Hungry Hyenas Help Human Health
Public health and economic benefits of spotted hyenas Crocuta crocuta in a peri-urban system (2021) Sonawane et al., Journal of Applied Ecology, https://doi.org/10.1111/1365-2664.14024
The natural world provides as with a laundry list of health services, from cleaning the water we drink to providing blueprints for cutting edge medicine. Yet on this list of ecosystem services, carnivores often get left by the wayside. One such carnivore is the spotted hyena, which can be found roaming the outskirts of many towns in eastern Africa. The hyenas are adept scavengers, and clear away massive amounts of discarded meat every year, potentially preventing the spread of carcass-borne diseases like anthrax and tuberculosis.
Yet as with many predators, hyenas have often been feared, whether as a result of their historical association with evil spirits or more recent unfavourable portrayals. In a world where carnivores like wolves, dingoes and bears are often feared and driven off, providing proof of the benefits they bring is crucial. So that’s what today’s researchers set out to do.
What They Did
The study took place around the edges of Mekelle, in northern Ethiopia. Over two months, the team observed hyenas feeding around the outskirts of Mekelle, and recorded the amount of flesh consumed by and number of the hyenas, enabling them to estimate the weight consumed per hyena per night. They then estimated the total population of hyenas that could potentially operate in Mekelle by performing call-in surveys. These involve broadcasting hyena distress signals outside of the city and observing the number of hyenas that turned up. Once the team had an estimate of the total number of hyenas active around Mekelle, they could estimate the total number of carcasses eaten by them.
That’s when things got tricky. From that point, the team had to simulate a variety of occurrences: the discarding of anthrax or TB-infected carcasses appearing throughout the city, hyenas consuming (or not consuming) the carcass, the movement of both people and cattle throughout these areas, and subsequent infections from that movement. This allowed them to estimate the number of anthrax and TB infections on a yearly basis, with and without hyenas.
Did You Know: Zoonotic Transmission and COVID
We’re all too familiar with zoonotic (animal-to-human transmitted) diseases these days, and our constant encroachment on the natural world is only increasing the frequency of such transmissions. While the ones here are a product of domestic cattle, many transmissions happen as a result of our increasing contact with wild animals, which when stressed, are much more likely to transmit viruses or other pathogens to humans. Read more about deforestation’s contribution to the pandemic below.
What They Found
The team estimated that a Mekelle hyena removed close to 1,000 kilos of meat from Mekelle each year, mostly from equids. The population estimate of hyenas around mekelle was just over 200, meaning they probably removed around 210 tons worth of carcasses annually. This equates to roughly four percent of the carcasses that humans or cattle are likely to come into contact with.
This was calculated to result in three less cases of anthrax and two less cases of TB in humans annually, and around 140 less infections in cattle. The combined cost of treating the humans and the cattle would potentially come to a little over 50,000 USD, with most of those costs being derived from the treatment of cattle.
This study relies on a lot of estimates with potential for uncertainty – the number of hyenas based on call-in surveys, the weight of carcasses consumed, the likelihood of a human to come into contact with a carcass. There’s nothing inherently wrong with this, but a paper like this would usually qualify that uncertainty. I would be interested to know whether or not the cost estimates here are a conservative estimate, and what the upper bound of such a cost could be.
50,000 USD might not seem like that much to a city the size of Mekelle, but as the author puts it, the services hyenas provide are “particularly valuable in a region with limited healthcare facilities and one of the highest zoonosis burdens in the world”. Admittedly, the benefits gained here need to be compared to the ‘costs’. There are (very infrequent) attacks on cattle by hyenas throughout the year, and people are often wary of them.
Carnivores like hyenas can provide enormous benefits to ecosystems, promoting biodiversity and improved ecosystem functioning. Yet we need to provide ways to demonstrate more tangible benefits to humans if we’re to move ahead with rewilding programs around the globe. Studies like these are a great starting point.
Dr. Sam Perrin is a freshwater ecologist who completed his PhD at the Norwegian University of Science and Technology who thinks hyenas are actually very good bois and would love one as a pet (obviously not for very long though). You can read more about his research and the rest of the Ecology for the Masses writers here, see more of his work at Ecology for the Masses here, or follow him on Twitter here.