It's not unusual for monkeys to be gobbled up by birds of prey. These predators are excellent at hunting small creatures from the skies.

However, it's less common for them to fall victim to an opportunistic "sit and wait"-style attack from a predator like a snake.

Now for the first time, researchers have witnessed just that. A boa constrictor struck and killed two baby marmosets at the same time. 

The attack happened in a forest in the Cerrado region of central Brazil. 

A 2m long boa constrictor fell to the floor with two young marmosets coiled in its grasp

Researchers spotted a group of eight black-tufted marmosets walking from branch to branch, 5m above the ground.

Soon after, researcher Danilo Simonini Teixeira of the University of Brasília in Brazil heard several distress "tsik tsik" calls.

The attack was quick. Only two minutes after the marmosets were first spotted, a 2m-long boa constrictor fell to the floor with two young marmosets coiled in its grasp.

The other adults in the group reacted immediately. They jumped onto the snake, striking it with their hands and then fleeing just out of danger, a pattern they repeated several times. They also continued to make distress calls. 

"The marmosets were always seen to jump and or attack the same middle portion of the snake's body," the researchers say. "Due to the coils around the two victims, this was its most exposed area."

Five minutes later, the monkeys gave up and moved away from the snake.

The snake did not seem fazed by any interference. In fact, it strengthened its grasp on one of the marmosets. Once its body was lifeless it did the same for the second one.

The incident has been outlined in the journal Primates.

Ten minutes after the snake was first seen by Teixeira, it loosened its coil on the marmosets and travelled to a nearby tree, leaving their carcasses dead on the ground. It is not known whether the snake returned to eat them.

The marmosets were both only about half a year old.

It's unclear how frequent such an attacks are, but the researchers note that "while in motion, marmosets may become less vigilant of their surroundings." Juveniles are also clearly more at risk.

These attacks therefore depend on "stealth and surprise… so the threat level it poses to primates depends on detection evasion, distance to potential prey."