In the languid news week after Christmas, hungry media outlets swarmed over a report of piranhas attacking swimmers on a river in Argentina. “Massive Piranha Attack” cried The New York Post. “70 Christmas Day Bathers Are Savaged” added The Daily Mail, promising “the truth about the fish with a bite more powerful than a T. rex.” ABC News called it a “Christmas Day feeding frenzy.” In fact, the injuries ranged from minor cuts to at least one missing finger part — not exactly as newsworthy as, say, the 800,000 Americans who require medical treatment for dog bites each year.

Piranhas have always been among our favorite subjects for sheer, sputtering nonsense. Theodore Roosevelt, on a 1913 expedition in South America, called piranhas “the most ferocious fish in the world.” More recently, multiple “Piranha” movies have ridden this hysteria to the bank.

This is an awful lot of hype for piranhas to live up to, and predictably, they disappoint. To test the colorful mythology of the ferocious piranha, I once climbed into a tank of hungry red-bellied piranhas at the Dallas World Aquarium. (They fled to the opposite corner.) In the Peruvian Amazon, I stood waist-deep in the Rio Napo while catching and releasing piranhas on a hook-and-line. (The nibbles were strictly of the usual kind.) In the flooded grasslands of Venezuela, I drove around tossing a chicken carcass into various bodies of water to time how long it took for the flesh-maddened swarms to strip it to feathers. (There was enough chicken left at the end of the day to feed a family of four.)

The point of this exercise, recounted in my book “Swimming With Piranhas at Feeding Time,” was that piranhas do that swarming, blood-crazed, flesh-ripping thing only in a couple of rare circumstances, both involving a highly concentrated food source: They will swarm around bird rookeries, where the fledglings leaving the nest often tumble straight down into the water. And they’ll do it around docks where fishermen clean their catch and heave the guts into the water.