The Mental Life of Monkeys and Apes: A Study of Ideational Behavior — Context and Discussion

Multiple-Choice Problems and the Emergence of Tactics

The core of the study is a series of multiple-choice experiments in which the animal must learn to select the correct box from a row. Yerkes meticulously catalogs the “reactive tendencies” each subject displays. For Julius the orangutan, these include a left-to-right scanning method, a later right-end preference, and a peculiar feint: approaching the box at the right end, pretending to enter, then turning to the correct box second from the right. Yerkes notes that this feint “is peculiarly interesting, and its origin and persistence are difficult to account for.” The detailed day-by-day records show how punishment (confinement) and reward (food) were adjusted when the animal became discouraged. The tables of conditions reveal a flexible, responsive experimental design rather than a rigid protocol.

Supplementary Tests: Box Stacking, Draw-In, and Lock and Key

Beyond the multiple-choice setup, Yerkes devised tasks requiring tool use and sequential action. Julius was given a box-stacking problem, a draw-in experiment (pulling food within reach using a stick), and a lock-and-key test. The descriptions are terse but revealing: the orangutan’s successes and failures are recorded without anthropomorphic gloss. For instance, in the draw-in experiment, Julius initially failed to use the stick as a tool but later succeeded after repeated trials. Yerkes also notes moments of distraction—Julius playing with sawdust “until all had slipped through his fingers”—which he treats as behavioral data rather than mere anecdote. These supplementary tests are presented as converging evidence for ideational behavior, though Yerkes is cautious about inferring internal mental states.

Individual Variation and the Role of Emotion

Yerkes pays close attention to differences among his three subjects. Skirrl, a crab-eating macaque, shows distinct handedness and emotional responses; Sobke, a rhesus macaque, exhibits fear and maternal instinct in miscellaneous observations. The section on instinct and emotion includes brief notes on fear reactions and sympathetic behavior, though the excerpts provide only fragments. Yerkes’s approach is comparative: he does not treat the orangutan as a scaled-up monkey but documents species-specific tendencies. The historical and critical discussion that follows reviews prior evidence for ideation in monkeys and apes, situating his own work within a broader scientific conversation. This comparative lens is central to the monograph’s structure.

The Case for a Primate Research Station

The final substantive section shifts from experimental results to institutional advocacy. Yerkes argues that the study of primates requires “adequate and permanent provision” in the form of a dedicated research station. He outlines a plan for such a facility, emphasizing the need for long-term, uninterrupted observation. This section is not an afterthought but a culmination: the experimental data are presented as evidence of what could be achieved with better resources. Yerkes’s tone is pragmatic, not utopian. He notes that the task of “prospecting for an anthropoid station” may prove more important than his own experiments. The bibliography that follows includes references to contemporary work in comparative psychology and primatology.

Readers should approach this monograph as a primary source in the history of comparative psychology. Yerkes’s detailed reporting of trial-by-trial data, including failures and distractions, offers a window into early twentieth-century experimental practice. The work is as much about method as about results, and the advocacy for a primate station frames the experiments as a proof of concept. Those interested in animal behavior, the history of science, or the origins of primatology will find this a valuable document.

There’s something touching about Yerkes watching that orangutan pause, thinking before choosing—the quiet weight of another mind working. It makes me think of the little chipmunk study, A New Chipmunk (Genus Eutamias) from the Black Hills — Themes and Context, where a creature gets noticed for being itself, not just a specimen. Both feel like patient glances at small, earnest lives.