Cup tool use by squirrel monkeys

Abstract

Captive-born male and female squirrel monkeys spontaneously 'invented' a cup tool use technique to Contain (i.e., hold and control) food they reduced into fragments for consumption and to Contain water collected from a valve to drink. Food cup use was observed more frequently than water cup use. Observations indicate that 68% (n = 39/57) of monkeys in this population used a cup (a plastic slip cap) to Contain food, and a subset of these monkeys, 10% (n = 4/39), also used a cup to Contain water. Cup use was optional and did not replace, but supplemented, the hand/arm-to-mouth eating and direct valve drinking exhibited by all members of the population. Strategies monkeys used to bring food and cups together for food processing activity at preferred upper-level perching areas, in the arboreal-like environment in which they lived, provides evidence that monkeys may plan food processing activity with the cups. Specifically, prior to cup use monkeys obtained a cup first before food, or obtained food and a cup from the floor simultaneously, before transporting both items to upper-level perching areas. After food processing activity with cups monkeys rarely dropped the cups and more often placed the cups onto perching. Monkeys subsequently returned to use cups that they previously placed on perching after food processing activity. The latter behavior is consistent with the possibility that monkeys may keep cups at preferred perching sites for future food processing activity and merits experimental investigation. Reports of spontaneous tool use by squirrel monkeys are rare and this is the first report of population-level tool use. These findings offer insights into the cognitive abilities of squirrel monkeys and provide a new context for behavior studies with this genus and for comparative studies with other primates.

Keywords: Saimiri; planning; squirrel monkey; tool transport; tool use.

Fig. 1.

Food cup use by an adult female squirrel monkey (Meg) in her home environment (A) Transporting a cup that she began using moments earlier to Contain chow (B) Removing the chow from the cup (C) Placing the chow in her mouth (D) Holding the cup next to her mouth to Contain chow fragments as she crushes the chow with her teeth (E) Removing the food fragments from the cup (F) Eating the food fragments. See Supplementary movie.

Fig. 2.

Water cup use by an adult male squirrel monkey (Isaac) in his home environment (A) Approaching water valve with a cup in hand (B) Applying the cup horizontally onto the valve (C) Removing the cup from the valve and looking into it, putatively monitoring progress collecting water (D, E) Applying the cup horizontally onto valve again (F) Removing the cup from the valve again and looking into it, putatively monitoring for collected water (G) Transporting the cup with the Contained water to the back of the home environment (H, I) Preparing to sip water from the cup (J) Inverting the cup at the mouth and sipping the water from it. See Supplementary movie.

Fig. 3.

Cups that adult female squirrel monkeys used to Contain chow processed into fragments for consumption. (A) ‘Ingrid’ holding a cup in her left hand with processed chow fragments in it (B) A cup with leftover chow fragments that was placed onto high-level perching by ‘Greta’ after she finished using the cup to Contain the chow.