Accident that Killed 21 Pronghorn Not All that Uncommon

Carnage Abounds on Western Wyoming Roads

By Brian Maffly, 2-21-07

	No Place for a Picnic: Highway workers dump big game and livestock carcasses at this site near Boulder in western Wyoming's Upper Green River Valley.

Last month’s deadly encounter between a truck and a herd of pronghorn that killed 21 of the animals in a Wyoming gas field was hardly the fluke it initially seemed. At least four other winter-time mass-casualty roadkills have occurred in southwest Wyoming since 2003, including a train incident that killed 41 pronghorn near Granger.

I spoke with four field biologists about these accidents, and all speculated that the flight behavior of pronghorns played a role. For millennia, pronghorn and other ungulates have evolved to avoid predation by traveling in herds. While running from a perceived attack, the pronghorn, the continent’s fastest land animal, will sprint and juke in tight groups.

“It confuses the predator when they move in unison. It makes them seem like a bigger creature,” says Therese Hartman, a Pinedale-based biologist with the Wyoming Department of Game and Fish.

This behavior may have worked as a survival strategy for evading the fast feline predators pronghorn co-evolved with 2 or 3 million years ago. But in the age of automobiles it has become a prescription for mass suicide when animals’ group flights intersect roads.

On Jan. 15, a woman driving a gas-field service truck collided with a band of pronghorn that darted en mass onto the road, according to an investigator with the Wyoming Department Game and Fish. It was 2 p.m.on a clear day and the unfenced dirt road was free of snow. The driver reported she was traveling within the 35 mph speed limit when she struck the animals near a compressor station on the edge of the Jonah gas field south of Pinedale, wildlife officials say.

A similar accident occurred the winter before, killing at least seven pronghorn on the Paradise Road in the neighboring Pinedale Anticline gas field. That paved road was fenced but with one that is not “wildlife-friendly,” said Hartman, who serves as the Game and Fish’s oil and gas coordinator in Sublette County.

Other recent episodes of pronghorn carnage in western Wyoming include:

• Seventeen pronghorn died Jan. 21, 2006 on a county road in Sweetwater County when they were struck by a pickup driven by a trona mine worker northwest of Green River.
• On Jan. 17, 2004, 17 pronghorn were killed in the area on State Highway 372.
• A train killed 41 pronghorn near Granger on Nov. 7, 2003.

Game and Fish issued press releases regarding these earlier accidents, but not for the ones in the gas fields. Those incidents came to light when photos of the carnage began circulating among conservationists. Since the late 1990s, a drilling boom in southwest Wyoming has industrialized important winter range for the state’s big game. These public lands overlie vast reserves of natural gas in the Upper Green River Valley, home to the Jonah and Pinedale Anticline gas fields, both of which are undergoing rampant development.

Hartman said the 35 mph speed limit on dirt service roads on the gas fields is routinely violated. Game and Fish officials have no enforcement authority and the BLM has only one enforcement officer to cover the entire region, she said.

Wildlife officials report anecdotal evidence that mule deer and pronghorn road mortality has climbed in the Upper Green River Valley since the onset of the natural gas boom, but no effort has been made to systematically measure the problem. Highway workers dump roadkill carcasses by the hundreds at a site off Highway 191, only a few miles from the Jan. 15 accident.

So why are pronghorn so flighty, a behavior that forces them to waste precious calories and puts them at risk of lethal injury?

One wildlife biologist told me herding ungulates, pronghorn in particular, are subject to “mass hysteria” when they perceive the presence of a predator, resulting in extreme responses to imaginary threats.

In the winter of 2003, Joel Berger, who studies Wyoming pronghorn migration for Wildlife Conservation Society, observed ravens triggering a mass flight in which the line of dashing pronghorn stretched for a mile in the Upper Green. One January day, Berger was examining the carcass of a young female pronghorn, freshly killed by golden eagle. Nearby was a massive group of pronghorn, numbering some 775 animals—three times larger than any other pronghorn congregation he had ever observed. Every time ravens flew overhead, this group of pronghorn would run.

“Following the 6th pass by a raven, in which the raven vocalized, the pronghorn began to run at top speed in a single-file through the snow in a 3.5-mile straight-line sprint,” Berger wrote in a scientific article about the incident.

Berger’s article hypothesized that the eagle kill primed the herd for panic. Pronghorn have developed a hypersensitivity to potential predatory attacks, which “may be a maladaptive by-product of a generalized anti-predator response,” because they cannot distinguish between creatures that pose a genuine or a marginal threat, Berger wrote.

So perhaps pronghorn may regard a moving car as a predator to be evaded, especially following an attack by an actual land-based predator. Biologists have observed bands of pronghorn running along a road in an effort to outpace a car, and then cutting suddenly across the vehicle’s path. This movement is similar to the way pronghorn flee a pursuing predator.

“If you speed up, the pronghorn speed up until they get out front,” Hartman said. “The prudent thing to do, if you understand pronghorn behavior, is to stop.”

Click deadly encounter between a truck and a herd of pronghorn that killed 21 of the animals in a Wyoming gas field was hardly the fluke it initially seemed. At least four other winter-time mass-casualty roadkills have occurred in southwest Wyoming since 2003, including a train incident that killed 41 pronghorn near Granger.” title="here to read Brian’s first story">here to read Brian’s first story about the Jan. 15 incident.

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