nature.net
June 2007
Free Lunch
By Robert Anderson
My Los Angeles neighborhood teems with scavengers: coyotes, crows, raccoons, and rodents. Recently I watched as several dozen vultures, those icons of the ilk, circled lazily overhead. Eyeing them suspiciously, I wondered if their Hollywood image was true—had something large died nearby? Later, an ornithologist friend reassured me that the birds were not hovering over a carcass. The “Questions and Answers” section of the Turkey Vulture Society’s Web site confirmed my friend’s assertion: the flock of vultures, known as a “venue,” had no immediate plans to dine. Instead it had formed a “kettle,” so named for the birds circling upward on a thermal of rising hot air, reminiscent of bubbles that rise in a kettle of boiling water.
Inspired to delve more deeply into the lives of scavengers, I learned that among the vertebrates there are few true scavengers—animals that feed solely on carrion. But other animals populate a vast gray area. Predators such as lions readily chew on someone else’s abandoned kill; animals such as raccoons, regarded mainly as scavengers, also hunt prey. True scavengers are common, however, in the invertebrate world, particularly on the deep ocean floor.
All diners on the dead perform a valuable service by cleaning the planet of rotting and often diseased flesh, thereby recycling nutrients into the food web. The adaptations that enable them to fill their niche are as remarkable as anything in nature.
The Snake River Birds of Prey National Conservation Area in Idaho has more information on turkey vultures, including a photo of a “kettle.” Unlike Old World vultures—and birds in general which have a very poor sense of smell—the turkey vulture excel in detecting animals a day or so after death, relying largely on the odor. Other new world vultures are less adept at smelling their meals and depend more on sight, often following turkey vultures to a corpse. At a Cornell University page on avian olfaction, I was surprised to find that certain seabirds—albatrosses and petrels—use scent in a similar way to hone in on dead fish. (Kiwis, which have the best bird noses, use olfaction to forage for insects and worms in the soil.) Vultures, like many carrion eaters, have evolved a number of tricks to cope with their putrid diet. Jim Caryl, a research scientist with a PhD. in biochemistry and molecular biology, explains how vultures neutralize the toxins and microbes in their food at the MadSci Network.
On the open plains of Africa, the competition for carrion is fierce. Go to Wildcast to watch a video clip (Flash player required) of African vultures waiting for spotted hyenas to leave a lion kill. Even among the avian scavengers there is a pecking order. Go to Wildwatch for a rundown on which bird species are first in line.
Vultures and other birds are so efficient at cleaning up the dead that some religious sects, most famously the Zoroastrians in south Asia, rely on the scavengers to dispose of their departed, who are placed in open areas rather than cremated or buried. At Death the Last Taboo, a Web site created by the Australian Museum about human death rituals, learn how an alarming ninety-eight percent decline in India’s two most common vulture species is threatening the tradition.
In the United States, the importance of wolves to a whole chain of scavengers has only recently been discovered by two researchers from the University of California at Berkeley, Christopher Wilmers and Wayne Getz. Go to the UCBerkeleyNews press release to learn how scavengers benefit from the reintroduction of wolves in Yellowstone National Park. For a related article on this research see the Public Library of Science article “Gray Wolves Help Scavengers Ride Out Climate Change.”
My favorite scavenger is the Tasmanian devil, the marsupial equivalent of a raccoon with attitude. You can listen to the devilish vocalization that gave them their name at the Web site of the Tasmania Parks & Wildlife Service. Click on the “Save the Tasmanian Devil” link to see how people are trying to save this largest marsupial carnivore, which is imperiled by a kind of contagious cancer. More on devil habits can be found at this San Diego Zoo page.
Highlighting the gray area between predator and scavenger behavior are the eating habits of Tyrannosaurus rex. You can find a synopsis of the debate about how this giant meat-eater got its food at Dinosaur-World.com. More can be found at the University of California Museum of Paleontology.
Fulltime scavengers are more easy to come by on the deep ocean floor, where, aside from methane seeps and hot volcanic vents, the only nutrients to be found are the dead organisms floating down from above. Researchers at the Monterey Bay Aquarium Research Institute (MBARI) have been studying the ultimate marine bonanza scavengers: a dead whale resting nearly 9,000 feet beneath the surface. According to a MBARI news release the nutrients from such a find can support a rich oasis of life for decades after the animal has bee reduced to bones, a process that takes several months. Go to the Whale Fall Cruise Logbook for more on the researchers’ activities.
Time-lapse video is ideal for watching scavengers do their work. Go to this amazing clip on YouTube to see a gang of crabs, eels, and giant isopods swarm over a fish corpse on the ocean bottom. A similar feeding frenzy of colorful nemertean worms was photographed in the Antarctic waters in 2003 by researchers with ASPIRE—the Antarctic Sewage Pollution Impact and Recovery Experiments (scroll down to the bottom of the page).
At OceanLink (http://www.oceanlink.island.net/oinfo/hagfish/hagfish.html) you can learn all about hagfish, marine scavengers belonging to a lineage of primitive vertebrates. They are infamous for producing copious amounts of slime as a defense against predators. To learn more about the giant, scavenging isopods, marine relatives of the common pill bug, go to New Zealand’s National Centre for Aquatic Biodiversity and Biosecurity.
Insects are the ultimate scavengers. One of the most persistent is the cataglyphis ant found in the Saharan Desert, which makes the longest overland forays of any insect. On YouTube you can watch David Attenborough describe how this tiny ant scours the desert for other insects that have succumbed to the mid-day heat and manages to find its way directly back to its nest. Go to the Science Museum of Minnesota’s Dermistid cam to see a colony of dermestid beetles busy cleaning the skeletons of museum specimens.
Another clip on YouTube reveals the remarkable courtship of the burying beetle, which revolves around death. When this scavenger locates a small vertebrate corpse, it buries the treasure and transforms it into a sort of edible nest for its young. It sounds gruesome, but the BBC segment treats these recyclers with the admiration that all scavengers deserve.
Robert Anderson is a freelance science writer living in Los Angeles.
Copyright © Natural History Magazine, Inc., 2007