes ecosystem.

She said they are already throwing their weight around, outcompeting native species such as bass and salmon for food and shelter, and eating the eggs and young of other species.

"They're a tricky fish, all around," she said.

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dirty little things, sneak out the window when they hear the door open
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