For conservation scientists, the proof isn’t in the pudding—it’s in the poop. It turns out that wild zebras have been dropping vital clues about their stress levels in the form of feces, and researchers are now beginning to unravel these pungent piles of data to glean important clues to the animals’ well being.
“Poop allows us to get into the inside of the animal,” says Rachel Santymire, director of the Davee center for Epidemiology and Endocrinology at the Lincoln Park Zoo in Chicago. “I always say they can’t lie to me.”
Poop-scooping has proved especially useful for cape zebras, a once-precariously endangered species that today inhabits the eastern and western capes in the southern end of the continent. Compared to their cousins, the more populous plains zebra, they are thinner, have narrower hooves, and sport a distinctive barcode-like stripe pattern on their backs.
Their unusual looks, however, have gotten them into some trouble.
Between 1960 and 1980, populations tanked thanks to habitat destruction, fencing and unregulated hunting for their distinctive pelts. At its lowest point, the population dwindled to just 80 animals stranded on three separate mountaintops. Despite the recovery in recent years, scientists remain uncertain whether current numbers can survive new pressures, including habitat destruction due to human development and a changing climate that may turn their last hold-outs into unsuitable habitat.
To track the success of past conservation efforts, biologists are now analyzing zebra poop for hormones that indicate high levels of stress. Glucocorticoid hormones in particular can help regulate stress responses in animals that influence whether they will fight or flee. Traces of these biochemicals can be found in droppings, skin, nails and hair—not just in zebras, but in humans and most other mammals.
Researchers have certainly used poop to determine animal stress before. But until now, nobody has used poop-embedded stress hormones to compare animals across a population, nor or to compare the relative stress between populations living in different habitats, says Susanne Shultz, a university research fellow in evolutionary biology at the University of Manchester and one of the coauthors of a study recently published in Functional Ecology.
Given how much long-term population data researchers have on them and their remarkable comeback, cape zebras might make the perfect subject.
Today there are between 4,000 and 5,000 in the wild, spread across 75 different populations— thanks in part to the establishment of Mountain Zebra National Park in 1937 meant to protect some of the last remaining savanna in the southern part of the country. But some 95 percent of these are derived from one of three populations that survived the tough times in the 20th century, meaning they have low genetic diversity and could more easily be wiped out by disease.
(Interestingly, the reinstating of hunting, which has been permitted in limited numbers since the species was downlisted in 2016 in the Convention on International Trade in Endangered Species of Wild Fauna and Flora, might be aiding in their recovery. Some landowners may prefer to keep viable populations on their land with the idea of selling some of the animals for trophy hunts. Their distinctive stripe pattern makes them more valuable than plains zebra skins; Shultz says trophy hunting cape zebra can cost far more than what a plains zebra costs.)
Moreover, Shultz says that the some aren’t doing well possibly due to the fact they’ve been relegated to sub-optimal habitat. “You basically have increasing aridity due to more bushveld”—the woodland ecosystems common in some parts of South Africa—“for them, which is poor habitat for zebra,” she says. Climate change may be further complicating the issue, Shultz says, as it is causing rainfall to become less predictable in the area, which can affect suitable habitat for zebras.
“It’s perceived that that’s where the mountain zebra want to be rather than that’s where the mountain zebra were left,” she says. “What seems like a good outcome might not be as good as we think it is.”
Shultz and her colleagues used poop samples to track the success of management efforts in real time. Starting in 2011, they took six sampling trips to six different populations over the span of two years. They examined the poop for glucocorticoid hormones and checked the males for the testosterone hormone levels to check male health and compared their findings with general species information.
In the populations that did well, they found high levels of testosterone only in mating seasons and high stress only during the cold season. But in the struggling populations, the animals had more constant testosterone and stress levels. “They never had a break, essentially. Over the two year period they showed evidence of chronic stress,” Shultz says.
“The inference is if you have animals in habitats that don’t have sufficient resources, you are physiologically stressing the animals. But also if the structure of the population is imbalanced, that adds an additional stress," she says. Shultz adds that the animals also suffer high levels of male competition, which can harm female reproduction by leading to more harassment of females and a frequent turnover of stallions in breeding groups.
Tracking stress responses by using poop is a game-changer, because it’s non-invasive and the raw materials aren’t exactly hard to come by, says Santymire, who wasn't involved in the recent study. Shultz and her coauthors did a good job of tracking individuals through poop samples, she adds. “They looked over time, they controlled for individual and they really controlled for the seasonal effect, which is the diet effect, by controlling for the moisture in the feces,” she says.
A few years ago, Santymire—whose colleagues know her as “Dr. Poop”—also published a poop study, which examined how increased predators and competition impacted the health and reproductive rates of black rhinos in South Africa. However, she and her colleagues are now starting to move away from using feces in favor of things like keratin from claws or nails and even snake skins, which she says are better for determining chronic stress levels.
Since the biochemical residues in these materials don’t change as much day by day, these samples can provide more accurate readings, she says. Yet as far as Shultz is concerned, there is a bright future in scooping up poop from all kinds of animals—not just zebras.
“The zebra are a really good test case because we have long-term data,” she says. But “the idea is to prove this concept with this species.”