primates are more cognitively advanced than other mammals and that the degree of cognitive awareness and ability grows significantly from prosimians to humans. However, researchers still debate which parameters should be used to define and compare intelligence as well as the causal factors leading to this cognitive growth. Intelligence is a concept hard to define and even more difficult to test for in living species. In studying how human intelligence evolved, scientists are faced with a dilemma; how to determine the intelligence of individuals that no longer exist?
Methods that have been used to determine the level of intelligence in animal ancestors include the comparisons of the volume of the braincase, brain size to body weight and neocortex development. Studying social behavior also gives clues to cognitive level. Anthropologists, for example, have a variety of theories on what caused the significant leap in intelligence from the prosimians to humans. Which is correct: Arboreal life, foraging, meat-eating, change in visual patterns?
Studying today’s population of primates can shed some light on cognitive evolution. It can also provide additional information on what makes humans unique and what can be done to eliminate primate extinction and enhance human evolution.
Prosimians, including lemurs, lack many typical primate characteristics and behave differently from monkeys and apes. The cranial capacity of a lemur is only about 24 cubic centimeters, compared to the gorilla of 505 cc and orangutan of 497 cc. The scanty evidence from prosimian fossils as far back as 55 million years ago shows relatively large occipital and temporal lobes and relatively small olfactory lobes (Noback & Montagne, 1970, p. 220). Expansion of the frontal lobe lagged behind that of the rest of the cortex, since even by 35 million years ago, frontal lobes were smaller than present- prosimians (except for tarsius). The prosimian neocortex, where planning, reasoning, and language take place, averages 14.5 times larger than the basal insectivores (ibid, p.113).
In many mammals, smell is the dominant sensory mode. Smell is less important with the higher primates. In lemurs, the nerves responsible for olfaction pass between the orbits from the internal cavity to the brain. In addition to their sense of smell, lemurs and tarsiers have another sense that is important in sexual communication — Jacobson’s organ, stimulated by substances found in urine of female primates and permits other individuals to determine chemically the reproductive status of a female (Fleagle, 1999, p. 24).
Much variability exists in the social behavior of the Malagasy lemuroids of Madagascar. In addition, they are difficult to know because so little information is available about subfossil species. Many species, especially nocturnal ones, have very primitive, relatively basic social structures; others are monogamous; and some live in larger groups with many males and females (ibid., p.109). The organization of larger groups also vary, with matriarchy and clusters of monogamous units in lemur cata. Unusual is the frequency of female feeding dominance over males. Ring-tailed lemurs do not mate for life. After attracting a female and mating, a male looks for another female.
Most lemurs live in treetops, but lemur catta spend more time on the ground when foraging (ibid). Lemurs have scent glands on their arms that produce a smelly odor. They rub their long striped tails on these scent glands and then wave them in the air when they meet an opponent. This smelly duel is known as a “stink fight.”
Lemur females usually give birth first at three years of age and produce offspring annually. In the wild, mating begins in April/May with birth in August and September. Single infants are most common, but twins are frequent when food is plentiful. Over the next five months, infants spend increased time alone, returning to the mother to nurse or sleep until weaned at five to six months of age (Jones, Martin & Pilbeam, 1992, p. 67).
When the lemurs move to different grounds, they use their tails as flags to direct one another to the right destination. Each lemur has a high piercing scream, which is used to alert one another of potential danger.
Tarsiers of Southeast Asia are among the smallest living primates. Once considered a true prosimian, they are now placed on a separate radiation leading to the lemurs. Although nocturnal, tarsiers share several traits with anthropoids such as a dry nose and lack of a tapetum lucidum, another layer to the retina in the eye. However, they also have unique traits such as each eye larger than its brain. Their brains are only 3cc vs. The lemur 24cc and the human 1,400cc (Fleagle, 1999, p. 119).
The nocturnal tarsier is arboreal, spending its entire life in trees. It travels by leaping from tree to tree. It even sleeps and gives birth clinging to a tree trunk. Tarsiers hop rather than walk on land. Similar to the lemur, they mark their territory. Unlike the lemurs, tarsiers are carnivores; they eat mostly insects, lizards, worms, and other very small animals. They do not probe for food sit and wait for prey (ibid, p. 55).
Both studies in the field and of captive tarsiers show that social behavior differs among and within species. In some areas these animals have been seen living in individual territories that overlap with males, family groups with mated pair and offspring, or sometimes larger polygynous groups. Some groups sleep together. In other areas, the males and females have separate territories. They groom when relaxing (ibid).
Tarsiers have very long six-month gestation periods and one of the lowest rates of fetal growth among mammals. Infants weigh about 30% of the mother at birth and suckle two months while they learn prey-catching (Jones, Martin & Pilbeam,1992, p. 43).
Old world monkeys fill an array of ecological niches in Africa, Asia and parts of Europe and the new world monkeys live in South America. They can be either arboreal or terrestrial. They are differentiated by the increase in brain size, both in relative and absolute terms, especially the cortex that increased in size and complexity. It is more folded than the smooth surface in other mammals. They have a greater reliance on vision rather than smell, reflected in the size of area of brain connected with vision, as well as in the forward positioning of the eyes (Ciochon, 1985).
Many of the old world monkeys live in large, complex groups known as troops. Life within these troops is often highly regulated by rules of etiquette, social ranking, and political and social groupings. Dominance is usually gained through fighting and making alliances. Baboons’ social organization includes close male/female friendships. Male baboons leave their birth troops and gain admission to and acceptance within a new troop through the help of female friends (ibid).
New world monkeys spend most of their time in the treetops. Living at such heights, they feed almost entirely on leaves and fruit. There is ample food and few large predators, aside from larger snakes and some birds of prey. New world monkeys have not developed the complex communities and family groups found among old world monkeys and apes. Most species give birth to twins and live in groups of 4-20 individuals. Social structure varies from one male-one female to multi-male-multi-female groups, with usually only one breeding adult female at a time. Groups are territorial and defend their home range through calls, fur displays, scent marking and facial expressions (ibid).
Apes have much larger and more complex brains. The areas of the brain associated with creativity and data integration are especially enlarged. A chimpanzee brain is approximately 380cc and a gorilla, 550cc. The simian’s neocortex is 45 1/2 times as large as the basal insectivores (Noback & Montagne, 1970, p.113). Modern apes use cognitive abilities with tools, hunting and forming alliances. They are located in parts of Africa, Asia, Malaysia and Indonesia.
The orangutan gives birth to a single offspring about every five years. Puberty in both sexes occurs at about seven years of age. Females raised in captivity begin mating at eight to nine years and give birth for the first time at ten to eleven years. Wild chimpanzee females mature three to four years later. Females are not receptive for three to four years after giving birth, and then resume sexual activities for one to six months until conception. Gestation lasts 230 to 240 days. There is no regular breeding season, but females only mate during heat, which lasts two to three weeks or more and occurs every four to six weeks. Puberty in both sexes occurs at about seven years of age. Females raised in captivity begin mating at eight to nine years and give birth for the first time at ten to eleven years. Wild chimpanzee females mature three to four years later. Females are not receptive for three to four years after giving birth and resume sexual activities for one to six months until conception. Gestation lasts 230 to 240 days (Fleagle, 1999, p.41).
The chimpanzees have male-bonded societies where females migrate between troops and individuals leave and rejoin the group, meaning an individual potentially has private information it could share or withhold. Vocalizations of apes contain semantic detail about social relations as well as external threats. Chimpanzees give food-calls in the wild that attract others; in captivity they can lead others to hidden food, and convey its quality. Apes deliberately deceive others, concealing both food and sex and even fake facial expressions or erections. The capacities to give or withhold information and to be aware of others’ intentions may be pre-requisites for the capacity to form ideas (ibid).
The most primitive hominids had a brain size of 400 to 600, much less than was previously expected. Hominids from two million years ago shared food between the sexes and ages, had a sexual division of labor, and a family main unit. Men hunted and scavenged meat.
In 2001, researchers found a six- to seven-million-year-old skull in Chad that sends back the earliest hominid millions of years. They designated Touma, which displays a mix of both chimpanzee-like and human-like characteristics, a new species and genus of hominid, Sahelanthropus tchadensis. The most striking features of S. tchadensis are its tiny brain combined with a huge brow ridge and a very short, non-snouty face — unlike the face of a chimp or australopithecine (Meek).
Touma likely lived between a lake and desert, moving through a diverse landscape of grassland and forest in search of food. Questions still exist on whether it was bipedal or if it was a hominid dead-end, type of ape or truly a homo sapiens ancestor.
There is a wealth of information on primate cognitive evolution, including that which was used for this paper. From the present information on primate evolution, it is possible to see a continuum in brain and social development from the prosimians to the hominids and to today’s humans. However, it must be stressed that little is known about the cognitive capacities of the majority of primate species (Tamasello, 1997, p.17). There are 180 different species and only a few have been researched. Add to this the fact that there is great diversity between and among species. A related problem is that primates are studied by fossils, in the wild and in labs (ibid). These approaches are very different. When going to the zoo, I gave much thought to how similar or different the animals’ behavior would be in the wild if studied today or if studied millions of years ago.
Other problems include the definition of intelligence and to what degree each of the primates have shown cognitive abilities. Some individuals in this field of study have come up with a universally acceptable definition of intelligence such as the ability to use language or create tools to manipulate the environment. By placing parameters on intelligence that only humans meet, and lower primates fit to varying degrees, it is inherently impossible to find intelligence in any other species. Another problem with this humanist definition of intelligence is that it is based largely on human introspection and the knowledge that humans are conscious, rational, linguistic animals. Kenneth Marable argues “if the same criterion that are used to rule out non-human intelligence were applied to humans without the benefit of introspection, we would doubt even our own intelligence”
Using the size of the brain either in cubic centimeters, encephalization quotient or in terms of neocortex development also presents a problem. A particular example is the spiny anteater with a neocortex that is relatively much larger than a human’s. Cortical folding has the same problem, with the dolphin having more folding than humans.
Another concern is trying to find one cause for cognitive growth. Evidence can be found for supporting a number of theories from foraging to tool use and meat eating.
It appears evident there cannot be one definition of intelligence or one causality theory.
Individuals such as Tomasello agree with this paper’s thesis that it is important to look at a number of parameters when defining intelligence and determining cause for cognitive growth. For future study, Tomasello suggests (431): Instead of looking at a unidimensional definition of intelligence, have an approach of cognitive ethology “in which we look first at the adaptive problems faced by a particular species or set of species and then at the ways they have evolved to face them.” Additionally, researchers should look at both the physical and social theories of cognition. Those working in the field as well as those in the laboratories must compare their findings and understand the benefit of both approaches, researchers should broaden the species that are studied and that consumers, educators and scientists alike find ways to eliminate possible extinction of today’s primates so the study of these animals can continue.
Ciochon Russel L. & Fleagle, John G. (1985). Primate Evolution and Human Origins. Menlo Park, CA: Benjamin/Cummings Publishing.
Else, James G. & Lee. Phyllis C. (1986). Primate Evolution. Cambridge: Cambridge University Press.
Fleagle, John G. (1999). Primate Adaptation and Evolution. New York: Academic Press.
Jones, Steve, Martin Robert & Pilbeam, David (Eds). (1992). Human Evolution. Cambridge: Cambridge University Press.
Marable, Kenneth. (n.d.). The Neurological and Environmental Basis for Differing Intelligences: A Comparison of Primate and Cetacean Mentality. Retrieved May 22, 2003, at http://www.msu.edu/user/marablek/whal-int.htm
Meek, James. (October 2001). Monkey or man? Toumai, hailed as our oldest ancestor, is stirring ancient scientific rivalries. Retrieved May 20, 2003 at http://www.guardian.co.uk/uk_news/story/0,3604,808955,00.html
Noback, Charles & Montagna, William (Eds). (1970). Advances in Primatology — Vol. 1: The Primate Brain. New York: Appleton-Century-Crofts.
Tomasello, Michael & Call, Joseph (1997). Primate Cognition. New York: Oxford University Press.
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