In contrast, younger infants are more influenced by their hand preference than are older infants when grasping a tool 8, 26. When older human infants grasp a single tool, they use the hand contralateral to the position of the target relative to the tool to manipulate the tool 8, 26. In experimental settings, some studies have reported that human infants and common marmosets exhibit handedness at the group level depending on the direction of tool manipulation required in each experimental situation 8, 21, 26. Wild chimpanzees are significantly more left-handed for termite-fishing, but they are significantly more right-handed for nut-cracking when using anvil and hammer stones 25. Analysis of the direction in which the tool is manipulated would enable the detection of trials in which the subject understood the appropriate direction in which to move the tool to obtain the reward but was not successful because of insufficient motor ability.Ī previous study reported that wild chimpanzees exhibit handedness at the group level depending on the type of tool-use 25. It is possible that subjects may have understood the appropriate manipulation method but may not have had sufficient motor capability to appropriately manipulate the rake to obtain the reward, leading to a failure to obtain the reward at the beginning of the training. However, it cannot be concluded that subjects did not understand that the rake required manipulation in the direction of the reward and not in the direction opposite that of the reward to obtain it. Other studies did not report performance at the beginning of the training 9– 13, 15– 20. 24 reported that degus waved the rake around the food for a short period of time but failed to obtain the food at the beginning of training. 5 observed that human infants touched or pushed the toy with the rake but failed to obtain the toy. Some studies have reported that subjects could not actually obtain the reward using the rake at the beginning of the training, when the reward was placed at the side of the rake 5– 8, 14, 21– 24. However, no research has thus far been conducted on whether rats ( Rattus norvegicus) can manipulate tools to obtain food in such a manner. These studies of nonhuman animals show that subjects could be trained to manipulate a rake-shaped tool laterally before pulling on it, allowing them to obtain a food item beyond their reach in situations in which they could not obtain the food by pulling the tool perpendicularly to themselves without further manipulation. Similar tool-use tasks have also been carried out by Japanese macaques ( Macaca fuscata) 10– 19, rhesus monkeys ( Macaca mulatta) 20, common marmosets ( Callithrix jacchus) 21, 22, and degus ( Octodon degus) 23, 24. These studies reported that infants aged 18 months and older could move the rake laterally before pulling on it to obtain a toy under conditions in which the toy was placed to the side of the rake 5– 9. In these studies, various conditions for the arrangement of the rake and the toy were adopted, and the distance between the rake and toy was gradually extended. A shared feature of these studies is that the subject was required to manipulate a rake-shaped tool to obtain a reward beyond its reach via a step-by-step protocol. ![]() ![]() In previous studies, 14–22-month-old human infants were observed to use a rake-shaped tool to obtain a toy beyond their reach 5– 8. In experimental settings, some studies have reported that human infants and nonhuman animals can be trained to use a rake-shaped tool to obtain a toy or food beyond their reach in situations in which they cannot obtain the reward by pulling the tool itself towards them. In the wild, chimpanzees ( Pan troglodytes) can hold a stick with their hand and dip it into and around the entrances of underground ant nests to obtain ants for consumption 4. For instance, wild woodpecker finches ( Cactospiza pallida) can hold a twig or cactus spine in their beak, insert it into an opening in the bark of a tree and pry out arthropods as a source of food 3. Tool use is indispensable for humans, and it is an important component of the survival and fitness of several non-human species as well 2. ![]() Tool-use is the exertion of control over a freely manipulable external object to generate a mechanical dynamic interaction between the user and an object or to affect the transfer of information between the user and the environment. St Amant and Horton 1 provided the following definition of tool-use.
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