The Evolution of Primate Sexuality and Social Organization

Primates evolved dynamic social mechanisms and reproductive functions that were critical to their existence and domination in their respective environments. Adaptation is a transition in an organism's characteristics that allows it to live and replicate in an environment that would be unsuitable for the organism otherwise (Fleagle, John G). Adaptation is a critical approach to studying primate social organization and sexuality. Natural selection is the primary cause of adaptation in animals, and it is one of the fundamental tools of evolution, along with migration, mutation, and genetic drift. Natural selection occurs when features of the organism which increases its chances of survival, are selectively passed on to the next generation. The features may be behavioral or anatomical. “Evolution is the genetic change that occurs in an organism through time it can be because of natural selection or other non-Darwinian processes such as genetic drift” (Kappler, Peter M., and Carel P. Van Schaik).

Primate societies have been traditionally characterized based on their social organization which can be fundamentally differentiated as pair-living, neighborhood and solitary, and group -living species. A social organization encompasses the sexual composition, size and the spatiotemporal cohesion of a primate’s society A social component of primate’s organization exists in mating (Hemelrijk, Charlotte K). Animals are sometimes forced to move to different areas to look for mates, and this leads to an inevitable social interaction between these primates and their potential mates. In a social organization, animal lives and acquires food either alone or with a single or many members of opposite or the same sex. Primates have evolved to the most intelligent Order in the animal kingdom; this intelligence makes them far more capable of creating complex social systems and sexual behaviors that guarantee them survival in their habitats.

Primates have evolved into two main phyletic groups, the strepsirrhines, And the haplorrhines which include the tarsiers and the cosmopolitan anthropods.



Grade 1- The Lemuroids

There are many species of lemuroids characterized by three uniquely specialized features that they possess. The first skeletal feature that can be used to identify a lemuroid is that they have a single claw on their feet’s second digit exclusively used for grooming. Secondly, their dental tooth comb is relatively unusual because their lower incisors and canines are kept separate by a large cleft, and finally, they have a lateral flying talus.

An example is a Malagasy lemur, which usually weighs below 5kg. Their social structure is group based even though they don’t build nests. Some are more active during the day while others are active at any time of the day or night. Lemur catta which dwell in the south of Madagascar are the commonly known Malagasy mammal; their long-stripped tail is almost iconic hence the name ring-tailed lemur. The ring-tailed lemur mainly uses their four limbs in walking and running, and they are mostly terrestrial spending more than 60% of their time on the ground rather than on trees. They have adapted well to their diet which is primarily fruit and leaves (Fleagle, John G).

They live in enormous social groups which contain around 20 individuals with a male to female ratio of 1 to 1. The large social group provides a great sense of security which makes it possible for them to travel for more than half a mile in one day and occupy a territory ranging from 10 hectares to 32 hectares. In a lemur society, the female often exercises dominance over their male counterparts with females often staying with their natal groups from their birth while males migrate. The common pattern of male migration is once every three years; this makes the average lemur social groups’ composition have both natal and immigrant males that occupy peripheral positions around the core of the social group which is the females.

Lemuroids have evolved into primates capable of intricate sexual patterns, mating rituals, and social organization. The adult males are constantly showing submissive behavior evoked by the males as illustrated in Lemur catta, which portrays female dominance. Except for a few species such as Eulemur fulvous, female dominance occurs in most Malagasy primates. The adaptiveness of female dominance in the lemuroids is the result of biology. However, the role played by the males in the female dominance evolution is not fully grasped and can’t is unexplained by Darwin’s sexual selection theory. “The sexual selection theory explains the extent to which secondary sexual characters adapt. An animal’s reproductive success according to the theory is due to the evolution of secondary sexual characters” (Hemelrijk, Charlotte K). The theory also states that the evolution takes place so that an animal can increase its attractiveness to the opposite sex or compete with same sex members to find a mate. Males are subjected to intrasexual selection because they are forced to compete amongst themselves in to be accepted by a fertile female.

There is an occurrence of strong social bonds between certain pairs of male and female individuals in lemuroid groups; this often increases the probability of the males in a bond group to reproduce with the female they have bonded. The pair bonds create a disadvantage to the males which have not acquired a bond and devalues the natural competition to find a mate which makes lemuroid sexual traits appear monogamous. The lemuroids, social organization, and social structures which predicate on two main principles, their activity period and their fundamental units, such as the pair bond which mainly consist of a male, a female, and their offspring.



Grade 2- The Tarsiers

The tarsiers are among the smallest primates, they also the most unusual in the primate order. They exhibit both anthropoid and prosimian features. Tarsiers have large eyes which are larger than their brains; they have a retinal fovea which makes them resemble diurnal primates. Their eyes which occupy a large surface area of their heads remain protected by a bony socket that is like those of higher primates. Their noses are minus the attached upper lip, like those of higher primates. Tarsiers are well adapted to jump high; they have very long legs and ankle regions which contribute to this (Kappler, Peter M., and Carel P. Van Schaik).

Their reproduction process is also similar to those of higher primates with the inclusion of a hemochorial placenta like in apes, monkeys, and humans and unlike lemurs, they undergo monthly sexual cycles. Tarsiers dwell in secondary forests, and they are completely nocturnal. Their day is spent sleeping in trees or on grass vines, and they feed very close to the ground, their method is moving is primarily leaping where they can leap up to 3 meters which are very impressive considering their relative size. Their diet consists mainly of insects and other small animals such as lizards and birds.

They are usually in groups of below ten animals. They for pairs whereby a pair can occupy a territory of up to two hectares marked with sweat and urine. Tarsiers are small animals, but they have an unusually long gestation period which lasts for six months, and when the child is born, its eyes are already open, and it has the capabilities of vertical leaps. Females will stay with their parents until they are adults but males go out on their own when they are still young juveniles. Sexually mature tarsiers are monogamous, once they chose each other as mates, they might stay together for as long as seven years in their small family groups, usually with their still immature offspring. Social behavior in tarsiers and other primates primarily evolve to defend their territories and infants from outsiders that pose a threat. Another reason why a social behavior evolved in tarsiers is for food security when resources are depleting; relatives come together to share food and guard the remaining resources against being compromised and stolen by the outsiders. Infanticide is common in tarsiers in the wild and captivity, and it is because of this that the male stays close to their offspring to protect it from being attacked and killed by the outsiders. The male stays close to the females, and they travel together for protection (Fedigan, Linda Marie).



Grade 3- The Monkeys

Monkeys rank higher in the primate hierarchy than lemuroids and tarsiers. Platyrrhines or New Word monkeys have similar characteristics and anatomical features that enable categorization in the same taxonomical groups. “They have an evolution history of nearly 30 million years with origins from South and Central America where they flourished due to the lack of other primates that would’ve created competition for the available resources” (Fleagle, John G) . They are small sized primates with the common maximum weight of 10 kg; another distinct feature is that they have narrow nostrils, unlike apes and humanoids. Their physical features are primitive compared to those of Old World catarrhines. For example, they have the tympanic ring fused to the auditory bulla, and they have three premolars. Platyrrhines have unique and specialized features such as the lack of hypoconulids in their two molar teeth, they also have evolved separate sphenoid and the frontal bone which are kept separate by a parietal zygomatic bone, and they have long and narrow skulls (Fedigan, Linda Marie).

Most monkeys, for example, the crested capuchin are very intelligent animals that use social behavior to enable them to flourish in their natural habitat. The social behavior is however not absolute; it varies depending on the species and the environment that it dwells. Vocalization is the primary method that monkey use to send messages, they have a variety of sounds that can be used to express different messages, for example, warning if a potentially harmful animal approaches. They also use vocals to form bonds with the young infants.

A clear majority of monkey species live in bisexual groups of more than three adults which are polygynous, multimale and multifemale groups. Sometimes in polygamy societies, the male monkey engages in fierce competitions in to find receptive females willing to mate with them. In other species, males are prone to defend territories which have more females and potential mates. A male’s ability to defend their females usually determines primate mating. When there are many females in a smaller territory or their reception to the males’ advances are not synchronized then monopolization is more likely to occur than when the females have synchronized periods and are not solitary (Hemelrijk, Charlotte K).

Male and female relationships get determined by sexual conflicts and sexual selections. Other factors that influence male and female relationships in monkeys are; how the monkey ranks in the dominance hierarchy, the risk of infanticide, the degree of sexual dimorphism and the length of time that a male monkey resides in a group. Other males and female relationships occur as the females do most of the grooming in the groups, while the males offer protection to the females and their infants.







Grade 4- Apes and Man

There are two different radiations that make up higher primates, the Cercopithecoidea, commonly known as Old World monkeys and the hominoids. Hominoids lack diversity as there are only five extant hominoids, ranging from a 200-kg gorilla to a 4kg gibbon. Humans are the only hominoid species to be geographically present in all regions of the world, and this is because of the superior intelligence which has enabled humans to travel to and survive most climatic conditions. The remaining hominoid species are note widely distributed. Instead, they scatter in Southeast Asia and Africa (Fleagle, John G).

Hominoids have relatively large brains, broad nasal regions, and broad plates. They do not have a tail, not to mention their expanded sacrum. Their wrists have a fibrous meniscus instead of the ulna and carpal bone articulation. The hominoids behavior varies greatly with that of Old World monkeys because of their ecology and world history. They have relatively longer gestation periods than similarly sized animals; they also take a longer time to reach sexual reproduction maturity when compared to other animals of the same size. Their social organization is not female philopatry based which makes their societies not organized around females, Unlike many other primates. Hominoids categorization occurs in two families: the hominids and the hylobatids. Gibbons are the only species in the hylobatids’ family. They are the most diverse and the most primitive of the apes, with many anatomical features like the monkey such as the limb proportions. Gibbons weigh between 5 and 11kg, both male and female might weigh the same there’s no sexual size dimorphism. They are unique such that they have the longest forelimbs to body ratio of all primates their legs are also length. Their females are the only ones of the apes that have sexual swellings which change color and shape depending on the time of the estrous cycle. Gibbons live in social groups. The groups’ composition is female, one male, and their offspring. Some populations may, however, have more than one adult male or adult female. They are territorial primates that defend their territories which usually results in conflicts with outsiders (Kappler, Peter M., and Carel P. Van Schaik). Their rate of reproduction is relatively low with females having single births after every couple of years. The young spend a considerable amount of time with their families, usually around ten years before they are hassled out with the same sex parent.

The remaining hominid genera are the humans and the great apes which consist of the Gorillas, orangutans, bonobos and the chimpanzees. Except for the Orangutans, all great apes have origins in Africa. Great apes are relatively larger in size, and they possess more advanced and detailed anatomical features. The most distinct of these features are; a broad ilium, broad premolars, and a robust fibula. Great apes primarily dwell in the forests where they build sleeping nests.

Humans have the most diverse social organization of all the primates and arguably all the animals in existence, human societies have a lot of small structures which evolve into large complex multilevel societies. Communication takes place by using a language with is a lot more advanced that the method of communication that other primates used, although many verbal communication patterns and signs in chimpanzees can be recognized by humans.



In conclusion, this paper has established that primate sexuality and social organization has greatly evolved, and this primarily to increase their chances of survival in their environment. Many social groups are formed to protect the animals from attacks by predators, to protect the young from being attacked by outsiders and other species, and ensure that they secure a mate and sire off-springs. Primates are greatly diverse in all aspects of size, diet, and social interactions but they have common features that make them categorized into one taxonomical group.



Works Cited

Fedigan, Linda Marie. Primate Paradigms. Chicago, Univ. Of Chicago Press, 1992,.



Fleagle, John G. Primate Adaptation And Evolution.



Hemelrijk, Charlotte K. Self-Organisation And Evolution Of Social Systems. Cambridge, Cambridge University Press, 2011,.



Kappler, Peter M., and Carel P. Van Schaik. "Evolution Of Primate Social Systems". International Journal Of Primatology, vol 23, no. 2-35,.