Chimpanzees in the West African nation of Senegal take shelter from the scorching heat in caves, a new study has found.
Coincidence or not, the unusual behaviour, the first known case of regular cave use by an ape species, is found among the same chimpanzees that were recently found to hunt small mammals using sharpened sticks.
Jill Pruetz, an anthropologist at Iowa State University, who began her fieldwork in 2001, at a site known as Fongoli, said local Malinke people showed her the caves, which were often occupied by chimpanzees during the hottest part of the year.
"It took years and years for the chimpanzees to get habituated [to the researchers' presence. As soon as we would walk anywhere close, it would scare them out of the caves," National Geographic quoted Pruetz as saying.
Even with few direct observations, Pruetz's team was able to assess the extent of cave use using clues left behind on sandy cave floors: tracks, droppings, and food remains.
Intrigued at first, she observed the chimpanzees' odd behaviour. Research showed that the cave use was concentrated at the end of the dry season in May and June.
"The behaviour appears to be an adjustment to heat stress," said Pruetz.
"No one has ever before published reports of apes in caves. This is one of those cases in which the apes genuinely surprise us, exceeding our expectations and imaginations," added William McGrew of Cambridge University in England.
According to Pruetz, cave use is just one of several strategies chimpanzees use to cope with their difficult environment, where both shade and water are critical resources.
In April and May maximum temperatures in open grassland near the caves can reach 107.6 degrees Fahrenheit (42 degrees Celsius).
Findings revealed that the temperature in the largest of the three small caves used by the chimpanzees never exceeded 84.2 degrees Fahrenheit (29 degrees Celsius).
Pruetz said that while her new paper is based solely on data collected at the Fongoli study site, her team has also observed cave use in other nearby chimpanzee populations.