Three Causes of Death for Pronghorn

When the naturalist and ornithologist George Ord formally named pronghorn in 1815, he was unsure whether the species was an antelope or a goat. In the journals of their famous expedition from 1804-1806, Lewis and Clark made over 200 references to what they described as “wild goats or antelopes”. Pronghorns are also featured in the petroglyphs and pictographs of Indigenous nations throughout the continent. Over the years, and perhaps serving as a measure of the cultural importance and sense of mystery they inspire in humans, pronghorn have acquired a number of nicknames, from the more colloquial speed goat to the poetically ethereal imagery of the prairie ghost. As it turned out, George Ord would compromise and give the species a scientific name that split the difference between his uncertainty, Antilocapra americana, or “American antelope goat”. 

American Antelope Goat

In fact, though, pronghorns are neither antelopes nor goats, but a 25 million-year-old North American species whose closest living relative is the giraffe and are the only surviving pronghorn species from about a dozen during the Pleistocene. They are the only species in North America to have branched horns (antlers, like those on deer and elk, are shed and regrown each year; horns, like those on wild sheep and goats, continue to lay down successive growth rings throughout the individual’s life, like rings of a tree). Unlike other horned animals, however, pronghorn horns have a boney core that is attached permanently and surrounded by a keratinous sheath that is shed and regrown annually. They are a species of unparalleled fascination and uniqueness.

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Pronghorn skull with boney core and keratinous sheath.

The interactions between humans and pronghorns over the last two centuries is a history of both tragedy and triumph and one shared by much of the continent’s wildlife. The most well-known story of a wildlife species declining from massive abundance to disastrous near-extinction is, of course, the bison; however, the story of pronghorn conservation also deserves a spot on the shelves of conservation shame and success.

Pronghorn are a western grasslands and sagebrush plains species. As European settlement expanded westward across North America, pronghorn populations were decimated by overhunting and associated impacts of human expansion. In the 1800s, it is estimated that there were more than 35 million pronghorn in North America. As with many other large mammals, pronghorn were considered at high risk of extinction throughout the first half of the 1900s (the IUCN reports that by 1924, there were less than 20 000 individuals).

Smithsonian National Museum of Natural History

North American pronghorn range. Source: Smithsonian National Museum of Natural History

Currently, there are approximately 1 million pronghorn in North America, with roughly 50% of them living in Wyoming. Pronghorn populations are generally stable and healthy across the continent; however, as a species that inhabits vast tracts of open habitat, they are vulnerable fragmentation and anthropogenic infrastructure such as roads and fences. (As a grasslands species, pronghorn never needed to jump over large natural barriers, so they did not evolve this ability, unlike deer. Fences, therefore, pose a particular problem for them.)

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Pronghorn Reproduction

Among the many ways in which pronghorn are unique among other ungulate species, such as deer and elk, are their reproduction characteristics. Generally speaking, ungulate species mate in the fall, during which time they tend to sacrifice eating for breeding. Therefore, the health and body condition (e.g. amount of fat) an individual has going into winter will be critical in influencing how well it survives the winter. Generally speaking, winter is the season during which most individual ungulates die; it is certainly the season that has the greatest influence on the health of individuals, who often come out of winter in worse condition than going into the season. Females give birth in the spring, and the health of the young will be directly influenced by that of their mothers.

Now, pronghorns differ a little from other ungulates in the way their environment influences their yearly cycle. Typically for ungulates, spring and summer are critical feeding times to restore body fat and prepare for the fall mating season. Most ungulates heavily rely on stored body fat during mating season so the health of an individual going into the mating season will play a large role in determining its reproductive success. Pronghorn, on the other hand, fuel their mating by the current intake of food, relying less on stored fat reserves. Therefore, the surrounding environmental conditions may have a stronger influence on reproductive success for pronghorns than other species.

Another way that pronghorns differ from other ungulates is in how much energy they put into giving birth in the spring. Pronghorns have a longer pregnancy period than many other ungulate species, and the young are larger relative to the mother than all other ungulate species. What all this means is that female pronghorn have their lowest levels of body fat in June, after they give birth and lactate. As a result, female pronghorns have higher death rates in summer than winter. In New Mexico, for example, research has found that 73% of female mortality occurs during the summer between giving birth and weaning young.

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Causes of Death in Summer

While pronghorn populations are healthy in Wyoming, the state has seen a 30% decline in pronghorn over the previous 20 years. Understanding the different causes of death among adult females, particularly in the summer when mortality is highest, helps managers better understand factors that affect pronghorn populations. Recent research published in The Journal of Wildlife Management by a group of authors led by Adele K. Reinking at the University of Wyoming examined three broad causes of death among female pronghorns: intrinsic (factors related to the individual, such as age and body condition), environmental (factors related to climate and land cover), and anthropogenic (factors related to humans, such as roads, wells, and fences). The researchers wanted to better understand the mortality risk to pronghorn from these three sets of factors. In 2013 and 2014, the researchers captured 151 female pronghorns in Wyoming and fitted them with GPS collars that tracked movement and sent a signal when the animals died.

Elizabeth McClelland from Kays and Wilson's Mammals of North America, © Princeton University Press (2002)

Credit: Elizabeth McClelland from Kays and Wilson’s Mammals of North America, © Princeton University Press (2002

To understand the influence of intrinsic factors, the study assessed body condition. Scientific studies of body condition generally focus on measuring fat content (this is the method for studies on species ranging from pronghorn in Wyoming to polar bears in the Canadian Arctic): less fat means lower energy reserves and therefore corresponds to poorer body condition.

In terms of potential environmental factors, over the last 20 years, Wyoming experienced 11 years of drought, with five of those years characterized as extreme drought.

Anthropogenic factors are largely related to the expansion of road and fence networks that may affect wildlife movement, largely due to increased interest in energy development in Wyoming and elsewhere across pronghorn range.

Body Condition and Snow Depth

The research found that the greatest influences to pronghorn summer mortality were a combination of poor body condition and increased snow depth during the previous winter. In contrast to some previous assumptions, the study did not find that anthropogenic features had an influence on summer mortality. Nevertheless, the researchers point out that increased infrastructure associated with oil and gas development in Wyoming has had impacts on other species such as sage-grouse, mule deer, and elk. Further, resource development has been shown to affect pronghorn resource selection and movement – pronghorn may avoid areas that are disturbed or fragmented by roads, wells, and fences, potentially avoiding areas with higher quality food. Although these behavioural impacts have not been shown to immediately affect pronghorn survival, it is possible that as these changes in behaviour accumulate over time, they might impact pronghorn survival.

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Pronghorn rely more heavily on immediate food intake during mating than other ungulates and there is potential for the increased prevalence of drought conditions with climate change. As pronghorn avoid anthropogenic infrastructure and potentially choose areas with lower food quality, this could impact reproduction. The inability to access areas with high food quality due to avoiding roads and fences may also cause pronghorn to enter winter in poorer body condition, a primary influence of summer death. Though the researchers did not find evidence of this yet, fences may also prevent pronghorn from freely choosing areas during the winter season that have more favourable snow conditions (pronghorn have a difficult time when snow is deeper than 25 cm). Therefore, the effect of anthropogenic factors may influence and compound other, more directly important factors, such as body condition and snow depth.

Management Implications

In terms of management implications, it is important that all factors that influence a species’ survival are addressed together. As this study shows, separating individual, environmental, and human-caused risks of death neglect the effects that these factors have on one another. At the very least, lessons from other contexts, including our collective recent failures with caribou management, should demonstrate the critical importance in taking a precautionary approach and to reduce the potential for our own impacts on wildlife.

The pronghorn is a species with unparalleled fascination and uniqueness. They have evolved over 25 million years. They are the last surviving species of its kind. They emerged from the Pleistocene and persisted while countless other species disappeared. They also stand as a powerful conservation success story in North America. It is perhaps somewhat ironic that one of their nicknames is the prairie ghost; as though already a warning about what might happen if we become complacent or take for granted the successes we have enjoyed with their recovery.

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