Instead of tracking wolves to prey, ravens remember — and revisit — common kill sites

(Press-News.org) Stark black against an open sky, common ravens are often spotted soaring above wolves in Yellowstone National Park. Researchers assumed that the notorious scavengers were following the wolves to get their scraps, but new research reveals a twist: Ravens don’t follow wolves, they remember common hunting grounds and regularly check back for fresh meat.

When food is easy to find, animals save energy by memorizing the path to retrieving it. Because scavengers rely on other animals to eat, their meals are less predictable. Some scavengers contend with this insecurity by tailing predators, but as this study shows, ravens don’t. Researchers tracked 69 ravens and 20 wolves across Yellowstone National Park for two and a half years and found that the ravens knew where to go without cues from the wolves.

“Scavengers are not quite as glorious as predators, and have traditionally been understudied by comparison. Getting a better understanding from the scavengers’ viewpoint might give us insight into sensory abilities, underappreciated environmental cues and spatial and temporal memory,” said John Marzluff, a University of Washington professor emeritus of environmental and forest sciences and the study’s senior author.

The study was published March 12 in Science.

The mutualistic relationship between ravens and wolves has fascinated humans for centuries. According to Norse mythology, the god Odin created two ravens — Huginn and Munnin — to travel the world gathering intelligence for him. Odin sent his two wolves, Geri and Freki, with the ravens to ensure they remained fed.

“This tight coevolutionary relationship between predator and scavenger has persisted in human thought for millennia,” Marzluff said.

Modern scientific research documents a similar relationship between the two species. Ravens have been known to follow wolf tracks through the snow and respond to hows. After wolves were reintroduced to Yellowstone National Park in 1995, ravens were more likely to be spotted near a wolf than anywhere else in the park. The odds of seeing a raven further increase when wolves are hunting.

Marzluff, who is well known for studying crows and ravens, teamed up with lead author Matthias-Claudio Loretto, an assistant professor at the University of Veterinary Medicine Vienna then the Max Planck Institute of Animal Behavior, to study how ravens track wolves so well.

The wolves in Yellowstone are already closely monitored, but the researchers needed data on the ravens to compare. Over a few months, Marzluff and Loretto trapped 69 ravens and outfitted them with small GPS trackers. For two and a half years, the researchers monitored where the ravens and wolves went, which routes they took and when their paths crossed. 

They only documented one instance of a raven following a wolf for an extended period of time, yet overall, ravens still managed to arrive promptly after the wolves made a kill. Ravens were spotted at nearly half the observed wolf kills within seven days and some flew more than 150 kilometers to reach a kill. Their flight patterns also suggested that the ravens were making a beeline instead of conducting a sweep. 

Ravens were also far more likely to visit areas where wolf kills were more frequent, per the researchers’ “carcass abundance map,” which split the territory into nine square kilometer parcels and plotted kill sites.

The authors propose that ravens rely on spatial memory — the brain’s ability to follow directions — to monitor the wolves’ favorite hunting grounds. Their hypothesis is further supported by data showing that ravens fly over common kill sites en route to other food sources, including areas where humans hunt wild game. 

“We already knew that ravens can remember stable food sources, like landfills,” Loretto said. “What surprised us is that they also seem to learn in which areas wolf kills are more common. A single kill is unpredictable, but over time some parts of the landscape are more productive than others — and ravens appear to use that pattern to their advantage.”

Additional co-authors include Kristina B. Beck and Thomas Müller from the Senckenberg Biodiversity and Climate Research Centre; Douglas W. Smith, Daniel R. Stahler and Lauren Walker from National Park Service and Martin Wikelski and Kamran Safi from ​​Max Planck Institute of Animal Behavior.

This study was funded by the European Union, the National Geographic Society, the German Research Foundation, the James W. Ridgeway endowment to the School of Environmental and Forest Sciences at the University of Washington and Yellowstone Forever.
For more information, contact Marzluff at corvid@uw.edu or Loretto at  matthias.loretto@vetmeduni.ac.at.

END