Chimp Mating Habits

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Chmpanzee Mating Habits
  Chimpanzee Pan troglodytes   SOCIAL ORGANIZATION AND BEHAVIOR Primate behavioral ecologists have long debated the costs and benefits of group living, but some of the factors that affect chimpanzee social structure include decreased likelihood of predation, resource defense and feeding efficiency, and higher copulatory success because of access to mates (Sakura 1994; Boesch 1996). Chimpanzees live in a fission-fusion social group consisting of a large community that includes all individuals that regularly associate with one another (up to a few hundred individuals) and smaller, temporary subgroups, or parties. These subgroups are unpredictable and can be highly fluid, changing members quickly or lasting a few days before rejoining the community (Goodall 1986; Chapman et al. 1993; Boesch 1996). At Taï, the average party size is between five and eight individuals (Boesch 1996). Party size greatly increases when food availability increases, though, and at Kibale, average party size is 10 but ranges from one to 47 individuals during periods of highest food availability (Mitani et al. 2002). Party size also increases when the estrous females are present (Matsumoto-Oda et al. 1998; Mitani et al. 2002). Party composition is variable, including unisexual and bisexual parties of adolescents or adults, parties of adult females and infants, lone adult females and their offspring, and mixed age and sex parties (Boesch 1996). The most frequently observed party across all chimpanzee communities is the mixed-sex party, though Taï males are the most inclined to associate with females, and Gombe males are the least inclined to do so. At Bossou, though, where the adult sex ratio is skewed, the high number of females relative to males means the most common party composition there is mother-infant (Boesch 1996). Photo: Frans de Waal There is a distinct linear dominance hierarchy in male chimpanzees, and males are dominant over females (Goldberg & Wrangham 1997). Males remain in their natal communities while females, in general, emigrate at adolescence, between nine and 14 years old (Nishida et al. 2003). The complete transition between groups may take up to two years, though, and is characterized by vacillating between their natal group and new community (Goodall 1986; Hasegawa 1989; Pusey 1990). Rates of female transfer are much higher at Mahale and Taï than at Gombe and Bossou. This may be attributed to the smaller population size and isolated conditions at Gombe and Bossou; with fewer options, it is more beneficial to remain in their own group and take behavioral precautions to avoid inbreeding (Goodall 1986; Gagneux et al. 1999; Nishida et al. 2003). For female chimpanzees that do emigrate, though, they are not likely to be related to other adult females in their new  community and the dominance hierarchy is linked to age (with younger immigrant females ranking the lowest) and the status of their offspring (Nishida 1989). Lactating females generally spend most of their time with their own offspring, though they may be seen with other lactating females in nursery groups (Pepper et al. 1999). Females become very sociable during estrus, though, and are seen mostly in bisexual parties (Pepper et al. 1999). Given the female-biased dispersal pattern, male chimpanzees in a community are more likely to be related to one another than females are to each other, but matrilineal kinship does not always strongly influence patterns of male chimpanzee social behavior. In studies at Kibale, genetic analyses of males support the theory that cooperation is of greater evolutionary significance than kinship affiliation (Goldberg & Wrangham 1997; Mitani et al. 2000). Research at Gombe, on the other hand, has consistently emphasized kinship as the most important underlying factor of the strong social bonds (Goodall 1986). Close relationships between males serve two purposes within chimpanzee communities: inter-community interactions and intra-community politics (Goldberg & Wrangham 1997). Some examples of inter-community interactions include hostile attacks by groups of males and cooperative boundary patrol parties. Intra-community interactions that are dominated by male cooperation include securing and maintaining dominance, mate guarding, and group hunting and meat sharing (Goldberg & Wrangham 1997; Mitani et al. 2000). First seen at Gombe in 1963, chimpanzee hunting behavior probably evolved because of the direct benefits of a protein source in their largely frugivorous diets, but it is more than nutritionally important; meat is socially important as well (Mitani & Watts 2001). Meat is social currency used to develop and maintain alliances between adult males; it is usually shared reciprocally and non-randomly (Mitani & Watts 2001). Hunting is cooperative in the sense that multiple males are involved in cornering and capturing prey, though there is debate among researchers if this is true cooperation (Videan pers. comm). Members of the hunting party are spread out widely on the ground and in the trees (if hunting arboreal prey such as colobus monkeys), and other members of the community often observe and vocalize excitedly throughout the pursuit (Watts & Mitani 2002). Hunting success increases with group size, and chimpanzees are more successful where the canopy is broken and open. The male chimpanzees in Kibale are the most successful hunters with an average success rate of 84%, though at other sites hunting parties have success rates above 50% (Watts & Mitani 2002). The influence on red colobus ( Procolobus badius ) populations because of high success rates of chimpanzee hunters should not be ignored. Chimpanzees are contributing to population declines of red colobus monkeys in multiple sites across Africa, especially at Gombe (Struhsaker 1999). REPRODUCTION Males reach adolescence between nine and 15 years of age and are capable of reproduction at 16 years or older. First estrus is seen in females at 10 years of age and is characterized by anogenital swelling. Menarche occurs a few months after the first swelling and continues on a cycle of about 36 days (Goodall 1986). There is a period of adolescent infertility in female chimpanzees that usually coincides with permanent emigration from their natal groups (Goodall 1986; Nishida et al. 2003). During the transition period, females still exhibit sexual swellings that may serve as a passport to gain males' tolerance in their new social communities (Boesch & Boesch-Achermann 2000). Once established in their new communities, young females cease cycling for two to four years but continue to attract adult males and mate promiscuously. First parturition occurs, on average, between 13 and 14 years and the interbirth interval is between three and five years (Goodall 1986; Boesch & Boesch-Achermann 2000; Nishida et al. 2003). The lag time between menarche and first parturition may have adaptive significance for emigrating females. Having offspring before being accepted by the community could  jeopardize both the mother and infant (Boesch & Boesch-Achermann 2000). Infanticide has been documented at Gombe, Mahale, and Kibale study sites and is often attributed to sexual selection theory; males kill infants unlikely to be their own, infanticide shortens interbirth intervals by inducing cycling in females that lose infants, and infanticidal males thus increase their chances of siring offspring. Infanticidal behavior by females has also  been observed, though there is some question as to whether these were isolated incidences of pathological behavior, or if it could be related to dominance rank in females (Goodall 1986; Pusey et al. 1997). Photo: Frans de Waal Mating occurs throughout the year and there is no evidence of a birth season, female chimpanzees do exhibit seasonality in the number of estrous females within a group (Wallis 1995; Boesch &Boesch-Achermann 2000; Wallis 2002). The number of estrous females is positively related to food abundance; because of the energetic requirements of ovulation and mating, female chimpanzees are more likely to come into estrus during times when food is readily available (Anderson et al. 2006). The majority of chimpanzee reproductive behavior is promiscuous, with females mating with multiple males opportunistically during estrus, though the majority of copulation occurs during the 10-day period of maximal tumescence (Goodall 1986). There are other types of reproductive strategies that are recognized as well. Restrictive mating, where the dominant male restricts other males from mating with estrous females in the community, consortship mating, where an adult pair leave the community for several days to weeks, and extra-group mating, where females leave their communities and mate furtively with males from nearby communities (Goodall 1986; Gagneux et al. 1999). Chimpanzee social and mating groups do not always overlap, given the variety of reproductive situations. This may have evolved because females have limited choice in mates after committing to a community, and the dominance hierarchy of males often dictates which males an estrous female will mate with. By having multiple strategies, females can expand the pool of males from which they choose while not losing the important support of the males in their communities (Gagneux et al. 1999). Having multiple strategies also maximizes the chances of males' reproductive success; they are able to vary, throughout their lives, their mating strategies with depending on their position in the dominance hierarchy. PARENTAL CARE In chimpanzees, the majority of parental care is the responsibility of the mother and is critical to the survival and emotional health of youngsters (Goodall 1986). Chimpanzee infants and juveniles benefit from the close relationship with their mothers in terms of food, warmth, protection, and the opportunity to learn skills. There is also some evidence that a young chimpanzee achieves rank according to his or her mother's status (Goodall 1986; Boesch & Boesch-Achermann 2000). Chimpanzee newborns and mothers are in constant ventral-ventral contact for the first 30 days of life. During the newborn period, chimpanzees are helpless to survive without maternal support and though they have a firm grasping reflex, it is not strong enough to support the infant for more than a few seconds at a time. In fact, for the first two months of life, chimpanzees are unable to support their own weight and depend entirely on their mothers' support (Bard 1995). After five or six months, chimpanzee infants ride dorsally on their mothers' backs. During the first year of life, infant chimpanzees maintain almost continual contact with their mothers. By the age of two, they begin to travel and sit independently within five meters of their mothers and this corresponds to a decrease in nursing and the onset of independent eating and play behavior (Bard 1995; Coe  1990). Not until three years of age do young chimpanzees venture more than five meters from their mothers, and between ages four and six, the period of infancy ends with weaning (Bard 1995). During the juvenile period, from about six to nine years old, chimpanzees remain close to their mothers but play independently and have greater social interactions with other community members. Adolescent females spend some of their time moving between groups and are supported by their mothers during agonistic encounters while adolescent males spend more time with adult males in social activities such as boundary patrols and hunting parties (Bard 1995). COMMUNICATION Photo: Roy Fontaine Visual and vocal communication are important to chimpanzee society. A suite of facial expressions, postures, and sounds function as signals during interactions between individuals and groups (Goodall 1986). Chimpanzees have particularly expressive, hairless faces and facial expressions play an important role in close-up communication between individuals. For example, a full closed grin is in response to an unexpected and frightening stimulus and evokes an instant fear response in others. Other facial expressions include lip flip, pout, sneer, and compressed-lips face (Goodall 1986). Body position and stance also convey information to other individuals. Submissive positions include extending the hand, crouching, and bobbing while aggressive positions usually involve an individual trying to appear larger than he is by swaggering bipedally, hunching his shoulders, and waving his arms (Goodall 1986). Adult chimpanzees will also drum on the trunks of trees, by beating their hands and feet against large trees, for a dramatic display. Drumming can be heard in multiple contexts including while traveling, during displays, encounters with other chimpanzee communities, and when arriving at large food sources (Crockford &Boesch 2005). LISTEN TO VOCALIZATIONS ã   Pan troglodytes: Scream (African Primates at Home/Indiana University) ã   Pan troglodytes: Vocalization ( Vocal communication also conveys a wide variety of emotional states and intentions and often serves to affect the behaviors of those that hear the calls. One important vocalization is the pant-hoot, which is the most commonly heard call of adult individuals and is used to signify food enjoyment, social excitement, and sociability feelings (Goodall 1986). One of the best ways to assess dominance rank is to listen for pant-grunts, which are directed towards dominant individuals by submissive individuals (Goodall 1986; Pusey et al. 1997).
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