<p>Chimpanzee reproduction is very complex, and many misconceptions arose early in the study of these animals about the nature of their mating system. Both males and females are known to mate with multiple partners, so they can be considered polygynandrous. However, at times a male may control sexual access to a female, preventing other males from mating with her. A male may do this either through force and dominance in a group mating situation, or by taking the female on a consortship away from other males and thereby securing exclusive sexual access to her. Each of these situations will be discussed at length below.<span> (Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p>It is important to note that copulation may serve a number of social functions in this species. Females and males mate more often than would be necessary to ensure impregnation. Copulation may help to develop bonds between males and females. It may funtion in establishing and maintaining group unity.<span> (Goodall, 1986)</span></p> <p>Females have an estrus cycle which lasts approximately 36 days. During the course of this cycle, as her hormone levels change, a female experiences changes is the size, shape, and color of her genital skin. As circulating estrogens increase during the follicular phase of the cycle, the size of the swelling increases. When the anogenital skin is fully engorged, it is typically bright pink, and can measure from 938 to 1400 cc. The state of maximal tumescence is of variable length in different individuals and at different stages of maturity, but lasts an average of 6.5 days. It is during this time that females are sexually receptive and that the bulk of copulations with mature males occur.<span> (Goodall, 1986)</span></p> <p>The anogenital swelling of females is very important in the sexual behavior of these animals. Most copulations involving mature males and females (96.2%) seen at the Gombe Stream National Park in Tanzania were observed with females who were maximally tumescent. Of the few copulations observed when females were not maximally swollen, almost 75% were performed by one of two adult males, indicating that the propensity to copulate with females who are not at their peak swelling may be something of an individual idiosynchracy.<span> (Goodall, 1986)</span></p> <p>The role of the anogenital swelling is many-fold. First, it serves as a signal, visible to males from a great distance, that a female is sexually receptive. Since females tend to be relatively solitary, advertizement of their sexual state to potential mates is essential for reproduction to occur. Males are very interested in the condition of the genitals of each female they encounter. Second, the anogenital swelling may aid females in obtaining food resources including meat. Females who are maximally swollen are often able to supplant more dominant animals at a food source, and are more successful at begging food from males than are unswollen females. Finally, because the males find sexual swellings so attractive, having a maximally engorged genital region may help stranger females to interact peacefully with unfamiliar males as they disperse into new areas.<span> (Goodall, 1986)</span></p> <p>There are several possible mating scenarios that males and females may encounter. Each of these is based in part on the phase of the female’s cycle. A female may experience one or more of these scenarios during a particular cycle. The types of situations she encounters depend upon a female’s popularity as a sexual partner, how many other females are in estrus at the same time, how popular those females are, and how attractive the female is to the dominant male.</p> <p>First, during early tumescence, females are mated by infants, juveniles and early adolescents. Infants and juveniles are probably gaining experience through the copulation, they are unlikely to sire offspring. Mature males do not typically copulate with females until they are maximally tumescent, although exceptions to this rule have been observed.<span> (Goodall, 1986)</span></p> <p>In the second sexual scenario, a female who has achieved maximal tumescence becomes the nucleus of a multi-male party. Other estrus females may travel in the same sexual party. These parties can include some or all of community males. During this phase of a female’s cycle, mating can be promiscuous. The males are typically not comepetitive in this situation, and different males may mate with the female in rapid succession.<span> (Goodall, 1986)</span></p> <p>The third situation a female might encounter occurs during the second half of maximal tumescence. As the timing of ovulation approaches, dominant males may become possessive and prevent subordinates from copulating with the female. This may involve outright conflicts or, because the dominance relationship between males is well established, may be as simple as the dominant male maintaining close proximity to the female, thereby communicating to his subordinates that the female is no longer up for grabs. Inhibition of the copulations of other males may also occur through threats or attacks. Interestingly, these attacks and threats are often directed at the female, should she express sexual interest in another male. Directing aggression toward the female benefits the male in several ways: 1) it prevents potentially costly fights with other large males, 2) it teaches the female not to copulate with anybody else, and 3) it prevents a third male from mating with the female while the possessive male is punishing another sexual rival. If the possesive male is the highest ranking male in a party, he can inhibit copulations between the female and all other males.<span> (Goodall, 1986)</span></p> <p>The result of this restriction of mates late in the course of maximal tumescence has the effect of reducing the number of potential sires for any offspring conceived during that estrus cycle. Since sperm remain viable in the fallopian tubes for 48 hours, only males copulating with a female during the last four days of her swelling could fertilize an egg. Even though a female may mate with many males during any particular cycle, not all of these matings have the potential of resulting in impregnation.<span> (Goodall, 1986)</span></p> <p>The fourth sexual mating situation is the consortship. During a consortship, the female may be led away from the group by a particular male. When consorting, male/female pairs often move to the periphery of the community range. Pairs can stay together up to 3 months. During consortship, both members of the pair maintain relative vocal silence, helping to avoid the attention of other community members, as well as attention from the males of neighboring communities who might behave with hostility toward the pair. Consortships inherently involve the cooperation of female.<span> (Goodall, 1986; Nowak, 1999)</span></p> <p>Whether males engage in any of these sexual scenarios is highly variable among individuals. A male's preference for, or success in, group mating versus consortships may change depending upon the rise and fall of his fortunes in the constant struggle for dominance between community males. Males who are actively moving up the dominance heirarchy may not spend time away from other males frequently, as in doing so, they may sacrifice social status. Such males, who are in their prime, are more likely to be able to monopolize sexual access to a female in a group situation. High ranking males, especially the alpha male, may take females on consortships, but because of the need to maintain their social standing, these consortships tend to be short in duration.<span> (Goodall, 1986)</span></p> <p>It may benefit lower-ranking males to initiate consortships when possible, as in consortships there is no mating competition from other males. This may represent the most likely chance the male has to sire offspring. However, it is harder to entice a female to come on a consortship if she is close to ovulation because of competition/possessiveness of other males. A female may benefit from consortships by being able to choose the male with whom she mates. There may also be better access to food and reduced aggression during a consortship as compared to a group mating situation. However, these benefits must be weighed against the potential cost of encountering hostile neighboring chimpanzee groups when spending time in the periphery of the range.<span> (Goodall, 1986)</span></p> <p>Most consortships (40%) at Gombe Stream National Park were initiated when a female was maximally swollen. Only 16% of consortships were intiated when the female was at variable tumescence, and even fewer consortships began when females were flabby (12%) or pregnant (12%).<span> (Goodall, 1986)</span></p> <p>To initiate a consortship, a male may gaze toward the female he desires to consort with. This is often accompanied by piloerection (fluffed-out hair), branch shaking, arm stretching, and rocking. If the male succeeds in getting the female to follow him away from the group, he will often walk while looking over his shoulder to make sure she’s still tagging along. This sequence of behaviors may be repeated until the female follows him. If the female does not comply with the male’s wishes, he may become hostile, using aggression to force her to follow him.<span> (Goodall, 1986)</span></p> <p>During just over half of fertile cycles, females are confined to multi-male groups. About 21% of fertile cycles occur on consortships. The remaining 15% of cycles occur when young females visit males in other communities. In spite of the numbers of fertile cycles which fall under each mating situation, females are disproportionately likely to conceive during consortships. The exact mechanism of this is not understood.<span> (Goodall, 1986)</span></p> <p> <strong>Male mate choice</strong> </p> <p>Because males become possesive of females only late in the course of maximal tumescence, it appears that they have some ability to discern the fertile period of females. The ability of male chimpanzees to gauge the potential fertility of a given female can unquestionably be inferred from patterns of copulations. The increase in the copulation frequency of dominant or older males as ovulation approaches demonstrates that males do not respond the same to females throughout the duration of maximal tumescence. Copulations increase as fertilization and impregnation become more likely. In addition, females who were presumed to be undergoing nonfertile cycles (such as during pregnancy and early in the postpartum period) are typically not sexually popular with mature males.<span> (Goodall, 1986)</span></p> <p>Aside from potential fertility, one characteristic involved in male mate choice is the age of the female. When presented with two receptive females, males typically show a preference for copulating with the older of the two. Personality traits of individual females may also contribute to males favoring them. A female who is relaxed in the presense of males may be prefered over a more skittish female. Novelty can also play a role in attracting males, since they seem to prefer unfamiliar females over those with whom they have longstanding relationships.<span> (Goodall, 1986)</span></p> <p> <strong>Female mate choice</strong> </p> <p>Females have some ability to choose the males with whom they mate. They may choose to accept or decline a male’s invitation to consortship. This may allow a female to ensure that a particular male who is low in dominance standing, and therefore is less successful in group mating competitions, sires her offspring. The characteristics of males with whom females consort may vary. It seems that the overall “friendliness” of a female’s relationship with a male may play some role in her choice of him as a consortship partner. Whether the male has played with her, groomed her, or engaged in other friendly behaviors with her as she matured or when she is not maximally swollen, may play some role.<span> (Goodall, 1986)</span></p> <p>Although consenting to a consortship clearly demonstrates choice on the part of the female, it should not be assumed that by staying in a multi-male mating party as ovulation approaches, a female is relinquishing her mate choice. She may be choosing to mate with particular dominant individuals. Or, she may be enhancing her social status and familiarity to all the community males by remaining in the group.<span> (Goodall, 1986)</span></p> <p>That females discriminate between various males in mating situations is clear. Females avoid copulations with their mature sons and their brothers. There is also some evidence that young females avoid copulations with the older males in their communities (who may potentially have sired them). Although matings do occur between siblings, and occasionally between mothers and their mature sons, the frequency of such matings is much less than would be expected by random pairings of adults within the community.<span> (Goodall, 1986)</span></p> <p> <strong>Initiating a copulation</strong> </p> <p>Copulations are typically initiated by males. The male sits in what is called the “male invite” posture, with his legs flexed and slightly splayed. This displays his erect penis to a potential mate. A male chimpanzee’s penis is bright pink, thin, and tapered to a point. It is very visible against the black hair and pale skin on the male’s lower abdomen and thighs. The value of the erect penis as a signal may be enhanced as the male “flicks” it-- causing the penis to make a rapid “tapping” movement.<span> (Goodall, 1986)</span></p> <p>In addition to displaying his penis, a courting male may show piloerection (fluffed-out hair). A male may gaze directly at a female. Such a gaze directed at a male rival is an unambiguous threat, but in a sexual context appears to serve as an invitation. He may place his raised hand on a branch overhead, and he may shake the branch. This is all a low-key invitation to the female to present her hindquarters to the male for copulation. If he fails to attract the notice of the female, he may incorporate one or more of the following behaviors into his display: arms outstretched toward the female, a bipedal swagger, a sitting hunch, side to side rocking, swaying of vegetation, or stamping with the foot or knuckes.<span> (Goodall, 1986)</span></p> <p>Copulation usually occurs in a squatting position after the female crouches and presents her rump to the male. Often, there is no contact between the participants in the mating except at their genitals, although sometimes the male may hold the female.<span> (Goodall, 1986)</span></p> <p>Ejaculation is usually achieved within 8.8 thrusts. The copulation is ended as the male scoots back or the female darts forward. Males and females have been seen to clean themselves with leaves after copulating. Females not infrequently consume the vaginal plug (congealed semen) after mating.<span> (Goodall, 1986)</span></p> <p><strong>Mating System: </strong>Polygynandrous (promiscuous); Cooperative breeder</p> <p>There is no clear seasonality in the reproduction of chimpanzees. Females cycle throughout the year, and males copulate with them when they are receptive. Females may have infertile cycles, such as are seen during the period of adolescent sterility, during pregnancy, and early in the postpartum period. Males copulate with females during infertile cycles as well as during fertile cycles. This indicates that there are social functions other than reproduction related to sexual behavior in these animals.<span> (deWaal, 1982; Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p>The female reproductive cycle lasts an average of 36 days. As hormone levels change during their cycle, so does the size of the female’s anogenital swelling. There are four main phases to the cycle, including <em>inflation</em>, as the size of the swelling increases, <em>maximal tumescence</em>. when the sexual skin is fully distended, <em>detumescence</em>, when the previously swollen skin looses all turgidity, and <em>flat</em>, when there is no sign of swelling in the anogenital area. Menstruation occurs about nine days after detumescence begins, and lasts for about three days. Ovulation typically occurs on the last day of maximal tumescence.<span> (Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p>Females do not reproduce frequently. There is a prolonged period of juvenile dependence during which the offspring relies on the mother for milk, protection, and education. Becuse of the care required by a single offpspring, females cannot produce offspring frequently. The duration of the interbirth interval varies from population to population. Some of the variability may be due to ecological factors (highly productive habitats may allow females to wean their young sooner, or may result in higher rates of infant survival, both of which would affect interbirth intervals). Because different populations may also represent different subspecies, genetic differences in the timing of reproduction may also be involved.<span> (Goodall, 1986)</span></p> <p>Average interbirth intervals range between 3 and 6 years. Gestation lasts from 202 to 260 days, with a mean of 230 days. Typically, a single young is born, weighing about 2 kg. Twinning is rare, but may be more common than in humans. The infant is carried ventrally by the mother for about 3 to 6 months, after which infants may either ride on their mother’s ventrum or on her back. As the young chimp grows, it increasingly rides on its mother’s back during travel. Although young chimps sometimes walk on their own, they regularly ride on mom until the time of weaning at 3.5 to 4.5 years.<span> (Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p>The age of independece is somewhat difficult to judge in this very social species. Young chimps can survive without their mother after they are weaned. However, orphaned chimps are often “adopted” by an older sibling or another close relative, who provides the young chimp with care similar to that which the mother would provide. Young typically travel all the time with their mother until they reach puberty. At puberty, females may become the focus of sexual parties, and males become very interested in establishing themselves in the dominance hierarchy. The activities of maturing chimps around the age of 10 years lead to the parting of ways between a mother and her son.<span> (Goodall, 1986; Nowak, 1999)</span></p> <p>Females and males enter puberty around the age of 7 years. Females experience a period of adolescent sterility of about three years, during which they cycle, but do not ovulate. During this period, females may transfer into a neighboring chimpanzee community.<span> (Goodall, 1986)</span></p> <p>As in humans, there is a great deal of variability between different populations and between individuals within populations in the timing of first birth. Female chimpanzees in the wild give birth to their first offspring between the ages of 11 and 23 years. In Tanzania, the average age at which a female first gives birth is between 14.5 and 15 years. Captive chimpanzees reach sexual maturity at younger ages, and have been known to have babies at ages as young a 7.5 years. However, even for well-fed captive animals, the average age at which a female has her first offspring is between 10.5 and 11.15 years.<span> (deWaal, 1982; Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p>Like females, males enter puberty around the age of 7 years. Males of any age, including infants, may mate with females, but these copulations are unlikely to result in impregnation of the female. It is not until males attain social maturity that they can effectively compete for access to females who are fertile. In the wild, males are first seen to ejaculate around the age of 9 years. They do not reach adult weight and social maturity until they are about 15 years of age.<span> (Goodall, 1986; Nowak, 1999)</span></p> <p>In general, chimpanzees can be classified into age categories that represent developmental stages. Until the age of 5 years, chimpanzees are infants. From 5 to 7 years of age, chimpanzees are called juveniles. From 7 to 10 years of age, females are called adolescents. Similarly aged males, from 7 to 12 years are also called adolescents. Females aged 10 to 13 years are considered subadults, as are males aged 12 to 15 years. Females are considered fully adult around the age of 13 years, whereas males reach maturity later, around 15 years.<span> (Goodall, 1986; Jones et al., 1996; Nowak, 1999)</span></p> <p><strong>Key Reproductive Features: </strong>Iteroparous; Year-round breeding; Gonochoric/gonochoristic/dioecious (sexes separate); Sexual; Viviparous</p><p>The breeding interval varies. Female cycles last about 36 days, and females mate during each cycle. However, if pregnancy ensues, a female may not begin cycling again for 2.5 to 5.5 years.>p<p>Chimpanzees may breed throughout the year.>p <p>As is true for most mammals, females provide the bulk of parental care. They carry their offspring, groom them, nurse them, and provide them with opportunites to learn all of the complex behavioral patterns of the species. Young are completely dependent upon their mother until weaning at 3 to 4 years of age, but continue to travel with her and rely heavily upon her for support until they reach adulthood. Bonds with the mother extend throughout an individual’s life. In spite of having achieved independence, both males and females may maintain social bonds with their mother for the remainder of their lives. Although females sometimes emmigrate into a new community of chimpanzees, thereby severing ties with their mother, females may also stay in their natal community as adults. In this case, they may occasionally travel with their mother. Males often use their mother for emotional support when establishing themselves in the male dominance hierarchy. When things are not going particularly well for them, some males may seek the comfort, stability, and quiet that only their natal family can provide.<span> (Goodall, 1986; Nishida and Hiraiwa-Hasegawa, 1986; Nowak, 1999)</span></p> <p>Because multiple young of different ages may be traveling with their mother at any time, bonds between siblings are also strong. These bonds may remain strong during adulthood, and brothers are frequent allies in intragroup intrigues. Older siblings frequently help to carry infants and play with infants. If the mother should die, older siblings will often assume the care of their immature, weaned siblings.<span> (Goodall, 1986)</span></p> <p>Males do not provide any direct parental care for young, although they can be quite gentle and playful with young members of their community, especially those still possessing a white tail tuft. Males may indirectly provide protection for their young. Adult males in the community engage in border patrols, which may help to protect the young from potentially dangerous stanger males.<span> (Goodall, 1986)</span></p> <p>The relationship between a mother and her offspring can have many repercussions during the life of the offspring. Although rank is not technically inherited from the mother, the rank of a female does affect her offspring. A mother who has a high rank, who is confident and relaxed in dealing with other chimpanzees, is likely to have offspring who behave in a similar fashion. Nervous mothers may produce offspring who are fearful of other chimpanzees, and who may not do well in dominance competition.<span> (Goodall, 1986)</span></p> <p>Because young males do not emmigrate at maturity, they inherit the home range of dominant males. There is no certainty of paternity in this polygynandrous species, so transmission is not directly from father to son and it is unlikely that such relatives would recognize one another as such. Females may remain in their natal community also, although they may transfer to a different chimpanzee social unit upon reaching maturity.<span> (Goodall, 1986; Nishida and Hiraiwa-Hasegawa, 1986; Nowak, 1999)</span></p> <p><strong>Parental Investment: </strong>Altricial; Pre-fertilization; Pre-fertilization :: Provisioning; Pre-fertilization :: Protecting; Pre-fertilization :: Protecting :: Female; Pre-hatching/birth; Pre-hatching/birth :: Provisioning; Pre-hatching/birth :: Provisioning :: Female; Pre-hatching/birth :: Protecting; Pre-hatching/birth :: Protecting :: Male; Pre-hatching/birth :: Protecting :: Female; Pre-weaning/fledging; Pre-weaning/fledging :: Provisioning; Pre-weaning/fledging :: Provisioning :: Female; Pre-weaning/fledging :: Protecting; Pre-weaning/fledging :: Protecting :: Male; Pre-weaning/fledging :: Protecting :: Female; Pre-independence; Pre-independence :: Provisioning; Pre-independence :: Provisioning :: Female; Pre-independence :: Protecting; Pre-independence :: Protecting :: Male; Pre-independence :: Protecting :: Female; Post-independence association with parents; Extended period of juvenile learning; Inherits maternal/paternal territory; Maternal position in the dominance hierarchy affects status of young</p>
- Nowak, R. 1999. Walker's Mammals of the World, Sixth Edition. Baltimore and London: The Johns Hopkins University Press.
- Goodall, J. 1986. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, Massachusetts: The Belknap Press of Harvard University Press.
- deWaal, F. 1982. Chimpanzee Politics: Power and Sex among Apes. New York: Harper and Row.