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Title of Project: Sex differences in infant chimpanzees just like humans infants


III. Main Body


  1. Abstract

The topic of sex differences in human children social behavior has for decades been hotly debated and is among the most controversial topics. However, plenty of studies have recently proved that the biological basis of humans plays a significant aspect in determining the social behavior of a child. The exploration of the evolutionary basis of sex differences, the aim of this study is to find out the sex differences regarding sociability among the wild chimpanzee’s infants’. The chimpanzee’s infants’ are the Pan Troglodytes schweinfurthin, in Nyungwe National Park in Rwanda. The researchers use a long –term set of data on infant-mother behavior in analyzing the diversity of social partners among infant chimpanzee aged from 30 to 40months. The study shows that female infants (N=9) interacted less compared to the male infants (N=11) in interacting with other individuals after the control of maternal sociability. Also, the study shows that the male chimpanzee infants did interact more with male adults than the way the female infants did. Thus, the study uses well-documented data of sex differences in social tendencies of chimpanzees at the adult stage that starts to show at the early stages of development. The data also suggest that human children show behavioral sex differences that are from both our evolutionary and biological heritage.

  1. Introduction


The topic of sex differences in the social behavior of human infants is always a controversial and hotly debated topic (McIntyre & Edwards, 2009). Evidence from various studies on sociability of human children suggests that girls are more keen in social understanding and constant eye aspects as well as in prosocial behavior than boys (Eisenberg, Fabes, & Spinrad, 2006).

Other studies have, however, shown that boys get involved in highly dyadic activities that have larger group[s than girl’s play (Fabes, Martin, & Harnish, 2003). The primary assumption is that parents and teachers do apply differential treatment top boys and girls, and this is the main factor that leads to sex-typical behavior. Various other studies have also indicated that the main aspect that determines human children social behavior is their biological basis. For example, the study by Connell, et al. (2000) on human neonates shows that the female infants are using the social object of looking at the face while male infants’ preferred the mechanical object of looking at a mobile object.

Also undertaking the same investigation into the biological and evolutionary basis of sex differences regarding behavior we can analyze one of our closest nonhuman primates, the Pan Troglodytes, the chimpanzee. Usually, the wild chimpanzees live in communities comprising of 20 to almost 2000 members. The communities are also multi-female, and multi-male and are characterizes hierarchy of male dominance whereby the philopatric males are the community’s stable core responsible for defending the group’s territory. In these stable communities, there are also grouping patterns that represent a social system of fission-fusion while the temporary subgroups also form because of various factors such as social relationship to others, food availability and the females sexual state.

Also, other neighboring communities of the chimpanzees have a high level of intercommunity and territorial interactions which can sometimes lead to fatal outcomes (Wilson & Wrangham, 2003). The central Africa chimpanzees P. t. Schweinfurthii, comprise of the adult females who are usually the less gregarious compared to their male counterparts. They spend most of their time under the accompany of their dependency offspring (Murray, Sandeep, & Pusey, 2007). Furthermore, the indices between female-female dyads are far much weaker compared to the male-male dyads (Glibly & Wrangham, 2008). Females are usually social at the time when they are sexually receptive and less social during pregnancy (Pusey, Murray et al., 2008). Also, mothers are less social compared to the non-mothers. Also, another sex differences behavior among adult chimpanzees is their differences in ranging and feeding patterns. The females are biased in their use of tools while male have biases ion their vertebrate hunting as well as territorial defense (Muller & Mitani, 2005).

There are reports about theBehavioral based sex differences among the young wild chimpanzees despite the few available data. The bias for tool use in an adult female is also among the youngsters as the young female chimpanzees gain their skills on termite fishing two years earlier than their male counterparts. Also at this period of this skills development, the young females are keen in looking at how their mothers use tool while the young males are keen spending more time playing. Also, the young females spend more time in ‘nest making’ and ant dipping’ than the young males. The intriguing aspect is that the young females in a study site also carry the stick for cradling and a form of taking the mothering role in a significantly more way than the young males (Kahlenberg & Wrangham, 2010).

The mother-infant relationship for young chimpanzees is of great importance since they need long periods of social and nutritional dependency. They also do not have a direct paternal care. Thus, the physical contact between the infant and mother are part of the young chimpanzee lives in their first two years of life. The offspring append most of their time at their mother’s arm’s reach or physical contact up to they reach three years. The offspring are mainly depending on their mothers from infancy up to the time they start weaning at 3 to five years. Behaviorally they remain dependent on their mothers in socializing and traveling through the jungle up to the time they are at least eight years. It is after they reach ten years that most of the chimpanzees start to spend time way from mothers. Studies indicate that the sociality of an infant has to tease apart from the sociality of the mother’s effect for the infant to develop his or her social behavior.

The predictions given by the sociological theory are that the differencing correlates on successful reproductions require adult female chimpanzee’s motivativation in maximizing her feeding efficiency. On the same note, the adult male has to be motivated towards socially establishing themselves so that they can have access to dominance hierarchy, more alliances, and access to female (Muller & Mitani, 2005). Thus, it is not surprising to find that the female versus male-biased sex differences among youngsters is at the center of the caretaking behavior and food-finding methods. Also not surprising is the fact that sex differences in patterns of associations among the sub-adult chimpanzees identification whereby the adolescent male chimps usually leave their mothers sooner than adolescent female and are therefore more gregarious. However, studies on sex differences in chimpanzee infant social behaviour have been few especially on their typical stages of social and behavioural development among the wild infant chimpanzees by Bloomsmith, and Blangero (1997). The study investigates the sibling’s effect on social relationships among the infant chimpanzees from birth to the 24 months. They found no significant and suggestive differences on social behaviour whereby the males are more social than the females.


This study aims to establish the sociability of sex differences among infant chimpanzee at Nyungwe National Park in Rwanda.

Action Plan

The aim is to explore sex differences in chimpanzee infants’ social behavior.

Data Gathering

Nyungwe National Park IN Rwanda is a vast and untouched tropical forest that has sense and high canopies. It has a spectacular biodiversity that hosts the country’s mountain gorillas and chimpanzees. It has tall old ebonies, mahoganies, and giant tree ferns above the ground with epiphytes and orchids clinging to each branch. The park is home to chimpanzee among other twelve species of primates. The chimpanzee is easily recognizable and habituated to human observers. They have matrilineal kinships that can have up to four generations. The focus of the study will be a subgroup of the Rwandan chimp population comprising of a small community in Cyamundongo Forest that ranges from 40 to 60 individuals. The age and sex class of this group compose of 6-14 adult males, 6-14 sub-adult and 12-25 adult females and 7-15 sub-adult males.

Data from study subjects and behavioral aspects

There will be the collection of a detailed mother-infant data on behavior among members of the Cyamundongo Forest community. The researchers will record data from each infant and their mothers and also when possible other older siblings in minute samples. The data will have information about the proximity, behavior, vocalizations, locomotion, interaction and play with other chimpanzees. The dyad of infant-mother is the focal unit. The researcher will also collect data on group composition from individual’s parent within five minutes of the focal point. This analysis will only include infants with a ten-hour digitized data ages between 30 to 36 months collected within five different days of observation. Using the same criteria it w as also possible to include data from twenty different infants (N-= 11 males and N=9 females) from ten different mothers. The collection of data will be from 2016 to 2019. The protocol for observation will adhere to all legal and welfare requirements of the hosting country Rwanda, and the study will be approved as well as permitted by the Rwandan Wildlife Research Institute, Nyungwe National Park and the Commission of Technology and Science of Rwanda.


The variable of interest to the response is to find out the number of social partners of which the infant chimpanzee had direct interaction and constant. Thus, it will be possible to count the type and some individuals that the infant groomed, played with or was in physical constant within the duration of interest. The individuals fell into various categories such as 1. The maternal family who are the matrilineal relatives such as cousins, grandmothers, and adult siblings. 2. The immediate family is comprising of nonadult siblings and mothers. 3. Others that are the non-maternally related chimpanzees. These social partners were also according to age class, juvenile; adult; adolescent or regarding sex as female or male. The main interest was about infant interactions especially outside their immediate family and also the need to avoid biases relating to family size variation such as the number of other siblings. Thus, the main variable responses for our model will be the number of social partners for each infant outside their immediate family.

Also recognizing the young chimpanzee’s importance of the maternal relationship, it will be essential to investigate and control for the social partners number for the mothers. So to eliminate any variability biases regarding offspring number that the mother will have it will be important to count the non-offspring social partners number for each mother.

Also having a long period of observation time is essential for this study since it will provide opportunities to look at the social interactions that will be part of this study according to the statistical models within control duration for data collections. The data will, therefore, cover a long period from 2016 to 2019 as well as a year term for investigating the anomalous year’s effect and the collection of this anomalous data or the temporal patterns in the long term.

The study will also investigate as well as set control to various factors that may impact the sociality of a chimpanzee. Thus, it’s crucial to investigate variables such as the following: a.) The community size; where we find out the total number of chimpanzees living in Cyamundongo Forest at the period of data collection. 2. The season. The party sizes of the chimpanzees usually vary depending on the season. Thus, it will be crucial to investigate the month effect for a six-month interval for each infant. 3. The median group sizes will be analyzed. 4. The average number of other individuals present for analyses.

Statistical analyses

The study will use the SAS version 9.2 for doing all the statistical analyses. Due to the high chances of having multiple offspring from the same mother, we will use linear regression for the data set that uses a Mixed Model format that has a random effect for the mothers. The bivariate regression will first be undertaken to identify the covariates that demonstrate a significant outcome relationship. There will also be the use of a forward selection in finding out which is the best fit model such as the likelihood ratio tests and the covariant for interaction.


The study will show whether there is a significant sex difference regarding the type and number of social partners among the infant chimpanzees. It will also show that the best fit model or offspring number of social partner’s interactions as one that included the term of the mothers nonoffspring social partners and the interaction between offspring sex and the number of social partners for the mothers. The study will also show the significance of other variables that can affect results such as the observation period, season, and year, a total number of individuals, community size, median group and an average group as being the less significant. The researcher will also investigate the interactions between sex-age classes and show they’re significant in impacting on nonadult social patterns. Thus, the outcome of the study will demonstrate the social behavior differentiation of females and male as well as infant chimpanzees and their social group. Most likely the young male’s chimpanzees are the ones with a higher level of interaction with other individuals even after having controls for group size and mother’s interactions. The essential aspect of this finding is that it is not affected by the family size since the study removes interactions with dependent siblings and other siblings. For this study, the male infants were the ones with the highest level of interactions with other individuals compared to the female infants.

From this argument, it is possible to predict the fact that the mothers of female infants with have fewer interactions with adult males than mothers of infant males. However, this may not be the case since earlier studies have proved that mothers of infants irrespective of their sex will not frequently interact as compared to infant males. It is because a stable social bond of male and female among the chimpanzees are rear due to their promiscuous mating systems. However, they have an enduring male-male bond which yields to increase in reproductive success and dominance rank (Gilby et al., 2013). Thus, the results are most likely to show that even the young age chimpanzees of 30 to 36 years will show social, behavioral patterns that match to the sex-specific social roles of adults.

Thus, the results of this study are mostly likely to complement other studies on primates that display a distinct sex difference of the youngster’s behavior that is similar to the different sex roles for their adulthood. Thus, this study also contributes to answering the question of the similarity of primates to human behavior regarding their sex roles that begins from a young age. Thus, the accumulating and consistent reports on sex differences on a development of behavior among the nonhuman primates do suggest that while the human’s gender socialization plays a role in magnifying the differences between females and males theses sex differences in behavioral terms are rooted in our evolutionary and biological heritage.


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  1. Recommendations

Similar studies should be with bonobos and gorillas which are also primates with close behavioral similarities as humans

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