Size and Form of the Penis in Orang-Utans

Abstract

Size and form of the external penis, or pars libera, was documented for 17 living orangutans ( Pongo pygmaeus ) of varying age. The pars libera was much longer than previously represented in comparative analyses, and most were filiform (as in Pan ), but form and color was variable: 1) a partially or completely demarcated glans-like structure (as in Gorilla ) was present in a few subjects; 2) epidermal specializations of the pars distalis had either a hinge-like or flap-like form and might be homologous to the keratinized spines seen in other species; 3) the pars libera was not a uniform pink in all cases as the pars medialis often was black or purple. Comparisons of the length of the pars libera relative to size of females across the great apes and humans ( Homo ) reveals that the pars libera of Pan is much longer than that of Homo which is longer than but similar to that of Pongo , which is much longer than that of Gorilla . Relative to size of males, however, Homo have pars libera that are twice as long as those of Pongo . Similarities and differences in penile structure between Pongo , which has a non-gregarious (dispersed) mating system, and Pan (with a multimale system) may be understood in relation to the form of the female genitalia; this is consistent with the notion that sexual selection by female choice may be an ultimate cause of sexual characteristics of hominoids. Hypotheses about causation, however, require scrutiny with a more complete dataset from living subjects.