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.

Eating for Two (At Least)

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.

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Rivas says Candisani's photos represent only the fourth reported instance of a female anaconda strangling her mate—and the first caught on camera.

Big and Beautiful

Green anacondas are especially suited to sexual cannibalism because of the massive size difference between sexes.

Males average around nine feet and females average 12, although up to 17 feet is common, Rivas says. Candisani estimated the snake he saw at 23 feet long, which would be unusual. (See "Giant Python Meals That Went Bust.")

However, Rivas noted that the area where Candisani captured the photo, not far from Bonito City in the state of Mato Grosso do Sul, is consistently wet.

“In areas with a strong dry season, [anacondas] can hibernate for months under the mud because life is hard,” he says.

“But in places that are flooded all the time, they probably have a better food supply, and so might grow larger.” (Read how snakes know when to stop squeezing their prey.)