When I initially went through some of the materials in the Fossey Collection, one of the items that caught my interest were descriptions of tool use in the mountain gorillas. These descriptions were within the “Manual for Census Workers”. Although this manual addressed daily research procedures, it also providing descriptions of the gorilla groups and behaviors. Under the heading “Application and reactions to surrounding objects” Fossey described several different behaviors involving the use of unnattached objects that could potentially qualify as tool use. These included descriptions of both directed and undirected throwing of foliage, and the use of a plant as a toy to tickle an infant.
This was particularly surprising, as I had thought that tool use in wild gorillas had not been documented until the 2000s. Breur and colleagues (2005:2041) explain “there has been to our knowledge no reported case of tool use by wild gorillas, despite decades of tool use.” However, two years later, Wittiger and Sunderland-Groves (2007) note that early accounts of gorilla behavior from Merfield (1957) and Schaller (1963) include descriptions of object throwing. Schaller (1963) described object-throwing as one of the nine components of gorilla displays. Clearly, these behaviors had been observed multiple times in the earliest gorilla studies. How did these observations get overlooked?
One reason might be that uncertainty in whether these behaviors count as tool use. The behaviors Fossey described involve using objects, but not as a means to manipulate food or other items. There is some debate over whether object-throwing fulfills the definition of tool use. The now-standard definition of tool use was not articulated until 1980. Beck (1980:10) defines a tool use as “the use of an unattached object to alter more efficiently the form, position, or condition of another object, another organism, or the user itself.” Beck (1980) specifically includes the throwing of objects toward predators or rivals as one of the six main categories of tool use. However, it only qualifies as tool use if the objects are intentially directed at recipients to induce a change in the recipients’ behavior. For example, throwing a piece of food after most of it has been consumed is not tool use, but throwing it at another individual in an aggressive encounter would be. However, determining intention, and whether or not it is a directed, can be difficult. Some discussions of tool use exclude object-throwing because of this ambiguity (Panger 1998).
Fossey’s descriptions include both directed throwing of foliage, and non-directed tossing. Directed throwing involved an overhanded throwing motion, and were directed to either Fossey or other gorillas. Conversely, non-directed tossing involved an underhanded throw. She states that “aimed throwing is done approximately 30% of the time.” This aimed throwing would fulfill the definition of tool use, whereas the more common under-handed tossing of foliage would not. However, the total frequency of foliage-throwing is not given, so it is unclear how often this behavior occurred. More recent reports of object-throwing, including foliage or branches, in wild Cross river gorillas all describe underhanded throws (Wittiger and Sunderland-Groves, 2007).
The other description of tool use is within the context of play. A female gorilla picked a flowering helichrysum (an herbaceous shrub) and used it as a “play object” to tickle an infant. Given that the object was directly used to induce changes in another individual’s behavior, it fulfills the definitions of tool use. However, unlike the more common foliage-throwing, this was a single ancedotal observation. Furthermore, since many early descriptions of tool use focused on using tools in the context of feeding or foraging, tool use in a social context may have been overlooked.
All other descriptions of tool use in wild gorillas (to my knowledge) involve using objects to facilitate movement in the environment. Breur and colleagues (2005) describe the use of a stick to probe water depth, and to assist in crossing, as well as using a tree trunk as a bridge to cross the swamp. More recently, Grueter and colleagues (2013) reported an observation of a female gorilla using a bamboo stalk as a ladder for her infant to climb vegetation that would not support his weight.
Given that the descriptions of tool use were in the Manual for Census Workers, I’m surprised that Fossey’s observations have not been reported in the published literature. However, it is a reminder that sometimes important behavioral observations are noted, and then remain hidden in unpublished accounts. I thought this was a particularly interesting find, and that was only after a short time going through the materials. I look forward to spending more time looking at the materials in greater detail to see if there are other interesting behaviors that may have been overlooked.
Fossey D. Unpublished. “Manual for Census Workers, ” in Fossey’s Notes on the Gorilla Project (1969-1971), Box 15. Harold TP Hayes Collections, Z Smiths Reynolds Library. Wake Forest University, Winston-Salem, NC. Accessed on November 13, 2013.
The inventory of the collection can be found here: http://wakespace.lib.wfu.edu/handle/10339/36412.
Beck, BB. 1980. Animal Tool Behavior: The Use and Manufacture of Tools by Animals. New York: Garland Press.
Breuer T, Ndoundou-Hockemba M, and Fishlock V. 2005. First observation of tool use in wild gorillas. PLoS Biol 3:2041–2043.
Grueter CC, Robbins MM, Ndagijimana F, and Stoinski TS. 2013. Possible tool use in a mountain gorilla. Behav Processes 100:160–2.
Merfield F. 1965. Gorillas Were My Neighbors. London: The Companion Book Club.
Panger, MA. 1998. Object-use in free-ranging white-faced capuchins (Cebus capucinus) in Costa Rica. Am J Phys Anth 106: 311-321.
Schaller, GB. 1963. The Mountain Gorilla: Ecology and Behaviour. Chicago: University of Chicago Press.
Wittiger L, and Sunderland-Groves J. 2007. Tool Use During Display Behavior in Wild Cross River Gorillas. Am. J. Primatol. 1311:1307–1311.