Males are "still needed" even when females can reproduce without them in some species, a study has found.
Scientists at the Okinawa Institute of Science and Technology Graduate University (OIST) were looking at species in which females are able to clone themselves and perpetuate offspring without males.
They note that species can grow their populations faster in harsh environments when females do not have to worry about finding a worthy mate.
It is thought this ability arose independently in certain species - including amphibians, reptiles, and fish – because of either a conflict between the sexes or to ensure survival when there are few mates available. In many of these species, populations are composed almost entirely of females.
Published in the journal The Science of Nature, the authors looked at the early evolutionary transit from sexual reproduction to that of cloning in the little fire ant – a species where some populations reproduce through sex and others through cloning.
Despite not necessarily needing males, all populations still have males. As with other ant populations, males fertilise queens to produce worker ants, which are all sterile, and have genetic make-up from both mother and father.
However, they are unusual because fertile males hatch with no genetic contribution from the queen, while new queens hatch with no genetic contribution from the father. Previous research has also shown that workers can be produced clonally by the queens.
Why males still exist remained a mystery, because queens can create new queens and workers without them.
By studying both sexually and clonally reproducing populations, the authors found many of the eggs laid by the virgin birth queens did not survive, with most not making it past early stages of embryo development. In comparison, the inseminated queens had a near 100% success rate for hatching and the queen laid eggs more often. Researchers believe mating increases fitness and has healthier offspring
The mechanism that stops sex from being completely eliminated is still a mystery, but the researchers say their findings suggest there is an evolutionary constraint that keeps males in the game – with female reproductive systems not performing as well without them.
"Although clonal queens can indeed produce both workers and queens without mating, the hatching rate is far below the level necessary to maintain functional colonies," the authors conclude. "On the other hand, virgin queens from populations exhibiting the original ... reproduction system also show low hatching rates, but produce only haploid male eggs.
"Reasons for the existence of [little fire ant] males have been disputed. However, our data suggest that physiological constraints, such as the requirement for insemination, must be considered in regard to evolution of reproduction systems, in addition to ecological data and theoretical considerations of fitness."