Why did Serengeti’s wild dogs disappear? Study challenges controversial hypothesis

In 1991, all of the African wild dogs that previously lived in Serengeti National Park in Tanzania completely disappeared. In the debates that followed, one hypothesis became particularly controversial: in a series of papers, Roger Burrows, a researcher from the University of Exeter, U.K., contended that the “handling” of Serengeti’s wild dogs by researchers had led to the animals’ deaths.

In correspondence published in Nature in 1992, Burrows wrote that the blood serum of some of the African wild dogs (Lycaon pictus) had shown rabies antibodies, suggesting they had been exposed to the rabies virus at some point. He said it was possible that the wild dogs had some degree of natural immunity to the rabies virus, and that darting the wild dogs with anaesthetic and handling them, either to take blood samples from them, to vaccinate them, or to put radio collars on them, was extremely stressful to the animals. This “invasive” handling increased stress hormones in the animals, which then compromised their immunity and reactivated the latent form of the rabies virus within their bodies, leading to the death of the wild dog packs — although it was never confirmed if all the Serengeti packs had died or if many had simply moved away from the park. For conservationists and researchers who at the time were increasingly relying on radio collars to track wild animals, though, this hypothesis came as a rude shock.

Many researchers offered counterarguments. They published data from other African wild dog habitats in Africa, which suggested that capturing and handling of the animals was not harmful to the species. But Burrows and other proponents of his hypothesis remained unconvinced. They said the data presented from other ecosystems in Africa were flawed: there was often little background information on whether the wild dogs in the other ecosystems had been exposed to rabies or other infectious diseases, which meant the researchers couldn’t know if those animals carried a latent form of the rabies virus. If there was no latent virus lurking in the animals, no amount of handling would reactivate them.

Now, in a new study published in Ecology and Evolution, Craig Jackson, a postdoctoral researcher at the Norwegian Institute for Nature Research (NINA), and his colleagues have challenged Burrows’ hypothesis, this time by presenting data from the Serengeti ecosystem itself.

African wild dogs never returned permanently to the plains of the Serengeti since their disappearance in 1991. But the animals are present to the east of the park, in Ngorongoro Conservation Area and Loliondo Game Controlled Area. Researchers at the Tanzania Wildlife Research Institute (TAWIRI) have been monitoring 121 of these wild dogs since 2005 and 2006, and they think these animals satisfy the conditions laid down by Burrows and his colleagues.

All of the 121 wild dogs have been “handled” by researchers, either for radio collaring or during an attempt to reintroduce some of them to Serengeti National Park. There is also evidence of rabies and canine distemper virus within the ecosystem: there have been numerous cases of rabid domestic dogs in Ngorongoro and Loliondo; the domestic dog population has increased over the years; and the wild dog populations occur alongside those of domestic dogs. This, the researchers say, points to high rates of exposure to the rabies virus within the current wild dog population.

“Those diseases are present in the ecosystem, the wild dogs are present in the ecosystem, and there was handling in the same ecosystem,” Jackson told Mongabay. “So all those conditions were met, and we could then evaluate whether or not it resulted in mortality.”

The team says it found no evidence to support Burrows’ hypothesis.

According to Burrows, “handled individuals were significantly less likely to survive for 12 months after the date of first handling.” But of the 121 wild dogs that Jackson and his colleagues had captured and handled, 106, or nearly 88 percent, survived for more than 12 months. In fact, of the 67 wild dogs that had been immobilized, captured, placed into containers and moved from Loliondo into Serengeti National Park as part of a reintroduction project in 2012, where they were housed in the enclosure for several months — which the researchers say would amount to longer-term stress for the animals — nearly all survived for more than 12 months. They had even higher survival rates than the wild dogs that had been handled briefly and released.

“That’s basically because they were kept in an enclosure for several months before being released and in that enclosure they were not exposed to lions and hyenas and other things out there that they would generally bump into during their daily activities,” Jackson said. “So that was also further strong evidence against the effects of stress and the reactivation of the virus.”

Marion L. East, a senior scientist at the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin, who has co-authored some papers with Burrows, said she wasn’t convinced by the new study. Their results would be persuasive, she said, if they had rigorously collected and screened blood samples from the 121 handled wild dogs to confirm disease exposure, infection and stress. Had they done this, “Jackson et al. would have provided useful and interesting findings,” East said. “Unfortunately, the study lacks a rigorous analysis of demography, and exposure to disease and levels of ‘stress’ are based on assumptions not relevant data.”

Jackson and his colleagues, however, think the African wild dogs went missing from the Serengeti plains because of a very different reason: competition with lions and hyenas.

While wild dogs haven’t established packs inside Serengeti National Park since 1991 — even the reintroduced packs eventually moved back outside — they have grown in number outside the park, they say. The wild dogs have also been known to travel long distances and establish new packs over the years. Yet they’ve failed to recolonize the national park.

“Inside the national park you don’t have people, livestock, conflict, and persecution, and you’ve got a lot of prey and food available,” Jackson said. “None of the six packs that had been reintroduced into Serengeti National Park utilized the former area in the Serengeti plains either. They just avoided those areas and our reasoning is that in those areas they are exposed to lion and hyena populations.”

The numbers of both hyenas and lions have increased in Serengeti National Park over the decades, the researchers say. During the 1960s and 1970s, for example, hyenas would be present at fewer than half of Serengeti wild dog kills. But in the late 1980s, there would be at least one hyena at 85 percent of wild dog kills, waiting to snatch their prey.

“So it’s not just a direct competition or mortality risk to the individuals, but it is also the competition from kleptoparasitism, that is the loss of their kills to other carnivores, especially on the plains where you have large clans of these hyenas,” Jackson said.

Wild dogs are also known to avoid lions, he added. The packs that were reintroduced to Serengeti from 2012 onward showed similar patterns: when the wild dogs were still inside the national park, they would spend most of their time on hilly, undulating, rugged areas and avoid the plains where the lions and hyenas typically moved around.

“This is not an isolated incidence or case study, we know that wild dogs are very vulnerable to interspecific competition with lions and hyenas elsewhere,” Jackson said. “So we suspect that something similar is happening here.”

While many researchers have rejected Burrows’ hypothesis over the decades, this is the first study to test the hypothesis with data from the Serengeti ecosystem, Jackson added.

“Having deployed quite a few radio collars myself and handled wild dogs, obviously you have to think about the animals’ well-being and whether what you’re doing is actually OK and not detrimental to them in the long run, especially when you’re dealing with an endangered species,” he said. “But the wild dogs never went extinct in the Serengeti ecosystem. And having been involved in the work in Serengeti, deploying radio collars, we knew that the packs were persisting for years and we didn’t see any negative effects. This was just a way to get data to test the hypothesis more explicitly.”

Citation:

Jackson, C. R., Masenga, E. H., Mjingo, E. E., Davies, A. B., Fossøy, F., Fyumagwa, R. D., . . . May, R. F. (2018). No evidence of handling-induced mortality in Serengeti’s African wild dog population. Ecology and Evolution, 9(3), 1110-1118. doi:10.1002/ece3.4798