Medieval manuscripts may have described “new” whale feeding trick centuries ago

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: 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. “I thought it was just an interesting coincidence at first," said McCarthy. "Once I started looking into it in detail and discussing it with colleagues who specialize in medieval literature, we realized that the oldest versions of these myths do not describe sea monsters at all, but are explicit in describing a type of whale. 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." Granted, the mention of a special emitted scent or perfume doesn't match what we know about humpback or Bryde's whales (although sperm whales do produce ambergris, a highly valued perfume ingredient). But McCarthy et al. believe this could be describing the way whales will sometimes belch out filtered prey to help attract fresh prey into their mouths. In fact, the Old Norse King's Mirror text quoted above specifically talks about the hafgufa expelling its own bile as bait. As for why scientists have only recently observed trap-feeding behavior in whale populations, the authors suggest that these kinds of niche hunting strategies may have once existed but then became unnecessary as whale populations steeply declined because there was less competition for fish. While today's whale populations are below normal, their protected status has boosted their numbers. So it's possible trap feeding has re-emerged along with those increased numbers. "We found that the more fantastical accounts of this sea monster were relatively recent, dating to the 17th and 18th 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," said Sebo. "In fact, the behavior described in medieval texts, which seemed so unlikely, is simply whale behavior that we had not observed but medieval and ancient people had.” DOI: Marine Mammal Science, 2023. 10.1111/mms.13009  (About DOIs).