Making Holes in the Dark

Sexual selection has played a major role in the evolution of the reproductive organs of animals. A full appreciation of this aspect of sexual selection is relatively new, however, and was unknown to Darwin at the time he formulated his theories concerning the pre-copulatory aspects of selection. Here, and in the following two chapters, I consider how sexual selection has affected the evolution of reproductive anatomy and physiology in mammals. Can meaningful correlations be established between...

Multiple Pattern ejaculations number

Figure 5.8 Dewsbury's (1972) classification scheme for masculine patterns of mammalian copulatory behaviour. thrusting by males during intromission (Figure 5.8). Where the primates are concerned, this decision is straightforward, because pelvic thrusting occurs in all species of prosimians and anthropoids studied thus far. It represents a primitive condition for the Order Primates as a whole. This is not the case for all mammals, however. In the laboratory rat and the rabbit, for example, males...

Rapid sperm transport the vas deferens and sperm competition

Gambar Vas Deferens

Given that sexual selection has influenced relative testes sizes and hence sperm production as well as the morphology of individual gametes (sperm mid-piece volume) in mammals, we may further enquire as to whether other components of the male reproductive tract have been likewise affected by sperm competition. Rapid transport of many millions of spermatozoa during copulation is a complex process, and especially so for those species where the numbers of gametes ejaculated and frequencies of...

Sexual dimorphism body weight and mating systems

Sexual dimorphism in adult body weight is most pronounced in those primates, such as geladas, hamad-ryas baboons and gorillas, which have polygynous mating systems (Figure 7.1). Competition between adult males for access to females is intense in such species, as is the case in other polygynous mammals (as, for example, elephant seals or red deer). Selection for increased male body size has also occurred in anthropoids such as macaques, chacma baboons and mangabeys, which have multi-male...

The Road to Truth

Much more is now known about human origins and evolution than was the case almost 40 years ago, when I began to study primate reproductive biology. In the final essence, it is the fossil evidence of hominid evolution, reviewed briefly in the first chapter of this book, which provides the backcloth against which the sexual behaviour and reproductive anatomy and physiology of the extant primates may be viewed in true evolutionary perspective. This has been the approach adopted throughout this...

Ill

Narrow Broad Broad waist, waist, waist, small large small breasts breasts breasts Figure 7.14 Relationships between waist size, breast size, and mid-cycle levels of salivary oestradiol in women. Women with narrow waists and large breasts have the highest oestrogen levels. Source From Jasienska et al. (2004). Members of the genus Homo, which preceded H. sapiens in Africa, had a very similar post-cranial anatomy, walked upright, and were probably subject to thermoregulatory or other selective...

Copulatory Pelvic Thrusting In Apes

Variable as human copulatory positions may be (due to cultural factors) they are fundamentally the same as those which occur in non-human primates. The, ventro-ventral, or 'missionary position' and female superior copulatory positions of humans are homologous with patterns described for the great apes. Elsewhere among the primates, distinctive copulatory postures are restricted to particular phylogenetic groupings. Examples discussed in this chapter include the inverted copulatory positions of...

The accessory reproductive glands and sperm competition

Only 5 per cent of the volume of human semen is made up of fluids contributed by the testes and epidi-dymis. Spermatozoa which leave the epididymis are transported rapidly through the vas deferens, and then mixed with the secretions of various accessory reproductive glands which provide the bulk of the ejaculate. Adult male mammals possess an impressive variety of these glands, including the seminal vesicles, prostate, and ampullary and bulbourethral (Cowper's) glands (Eckstein and Zuckerman...

Masculine secondary sexual adornments a comparative perspective

Nasalis Larvatus Skica

Many of the secondary sexual adornments that are visually distinctive and sexually dimorphic in primates are difficult to quantify. It is probably for this reason that most authors have concentrated on sex differences in more readily measurable traits, such as body weight or canine tooth size. Exact measures of masculine adornments, such as the lengths of capes of hair, beards, or crests, or quantitative data on the distribution and brightness of sexual skin colours, are difficult to obtain for...

Iiiiiiiii 2 4 6 8 10 12 14 16 18 Vaginal length

Figure 4.3 Relationship between vaginal length and length of the (erect) penis in nine species of primates, including H. sapiens. 1. Lemur catta 2. Callithrixjacchus 3. Lophocebus aterrimus 4. Mandrillus sphinx 5. Macaca mulatta 6. M. arctoides 7. Pan troglodytes 8. Gorilla g. gorilla and 9. H. sapiens. human penile length is exceptional among the primates. This is not the case. Does the thickness of the human penis have any significance as regards sexual selection during human evolution If...

Glance at the Terrain

'There is a gift of being able to see at a glance What prospects are offered by the terrain.' 'We see nothing till we truly understand it.' The purpose of these essays is to explore the evolutionary origins of human mating systems, the physical determinants of sexual attractiveness, mate choice, and patterns of copulatory behaviour. These are ambitious goals, given that sexual behaviour does not leave a fossil record. We are all the products of an unbroken chain of sexual and reproductive...

Human physique and sexual attractiveness

Somatotypes Figures

A number of studies have produced evidence that women find certain types of masculine physique more attractive sexually. A broad chest and shoulders, narrow hips, and a muscular torso are important traits which influence female assessments of masculine physical attractiveness (Lynch and Zellner 1999 Dixson et al. 2003). In order to quantify these preferences, some studies have made use of Sheldon's (1940) system of somatotyp-ing. This classifies human physique according its degree of mesomorphy...

Pliny The Elder

Sexual Dimorphism Humans

It was once thought that human evolution had progressed in a linear and stepwise fashion, from simpler to more complex forms, culminating in H. sapiens. However, as the fossil record has grown, it has emerged that a diverse assemblage of hominids existed in the past, some of which were ancestral to humans, whilst others occupied separate branches of an extensive evolutionary tree. The relationships between these fossil forms have occasioned much debate among palaeontologists and will,...

Penile morphology sexual selection and human evolution

Semen Covered Monkey Penis

It is an unfortunate circumstance that so many authors have applied hyperbole to descriptions of human penile morphology. Smith (1984) regarded the human penis as 'extraordinary relative to the other hominoids'. Baker and Bellis (like Smith) stressed that 'it is nearly twice as long and over twice as wide as that of the chimpanzee'. Jolly (1999) states that a 'peculiarity of humans is that the penis is twice the size for body weight as that of any other primate.' _ Human Gorilla -Orangutan...

Adams Apple

Membrana Thyreoidea

'It is interesting to conjecture as to the appearance and mode of life of the ancestors of present day Man, and in such a study much assistance may be derived from an examination of the larynx.' An obvious anatomical sex difference in adult humans concerns the size and shape of the larynx, with resultant differences in vocal pitch this is usually deeper in men than in women. These structural and vocal sex differences develop during puberty when, under the action of testosterone, the cartilages...