Huge herds of hairy, pig-like animals called peccaries suddenly vanish and don’t come back for years – and it seems to be because their populations are growing so large that they wipe themselves out.
It is the first example of natural population cyclicity documented in a mammal in Central and South America, and the largest in geography and timescale for any mammal in the world, says at the University of Brasilia in Brazil.
Fragoso was studying herds of white-lipped peccaries in the Amazon in 1991 for his PhD when they suddenly seemed to vanish. The disappearance of the creatures, which are hoofed and about a metre long, across tens of thousands of square kilometres sent him on a three-decade-long investigation.
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“It became a big detective story,” he says.
Fragoso suspected that the peccaries’ population size underwent cycles when he heard about the animals’ role in local mythology from Indigenous groups. Village leaders told him that their powerful shamans had died and taken the peccaries with them to the underworld out of anger, but one day they would return.
Studying records of the trade in peccary skins over the past century, as well as observations of population levels by Indigenous groups, helped provide evidence that the animals went through periods of boom and bust.
Fragoso and his colleagues documented 43 disappearances in nine countries, from Mexico to Argentina, revealing that the animals have a 30-year population cycle.
The researchers found that the populations grow until they reach a density of between about 25 and 100 peccaries per square kilometre, and then they collapse – similar to .
The peccaries vanish for between seven and 10 years across areas as large as 5 million square kilometres before regenerating slowly and spreading across the region once more.
The peccary herds probably grow so large that it leaves them vulnerable to outbreaks of disease, which wipe them out, says Fragoso. Changes in climate could also cause the plants, fruits and animals they feed on to fall below the levels needed to support large populations.
Other potential drivers of the cyclic phenomenon, such as predators or hunting by humans, are less likely, says Fragoso. “Predators do not control the peccary population. There are not enough jaguars,” he says. “Plus, research from Peru shows that it’s the peccary population that likely controls the jaguar population.” Peccary populations cycle like this even in areas of the Amazon with light or no hunting, he says.
The peccary is a keystone species, comparable to the bison in the US or the elephant in Asia, so its presence is important for the health of the Amazon rainforest, says Fragoso.
Peccaries are omnivorous and travel in herds of hundreds, transforming the forest and the animals and plants within it as they roam. “They basically eat all the seedlings and seeds in an area, so no one plant species can dominate,” says Fragoso.
Peccaries make up much of the diet of several Indigenous groups and are also key prey for jaguars, so their disappearance is likely to have a wide knock-on effect.
Deforestation is making it harder for peccaries to bounce back when their population drops, says Fragoso. In regions like the Cerrado savanna in eastern Brazil, the mammals haven’t been seen in decades, probably because deforestation left no forest corridors for them to follow back to the areas they had inhabited.
“Reintroduction programmes are really critical to getting them in the areas where they’ve disappeared, but [so is] creating connections through habitat corridors between existing parks,” says Fragoso.
PLoS One
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