The animal kingdom is chock full of nutcrackers and that skill can even vary between individual animals. Some chimpanzees appear to be more efficient at using tools to crack nuts than others within their groups. This efficiency could also give them an evolutionary advantage, since better nutcracking requires that these primates expend less energy and get more food in return. The findings are detailed in a study published December 23 in the journal Nature Human Behaviour.
Chimpanzees have long been observed using various stone tools. Biologists and anthropologists view tool usage as a key indicator of brain and cognitive development in animal species. Oldowan technology–three specific stone tools that date back roughly 2.9 million years ago from present-day Kenya–is considered the earliest known example of humans using stone tools.
“We had access to this incredible treasure trove of videos of our closest living relatives using stone tools that are known to be similar to hominin Oldowan technology,” Sophie Berdugo, a study co-author and evolutionary anthropologist from the University of Oxford in the United Kingdom tells Popular Science. “I’m personally interested in social and cultural learning, and I wanted to know whether variation in learning experiences had tangible effects on adult behaviour.”
In the new study, Berdugo and her colleagues analyzed footage of over 3,882 observations of 21 wild chimpanzees cracking nuts in Bossou between 1992 and 2017. Five different measures of efficiency were used to determine whether some individual chimps were more efficient than others. These included how long it took the chimpanzees to crack the nuts (bout duration), the number of strikes per nut, success rate, the number of times a strike moved a nut (displacement rate), and the number of times the chimpanzees switched their tools (tool switch rate).
They found consistent individual-level differences in efficiency for all of the measures except in tool switch rate. For example, some chimpanzees took twice as long to get to the hidden nutritious legume than others of the same age and sex. The four measures of efficiency also improved the older the chimps got, up to the age of 11. However, biological sex did not appear to play a role in nutcracking.
“It was really surprising to find no difference in efficiency between male and female chimps, because a female bias in tool use efficiency has been found in the past in chimpanzees and bonobos (who are both our closest living ape relatives),” says Berdugo. “Although it may be that the Bossou chimps are just a weird community that is bucking this trend.”
Berdugo believes that this lack of sex difference might be because their research looked at tool use over such a long time period. All of the previous research that saw a female bias used short term data of only a few years.
“This really illustrates the importance of long-term research for long-lived species like chimpanzees,” says Berdugo.
There is some evidence that suggests differences in tool use between some individual species, including in Burmese long-tailed macaques when cracking oysters. However, this type of research has typically relied on data collected over short time spans, not across the span of more than two decades like this study.
[ Related: Great apes may have cognitive foundations for language. ]
These findings suggest that some chimpanzees may have greater cognitive or motor abilities than others in their groups. Additional longitudinal research is needed to understand the causes and consequences of this variation. Understanding these differences in tool use could help the interdisciplinary field of primate archaeology, which examines the evolutionary role that tools have played in humans and non-human primates. This chimpanzee community has some human connections, as Bossu chimps use stone tools the way that scientists believe early humans may have used them.
“Primate archaeology is now a well established research field,” explains Berdugo. “There is now a growing recognition that some individuals may be contributing more to the material record than others, or that tools used by individuals with certain skills (or lack thereof!) could be easier to identify.”
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