As the rain begins to fall on Tanzania’s Tarangire National Park, thousands of zebra, wildebeest and giraffe will begin one of the world’s greatest migrations. But many of the herds trampling across the grass at the foot of the Rift Valley highlands are falling in number — and scientists do not know why.
To find out, they are using a new “photo-fit” system that can recognize individual animals by their unique skin patterns, which are as distinct as a human fingerprint.
The park, dotted with muddy water holes and ancient baobab and acacia trees, has the highest diversity of migratory hoofed mammal (ungulate) species in the world. The animals move out of the park as the wet season begins, in search of new feeding and calving grounds. They will make the return dry season journey in June, to take advantage of the permanent water of the Tarangire River.
Numbers of wildebeest have fallen from 50,000 to 6,000 in the past 20 years, and numbers of antelope species, such as hartebeest and oryx, have declined by 90 percent and 95 percent respectively. Confusingly some species — zebra, giraffe, gazelle and buffalo — have remained relatively stable. To understand such contrasting fortunes, scientists from the Dartmouth and Utah universities in the US are working to determine whether habitat loss, changed food sources, or hunting — or a combination of all — is responsible.
Traditionally animals have been tracked by being captured and tagged, but that is “expensive and invasive,” Dartmouth’s Doug Bolger said.
Scientists have turned to computer-assisted photo identification — the first time the method has been used on such a large scale.
“This new technology allows us to get the best handle on what the population is doing,” Bolger said.
“What we’d like to understand is why some migratory routes have shut down,” he said. “By analyzing the movement of certain species, we hope to understand what makes a landscape they will pass through and what won’t.”
The team identify an individual animal by its stripe width, color and shape, the distance between the marks and whether it has unique characteristics such as scars or, in the case of some wildebeest, broken or deformed horns.
Hundreds of digital images are collected in the field each day and the animal’s stripe or spot pattern is mapped using a 3D model. Custom-built software then extracts the pattern and compares it with a database of previously photographed individuals.
At a later stage the figures will be crunched through another computer program, to generate estimates of population size, survival and reproduction rates.
“We’re still at least a year away from being able to use all this data to come up with population estimates and survival rates,” Bolger said.
But as more and more animals are added to the researchers’ photo albums, the difficulties the herds face on their trek across the picture-perfect savannah come a little closer to being understood — and protection is a step closer.
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