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A Study on Aggression and Altruism in Humans and Non-Human Primates and Its Importance to Evolution

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What causes primate social behavior? According to American biologist Edward Wilson, social behavior in primates is caused by evolution (Larsen, pp. 194). However, despite the fact that aggression and altruism have opposite meanings, both elements are present in the primates’ world. What evolutionary aspects have caused two contradictory elements to be present in primates, including humans, to this day? The intention of this paper is to investigate the presence of altruism and aggression in human and non-human primates and to understand why it is considered important to evolution.

In the selfish gene theory, Richard Dawkins (1976) explains that human evolution is due to our genes’ selfish nature. He believes that our evolution is due to our genes’ drive to survive and reproduce. However, time has shown that humans care about more than themselves and have displayed altruistic behavior throughout time (Slyke, 2010). Altruism is defined as behavior towards a recipient that benefits the recipient at the cost of the actor who is performing the behavior (de Waal, 2008). Altruism is common in humans, even towards strangers where there is no foreseen benefit of acting altruistically (Fehr & Fischbacher, 2003). Reciprocal altruism is helping someone who has helped you in the past (Slyke, 2010). Indirect reciprocity is the act of helping someone when there are no direct benefits; however, as a result your reputation as an honest helper increases and thus increases your chances of receiving benefits in the future (Slyke, 2010).

Humans may also be motivated to act altruistically purely through empathy with or concern for the recipient (Silk & House, 2011). On the other hand, their altruistic behavior may be due to selfish reasons wherein the actor is expecting foreseen benefits (Silk & House, 2011). Competitive altruism has also been found to be present in humans during a study in which participants increased their generosity to increase their self-presentation to be chosen as a social partner (Barclay & Willer, 2007). Kin selection is also an evolutionary process of altruism in that humans will often help family members more often than strangers (Slyke, 2010). As you can see, altruism can be displayed by humans in many ways and for many reasons, from increasing ones’ reputation to empathizing with someone and understanding their need for help. How are these mechanisms of altruism important to human evolution?

According to a study on altruism, altruism toward non-relatives was actually seen as sexually selected human trait that people sought in their potential mates (Phillips, Barnard, Ferguson, & Reader, 2008). This supports the notion that altruism is a component of sexual selection which is important to the evolution in humans (Philips et al., 2008). Altruism, as a whole, also may contribute to our evolution in ensuring our survival. Many humans perform many altruistic acts every day, such as holding the door open for someone or letting them go ahead of them in a grocery store line. These small acts may not increase our chance of survival but bigger acts may. For example, a person may give some money to a person in need. The person may use the money to acquire food, which is necessary in order to survive. This act, and others like it, enables someone to survive and possibly to reproduce, which would contribute to our evolution as a whole. Altruism also increases social bonds and cooperation, which may also be seen as an important component to survival (Slyke, 2010).

Non-human primates are also prone to altruism (Warneken, Hare, Melis, Hanus, & Tomasello, 2007). Throughout my research, I have noticed that humans and non-human primates perform altruism for many of the same reasons. However, there are some differences as well. Among nonhuman primates, altruism can be expressed through grooming, food sharing, and other various situations for multiple reasons.


Allogrooming, or social grooming, is perhaps the most common altruistic behavior performed among primates (Schino & Aureli, 2010). A possible motivation of grooming of primates is kin selection, in which primates groom those they are related to (Schino & Aureli, 2008). For example, Rhesus and Japanese macaques spend more time grooming close kin than more distant kin (Silk, 2002). Reciprocal altruism is another explanation for grooming, as is the case with red-fronted lemurs (Eulemur fulvus rufus) (Port, Clough, & Kappeler, 2009). In fact, Schino & Aureli (2010) ran a study that found that grooming distribution among primates was greater explained by reciprocation than kin selection. However, this does not mean that grooming is reciprocated in equal amounts. In redfronted lemurs, female primates who ranked low in their group groomed others the most, relative to the amount of grooming they received (Port et al., 2009). Port et al. (2009) also found that on average, males reciprocated less towards females than they received from females.

What are some other reasons for the use of grooming in the primate world? Among chimps, grooming has been used to elicit future assistance from those they groom (Koyama, Caws, & Aureli, 2006). In addition, although it does not fit the usual definition of grooming, primates will often clean the wounds of conspecifics, or members of their species, in an effort to heal the wound (de Waal, 2008). Lastly, grooming may be exchanged for tolerance by outcasts, especially at feeding sites, where food is unlikely to be shared with them (Silk, 2002).

How is grooming important to evolution? Much like humans, it is important because it forms stronger social bonds. It also elicits cooperation for future conflicts, which helps to ensure the species’ survival (Koyama et al., 2006). Grooming may also be used in exchange for food, which of course is necessary for survival (Frank & Silk, 2009). Furthermore, the cleaning of wounds may ensure the individual’s survival who will then have the ability to reproduce. The next example of altruism displayed in primates is food sharing.

Food Sharing

Food sharing is perhaps the second most common form of altruism within the primate community (Silk, Brosnan, Henrich, Lambeth, & Shapiro, 2013). Jaeggi & van Schaik (2011) stated that among nonhuman primates, chimpanzees are the only species that shares food among adults in the wild. However, Silk & House (2011) found that marmosets, capuchins and tamarins are even more giving of food than chimpanzees are. While doing field observations at the Ellen Trout Lufkin Zoo, I witnessed food sharing in two white-faced sakis (Pithecia pithecia). I am unsure to the underlying cause of this sharing as I do not know the relationship of the two monkeys. The male would go to their food bowl and grab two food pellets and then give one pellet to the female. Patterns of food sharing among primates has been found to be due to reciprocity, kin selection, to strengthen social bonds, and to appease beggars (Silk & House, 2011). Meat sharing is common among chimpanzees after a group-hunt (de Waal & Suchak, 2010). Those who actually participated in the hunt have easier access to the food than those who did not participate (de Waal & Suchak, 2010). Food sharing has a very necessary need for evolution. Food. Food is necessary in order to survive and survival is needed in order to reproduce.

Other Examples of Altruism

Alarm calling, which involves a primate making a loud call to alert conspecifics that there is a threat approaching or nearby, is considered an act of altruism as it puts the actor at risk while benefiting others (Silk, 2002). Tree bridging, in which a primate uses his or her body as a bridge between two trees to assist younger ones over, is a common altruistic act among orangutans (van Schaik, 2004). During my field observations, I watched as a mother De Brazza’s monkey traveled around her cage with her young offspring clinging to her stomach. Acts like these show the strong kinship bond among primates. A study examining altruism in chimpanzees and human infants found that the chimpanzees and the infants helped human strangers without the expectation of a reward (Warneken et al., 2007). Warneken et al. (2007) also found that the chimpanzees also helped other chimpanzees who were unrelated and unknown to them.

Alarm calling is a motivation for evolution in that it ensures the survival of the many, although it may sacrifice the life of the caller (Silk, 2002). Alarm calling ensures that the group is made aware of a threat and can prepare for it (Silk, 2002). Tree bridging may be seen as helpful to survival of young chimpanzees in that it ensures that will make it to the next tree safely. It certainly doesn’t hurt evolution may be a better way to look at it. Chimps helping other chimps not related to them may be seen as helping the survival of chimps as a whole instead of just helping one’s blood line. The fact that both humans and non-human primates are willing to help conspecifics other than their kin shows how similar we are to other primates. Another similarity we share is aggression.

Aggression has been defined by many psychologists as any behavior that one person displays towards another with the intention to harm someone who does not want to be harmed (Baumeister & Bushman, 2014). Aggression in humans has been subdivided into reactive aggression, which is an emotional, unplanned response to a provocation, and proactive aggression, which is behavior that is planned, controlled and goal-oriented (Nouvion, Cherek, Lane, Tcheremissine, & Lieving, 2007). Males have been found to be more aggressive towards same-sex individuals, as well as opposite-sex individuals (Wolfer & Hewstone, 2015). However, women are more likely than men to display relational aggression, such as spreading rumors about someone or ostracizing them from their group (Baumeister & Bushman, 2014). In humans, most aggression involves familiar people (de Waal, 2000).

One major difference between aggression in humans and non-human primates is humans’ ability to perform aggression with the use of modern weapons (Cashdan & Downes, 2012). Aggression in humans ranges from spousal abuse to fighting in bars, to pushing someone or even to murdering someone, with or without a weapon. The reasons for aggression in humans are also complex and expansive. Some of the reasons are infidelity, lying, provocation, jealousy, to impress a potential mate, or having an incentive, such as money. How are primates different when it comes to aggression?

One of the theories designed to explain aggression in primates is the male mate-defense hypothesis, in which males’ aggression rises when there are females in heat present (Kitchen & Beehner, 2007). This theory has been supported by evidence found in chimps (Pan troglodyte), Japanese macaques (Macaca fuscata), bonnet macaques (M. radiate), and blue monkeys (Cercopithecus mitus) (as cited in Kitchen & Beehner, 2007). This has also been found to be true for white-faced sakis (Pithecia pithecia), whose main reason for intergroup aggression appears to be maintaining exclusive access to females (Thompson, Norconk, & Whitten, 2012). In female chacma baboons, oestrous females receive the most aggression, showing that females also fight for potential mates (Huchard & Cowlishaw, 2011). Furthermore, pregnant chacma baboons were found to initiate the most aggression, possibly due to competition over food resources involved in successful gestation and lactation (Huchard & Cowlishaw, 2011). In polygamous species, such as olive baboons (Papio anubis), males are chosen to be mates based on their competitive prowess, thereby having increased levels of male-male aggression (Sapolsky, 2006). Mountain gorillas also choose their mates based off of their fighting ability (Robbins, Gray, Uwingeli, Mburanumwe, Kagoda, & Robbins, 2014).

Aggression can be displayed in many different forms, including vocalizations, chasing, wounding, and sometimes killing conspecifics (Harris, 2007). Aggression in green monkeys (Chlorocebus aethiops) is displayed through threats, chases, blows, lunges, pushes and bites (Chalyan & Meishvili, 2007). In one species of macaques (Macaca mulatta), the intensity of aggression is high with lots of biting (Thierry, 1985). In green monkeys, males exhibited higher levels of aggression than females due to the competitive nature of achieving high status and gaining access to females (Chalyan & Meishvili, 2007). In marmosets (Callithrix iacchus), aggression increases in families that contain more than five individuals (de Filippis, Chiarotti, & Vitale, 2009). Hanya (2009) studied the occurrence of aggression during feeding time in wild Japanese macaques. He found that the smaller number of feeding sites there were, the higher the level of aggression displayed. However, aggression was only displayed during feeding of high quality foods, such as fruits and seeds, as opposed to low-quality foods, such as leaves and flowers (Hanya, 2009). Bwindi mountain gorillas also fight over high quality foods and foods that take longer to eat (Wright & Robbins, 2014). Kitchen found that when small infants were present, Black howler monkeys (Alouatta pigra) howled quicker and for a longer time period at potential infanticidal intruders (as cited in Wich & Sterck, 2007). While observing the lemurs at Lufkin Trout Zoo, all of the sudden, many of them started making loud noises. I am unsure of what started the noises, but felt they were a warning. From the research, you can imply that there are many factors involved in aggression, from food, to protection of young conspecifics.

The amount of aggression and to whom it is mostly directed varies among different species of primates. Monkeys of higher-ranking status in a group aggressed toward mid-ranking conspecifics in a significantly greater number of instances than vice versa (Chalyan & Meishvili, 2007). Bwindi mountain gorillas (Gorilla beringei beringei) also tend to aggress down the social rank (Wright & Robbins, 2014). In monkeys, conflicts usually occur between two individuals as opposed to between groups (Chalyan & Meishvili, 2007). In one species of macaques (macaca tonkeana), aggression is often bidirectional, meaning that both parties involved are participating in aggression. Wich and Sterck (2007) found that male Thomas langurs (Presbytis thomasi) acted more aggressively towards unrelated and unfamiliar conspecifics who were threatening them then towards kin. However, overall, physical fights in Thomas langurs are rare (Wich & Sterck, 2007). In contrast to Thomas langurs, female macaques fight more among their female kin rather than those unrelated to them (de Waal, 2000). Similarly, most aggression in marmosets (Callithrix iacchus) involves siblings (de Filippis, Chiarotti, & Vitale, 2009). Moor macaques (Macaca maura) exhibit low aggression, whereas crested black macaques (Macaca nigra) are more aggressive, which may be due to their lack of affiliative actions, such as grooming (Riley, Sagnotti, Carosi, & Oka, 2014). Bwindi mountain gorillas have lower aggression rates than the average of many species of primates which may be due to their large size which can inflict costly injuries (Wright & Robbins, 2014). Now that we have examined aggression in primates, let’s discuss how it is important to evolution.

One way aggression is important to evolution is that much of the research suggests that aggression is seen as a sexually selected trait. Females love a man with bravado. Sexual selection seems to be an important component of aggression for both humans and non-human primates. I hypothesize that it is due to males showing a strong side when exhibiting aggression, which could be a good indicator that he will be a strong protector. Aggression in primates is also used to protect or acquire food sources, which is important to survival. Coalitionary aggression, in which conspecifics join together to direct aggression toward one or more conspecifics, has been shown to increase reproductive success in chimpanzees (Gilby et all, 2013). Reproduction is, like Richard Dawkins proposed, one of our genes’ main objectives.

In conclusion, altruism and aggression are both important elements of evolution. Although aggression is useful in both humans and non-human primates, it seems to be more of an evolutionary drive in primates. In both humans and non-human primates, sexual selection appears to be a major underlying cause of altruism and aggression. Once again, this shows how similar we are to other primates.

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