Multiple Pattern ejaculations number

YES

1

NO

2

YES

3

NO

4

YES

5

NO

6

YES

7

NO

8

YES

9

NO

10

YES

11

NO

12

YES

13

NO

14

YES

15

NO

16

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 make rapid pelvic thrusts only during the pre-intromission phase of the mount; once intromission is attained, there is a single thrust only, and not the repetitive thrusting patterns which occur in the primates.

3. Multiple intromissions characterize the copu-latory behaviour of some mammals, such as the laboratory rat, although the number of intromissions a male makes prior to ejaculation is influenced by many factors, including age, and experiential and genetic variations (Larsson 1956). In the rat, the male's intromissions evoke neuroendocrine responses in the female which are important for the secretion of prolactin and for support of the corpus luteum during the ensuing pregnancy (Adler 1978; Freeman 1994). They also serve to dislodge copu-latory plugs deposited by other males during previous ma tings (Wallach and Hart 1983). The role played by sexual selection in the evolution of multiple intromission copulatory patterns is also apparent in some primate species. In various macaques and baboons, red colobus monkeys, and red uaka-ris, males make a series of mounts with intromission and pelvic thrusting prior to ejaculation. All these monkeys have multi-male/multi-female mating systems, in which males have large testes and engage in sperm competition (Dixson 1998a). However, human beings, like male gorillas, chimpanzees, mandrills, talapoins, howlers, and many others, usually ejaculate during a single intromission with pelvic thrusting. This is a relatively simple copulatory pattern which is much more widespread among the primates than the multiple intromission pattern. Single intromission copulatory patterns may equally occur in species which commonly engage in sperm competition (e.g. chimpanzees and bonobos) and those which do not (e.g. owl monkeys and marmosets).

4. The capacity to have multiple ejaculations is the fourth criterion used in Dewsbury's scheme (Figure 5.8).

It is usual for males to exhibit a period of reduced sexual arousal once orgasm has occurred; this is the post-ejaculatory refractory period. Dewsbury pointed out that whereas males of some species cease to copulate for extended periods, others are capable of mating again and of having a second ejaculation within 60 min. Yet this criterion, which results in the division of eight potential copulatory patterns into sixteen, is highly problematic. Among the primates, for example, Dewsbury and Pierce (1989) could identify only four species, out of thirty-three considered, which were not capable of multiple ejaculations. Even these four examples are contentious, however, and in two cases (Otolemur crassicaudatus and Cerco-pithecus (Chlorocebus) aethiops) it is likely that ejaculation can occur more than once per hour (Dixson 1998a). The problem is that almost all primate species which have been studied adequately are capable of ejaculating more than once per hour, at least when they are observed in captivity. The post-ejaculatory refractory period is more labile in monkeys and apes than in many rodents and other mammals commonly studied in the laboratory. Thus, classifying humans, chimpanzees, marmosets, and rhesus monkeys as multiple ejaculators serves no useful purpose, as it obscures very real differences which exist between them as regards copulatory frequencies in relation to sperm competition. The evolution of copulatory frequencies in humans and in other primates will be considered in the next section.

Unfortunately, none of the four criteria defined above are adequate to address the question of copulatory durations and their evolutionary basis. Whilst it is true that species which exhibit a genital lock tend also to mate for extended periods, it is the case that extended matings also occur in many mammals which lack a genital lock. Intromission duration is an important trait and cannot be ignored if we wish to understand the evolution of mammalian copulatory patterns. It has been argued that long intromission durations originated in nocturnal, ancestral mammals and may represent a conservative trait for the Class Mammalia as a whole (Ewer 1968; Eisenberg 1977; 1981). Traditionally, mammals have also been classified as being either induced ovulators or spontaneous ovu-lators (Asdell 1964). One notion uniting the evolution of copulatory and ovulatory mechanisms is that induced ovulation may also represent a primitive trait among mammals. Thus, prolonged matings, in early nocturnal mammals, may have been adaptive in stimulating the surge of luteiniz-ing hormone required to trigger ovulation (Eisenberg 1981). It has to be kept in mind, however, that among extant mammals which are induced ovula-tors, some species copulate only for brief periods (e.g. the rabbit and domestic cat), whilst others engage in prolonged intromissions (e.g. the ferret and other mustelids). Some authors stress that rather than a rigid dichotomy a continuum exists among mammals in the expression of induced and spontaneous ovulation (Conaway 1971; Weir and Rowlands 1973), so that both mechanisms may be expressed within a single species (e.g. among hystricomorph rodents: Weir 1974).

I emphasize that these interesting hypotheses still remain largely untested, due in part to the limited information on relationships between copulatory stimuli and ovarian physiology in the 4,500 species of extant mammals. For an authoritative discussion of mating-induced ovulation and its evolution in mammals, the reader is referred to the review by Kauffman and Rissman (2006). In order to make Dewsbury's scheme more applicable to assessments of evolutionary questions, it is important to include the presence (or absence) of prolonged intromission as a copulatory trait. Therefore, in Figure 5.9 prolonged intromission (defined as an intromission lasting for more than 3 min) replaces the largely redundant category of 'multiple ejaculations'. Sixteen copulatory patterns are thus defined, the same number as in Dewsbury's (1972) original classification. Just four of these copulatory patterns (nos. 3, 10, 11, and 12) are represented among extant primates, as listed in Table 5.1.

Prolonged single intromission patterns of mating, including a genital lock (pattern no. 3) or lacking a lock (pattern no. 11) occur in various nocturnal lopulatory Lock?

Pelvic thrusts during intromission?

Multiple intromissions?

Prolonged intromissions?

Pattern number

Examples

YES

1

NONE

NO

2

NONE

YES

3

Greater galago, Stump-tail macaque

NO

4

NONE

YES

5

NONE

NO

6

NONE

YES

7

NONE

NO

8

NONE

YES

9

NONE

NO

10

Rhesus macaque, Red colobus

YES

11

Woolly spider monkey, Orangutan

NO

12

Common marmoset, talapoin, chimpanzee

YES

13

NONE

NO

14

NONE

YES

15

NONE

NO

16

NONE

Figure 5.9 A modification of Dewsbury's scheme for classifying masculine copulatory patterns, to include intromission duration as an important evolutionary trait. Source: After Dixson (1998a).

Table 5.1 Classification of primate copulatory patterns, according to the criteria laid down in Figure 5.9

Pattern no. 3

Possible genital lock: a single prolonged intromission and pelvic thrusts Prosimians

Otolemur garnettii O. crassicaudatus Galagoides demidoff Daubentonia madagascariensis

New World monkeys

None described

Old World monkeys

Macaca arctoides

The apes and man

None described

No genital lock: multiple brief No genital lock: a single prolonged No genital lock: a single brief intromissions and pelvic thrusts intromission and pelvic thrusts intromission and pelvic thrusts

None described

Cacajao calvus Saimiri sciureus* Leontopithecus rosalia

Colobus badius Macaca silenus M. nemestrina M. nigra M. maurus M. fascicularis* M. mulatta M. fuscata M. thibetana Papio ursinus* P. hamadryas

None described

Microcebus murinus Galago moholi Arctocebus calabarensis Loris tardigradus

Nycticebus coucang

Lagothrix lagotricha Ateles belzebuth A. geoffroyi

A. fusciceps Brachyteles arachnoides

None described

Pongo pygmaeus

Lemur catta Varecia variegata Cheirogaleus major Tarsius bancanus

Callithrix jacchus Saguinus oedipus Cebuella pygmaea

Callimico goeldii Callicebus moloch

Aotus lemurinus Cebus nigrivittatus Alouatta palliata

Nasalis larvatus Miopithecus talapoin Erythrocebus patas Chlorocebus aethiops Mandrillus sphinx Cercocebus albigena Theropithecus gelada Papio anubis Macaca radiata M. sylvanus

Pan trogolodytes P. paniscus Gorilla gorilla Homo sapiens

*Pattern no. 12 also occurs in these species.

Source: Data are from Dixson (1998a) and references cited in the text.

prosimians (such as galagos, angwantibos, lorises, and mouse lemurs), and in a few large-bodied diurnal anthropoids, most of which are highly arboreal (e.g. spider monkeys, woolly monkeys, and orangutans). The presence of prolonged patterns of intromission in nocturnal prosimians may indeed represent a primitive trait, retained from nocturnal, arboreal stem forms of the Order Primates. Many of the extant species are non-gregarious, and the sexes associate only infrequently, during the female's oestrus. Prolonged patterns of mating may be advantageous on several levels: as a mate-guarding tactic by males; as a means of inseminating maximum numbers of sperm and gaining an advantage in sperm competition; or to impart tactile stimuli which influence female fertility (Dixson 1995a; 1998a). Natural selection as well as sexual selection may have played a role in the evolution of prolonged matings (patterns 3 and 11) in the nocturnal primates. Such prolonged matings are absent in all small-bodied diurnal primates, and virtually absent in terrestrial species (the exception being the stump-tail macaque: Table 5.1). It is thought that predation risk may have selected against prolonged matings in such cases (Dixson 1991).

Among the anthropoids, the occurrence of prolonged patterns of intromission almost certainly represents a derived condition, rather than a primitive retention from nocturnal ancestors. In the atelines (spider monkeys, muriquis, and woolly monkeys), it is also associated with the occurrence of the specialized 'leg lock' copulatory pattern described earlier, and with multiple partner mat-ings and sperm competition in social groups. The case of the orangutan is intriguing, as one wonders whether the dispersed nature of their social system, and infrequent contact between the sexes, may have facilitated the evolution of prolonged copulations during their consortships. All the primates are considered to be spontaneous ovula-tors. However, given the possibility of gradations between induced and spontaneous ovulation in mammals, it would be premature to deny that this might occur in orangutans and some other species. In greater galagos and ringtailed lemurs, for example, copulation is associated with a shortening of the female's receptive period (Dixson 1995a). In both these prosimians, males have very large penile spines. Stockley (2002) has produced evidence that the possession of such spines in male primates is associated with shorter durations of sexual receptivity in females.

Pattern number 10, which involves multiple intromissions with pelvic thrusting by males, occurs in a number of monkeys in which the sexes live in large social groups and in which large, relative testes sizes and sperm competition pressures are pronounced. Examples include many of the macaques and baboons (Table 5.1). This is not intended to imply that multiple intromission patterns of copulation are the norm in multi-male/multi-female primate groups, however. The reason why this pattern is present in some species, but not in others where females engage in multiple-partner matings, has not been explained. It is the case, however, that where multiple intromission copulations (pattern no. 10) and prolonged single intromissions patterns (nos. 3 and 11) do occur, they are much more likely to be found in species which engage in sperm competition (i.e. in multi-male/multi-female and dispersed mating systems) than in monogamous and polygynous species (Figure 5.10).

In the fourth and final primate copulatory pattern, males make relatively brief single intromissions with pelvic thrusting to attain ejaculation. This pattern (no. 12 in Figure 5.9 and Table 5.1) is shared by a large number of primate species, including some which have multi-male/multi-female mating systems (e.g. Pan, Mandrillus, and some of the macaques) as well as polygynous forms (e.g. Gorilla, Theropithecus) and monogamous New World monkeys, such as Callithrix jacchus and Saguinus oedipus. This pattern is therefore widespread among diurnal primates and appears to have been relatively little affected by sexual selection. It is more likely that natural selection and the requirement to avoid predators has led to shortening of intromission durations among the diurnal primates as a whole (Dixson 1991). Intromission durations in the pattern number 12 group of species vary from less than 10 s, (e.g. in C. jacchus and P. troglodytes), up to 1.5 min in the gorilla, and 2 min or less in human males. Further examples of brief intromission durations are given in Table 5.2, together with those exhibited by primates in which copulations are usually much more prolonged.

There are convincing reasons to include H. sapiens in the pattern number 12 category, as the fundamental human copulatory pattern involves relatively brief intromission times. This conclusion emerges from a consideration of the data collected by Kinsey, Pomeroy, and Martin (1948), as well as from cross-cultural studies that indicate that a large degree of cultural conditioning is involved in those societies where men copulate for extended periods.

Multiple brief I I Single prolonged

Multi-male / Multi-female Dispersed Polygynous

Mating system

Monogamous

Figure 5.10 Relationships between the occurrence of complex copulatory patterns (involving multiple brief intromissions, or single prolonged intromissions) and mating systems in primates.

There is also the interesting fact that the propensity for males to ejaculate quickly is identified as a common problem in clinical studies, where it is usually referred to as premature ejaculation. This phenomenon is more likely to occur in younger men. Masters and Johnson (1966), for example, comment that

Problems of premature ejaculation disturbed the younger members of the study-subject population. These fears of performance were not associated with problems of erection; rather they were directed towards the culturally imposed fear of inability to control the ejaculatory process to a degree sufficient to satisfy the female partner.

That these fears are partly culturally induced is supported by the observation that they tend to be greater in men who are more educated and aware that rapid ejaculation should be viewed as a problem (Masters and Johnson 1966; 1970). Bancroft (1989) comments that 'The impulsive, inexperienced young man may ejaculate rapidly and enjoy the experience.' Control develops as a result of greater experience, and 'because of the dampening effects of the ageing process and also because of the lessening of novelty that comes particularly with a stable relationship.' Bancroft also points out, however, that 'It may be that for some men this

Table 5.2 Intromission durations in primates: distinguishing between single brief intromission (SBI) and single prolonged intromission (SPI) copulatory patterns

Species

Intromission duration (s)

Copulatory pattern

Tarsius bancanus

60-90

SBI

Cebuella pygmaea

4-10

SBI

Callithrix jacchus

Mean 5 or 6

SBI

Alouatta palliata

Mean 32

SBI

Saimiri sciureus*

<30

SBI*

Nasalis larvatus

Mean 41.5

SBI

Miopithecus talapoin

<60

SBI

Cercopithecus

<60

SBI

aethiops

Cercocebus albigena

Mean 13.9

SBI

Mandrillus sphinx

<60

SBI

Macaca radiata

<60

SBI

Mean 10

Pan troglodytes

Mean 7.0

SBI

Mean 8.21

Pan paniscus

Mean 15.3

SBI

Gorilla gorilla

med 96

SBI

Homo sapiens

<120

SBI

Microcebus murinus

>180

SPI

Daubentonia

Mean 3720

SPI

madagascariensis

Otolemur garnettii

780-15600

SPI

G. moholi

420-720

SPI

Galagoides demidoff

3600

SPI

Arctocebus

240t

SPI

calabarensis

Loris tardigradus

600-1020

SPI

Nycticebus coucang

180-420

SPI

Brachyteles

100-496

SPI

arachnoides

Ateles belzebuth

480-1500

SPI

A. paniscus

Mean 600

SPI

Lagothrix lagotricha

180-840

SPI

Macaca arctoides

>180

SPI

Pongo pygmaeus

med 840

SPI

max 2760

'Multiple intromissions also occur in Saimiri sciureus. tOne observed copulation only.

'Multiple intromissions also occur in Saimiri sciureus. tOne observed copulation only.

Source: From Dixson (1998a), and based upon multiple sources cited there. Abbreviations: med = median duration; max = maximum duration.

aspect of autonomic function is peculiarly difficult to control—they have over-excitable and too easily triggered "ejaculation centres." '

Let us assume for a moment that the propensity of many human males to ejaculate quickly, once intromission occurs, is not abnormal but is indicative of an evolutionary background of relatively brief sexual intercourse in human ancestors. Given that the australopithecines, as the precursors of humans, were relatively small bodied, largely terrestrial, and diurnal in habit, it is possible that natural selection (predation pressure) also selected against prolonged matings. However, with the evolution of the genus Homo, the tendency for intercourse to occur in greater seclusion, at night, and in more secure surroundings might then have facilitated the cultural emergence of lengthier patterns of intercourse, and associated patterns of foreplay between the sexes. Prolonged sexual intercourse is probably not the result of sexual selection during human evolution, but represents instead a culturally based phenomenon. Many aspects of human behaviour have been modified by culture; it should not surprise us that the duration of sexual intercourse might be determined by cultural factors. Among the non-human primates, by contrast, the prolonged type of copulatory pattern has been greatly influenced by both sexual and natural selection. None of the non-human primates which copulate for extended periods have monogamous or polygynous mating systems, however, and this fact distinguishes them from the human condition.

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