The Problem with Pronghorns

The Problem with Pronghorns

Article by Russell A. Graves

Of all the Texas big game species, none is more emblematic of the wide open west than the pronghorn. Swift and fleet footed, the pronghorn covers distances faster than any other land mammal on the continent. Over the past decade or so, the pronghorn of West Texas and the Marfa Plateau couldn’t outrun enigmatic problems that plague the species. 

A Unique Species

Pronghorns are a biological enigma. While they are popularly called “goats” or antelopes, in reality, the species is unique and has no direct relatives in the animal kingdom. In fact, it is not even a true antelope. Instead, the species maintains its own ecological niche by being the only surviving North American member of its family, Antilocapridae.

About 3 feet high at the shoulders and weighing 90 – 150 pounds, the pronghorn is also North America’s fastest land animal. They can sustain speeds of more than 50 miles per hour in short bursts and can cruise for miles at a time at half that speed, fast enough to leave predating coyotes and bobcats behind.            

Both the males and females sport true horns that consist of flat bone covered in a keratinous sheath that is shed and regrown each year. While both sexes have horns, the males are characterized by a black swatch of hair that rings the upper throat. They breed in the fall and after a 235 day gestation, give birth to typically one fawn although twins are not uncommon.

On the range, pronghorns’ diets overlap somewhat with cattle, but they prefer forbs and some brush species in their diet. Although overgrazing and habitat destruction were once a huge problems for pronghorns, that does not seem to be the case these days. 

A Decade of Decline

The story of the pronghorn across the mid-continent plains is one of sheer abundance, followed by a mass slaughter that wiped out many of the continent’s species over a relatively short time span. On the heels of the Manifest Destiny doctrine, pronghorns were hunted for meat to feed construction crews and settlers as the United States expanded westward to the Rocky Mountains and beyond.

When Lewis and Clark first traversed the Missouri River basin in the early 1800s, an estimated 35 million pronghorns roamed the prairies that cut through the continent’s gut. Less than 100 years later, only 13,000 were left.

During the 20th century modern wildlife management practices ensued and those numbers stabilized. Changing land practices, however, extirpated the pronghorn from the mid-Texas prairies westward. At one time the range of the pronghorn in Texas stretched from around present day Interstate 35 westward. Now the species is relegated to the Panhandle’s upper reaches, pockets west of Lubbock and around San Angelo, and the Trans-Pecos—most densely in the Marfa Plateau.

Thirty years ago, estimates put the number of pronghorns in the Trans-Pecos area at about 17,000 animals. In 2012, after fawn production hit an all-time low in 2009, pronghorn numbers on the plateau plummeted to around 2,000.

Science versus the Decline

In 2011, Louis Harveson, director of the Borderlands Research Institute (BRI), said, “The jury is still out on this year’s pronghorn status as the Texas Parks and Wildlife Department is still surveying Trans-Pecos populations. I suspect that this year, we will be at all-time low numbers in most of the Trans-Pecos. The drought has really taken a toll on all wildlife in the region, but especially our pronghorn herds.”

Harveson’s words proved to be prophetic as the populations plummeted for a reason that wasn’t all that apparent. Scientists knew that the drought was the biggest culprit.

In 2011, fawns were being born malnourished, and there was little or no forage in which to support the adults’ nutritional needs. Two years prior, however, the rains did fall yet the pronghorn populations did not respond as anticipated. Scientists like Harveson knew something else was affecting the species aside from drought. So, scientific resources were deployed in earnest.

Besides the drought, there was also another suspect that appeared to be the source of the pronghorn decline—the increased presence of the barber pole worm in Trans-Pecos herds. 

The barber pole worm is a parasitic nematode that is commonly found in the stomachs of domestic livestock such as sheep and goats, but until recently, their presence in pronghorns were thought to be inconsequential—mainly because the arid Trans-Pecos is not  thought to have enough moisture to sustain large populations of the roundworms. 

In eastern climes, roundworms are more plentiful because their life cycles depend upon moisture in the green grass to complete their egg to larval cycle. While the adult worm is what infects the host animal, eggs are passed through the feces where they hatch into larvae, crawl up wet blades of grass, and wait to be ingested by grazing animals.

In the chronically dry Trans-Pecos climate, widespread barber pole worm problems haven’t traditionally been an issue. However, microhabitats like areas around water holes or troughs may have just enough suitable moisture to create habitat in which the worms can thrive.

Even in these extremely dry times, scientists have documented several adult pronghorn dying from barber pole worm infestations. 

In 2009, pronghorn necropsies revealed that the blood sucking worms were present in 95 percent of the 102 Trans-Pecos pronghorns sampled. While finding the worms in pronghorns is not unusual, finding the worms in such high numbers is. In the 1960s TPWD biologists concluded that barber pole worms were found so infrequently, that the herd was considered clean of the parasites. 

At the end of the 2009 hunting season, blood was also tested in the same pronghorns studied for the presence of worms. The resulting numbers showed that selenium and copper levels, important for reproductive efficiency, were comparatively low. In 2010, the working group collected samples from 95 pronghorns that were from the same general area as the 2009 samples. In that group, the roundworm infections decreased around 50 percent and the copper and selenium levels were increased yet the reproductive rate of the animals was still alarming low—deepening the mystery for West Texas scientists.

Following the decline, range conditions started to improve. However, with range conditions improving, populations did not increase noticably. So while barber pole worms were still thought to be one of the decline’s causes, it was just one piece of the puzzle.

Finding a Possible Solution

Once scientific investigations began in earnest in 2009, scientists began to paint a more complete picture of the pronghorn decline. To help supplement the declining Marfa Plateau population, pronghorns from the Panhandle populations which are stable and thriving, were trapped and translocated to the depleted rangelands near Marfa. 

According to Shawn Gray with TPWD, the objectives of the translocation project were to supplement severely depleted herds using healthy populations from the Panhandle, monitor survival and production of the translated pronghorns, and use their research findings to increase survival of transplants and production of supplemented herds.

Since 2011, pronghorns have been released five times and in all, 637 animals have been moved from north of Amarillo to their new home in the Trans-Pecos desert grasslands. According to press releases, the Trans-Pecos Pronghorn Restoration Project is a five-year, $1.4 million public-private partnership with the Texas Parks and Wildlife Foundation. To date, more than $900,000 has been secured. 

Each pronghorn that is captured undergoes a battery of tests to determine its overall health. Each one is then fitted with ear tags for identification and at the most recent release site, 40 animals were fitted with GPS collars that log the animal’s location every 15 minutes. Data that is s collected provides valuable insight to the animal’s travel patterns and its vulnerability to predators.

So far the project’s progress is promising. 

“It seems that barber pole worms were the symptom of larger issues like drought, barriers to movement, disease and predation,” said Dr. Whitney J. Gann, research scientist with BRI. “One of the things that stood out to us about these four factors that we thought could have a large influence on the other factors was the barriers to movement component.”

According to Gann, pronghorns have an aversion to crossing fences. As such they do not have an instinct to jump over the wired barriers. Therefore, when pronghorns encounter a barrier like a fence, they often wouldn’t cross in and would lose access to more productive habitat.

“One of the things that BRI and TPWD have focused on is modifying fences to make them passable to pronghorns,” said Gann. “So far, we have put in more than 1,500 of these modifications.”

The fence modifications are simply raising the bottom wire of fencing to a height of 18 inches in 20 – 30-yard stretches every half mile and especially in fence corners since GPS data shows that pronghorns may get to a pasture’s corner and remain there. With the fence modified, pronghorns can simply crawl under the wire. 

The early results are promising.

“We know that our translocatees are using our fence modifications, because we can look at locations recorded from their GPS collars and from our satellite collars,” said Gann. “Research from across the pronghorn range in North America tells us that fences dramatically influence pronghorn movement and potentially prevent them from moving in search of better forage during less productive periods. We think that fences prevented pronghorn in the Trans-Pecos from accessing better range-sites, which ultimately affected parasite loads, reproduction and survival.”

After working with landowners in the area and modifying hundreds of fences, Gann said her team is able to document pronghorn movement almost constantly within and between our restoration areas. This increased connectivity will allow pronghorn in the region to move as they wish and select preferred foraging areas, she said.

“But we will always need more fence modifications; the more corridors the animals have the better. And it’s worth noting that increasing accessibility of the rangeland for all nomadic animals is positive, mule deer included,” explained Gann.  

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