Where is the Human Baculum?

Abstract

Humans are unique among the primates in many characteristics, especially the primary and secondary sexual characters. One of these characteristics is that human males are the only member of the old world simian group (apes and old world monkeys) without a baculum. The loss of this bone in Homo sapiens is considered with respect to its function and appearance in other hominids. It is hypothesized that its loss in humans is related to upright posture, mating systems, and sperm competition. I predict that as more fossils are discovered or bones associated with current collections reexamined. all hominids before, and possibly including Homo erectus will be found to possess a baculum. Keywords: human mating systems, penis, sperm competition, bacula, human evolution 1. Introduction Modern man is anatomically unique amongst his near relatives the primates, most obviously in being hairless, erect and wholly bipedal. Sexual characters are especially different in Homo Sapiens (Short, 1981) and can be divided into two categories, primary and secondary. Primary characters are those directly related to copulation and are formed early in development. These include the organs of gamete transfer, as well as grasping appendages to secure the female. Secondary sexual characteristics, which become evident during puberty and adulthood, have received more attention, usually under the term sexual selection (Darwin, 1871; Trivers, 1972). Humans are distinctly different from other primates with regard to several primary sexual characters, including concealed ovulation (Benshoof and Thornhill,1979; Alexander and Noonan,1979; Burley, 1979; Smith, 1984, but see Sillen-Tullberg and Moller, 1993), possession of a hymen (Burley, 1979; Katchadorian, 1990; Hobday et al., in press), and permanent breasts (Short, 1980; Cant, 1981; Smith, 1984; Caro, 1987). An additional difference is that the human penis lacks a baculum (Hill, 1972; Short, 1979). In this paper I shall first explain the form and function of both bacula and penis and summarize likely functions, offer a possible explanation for the enlargement of the penis and loss of the baculum in H. Sapiens, and finally consider some comparative evidence to support my hypothesis. 2. The Baculum What is a Baculum?. The baculum (also called a penis bone, os priapi, or os penis) is a heterotypic hone present in the penis of various male mammals. Morphologically it is the most diverse of all bones and is often elongated. The distal end is complex in shape with great interspecific variability, implying that it has been subject to much selection (Eberhard, 1985; Dixson, 1987a, b; Fooden 1988). A female counterpart, the os clitordoris or baubellum, is present in some species (Hershkovitz, 1977; Hancox, 1987; Williams-Ashman and Reddi, 1991). Both form from ossification of the mesenchyme tissues within the organ. Because bacula growth occurs with maturity, it has been used to age males of some species (Hancox, 1987; Chen et. al., 1988). A baculum is found in 6 orders of Eutherian mammals (Table 1). It may have evolved separately in each mammal order in which it occurs (Hershkovitz ,1977). Of the non-primate orders, only in the Carnivore family Hyeniidae is the baculum absent. In the primates it is found in all the Catyrrhini (Hill, 1972), all prosimians but Tarsius, while in the Platyrrhini it is absent in several genera belonging to different families (Ateles, Lagothrix, Alouatta and some specimens of Chiropotes and Cacajao) (Hershkovitz, 1977; Dixson, 1987a, b). Among the apes and old world monkeys, H. Sapiens is the only member without a baculum (Table 1). These species are considered the most specialized members of their respective groups, and the lack of bacula is considered degenerative (Hershkovitz, 1977). In species with a baculum it is often a major element of the penis, and so it is logical to first consider the function of the penis. …