Masturbation

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True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
Speculative Difference
Human Universality: 
Population Universal (Some Individuals Everywhere)
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Masturbation, or autoerotic manipulation of the genitalia, is a ubiquitous practice across human cultures, though most empirical research on the subject is heavily biased toward Western sexual behavior. Though masturbation occurs in myriad animals, human masturbation is typically coupled with imagined or observed sexual behavior, which may introduce a unique quality to the human manifestation of this behavior, particularly given the exceptionality of human imagination. Human masturbation is also likely more ubiquitous compared to other species, and may display distinct patterns, though social taboo limits access to extensive data on the subject.

Research on masturbatory practice by non-human animals almost exclusively focuses on male masturbation. Such studies have nonetheless observed autoerotic behavior in a diverse array of mammalian species. Masturbation appears to be particularly prominent in several non-human primate species, but is largely restricted to Old World monkeys and apes, while remaining rare in New World monkeys and absent in prosimians. In the few New World species that have been observed masturbating, ejaculation during masturbation is exceedingly rare. While ejaculation has been observed during masturbation in nearly all Old World anthropoids (i.e. 91% of cercopithecoids and 100% of hominoids), ejaculation from masturbation has only been observed in one New World genus: Brachyteles, or muriqui (spider monkeys; n.b. Brachyteles, despite classification as a New World genus, are unique among New World primates in their maintenance of a highly interactive/cooperative, bonobo-like social structure). Importantly, there was no significant difference in masturbatory practices in captive or semi-captive and wild primates, insinuating autoeroticism as naturally occurring in many non-human primate species. Bouts of masturbation occurred most frequently in species possessing multi-male multi-female breeding systems and those with large testis volume relative to body size.

Though relatively poorly characterized, female masturbation has been observed in some non-human primate species, including chimpanzees, orangutans, golden lion tamarins, and Goeldi's monkeys, though the latter two (New World) cases were exclusively observed in captive groups. This practice often took the form of self-stimulation of the clitoris and perineal region using either the fingers or tail. In some rare cases, orangutans of both sexes have been observed fashioning leaves or twigs for use in genital stimulation. This is significant, as the use of objects to aid in masturbation appears exclusive to Old World apes and humans, and may reflect a unique cognitive component of the behavior. It should be noted that male dolphins, which demonstrate remarkable cognitive abilities, have also been observed utilizing objects such as the tank floor and the bodies of other males for genital self-stimulation.

Timing

Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are defined in 2017 as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25,000 - 30,000 thousand (25 - 30 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6,000 - 8,000 thousand (6 - 8 million) years ago
  • the emergence of the genus Homo was 2,000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 500 thousand years ago
  • the common ancestor of modern humans was 100 - 300 thousand years ago

Probable Appearance: 
150,000 thousand years ago
Definite Appearance: 
35,000 thousand years ago
The Human Difference: 

Human masturbation is typically coupled with imagined or observed sexual behavior (e.g. pornography), while masturbation in other primates typically occurs in isolation or independent of the sexual behavior of others, and likely does not involve developed mental sexual imagery given the presumed human uniqueness of this capacity. Human masturbation also seems to be more ubiquitous across individuals, and may display different patterns compared to other primates, though data on this subject is limited.

Universality in Human Populations: 

Masturbation is a common practice in all human populations. Though some form of autoerotic genital manipulation may likely be universal across individuals, sufficient data does not currently exist to stake this easily-falsifiable claim.

Mechanisms Responsible for the Difference: 

Human capacities for imagination/fantasy and empathy may play significant roles in the integration of sexual imagery into masturbation practice, the latter allowing for sexual arousal and satisfaction consequent to observed sexual behavior

Possible Selection Processes Responsible for the Difference: 

Whether or not masturbation confers a direct fitness advantage or emerged as an evolutionary byproduct is subject to debate. Myriad theories exist as to the adaptive function of masturbation, but three main hypotheses have been explored most extensively. The ejaculate-quality-improvement (EQI) hypothesis contends that masturbation eliminates degraded gametes and avoids polyzoospermia (excessive sperm concentration in ejaculate; decreases fertility due to compromised sperm motility and migration), increasing the quality of ejaculate and thus improving the probability of impregnation upon mating. The STI-reduction hypothesis suggests that masturbation cleanses the male reproductive tract, decreasing the likelihood of contracting sexually transmitted infections. Finally, the sexual-outlet hypothesis argues that masturbation does not provide a direct fitness benefit at all, but rather arose as a non-adaptive byproduct of neuroendicrine specializations for increased sexual arousal and performance, necessitating an alternative outlet to copulation when mating partners are not available.

Studies of male rhesus macaques in the wild correlated frequency of masturbation with low mating opportunity; regardless of rank, males were most likely to masturbate on days in which they did not mate, and lower-ranking males masturbated more frequently than higher-ranking males. Patterns of masturbation in Japanese macaques seemed to depend on preferred mating strategy: Guarders, which tend to be higher-ranking males who mate repeatedly, often consecutively, with the same females, exclusively masturbate only on days when no estrus females are available for mating. Conversely, sneakers, which tend to be lower ranking males who mate opportunistically and often covertly, regularly masturbate prior to mating. The latter practice may serve to eliminate lower quality ejaculate prior to mating, as theorized by the EQI hypothesis. Examination of ejaculate quality between these groups indicates larger volume and total sperm number, but also a larger presence of degraded or slow-swimming sperm, in guarders, in contrast to smaller but higher-quality ejaculate yields from sneakers. These results suggest that masturbation may be a mechanism by which lower ranking, opportunistically mating males more effectively capitalize on limited mating opportunities by increasing their quality of ejaculate and consequent probability of impregnation. Indeed, DNA-paternity-exclusion analysis of the population observed for this study revealed six of nine babies within the troop to be sired by sneakers.

Despite similar masturbatory practices, studies in rhesus macaques determined that only 15% of masturbation events ended with ejaculation, casting doubt on the EQI hypothesis as the sole explanation for the widespread emergence of autoerotic behavior. Furthermore, red colobus monkeys, another species of Old World monkey, masturbate very rarely, and autoerotic behavior is performed almost exclusively by alpha males during intergroup encounters. Male Cape ground squirrels, a highly promiscuous squirrel species, have been shown to masturbate primarily after copulation, and far more frequently on days of female estrus. This pattern aligns more logically with the STI-reduction hypothesis than the EQI hypothesis. In short, current hypotheses on the adaptive function of masturbation appear insufficient to universally address this question, and patterns of masturbation most relevant to human autoerotic behavior have not yet been resolved. Masturbation may be differentially adaptive depending on species, or may have emerged independently of any direct fitness benefit. Further research is necessary to better resolve the functional role of masturbation in humans and other animals. More comprehensive research into female autoerotic behavior in non-human animals may also better inform the emergence of this behavior.

Implications for Understanding Modern Humans: 

Sexual behavior is an extremely significant aspect of the lives of all sexually reproductive animals, including humans. Masturbation is an enigmatic yet highly ubiquitous practice, as it is a non-reproductive sexual behavior. Understanding the origins of masturbation may shed light on its evolutionary function and its consequent conservation in human behavior, elucidating possible explanations for some of our most primal innate behaviors.

Occurrence in Other Animals: 

Autoerotic behavior has been observed in a diverse array of mammalian species, including dolphins, lions, bats, deer, zebras, walruses, sheep, warthogs, hyenas, and elephants.

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Self-Injury Likely