While tracking down a particularly large snake in the swamps of Brazil, photographer Luciano Candisani got more than he’d hoped for: the first known image of a female green anaconda squeezing her mate to death.

As thick as a “truck tire,” the female was well known to local guides Juca Ygarapé and Daniel de Granville, who took Candisani to her usual haunt in the Formoso River.

They found her half-out of the water, entangled with a small male on the river bottom—perhaps, Candisani thought, a post-mating embrace. He watched the pair for a few hours, taking some underwater photographs from about three feet away.

“I couldn’t actually understand what was going on at first,” Candisani says. “But then she dragged the male’s body with her when she went into the grass.” (Read why we were totally wrong about how boa constrictors kill.)

Though he took the photo in 2012, Candisani says he is publishing the photograph with National Geographic now because the swamps in which these anacondas live are under increasing threat by wildfire and the proximity of agriculture. Environmentalists want the swamps protected by a conservation unit.

In early February, a large fire, probably started by lightning, took five days to extinguish, he says.

At the time, the guides were astonished by the anaconda’s behavior. So the photographer sought out the advice of anaconda expert Jesús Rivas, a biologist with New Mexico Highlands University who has studied the reptiles in Venezuela for over 30 years.

Rivas has documented a few cases of cannibalism in anacondas, in which females have regurgitated mates after eating them. It’s unclear if this female ate her mate; Candisani says they couldn’t see her after she pulled the male into the grass. (See “Cannibalism—the Ultimate Taboo—Is Surprisingly Common.”)

The reason is simple: The male is good protein for an expecting mother, especially one who fasts the whole seven months of pregnancy.

“A full 30 percent of her bodyweight goes into making babies. Getting an extra seven or eight kilos of meat before you go into that stage isn’t such a bad idea,” he says.