When Jared Towers and two of his colleagues set out to observe a group of orcas off the coast of Vancouver Island, they assumed it would be a routine excursion. In fact, Towers, a cetacean researcher with Fisheries and Oceans Canada, was already familiar with this particular pod of killer whales, having documented them on previous research trips.

For an hour, the trio of humans photographed and identified several of the whales — including an orca calf that appeared to be no older than a few days, or even a few hours, judging by the fetal folds still visible on its body and a dorsal fin that wasn’t fully standing up.

The December 2016 boat trip “wasn’t anything too out of the ordinary,” Towers told The Washington Post. “After about an hour, we were just about to go back to the office and call it a day.”

That was when the group of scientists witnessed something they say they’ll never forget.

Ahead of them, some splashing, noise and “erratic movements” caught their attention. Towers said they figured the group was attacking some prey.

“We got a little closer and realized that the baby whale we observed earlier wasn’t surfacing,” Towers said.

Moments later, a male orca swam by with something in its teeth.

“The baby was hanging out of his mouth,” Towers said. “I knew right off the bat — I study killer whales pretty intensively — that this was a ‘first of its kind’ kind of observation.”

Immediately, the researchers dropped a hydrophone into the water and into “data collection mode” for the next five hours. They watched with a mix of agony and awe as the male orca proceeded to drown the calf, all while its mother tried in vain to stop the killing of her baby, he said.

“We were a bit horrified, but more so I think we were fascinated,” Towers said. “We knew that it was time to just collect as much data as we could to accurately record our observations.”

Though the expedition took place more than a year and a half ago, the observations by Towers, Muriel Halle and Gary Sutton were published just this week in the journal Scientific Reports. It is the first recorded instance of an orca killing a calf of the same species, providing evidence that killer whales engage in infanticide, a behavior reported in many species.

Among land mammals, infanticide occurs among primates and rodents, the report noted. It also occurs in some dolphin species. It is often “a behavior that leads to sexual behavior,” Towers said.

In this case, he believes the male killer whale may have drowned the orca calf because he wanted to mate with the mother.

Unsurprisingly, the female fought back.

“I think it's pretty safe to assume she was pretty upset,” Towers said. “Typically, mammals, when they have their infants removed, they’re quite protective of those babies.”

For several minutes, the researchers documented the female orca chasing the male. At one point, she rammed him so hard that his body undulated, “sending blood and water into the air,” the report stated.

An older whale, the mother of the male orca, joined the fray at times, trying to come between the fighting whales and often swimming alongside her son — all while the helpless calf was hanging from his mouth, unable to surface for air and slowly being drowned.

Underwater noise recorded by the hydrophones — including whistles, pulsed calls and percussive sounds — indicated there was probably ramming happening beneath the surface as well, Towers said. The male orca emerged with fresh teeth wounds visible on his left and right flanks, parts of his head and the base of his dorsal fin. His mother, the older whale that got involved, also had teeth marks on her body.

All of the whales “settled down” after about 10 to 12 minutes. The male orca was still holding the dead calf in his mouth by its fluke, its body trailing in the water.

“I think it may be because the mother realized the calf was dead, so what was the point of continuing to fight after that?” Towers said.

(For more footage of these observations, including audio of the orcas' cries during and after the attack, see the scientists' video below.)

The researchers attempted to get a sample of genetic material from the male orca and to recover the body of the dead calf but ultimately had to turn the boat around because it was getting dark. Towers said they are uncertain whether any mating behavior between the male orca and the calf's mother followed.

One way to tell, he said, would be to see if the mother of the dead calf gives birth to another baby soon — since killer whales have a gestation period of 17 to 18 months — and find out if its genetic material matches that of the male orca.

But Towers said the encounter primarily revealed how much more there is to learn about killer whale behavior, especially regarding mating.

“Looking at the behavior we’ve observed, we’re now beginning to think that it’s quite possible that females don’t have a lot of choice when it comes to breeding,” Towers told Canadian Broadcasting Corp. News.

Correction: An earlier version of this report misidentified Canadian Broadcasting Corp. News.

Read more: