SIZE AND FORM OF THE PENIS IN ORANGUTANS

Size and form of the external penis, or pars libera. was documented fo r 17 living orangutans (Pongo pygmaeus) of varying age. The pars liber a 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 Go rilla) 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 o ften was black or purple. Comparisons of the length of the pars libera relative to size of females across the great apes and humans (Homo) r eveals that the pars libera of Pan is much longer than that of Homo wh ich 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 pa rs libera that are twice as long as those of Pongo. Similarities and d ifferences in penile structure between Pongo, which has a non-gregario us (dispersed) mating system, and Pan (with a multimale system) may be understood in relation to the form of the female genitalia; this is c onsistent with the notion that sexual selection by female choice may b e an ultimate cause of sexual characteristics of hominoids. Hypotheses about causation, however, require scrutiny with a more complete datas et from living subjects.