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Primate Social Behavior: Adaptation for Reproductive Success

This article examines two very different types of primate social structure, The Hamadryas Baboon and the Orangutans, and shows how their socialization is adaptive for reproductive success.
  "Primate social behavior has evolved through natural selection (Fleagle,1999)”. This is why many primates live in groups. Fleagle goes on to say "in evaluating the importance of different factors that might select {in a Darwinian sense} for group living in primates, we must try to determine how group living would effect the fitness of an individual (Fleagle, 1999)”. Fleagle determines that there are four potential advantages to group living: greater protection from predators, improved access to food, better access to mates, and assistance in caring for offspring (Fleagle, 1999). Not all of these advantages apply to every species. Depending on the different characteristics of the individual primates any number of the advantages may not be applicable. Let's discuss different types of social groups and the advantages they may or may not utilize.

      I will begin with one of my favorite primates, the Hamadryas baboons. Hamadryas baboons have  a unique social structure among primates. They live in large groups that can reach populations of over 1000 individuals in some instances (Kummer, 1995). That large of a group is rare and usually center on artificial food sources such as a city trash heap. Most groups average from 50 to 150 individuals. Within these groups the social structure breaks down into single male and unrelated female harem groups. This harem group is very interesting for many reasons. As a male reaches sexual maturity he begins to kidnap immature females from other groups in order to begin constructing a reproductive group. He then acts as a surrogate parent to the juvenile females until they reach sexual maturity (Kummer, 1995). This requires constant monitoring and behavioral adjustments. I have observed this myself in Saudi Arabia. A female who wandered to close to other males was dragged to the edge of a cliff by her patron male and made to standstill in a submissive posture (see figure 1). This is not copulation, note that the male, reckognized by his size and mane of hair, is standing on the female’s right leg a holding her low to the ground.

Figure1 . Hamadryas baboons picture by Creighton Smith 1992

      The question becomes, how is this behavioral adaptation designed to increase individual reproductive success? The Hamadryas baboon maintains his harem throughout most of his sexually active adult life (Kummer, 1995). Many years pass before another male might usurp his position as the harem master. This meets Fleagle's prediction concerning the advantage of better access to mates. One could even make an argument for this kind of control over mate availability as being a highly evolved. His access to sexually reproductive females is almost guaranteed. Another advantage of that seems clear from their overall social structure is large numbers of these harems moving and feeding cooperatively, provides mutual protection from predators. There are two possible reasons for this. Predators will often times think twice about attacking a large group of primates especially when they present a unified front and brandish their large canines which all the big males possess (Kummer, 1995). Also, and more widely accepted, is the idea that an increase in overall numbers reduces an individual's chance of being selected as prey. This is precedented throughout the animal kingdom. From schools of fish to flocks of birds to roaming herds of wildebeasts. We see this strategy of congregating in large numbers as a defense against predators. As for assistance in protecting and rearing offspring, Kummer suggests that where other females in the harem may not take an active role in raising the young, there are many pairs of eyes to watch each baby and raise the alarm in case either a predator or other male Hamadryas baboons should approach the harem. However a large group does tend to reduce access to food sources. Other Primate social structures are designed around maximizing this potential but one finds that those are generally small groups. Let's take a look at these advantages as they apply to a very different kind of social group, orangutans.

      Orangutans live in what is known as a noyau social group, according to Fleagle. That means orangutan males live a solitary existence with large home ranges and orangutan females, who are also solitary, have a smaller home range. This results in males home range overlapping many different females. This does provide the advantage of greater access to food sources (Fleagle, 1999). There's less competition from other individuals. Predator avoidance comes from the fact that orangutans are almost primarily arboreal, spending upwards of 99% of their time in the trees foraging for food (Galdikas,1995). Despite their large size they spend a good deal of time in the tops and near the terminal branches of trees that will support their weight. The primary predators, other than poachers, are tigers. Tigers have access to the lower branches of trees but very rarely if ever move into the top portions of the trees were the orangutans live. For female orangutans there is no assistance in protecting and rearing offspring. Each female must fend for herself. Access to mates is another question. As far as individual reproductive success goes, large home ranges with a single female is counterproductive to an individual's males mating success. Each female gets visited by different males with overlapping home ranges. When those males per chance to meet in a females home range, very often a fight will ensue that can be deadly to either party.

     The two preceding examples of Primate social structure are examples of niche’ development. One would assume niche’ utilization to be adaptive as it is necessary for the survival of a species. As the social systems exhibited him by these primates have one or more of the advantages that lead to reproductive success, we can infer that in fact social groups are adaptive and contribute in no small way to the continued survival of the species. However, in the case of the Orangutans, they are extremely endangered due to humans destroying their habitat of which they need a lot to maintain their solitary life styles. Conversely, the baboons are able to adapt quickly to human encroachment and as a result have thrived in the margins of human expansion.

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