Crack It Open! How Monkeys Use Tools to Get Food
Article Summary by Breanne Cyr, Primates Incorporated Volunteer
For a video demonstration of tool use in Burmese Long-Tailed Macaques, view:
Think of the various ways humans use forks and knives to eat their food: some use a fork in one hand and a knife in the other; some switch utensils around so that they both cut and eat with their dominant hand; some don’t use a knife at all and just use the side of their fork to cut. Some of this variation is individual and some is due to culture. Furthermore, the method one uses may differ depending on what kind of food is being eaten. Just as humans vary culturally and individually in how they use such tools, monkeys show similar differences in tool use.
One such example exists in a species of monkey from Thailand called the Burmese Long-Tailed Macaque (1). These monkeys use tools – namely stones and shells – to crack open a variety of sea foods such as oysters, snails, crustaceans, and plant seeds. Recently, researchers studied five groups of Burmese Macaques and were able to classify the monkeys’ tool use based on the type and size of the tool, as well as 17 precise action methods in which monkeys used these tools to hammer.
Researchers classified tools by the material they were made out of (stone or shell), the size relative to the size of a monkey’s hand, and whether the food being cracked open was attached to something or able to be picked up and moved. Researchers also examined whether a monkey used the point, edge, or flat part (face) of the tool. Finally, they noted several behavioral elements of using tools such as whether a monkey used one or two hands with the tool, and the manner in which the monkey struck the food.
After studying more than 100 monkeys, researchers found that 80% of the Burmese Long-Tailed Macaques used stones and shells as hammering tools. Each monkey used between one and four of the 17 different action methods to crack open food. Overall, monkeys were most likely to hammer with the point of a tool using one hand as well as hammer using the face of a tool using one or both hands. Monkeys rarely hammered using the edge of a tool. Additionally, individual monkeys used different methods to crack open the same foods.
In general, however, monkeys used different methods depending on various circumstances. For example, although hammering with the point of a tool was very common, the monkeys essentially only used this method when they were breaking open foods that were attached to a surface, such as rock oysters. There were also differences in which hammering methods were most common between the five groups of monkeys being studied. Some of these differences may be due to culture, or what individuals learn from their family and peers, but the difference also seems to be ecological. Monkey groups that lived closer to the mangroves, which contained more foods that could not be moved, tended to use hammering with the point of a tool more often.
Studies such as this one demonstrate how monkeys use tools in different ways for the same purpose, just as humans do. There are both individual and regional differences in methods of using tools, also similar to humans. This is all the more evidence that, as the original article’s title stated, “There is more than one way to crack an oyster!”
1. Tan A, Tan SH, Vyas D, Malaivijitnond S, Gumert MD (2015) There Is More than One Way to Crack an Oyster: Identifying Variation in Burmese Long-Tailed Macaque (Macaca fascicularis aurea) Stone-Tool Use. PLoS ONE 10(5): e0124733.