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Unique to humans

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Humans share a lot of features with other vertebrates, other mammals, and, of course, other apes. Presented here is a collection of features that in modern scientific literature are considered more or less unique to humans among all other animals or, sometimes, more specifically, among other primates. Pease note a difference between qualitative (truly unique) and quantitative (presented in varied degrees) characteristics.

Very important website: The Martix of Comparative Anthropogeny. This is a web-based collection of information comparing humans and our closest evolutionary relatives (great apes), with a specific emphasis on uniquely human features.

  1. Absense of baculum (os penis).
    Os penis, an isolated bone located above the distal end of the urethra in the penis of many mammals; it is present in most primates, carnivores, and rodents; humans differ from all Old World monkeys and apes in complete absense of the penile bone. The os penis serves to assure rapid erectile state in absence of developed erectile tissues. Comparative study of primates showed that baculum length is broadly correlated with breeding system. Briefly, the more sperm competition is expected the bigger the baculum. The smallest baculum is usually found in pair-forming primates.
    In some animals, os penis is a point of vulnerability, for example, when male otters fight, they try to seize and break external genitalia of their adversary. Can we explain absence of baculum in Homo sapiens by evolution of bipedality and upright posture? In our ancestors, who just started to walk on two hind legs, most coveted body parts, like belly and external reproductive organs, became suddenly exposed. The vulnerability of penis in bipedal locomotion and decreased sperm competition under conditions of monogamous relashionships could've applied selective pressures in favor of occurrence of genitalia less prominent at rest but greatly expandable on demand.
    Martin RD. The evolution of human reproduction: a primatological perspective. Am J Phys Anthropol. 2007;Suppl 45:59-84.; Gilbert SF, Zevit Z. Congenital human baculum deficiency: the generative bone of Genesis 2:21-23. Am J Med Genet. 2001 Jul 1;101(3):284-5.; BETT WR. The os penis in man and beast. Ann R Coll Surg Engl. 1952 Jun;10(6):405-9.
  2. Nakedness
    Although we are not literally hairless, having about the density of hair follicles expected for an ape of our body size, we are unique in that our bodily hair are so weak and short that they fail to protect our skin from sun, cold, winds, and injuries as in other animals. There are a few wild animal species that have skin unprotected by hair to the same extent as in humans. These are elephants, rhinoceroses, hippopotamuses, walruses, whales and naked mole-rats. These species are aquatic and semi-aquatic, live underground, or their skin is very tough. A hairless mutation introduced into the chimpanzee/human last common ancestor (CLCA) 6 million years ago (Mya) diverged hairless human and hairy chimpanzee lineages. What accounts for our naked state? A few theories were put forward:
    • The body-cooling hypothesis suggests that the combination of an upright posture and lack of hair made it easier to radiate heat back into environment or to lose heat in the wind.
    • The aquatic-ape hypothesis advocates semi-aquatic existence of ancestral hominins during 1-2 million period between 6 and 8 million ago. This idea is indirectly supported by antiquity of schistosomas (human-specific parasites with aquatic phase in their life cycle), wading hypothesis for development of bipedality, and distribution of subcutaneous fat depots.
    • Reduction in ectoparasite load hypothesis explains human hairlessness by selective advantages gained as a result of human ability to regulate their environments with use of shelters and fire, which afforded them to reduce hair coverage and minimize parasites that undermine fitness and transmit pathogens.

    Pagel M, Bodmer W. A naked ape would have fewer parasites. Proc Biol Sci. 2003 Aug 7;270 Suppl 1:S117-9.  
    Sutou S. Hairless mutation: a driving force of humanization from a human-ape common ancestor by enforcing upright walking while holding a baby with both hands. Genes Cells. Apr 2012; 17(4): 264–272.
  3. Propensity to vasovagal syncope (fainting) in a face of real or perceived danger.
    Freeze, Flight, Fight, Fright, Faint is one of popular descriptions of the sequence of states experienced by frightened or stressed organisms: even amoeba rectracts its pseudopodia ("freezes") when bothered. Many organisms, from ants to possums, take this initial reaction to the next level: they "play dead" when faced with inescapable life-threatening danger. While playing dead, these organisms are alert and aware of the situation, ready to take an adequate action at the earliest opportunity. This behavior is not inherent to humans but is often copied by trained individuals to the great advantage in combat, etc. "Normal" people, however, do not play dead as the animals and action heroes do, they loose their consciousness (faint), fall down on the ground where they lie in "blissful unawareness". In literature it is also often called a "welcome relief". It seems that in animal world only humans can blackout like that in a face of danger. Why? One theory is that the fainting is a protective reaction against possible PTSD (postraumatic stress disorder). What makes us so vulnerable to the PTSD that it became epidemic? Is it the complexity of our brains and psyches? Was the PTSD so deleterious to the fitness of our ancestors that the apparent disadvantage of being helpless in the critical survival moment was favored by evolution? These questions face us today, in times when it became obvious how many of us suffer from PTSDs.
    To add to the mix: can our addictiveness to drugs or our impulse to drink ourselves to stupor under stress be akin the fainting reaction? The former usually occurs under chronic stress, the latter is caused by acute fear. Maybe it is just a matter of available time?
    Acute stress responses in animals and humans, Freeze, Flight, Fight, Fright, Faint diagram
    Bracha HS. Freeze, flight, fight, fright, faint: adaptationist perspectives on the acute stress response spectrum. CNS Spectr. 2004 Sep; van Dijk JG. Fainting in animals. Clin Auton Res. 2003 Aug; Alboni P, Alboni M, Bertorelle G. The origin of vasovagal syncope: to protect the heart or to escape predation? Clin Auton Res. 2008 Aug
  4. Paradoxal addictiveness to psychedelic drugs
    Plants have to interact with outside world by producing wide range of chemicals to attract organisms to their fruits and flowers (to assist in seed dispesal or pollination) or to punish bulk eaters of vegetative parts (smell, poisons). The question is why in the vast arsenal of available weapons plants use neurotoxins so often? Why not choose something simpler, which, in many cases, can be even more effective and universal? Why neurotoxins which mess up our psyches are spread so widely? There is a lively discussion among behaviorists, psychatrists, and plant researchers about how chemically-loaded plants shaped animals in general and humans in particular. Humans react strongly and predictably to a variety of herbal drugs. The mechanism of the reaction is most likely an evolutional legacy from our herbivor ancestors. Most first-time drug users have a strong aversion reaction to most of the drugs. In majority of cases this reaction is enough to cause other animals never eat this plant again. Humans are paradoxal in a way that they will try it again and again until they finally find a method to extract a favorable mental stimulation, go on a "trip", or loose consciousness completely. The question remains whether this drive to mess up our minds by self-administration of chemicals gave us a slightest advantage in grand scheme of "survival of the fittest" theory. This enhanced "addictiveness" of humans is an intriguing question that researches still did not clarify with any certainty.
    Sullivan RJ et al. Revealing the paradox of drug reward in human evolution. Proc Biol Sci. 2008 Jun 7;275(1640):1231-41.
  5. Ability to actively colonize new habitats on a scale never shown by any other species
    As opposed to passive dispersal with subsequent adaptation as seen, for example, for rats.

    Expanding world of modern humans

    Wells JC, Stock JT. The biology of the colonizing ape. Am J Phys Anthropol. 2007;Suppl 45:191-222.
  6. Big neonates
    Human babies have brains and body fat each contributing to 11-14% of body weight, a situation which appears to be unique amongst terrestrial animals. For example, orangutan's, gorilla's, and chimpanzee's neonates are rather thin in comparison to humans: they are born with only about 2% of fat. Many mammals produce round and quite fat babies but human babies are exceptionally chubby.

    Big human baby Photo credit: Mead MN. Environ Health Perspect. 2009 January; 117(1): A33.

    Cunnane SC, Crawford MA. Survival of the fattest: fat babies were the key to evolution of the large human brain. Comp Biochem Physiol A Mol Integr Physiol. 2003 Sep;136(1):17-26.
  7. Prolonged postnatal brain growth
    This is one of the most dramatic distinction between humans and other mammals (including primates). In all precocial mammals other than humans, at around the time of birth there is distinct slowing down in brain growth relative to body growth. In altricial mammals, the switch to diminished brain growth occurs at a developmental stage comparable to birth in precocial mammals. In humans, substantial brain growth relative to body growth continues for approximately a year after birth before a marked slow-down occurs. Because of this human neonates are unusually dependent on parental care in comparison with other primates for the first year of postnatal life, and sometimes labeled as "secondary altricial".
    Martin RD. The evolution of human reproduction: a primatological perspective. Am J Phys Anthropol. 2007;Suppl 45:59-84.
  8. Extreme menstrual bleeding
    Menstrual bleeding in humans is extreme in comparison to other simians. The degree of blood loss (~35 ml on average) which may lead to temporary or persistent anemia implies on considerable selection pressure to maintain it.
    Martin RD. The evolution of human reproduction: a primatological perspective. Am J Phys Anthropol. 2007;Suppl 45:59-84.
  9. Fat depots
    Human fat depots are unique in their size and distribution, especially among females whose breast, hip and thigh subcutaneous depots are mobilized for pregnancy and lactation.
    Bellisari A. Evolutionary origins of obesity. Obes Rev. 2008 Mar;9(2):165-80.
  10. Asymmetry of the cerebral cortex
    Functional asymmetry of the cerebral cortex is one of the most-defined quantitative human-specific brain features. The fact is reflected in several human-specific characteristics, for example, predominance of right-handedness (~90%), and asummetric distribution of various cognitive functions: for example, language abilities are located in the left hemisphere.
    Creely H, Khaitovich P. Human brain evolution. Prog Brain Res. 2006;158:295-309.
    Sherwood CC1, Subiaul F, Zawidzki TW. A natural history of the human mind: tracing evolutionary changes in brain and cognition. J Anat. 2008 Apr;212(4):426-54.
  11. Learning and teaching techniques
    A typical teaching scenario is as follows: the mother first establishes eye contact with the infant, the mother than looks and points to an object and names it. The first signal (looking at the infant) is not only for attracting the infant's attention, but is also an ostensive gesture signaling that learning by instruction is about to occur. There is some evidence that this special kind of learning through instruction may be unique to humans. While apes can learn by observation, there is little evidence that adult apes use deliberate instructions and infant apes, in their turn, are able to recognize ostensive signals.
    Frith CD. Social cognition. Philos Trans R Soc Lond B Biol Sci. 2008 Jun 12;363(1499):2033-9.
  12. Language
    There are many controversies that pervade not only speculation about phylogeny of human language, but also characterization of what has evolved, the linguistic component of the human behavioral phenotype. There is at least this much agreement about human language - it is a form of communication that is unique in the natural world.
    Unlike systems of communication employed by other species, human language has the following unique characteristics:
    • Modality/stimulus independence - it can take oral, visual, gestural, and even tactile forms; same stimulus (for example, sexual drive), can be coveyed by many different, not fixed modalities.
    • Duality of patterning - human language is compositional at two levels: at the first level, neural representations of a finite set of meaningless gestures, known as "phonemes", can be systematically combined into a much larger set of meaningful units, like representations of words, called "morphemes"; at the second level, this set of morphemes can be systematically combined into an infinite set of larger meaningful units, e.g. phrases, clauses, and sentences.
    • Shared, arbitrary symbols, capable of displaced reference - human language has a capacity to refer to events and objects that are not perceptually present, like spatially or temporally distant objects and events.
    • Generalized systematicity/domain independence - firstly, human language is generally systematic: it can represent any object for which it has a term as possessing any property for which it has a term; this property gives language a boundless creativity in representing unobserved and unobservable situations, for example, cats that walk upright in boots and talk, etc.; secondly, human language is task-domain neutral; it represents information about the world as independent of any task to which it might be put (information that is of no immediate use), for example, "In Japan, cherry trees started to blossom".
    • Hierarchical/recursive structure or syntax - sentences of human language are composed of nested hierarchies of sub-sentenial units, for example, "The herd, beyond the woods, north of the plain, west of the hill is on the move", i.e. because phrases can be nested within other phrases, we can construct sentences of arbitrary length, conveying information of arbitrarily precise specificity.

    Sherwood CC1, Subiaul F, Zawidzki TW. A natural history of the human mind: tracing evolutionary changes in brain and cognition. J Anat. 2008 Apr;212(4):426-54.
  13. Immortalized communications
    Moreover, a written system of communication allows knowledge to be conveyed from one agent to another without them ever meeting each other, thus separating communication from the very stimuli that normally drive it. In this way, previous generations can influence later generations far into the future.
    Frith U, Frith C. The social brain: allowing humans to boldly go where no other species has been. Philos Trans R Soc Lond B Biol Sci. 2010 Jan 12;365(1537):165-76.
  14. Flexible behavior
    All mammals have complex behaviors but these are generally stereotyped in nature and lack flexibility of human behavior. The use of thought process enables us to be very flexible, allows us to self-reflect, to search for meaning and purpose of life, to set individual motivations and goals by envisioning benefits often in far future. This is not to say that thinking and self-motivated behavior does not occur in other animals, but if ever found and scientifically proven, their extent would be considerably less than in humans. The flexibility in behavior and decision making although of obvious evolutionary advantage also enables us to choose not to have children and to commit suicide.
    Neill D. Cortical evolution and human behaviour. Brain Res Bull. 2007 Sep 28;74(4):191-205. Epub 2007 Jul 5.
  15. Emotional crying accompanied by tears
    The human eye produces 3 types of tears:
    • basal tears, which lubricate the eyeball;
    • reflex tears, which protect eye from irritation and foreign bodies;
    • psychoemotional tears: (1) caused by negative emotions, for example, sorrow, desperation, etc.; (2) caused by positive emotions, for example, relief, happiness, help offering, etc.

    Basal, reflex, and psycho-emotional tear secretion appeared phylogenetically in sequence, and they appear ontogenically in the same order. Basal tear has existed in terrestrial vertebrates for hundreds of millions of years in urodele amphibians, and it appears in humans some months before birth. Reflex tears appeared in some amphibians and reptiles, adn they appear in humans some days or weeks after birth. Psychoemotional tearing as an appeal for help has been scientifically demonstrated only in humans, and ontogenically it appears some months after birth. Psycho-emotional offer-to-help tearing is the last phylogenetic and ontogenic acquisition, and it appears some years after birth.
    Several hypotheses have been postulated to explain evolutionary origin of psychoemotional tearing. One of them suggests that psychoemotional tearing evolved in humans to enhance communication.
    The existence of basal tear was discovered only three centuries ago. The only kind of tear our ancestors knew before developing emotional tearing was irritative reflex tear, which almost always associated with ocular pain or disease. Thus, in the same way that bared teeth became the symbol of aggression, irritative reflex tears became a symbol of suffering. Thus, in the evolutionary progress toward enriching the expression of suffering and need for help, tearing became enhanced to further express this need.
    Tearing to "offer help" (weeping to express empathy with the sorrows or happiness of others) probably evolved much later in the evolution and represents further enrichment of mental concepts and feelings based on existing pathway.  
    Fooladi MM. The healing effects of crying. Holist Nurs Pract. 2005 Nov-Dec;19(6):248-55; quiz 256-7.
    MONTAGU A. Natural selection and the origin and evolution of weeping in man. JAMA. 1960 Sep 24;174:392-7.
    Murube J. Basal, reflex, and psycho-emotional tears. Ocul Surf. 2009 Apr;7(2):60-6.
    Murube J. Hypotheses on the development of psychoemotional tearing. Ocul Surf. 2009 Oct;7(4):171-5.
    Murube J. Tear apparatus of animals: do they weep? Ocul Surf. 2009 Jul;7(3):121-7.
  16. Theory of mind
    Theory of mind is defined as the ability to attribute mental states — beliefs, intents, desires, knowledge, etc. - to oneself and others and to understand that others have beliefs, desires and intentions that are different from one's own. Currently it is controversial whether theory of mind is unique to humans because some mentalizing abilities were found in chimpanzee, corvids, elephants, and some other animals. What is not controversial, however, is that our ability to deliberately decieve and manipulate the minds of others far outstrips that of any other creature.
    Frith U, Frith C. The social brain: allowing humans to boldly go where no other species has been. Philos Trans R Soc Lond B Biol Sci. 2010 Jan 12;365(1537):165-76.
  17. Male beards
    Morris (1968), in "The Naked Ape" raised many intriguing questions. One of them is why humans, absolutely uniquely in the animal kingdom, have two sexually dimorphic hair patches that grow indefinitely. One of these patches, the beard, begins to grow during puberty, and is thickest in young adulthood and full maturity, tending to decline in old age. Although the definitive answer is still a mystery, there are at least two theories that are not mutually exclusive. Both theories make parallels between lion's mane and human beard.
    Like the lion's mane, male beard are considered to be a trait developed as a result of sexual selection providing a visual aid for identification of gender at a distance; advertising social dominance; or as being sexually attractive to women.
    What about a protective function, similar to that attributed to the lion's mane? Thick and long beard certainly can protect face and neck from agressive face-to-face confrontations accompanied with "blunt force" blows to the front and sides of the neck as well as to the area just under the nose. Both are particular targets of attack in unarmed combat techniques.
    Blanchard DC. Of Lion Manes and Human Beards: Some Unusual Effects of the Interaction between Aggression and Sociality. Front Behav Neurosci. 2010 Jan 20;3:45.
  18. Bipedalism
    The habitual orthograde human posture and locomotion using harmonic cycles of anatomical pendulums are unique among all mammals. None of extinct (except for hominins) or extant animal that is known to move on its hind feet exhibits an orthograde spine in locomotion.
    Niemitz C. The evolution of the upright posture and gait--a review and a new synthesis. Naturwissenschaften. 2010 Mar;97(3):241-63.
  19. Largest brain
    Compared to mammals as a whole, humans have the largest EQ (Encephalization quotient - brain-to-body mass ratio). The position of the human species as an outlier in the body x brain comparison is made clear if one considers that although gorillas and orangutans overlap or exceed humans in body size, their brains amount to only about one-third of the size of the human brain.
    Recent studies, however, challenged value of EQ as a predictor of cognitive abilities of a given species because it cannot be used as a proxy for the number of neurons in the brain. For example, rodent brain with 86 billion neurons, like the human brain, would be predicted to weigh 35 kg – a value that is way beyond the largest known brain mass of 9 kg for the blue whale.
    Primate brains, on the other hand, scale in size in a much more space-saving, economical manner than in rodents, in which larger numbers of neurons are accompanied by larger neurons.
    Although according to these studies the human brain is not exceptional in its cellular composition, as it was found to contain as many neuronal and non-neuronal cells as would be expected of a primate brain of its size, Homo sapiens has largest brain among primates (1,300 grams vs. only 300–500 grams in great apes), hence also containing the most neurons.
    Until the similar cellular scaling data are available for cetaceans and elephants, a burning question remains whether having much larger brains than humans, they also have much larger numbers of neurons.
    Herculano-Houzel S. The human brain in numbers: a linearly scaled-up primate brain. Front Hum Neurosci. 2009;3:31.
  20. Societal policing against adultery
    The human mating system is characterized by bi-parental care and faithful monogamy is highly valued in most cultures. Marriage has evolved as a social institution and punishment for extra pair mating (EPM) or adultery is common. However, similar to other species with bi-parental care, both males and females frequently indulge in EPM in secrecy since it confers certain gender specific genetic benefits. Stability of faithful monogamy is therefore a conundrum. Two types of measures against cuckoldry are seen in the human society, namely mate guarding, a trait shared by many species, and societal punishment if cuckoldry gets exposed which appears to be uniquely human. Although the probability of getting exposed is small its consequences are known to be severe in most human societies and exposed adulterous individuals generally receive punishment in some or the other form.
    Watve MG, Damle A, Ganguly B, Kale A, Dahanukar N. Blackmailing: the keystone in the human mating system. BMC Evol Biol. 2011 Nov 29;11:345.
  21. Children helping parents to raise younger children and working
    Unique trait of human children in majority of successful cultures is helping parents to raise younger offspring, which allowed humans to have more offspring by shortening interbirth intervals without sacrificing the offspring's survival.
    While human juveniles depend on others for some of what they need to survive, they also may be successful at procuring some of the resources they need and, indeed, some in excess of their own consumption needs. ... Children's contributions play a substantial role during the peak demographic squeeze during the mid-portion of the family life cycle when parents have seven to eight children living at home and are least able to support their dependent young. ... Differences in primate infant and juvenile mortality rates suggest that human mothers maintain relatively short birth intervals without increasing child mortality. Redistributing the cost of reproduction appears to be a key to holding in balance the human life-history combination of a relatively rapid rate of reproduction and a high probability of child survival. ... The relatively fast rate of human reproduction appears to be sensitive not only to children's self-sustaining activities but to direct help to mothers. In addition to their children, mothers receive help from their spouses, their own mothers, other female kin, and unrelated individuals. Flexibility in allocare strategies to alleviate constraints on maternal time and resource budgets under variable ecological and demographic conditions gives rise to a number of pathways to cooperative breeding. Kramer KL. Children's Help and the Pace of Reproduction: Cooperative Breeding in Humans. Evolutionary Anthropology (2005)
  22. to be continued...

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Not unique to humans

The following are features that are often misrepresented as being unique in humans but are actually shared with other animals (not necessarily even primates).

  1. Extended mating period
    It is commonly claimed that humans are unique among mammals in that copulation can take place virtually at any time in the ovarian cycle. Indeed, most female mammals exhibit restricted receptivity period of 1-3 days in each ovarian cycle known as estrus or heat. This is also generally the case with prosimian primates. However, in non-human primates (monkeys and apes) an extended mating is common. Examples of exceptions are squirrel monkeys, gorillas, and some others. While it is true that humans are extreme in showing very wide distribution of copulation across most of the ovarian cycle, an extended mating period probably developed in early simians around 40 million years ago.
    Martin RD. The evolution of human reproduction: a primatological perspective. Am J Phys Anthropol. 2007;Suppl 45:59-84.
  2. Reproductive crypsis
    The lack of any external visible sign of the time of ovulation (concealed ovulation or reproductive crypsis) is frequently claimed to be a unique human characteristic. However, sexual swellings are completely absent in prosimian and New World monkeys. Prominent swellings are found only in minority of Old World monkeys and in one ape genus (Pan). Despite of absence of visible signs of receptivity, there is evidence for both humans and non-human simians that both females and males may percieve internal ovulatory cues.
    Martin RD. The evolution of human reproduction: a primatological perspective. Am J Phys Anthropol. 2007;Suppl 45:59-84.
  3. Duchenne laughter
    Duchenne laughter is linked to genuine mirth, amusement or exhilaration and is caused by stimuli that can be collectively called nonserious social incongruity. Non-Duchenne laughter occurs in cituations than are not particularly humorous or amusing - in ordinary conversation, under stress, under embarassing circumstances, etc. This is "learned" type of laughter that can be invoked strategically as opposing to spontaneous and involuntary character of Duchenne laughter. Neurophysiological studies and brain imaging data have shown that the two kinds of laughter arise from separate neural systems. Despite of its advanced features, Duchenne laughter is not unique to humans. It is derived from the primate relaxed open-mouth display ("play face") and its associated pant-like vocalization, and so has gomologues in the other great apes. Chimpamzees cannot laugh like humans because their anatomy constrains their vocal abilities; these constraints vanished in bipedal hominids. Rudimentar laughter probably evolved at least 6.5 million years ago, before the hominid line split from the other hominoids, and most likely resembled the pant-like laughter in chimpanzees.

    Laughing chimpanzee Photo credit:

    Gervais M, Wilson DS. The evolution and functions of laughter and humor: a synthetic approach. Q Rev Biol. 2005 Dec;80(4):395-430.
    Meyer M, Baumann S, Wildgruber D, Alter K. How the brain laughs. Comparative evidence from behavioral, electrophysiological and neuroimaging studies in human and monkey. Behav Brain Res. 2007 Sep 4;182(2):245-60.
  4. Oral sex
    Although it is widely used in human foreplay, oral sex has rarely been recorded in animals and the behaviour has been considered largely specific to humans. There were few observations of fellatio in non-human animals mostly interpreted to be associated with play rather than with an adaptive behavior designed to increase a chance of fertilization because quantitative studies were not carried out. The rare examples include oral sex during play in juvenile males or between juvenile female and juvenile male bonobos and female orangutan's manipulating her mate's penis with her hand and her mouth during copulation. In general, many animals may lick genitals before and after copulation, for example, the male of ring-tailed lemur often licks the genitals of the female in order to judge whether she is in estrous, and after copulation, he also licks his penis. Both male and female Livingstone's fruit bats also licked the genitals of their partners during heterosexual interactions.
    The authors of the article below found that female short-nosed fruit bats C. sphinx lick their mate's penis regularly during copulation, and that each second of licking results in approximately 6 extra seconds of copulation. Copulations also last longer if licking occurs than when no licking takes place. These observations are claimed to be the first to show regular fellatio in adult animals other than humans that may have functional significance. Of course, adaptive benefits remain unproven until tested.
    Tan M, Jones G, Zhu G, Ye J, Hong T, Zhou S, Zhang S, Zhang L. Fellatio by fruit bats prolongs copulation time. PLoS One. 2009 Oct 28;4(10):e7595.
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