A new study suggests that climate change is interfering with the way researchers count bonobos, which could possibly lead to overestimating endangered apes.
Like other great apes, bonobos build high nests from tree branches and sleeping foliage. The count of these nests can be used to estimate the number of bonobos, as long as researchers have a good idea of how long a nest sticks before it breaks. by the environment, which is known as nest decomposition time.
New data on rainfall and bonobo nests show that nests persist longer in Congo’s forests, from about 87 days, on average, between 2003-2007 and about 107 days in 2016-2018, largely as a result of the decrease in precipitation. This increase in nest decomposition time could cause a dramatic distortion of the endangered ape population accounts and endangered conservation efforts, researchers reported on June 30 in PLOS ONE.
"Imagine going to that forest … you count nests, but every nest has more time than 15 years ago, which means you think there are more bonobos than there really are," says Barbara Fruth, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Konstanz, Germany.
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The low rainforests, south of the Congo River in Africa, are the only place in the world where bonobos (Pan paniscus) still live in the wild (SN: 18/03/21). Estimates suggest that there are at least 15,000 to 20,000 bonobos there. But there could be up to 50,000 individuals. “The potential distribution area is quite large, but there have been very few surveys,” says Fruth.
From 2003 to 2007, and again from 2016 to 2018, Fruth and his colleagues followed wild bonobos in the Congo LuiKotale rainforest, controlling 1,511 nests. “The idea is to follow (the bonobos) forever,” says Mattia Bessone, a wildlife researcher at John Moores University in Liverpool, England. "You have to get up early in the morning to be able to be in the place where the bonobos nested, in time for them to wake up, and then follow them until they nest again."
In doing so, day after day, Fruth, Bessone, and their colleagues were able to understand for the first time how many nests a bonobo builds per day, which is known as the nest building rate. “It’s not necessarily one because sometimes bonobos build day nests,” Bessone says. The team found, on average, each bonobo builds 1.3 nests a day.
Monitoring the time remaining in these nests revealed that the structures had an average duration of 19 days longer in 2016-2018 than in 2003-2007. The researchers also compiled fifteen years of climate data for LuiKotale, which showed a decrease in average precipitation from 2003 to 2018. That change in rain is linked to climate change, the researchers say, and helps explain why nests have become more resilient. .
© B. Fruth / MPI of animal behavior
By counting the number of nests and then dividing it by the product of the average nest decay time and the nest construction rate, scientists can get an estimate of the number of bonobos in a region. But if researchers are using obsolete and shorter nest disintegration times, those estimates could be severely devalued, overestimating bonobo accounts by as much as 50 percent, Bessone says.
“The results are not surprising, but they also highlight how indirect (and therefore error-prone) our methods of estimating the density of many species are,” Martin Surbeck, a behavioral ecologist at Harvard University, wrote in an email. .
Researchers say technologies such as camera traps can be used to directly count animals instead of using proxies as nests and are the way forward for animal population studies. But until those methods become more common, nest counts remain vital for scientists to understand bonobo numbers.
This phenomenon is probably not limited to bonobos. All great apes build nests and nest counting is also used to estimate the number of these animals. Thus, the researchers say, the new results could have implications for the conservation of primates far beyond bonobos.