Five gecko lizards put into orbit in a Russian satellite in an experiment to study the effect of zero gravity on their sex lives have died, Russian officials say.

The dead geckos were discovered after the Foton-M4 satellite was returned to earth and was recovered, Russia's Roscosmos space agency announced.

Russian media quoted experts as saying a failure of the satellite's heating system may have caused the lizards to freeze to death.

"We can say with confidence that they died at least a week before the landing because their bodies were partly mummified," an official from Russia's Institute of Medical and Biological Problems said.

The satellite carrying the geckos, along with fruit flies, launched on July 18 with experiments designed to study the effect of weightlessness on the mating habits of the passengers.

Although the geckos did not survive, the flies did, a statement from Roscosmos said.

"After extraction of biological objects from the lander to carry out the initial evaluation, it was found that the fly Drosophila moved spaceflight well, successfully developed and bred," the translated statement said.

In additional experiments aboard the satellite, plant seeds and mushrooms were monitored, while a furnace specially designed for space operations carried out melting and cooling of various metal alloys in zero-gravity conditions.

The mission had been intended to last for two months, but the satellite was returned to earth after only 44 days.

Shortly after the satellite arrived in orbit, ground controllers lost the communication link with it, although they managed to re-establish control several days later.

The geckos -- one male and four females -- followed previous missions that put other animals into space, including mice, frogs and salamanders, in experiments designed to assess the affect of weightlessness on reproduction.

Scientists have long wondered about the challenges facing human reproduction on long-term voyages to destinations beyond Earth -- trips that might involve multiple generations.

"Reproduction in space is a long-term goal that people would want to meet," Ruth Globus, a scientist at NASA's Ames Research Center, told NBC News. "Even if people were to say, 'We don't care about the long term, we only care about now,' it's important to understand what's happening to the ovaries and the testes [in the space environment], and the subsequent changes that may occur."

The gecko mission was not equipped with a live video feed, so scientists will have to study footage recorded by a camera on the satellite to see if the geckos survived long enough to even think about sex.