They are small, mice-like critters known for their marathon mating sessions, which can last up to 14 hours. And that may be their undoing.

The Australian government has added two species of antechinus, the black-tailed dusky and the silver-headed, to its endangered species list, saying all that sex is killing them.

During mating season, which lasts for several weeks each year, males and females move frantically from one mate to another. There’s no courtship, just sex — with as many partners as possible.

“They literally become a marsupial zombie in their pursuit,” Jeff Corwin, wildlife expert and TV host, told CNN.

Males only live about a year, and the females live up to three years, but on average, they both die after a litter is born.

So while the round-the-clock fornication stresses both sexes, only the males produce testosterone, said Andrew Baker, head of a research team that has discovered five new species of antechinus since 2012.

The constant high levels of testosterone keep the stress hormone, cortisol, from shutting off. Eventually, it reaches toxic levels and causes the animal’s immune system to malfunction. The animal then bleeds internally and dies.

It isn’t just sex

A species that purges half of its adult population every year is already in a vulnerable situation, but the antechinus also faces pressure from humans.

“The antechinus is doing what it’s been doing for millions and millions of years. This strategy to be very, very active sexually and very competitive with males coming in to take your female away, this is nothing new to this species,” Corwin said.

“It’s not too much sex. What’s killing the species is habitat loss, climate change; and perhaps the biggest impact are invasive predator species, like dogs, cats and rats. They’re outcompeting this species to extinction, and this species, as it shows us, just wants to have a good time.”

Baker says there could be as few as a few hundred animals of each of the two species left. In order to save them, he believes humans need to get the antechinus to migrate to southern Australia, where it’s colder. But researchers aren’t sure how to introduce them to those areas.

So what if researchers tried to keep males and females separated? Baker says the males would only make it if males and females were in total isolation and only if females were introduced one at a time.

In an experiment, castrated males did survive — but that doesn’t do much for reproduction.

“Life is incredibly valuable,” Corwin said. “It’s miraculous in the way that it survives. And the antechinus illustrates that, but unfortunately, it’s being pressured to the verge of extinction. We may lose this species before we ever know what its great natural value is.”

Death by sex may seem counterproductive to the survival of a species, but many animals have evolved the behavior to maximize reproduction. Another example: garter snakes, who engage in massive, tangled orgies so exhausting the males age faster and die sooner. Have fun Googling that one.