Hyena scavenging brings health and economic benefits

Spotted hyenas (Crocuta crocuta) are often maligned and feared because they scavenge carcasses and are associated with dark nights and eerie, whooping or cackling calls. Some even see them as the embodiment of evil. 

But in certain towns in Ethiopia – Mekelle and Harar, for example – hyenas are an accepted part of town life and may even bring benefits to the townsfolk. 

In a recent study, a University of Michigan conservation ecologist and two colleagues attempted to quantify the health and economic benefits brought by the population of hyenas that scavenge at the landfill outside the town of Mekelle, the capital of northern Ethiopia’s Tigray region. 

“This is an important contribution to a growing body of work that highlights the benefits of predators and scavengers, rather than focusing only on their costs to humanity,” said study senior author Neil Carter, an assistant professor at the School for Environment and Sustainability.

Mekelle has a population of 310,000 people and 120,000 livestock. Animals are slaughtered for consumption and their carcasses are mostly disposed of on the town’s landfill or simply on the roadsides. Hyenas visit these sites every night to scavenge and, in so doing, remove the unwanted waste. The researchers investigated whether the hyenas prevent the spread of pathogens to people and livestock by consuming these remains. 

The team first estimated the size of the spotted hyena population that visits the town by night. They did this by broadcasting pre-recorded hyena distress calls from the roof of a vehicle at five sites around the town. During each hour-long survey, hyenas that responded to the calls were counted using night-vision binoculars. The researchers estimated that 210 hyenas visit the town at night to scavenge. 

Next, the researchers used night-time observations of hyena feeding behavior at the landfill to determine the number and identity of carcasses scavenged. The results showed that each adult hyena consumes around 2,100 lb of carcass waste in a year, and that this mass is made up mostly from horse carcasses. 

Chicken carcasses are the most commonly eaten item, but waste from cattle, horses, sheep and goats is also consumed. The total mass of waste consumed by the hyenas is 207 metric tons per year, which represents 4.2 percent of the total animal carcass waste disposed of by the town’s residents and businesses.

The field data were collected and analyzed by study lead author Chinmay Sonawane, a former student of Carter’s who is now a doctoral student at Stanford University.

The data were then used in a model of disease transmission to predict the number of anthrax and bovine tuberculosis infections that would arise in humans and livestock due to contact with infected carcass waste. The model predicted that hyena scavenging prevents five infections of anthrax and bovine tuberculosis in Mekelle residents and 140 infections in cattle, sheep and goats, during the course of a year. This disease-control service potentially saves the city $52,000 annually in treatment costs and livestock losses avoided.

“Yes, the effects are modest. Even so, this study totally upends the traditional narrative around hyenas – that they are a nuisance and should be removed,” said Carter.

“In comparison to the large cattle enterprises in the Global North, the benefits that we found are quite small,” said Sonawane. “However, losing even a single cow to anthrax or bovine tuberculosis can cause significant financial stress to cattle owners in Ethiopia, who tend to own much smaller herds of cattle than cattle owners in other parts of the world.”

“In addition, we only estimated the benefits accrued from hyenas within 5 kilometers of the city. It is very, very likely that hyenas from much further than 5 kilometers away are also coming into Mekelle and consuming even more waste.”

The study authors note that the benefits of hyena presence must be weighed against the costs of hyena attacks each year. A 2011 study reported 10 nonfatal hyena attacks on humans annually in Mekelle, along with an estimated economic loss of $2,928 from 33 fatal attacks on cattle and small ruminants annually. It is clear that in order to preserve and extend the health and financial benefits provided by the hyenas of Mekelle, the population of these scavengers must be managed to minimize the inherent risks to the humans with which they coexist. 

“By maintaining the hyenas’ access to waste, human-hyena conflict can be mitigated in Mekelle, and the sanitation and disease control benefits provided by hyena scavenging can be preserved or even increased,” wrote the researchers. Excluding hyenas from scavenging the animal carcasses may force them to hunt livestock, which would increase the human-hyena conflict. 

Currently, the activities of the hyenas of Mekelle help fulfil three of the UN Sustainable Development Goals: they ensure good health, help provide clean water and sanitation, and promote terrestrial diversity. Furthermore, waste generation will increase in Mekelle as the population gets larger, and the sanitation situation is unlikely to improve, given the financial constraints in this low-income region. 

“Hyena scavenging may, therefore, be even more valuable in the future,” concluded the researchers.

The study is published in the Journal of Applied Ecology.

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By Alison Bosman, Earth.com Staff Writer