Experts are studying the "torrid and tangled" sex life of one of the UK's most popular pet fish as they try to learn how the battle of the sexes influences evolution.
The guppy is one of the most popular pet fish in the UK
Researchers at St Andrews University hope that the guppy's mating behaviour can help them unravel the mystery of how new species are formed.
They have put Trinidadian guppies under the microscope because the fish is in the process of splitting into two new species.
Two groups that have been living in separate river systems for about two million years were put together and DNA fingerprinting is being used to establish the parentage of baby fish.
The research team discovered a web of sexual conflict, interbreeding, sneaky mating behaviour and sperm competition.
Professor Anne Magurran said the study had found a number of ways in which "reproductive isolation" was being encouraged.
The definition of a species is a group of creatures that can breed with each other but not with others, even if they look similar.
The research team believes that this reproductive isolation was the key to understanding how the world's vast diversity of creatures evolved.
More than 140 years ago Charles Darwin predicted that a lack of sexual contact was what would drive species apart.
The Trinidadian guppies have divided into two groups which never normally meet.
Professor Magurran and her team have been introducing males and females from the different populations in the laboratory to gain an insight into how reproductive isolation develops.
Tools such as DNA fingerprinting have only recently become available to help scientists test their theories about species formation.
"One possibility is that female guppies prefer to mate with native males," said Professor Magurran.
"If animals always mate within their own group reproductive isolation is very quickly established."
A female will usually choose several male guppies to father her offspring in an attempt to ensure that her babies have the best genes possible.
However, the researchers found that wily males often sneaked up on unsuspecting females and mate with them by surprise.
"This sneaky mating tactic means that there are many matings between the two groups of guppies and probably counteracts the effect of female choice on reproductive isolation," said Professor Magurran.
To test the theory that sperm from the females' own species can outcompete sperm from other species at fertilisation, the fish were impregnated with equal numbers of sperm from a native and a foreign male.
The guppies live in separate river systems in Trinidad
Researchers are now performing DNA fingerprint tests - similar to those used in paternity tests in people - on the baby fish.
The initial results indicate that native males do sire more of a female's offspring.
The team has also found that hybrid offspring are worse at reproducing than those produced by parents from the same group.
"These unfit offspring are another bar to interbreeding, and it seems that this mechanism is evolving simultaneously with sperm selection to cause reproductive isolation," said Professor Magurran.
The team's findings, Guppy Love: Sex and Speciation, will be on show at the Royal Society's Summer Science Exhibition in London.