One of only two portraits of his subject, Monkey clearly alerts us to Bacon's fascination with the primeval physiognomic elements that underlie the human form.

In the series of heads that Bacon executed in the late 1940s and 1950s, he returned to his preoccupation with animal characteristics, searching for a way to compare human and animal traits, he began to draw particular attention to the portrayal of the mouth. Stretched in a grimace or a cry, or even perhaps in a smile, the mouth was often the most prominent or even the only feature in some of the earliest heads and figures. Bacon discovered that our most concentrated experiences of agony or ecstasy were physiologically expressed through the mouth and drew particularly close to that of the animal.

In Monkey, the chimpanzee shares not only its mouth but most of its anatomy with man. The animal traits in this portrait have a double significance: man becomes animal, but not without the animal taking on something of the esprit of man. There is, in other words, a zone of non-discrimination between man and animal.