About 10 years ago, marine biologists witnessed two different species of whales in different geographic locations engaged in a novel feeding strategy. The whales would position themselves at the water’s surface and stay motionless with their mouths wide open. Fish would swim into their mouths, and the whales would snap their jaws and swallow. It’s been dubbed trap feeding, or tread-water feeding. A clip of whales engaged in trap feeding even went viral on Instagram in 2021.
Yet this feeding strategy might not be as recent as scientists initially thought. Researchers at Flinders University in Australia have found striking descriptions of what sounds a lot like trap feeding in Old Norse descriptions of the behavior of a sea creature called the hafgufa, according to a new paper published in the journal Marine Mammal Science. That creature, in turn, can be traced back to medieval bestiaries as a type of whale called aspidochelone, first mentioned in a 2nd-century CE Alexandrian manuscript called the Physiologus.
“It’s exciting because the question of how long whales have used this technique is key to understanding a range of behavioral and even evolutionary questions,” said co-author Erin Sebo, a medievalist at Flinders University. “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.”
Whales deploy a variety of feeding strategies. For instance, lunge feeding involves charging at schools of fish with the mouth open, while whales engaged in bubble-net feeding create a round curtain of bubbles to concentrate fish before charging into the center to feed. Scientists first observed lobtail feeding in the 1980s, novel behavior that seems to have been driven by a sharp decline in herring populations because of overfishing. The behavior is culturally transmitted between associated groups of whales.
Trap feeding was first recorded in 2011 in a group of humpback whales feeding on herring off Vancouver Island. The discovery was published in 2018 after a separate 2017 paper reporting similar feeding behavior among Bryde’s whales feasting on anchovies in the Gulf of Thailand. Similarly to the emergence of lobtail feeding, some researchers surmised that the behavior developed in response to increased pollution, “dead zones,” algal blooms, and similar environmental challenges, which had driven the whales’ prey closer to the water’s surface. Others thought it might simply be a particularly energy-efficient means of feeding when the fish population is less densely clustered.
Co-author John McCarthy, a maritime archaeologist at Flinders University, thought the recent trap-feeding behavior was strikingly similar to Norse descriptions of the hafgufa, most notably in a 13th-century text called King’s Mirror (Konungs skuggsjá). It’s described as a sea creature of great size with an unusual feeding method:
[W]hen it goes to feed, it gives a great belch out of its throat, along with which comes a great deal of food. All sorts of nearby fish gather, both small and large, seeking there to acquire food and good sustenance. But the big fish keeps its mouth open for a time, no more or less wide than a large sound or fjord, and unknowing and unheeding, the fish rush in in their numbers. And when its belly and mouth are full, [the hafgufa] closes its mouth, thus catching and hiding inside it all the prey that had come seeking food.
That is a remarkably accurate description of trap feeding, and the key details are also found in earlier medieval bestiaries and the aforementioned Physiologus: a creature holding its jaws open, emitting a smell or scent that attracts small fish to jump in its mouth, with the creature snapping its jaws shut and swallowing when enough fish have amassed in the trap. “The tradition remained remarkably coherent and consistent over 1,500 years, with minimal embellishment or reinterpretation,” the authors wrote. Further, the creatures in those earlier source texts are identified as whales, not mythical sea monsters. Many of those texts also have illustrations showing sea creatures consuming fish in a manner reminiscent of trap feeding.