Vikings we terrified of a monster they called hafgufa. Hafgufa was thought to hunt around the Icelandic waters, using its own vomit as chumming bait to gather prey fish — or any foolish humans who ventured into its path. However, hafgufa may have been inspired by a very real feeding behavior.

Trap-feeding, as the behavior is called, is so spectacular that it stunned people in ancient times.

Different whale species eat all sorts of things. Some munch on algae, phytoplankton, or zooplankton. Others eat krill or fish. Unsurprisingly, they also rely on different feeding strategies to pursue their food.

In 2011, one such strategy went viral on Instagram. The video shows whales swimming at the surface of the water with their mouths open at a right angle, just waiting for fish to swim into their mouths. Normally, sharks lunge towards fish violently, but the whales’ silly technique also works because it tricks fish, who don’t realize they’re swimming toward danger.

Remarkably, this strategy was only described recently by researchers. It’s been described in several species, but only in the past few years. But centuries before researchers observed it, the behavior was described in ancient manuscripts. Sort of.

Norse monsters

John McCarthy, a maritime archaeologist in the College of Humanities, Arts and Social Sciences at Flinders University, first realized something was up when he was looking up information on Norse sea monsters.

“It struck me that the Norse description of the hafgufa was very similar to the behaviour shown in videos of trap feeding whales, but I thought it was just an interesting coincidence at first. Once I started looking into it in detail and discussing it with colleagues who specialise in medieval literature, we realised that the oldest versions of these myths do not describe sea monsters at all, but are explicit in describing a type of whale,” says Dr McCarthy.

“That’s when we started to get really interested. The more we investigated it, the more interesting the connections became and the marine biologists we spoke to found the idea fascinating.”

Hafgufa was a prominent sea monster, but it may have not been a purely Norse creation. In fact, the Norse manuscripts may have been drawing information from medieval bestiaries — works that describe a large number of real and fantastical animals. Such early bestiaries describe a mythical monster similar to the hafgufa, called the “aspidochelone.” The oldest such bestiary dates to almost 2,000 years ago.

“We identify striking parallels with the behavior of a sea creature named hafgufa in Old Norse sources. The hafgufa tradition can be traced back to the aspidochelone, a type of whale frequently described in medieval bestiaries, first appearing in the Physiologus, a 2nd century CE Alexandrian manuscript,” the researchers write in the study.

Both the hafgufa and the aspidochelone are described as having an ability to draw fish to their mouth, and they’re both described as swimming on the surface of the water. No matter what form it is, the beast is always described as being so huge that it is often mistaken for an island — though that part is likely an exaggeration. So the researchers argue that the bestiary entry, and the following Norse mythical creature, describe this trap feeding behavior.

Co-author Erin Sebo, an Associate Professor in Medieval Literature and Language at Flinders University, says this may be a striking example of knowledge about the natural environment preserved in forms that predate modern science.

“It’s exciting because the question of how long whales have used this technique is key to understanding a range of behavioural and even evolutionary questions. Marine biologists had assumed there was no way of recovering this data but, using medieval manuscripts, we’ve been able to answer some of their questions.”

“We found that the more fantastical accounts of this sea monster were relatively recent, dating to the 17^th and 18^th centuries and there has been a lot of speculation amongst scientists about whether these accounts might have been provoked by natural phenomena, such as optical illusions or underwater volcanoes. In fact, the behaviour described in medieval texts, which seemed so unlikely, is simply whale behaviour that we had not observed but medieval and ancient people had.” 

It’s not clear why this behavior was only truly observed recently. It could be that we’re keeping a closer eye on whales (with things like drones) or that the changing environmental conditions are making trap feeding more common.

“There may be much more to learn about ocean life from medieval and from historical documents that predate human pressures on the marine environment, such as biomass reduction, biodiversity loss, and climate change,” the researchers conclude.

The study was published in Marine Mammal Science.