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The primate brain refers to the brain that is found within members of the primate order, which includes "prosimians", "monkeys", lesser apes, greater apes and humans . The brain is a complex organ that is encased in a bone-comprised cranium situated at the anatomical superior position/top of the vertebral column . The internal architecture of the brain is intricate, but is organised into four distinct lobes, including the frontal, parietal, temporal, and occipital lobe. The specific expansion of the neocortex, however, is autapomorphic within the primates, in terms of neuroembryological development. The result of the increase in the volumetric composition of the neocortex has been an increase in the cognitive abilities of primates, which has been demonstrated indirectly through measures such as the Encephalization Quotient. The implications of primate brain expansion have been important in the social environment in which primates find themselves. An increase in cognitive abilities has been shown to affect primate foraging strategies to social interactions and life span. Although brain expansion is a costly event from multiple viewpoints, including metabolic, the benefits seem to have outweighed the expenses and continue to be favoured evolutionarily .

Classification/Taxonomy of Primates
Further information: Classification of living primates

Euarchontoglires
The order Primates is a subclade of Euarchontoglires, which is in turn a subclade of Eutheria of the Class Mammalia. Other Euarchontoglires include Dermoptera (Colugos) and Scandentia (Treeshrews), and more distantly, Rodentia and Lagomorpha (rabbits, hares, and pikas). The precise Linnaean levels of these clades are disputed; for instance, some scientists place Dermoptera within the Order Primates, and call "true primates" the suborder Euprimates.

Primates
Early primatologists, following in the footsteps of Linnaeus,, often arranged primates as a series of paraphyletic groups based on their distance from humans. This practice is frowned upon by most modern biologists, as it is thought to be misleading and based on an arbitrary characteristic such as "similarity to humans." Now, monophyletic classifications (in which groups are composed of all descendants of a common ancestor) are more preferred in terms of describing the phylogenetics of primates.

The group traditionally known as 'prosimians' includes the entire suborder Strepsirrhini, including Lemuriformes(lemurs, lorises, and bushbabies), Adapiformes (extinct lemur-like primates). It also includes those members of the suborder Haplorrhini that are not Simiiformes (Old World and New World monkeys, and apes)- namely tarsiers and their extinct allies.

The term 'monkey' refers to two living groups (and several of their extinct allies) within Simiiformes. The parvorder Platyrrhini comprises the 'New World Monkeys'(found primarily in South America), while the Parvorder Catarrhini contains both 'Old World Monkeys' (Ceropithecidae, found throughout Africa and Asia) and apes. Therefore, while the two groups of monkeys are each monophyletic, their combination (which excludes apes) is a paraphyletic one. As a result, the word "monkey" is not particularly useful for classification purposes.

Great Apes
The specific group of primates classically referred to as "Great Apes" corresponds to the taxonomic family Hominoidea, with four extant genera: chimpanzees and bonobos (Pan), gorillas (Gorilla), humans (Homo), and orangutans (Pongo). The family Hylobatidae (gibbons) are considered to be "lesser apes", a paraphyletic term that also includes several extinct lineages of apes.

The lineage of apes is thought to have separated from that of Cercopithecidae approximately 25 million years ago (MYA) .Proconsul, which was present between 23 and 25 MYA, is thought to either represent one of the earliest Hominoidea or to pre-date the split between apes and Old World monkeys.

When following bifurcations down the lineage that leads to humans, Hominoidea can be organized into the successive subclades of Hominoidae (which excludes gibbons), Hominidae (which excludes orangutans), Homininae (which excludes gorillas) and finally Hominini (which excludes chimpanzees and bonobos).

Hominini
The tribe of Hominini comprises the genus Homo, along with genera allied with them since the split with chimpanzees. Within this tribe, Homo sapiens is the only extant taxon. However, the fossil and genetic evidence shows a much richer heritage than this.

While evolution is by no means a teleological process that leads inexorably to anatomically modern humans, it is occasionally useful to model it in this way. Certainly, the progression of protohuman fossils through the Pliocene and Pleistocene eras show increasing similarity to modern humans. Morphological features that are particularly useful for classification include the size of the jaws and teeth, the size of the cranium, and particularly, modifications of the skeleton to accommodate a bipedal gait.

Molecular evidence suggests that the most recent common ancestor of gorillas lived approximately 6.2-8.4 MYA (million years ago), and the most recent common ancestor of chimpanzees lived approximately 4.6-6.2 MYA. The earliest fossils thought to belong to the human side of this division are Sahelanthropus tchadensis (7 MYA) and Orrorin tugenensis (6 MYA), followed by Ardipithecus (5.5–4.4 MYA), containing species Ar. kadabba and Ar. ramidus.

The genus Australopithecus includes Au. afarensis (with the famous 3.2 million year old specimen "Lucy"), Au. anamensis, Au. africanus, Au. bahrelghazali, Au. garhi, and Au. sediba. There is some debate as to whether certain other species belong within the genus Australopithecus or in their own clades, Kenyanthropus (which would contain (Kenyanthropus platyops), and Paranthropus (which would contain P. aethiopicus, P. boisei, and P. robustus).

Australopiths are thought to either represent the direct ancestors of the genus Homo (and thus of modern humans), or a close sister taxon. The fact that they show a significant number of adaptations towards bipedalism but relatively small cranial volumes (in the neighbourhood of 375-500 cubic centimetres, c.c.s) suggests that enlarged brains are a more recent development than bipedalism, and possibly a response to it.

Homo
The genus Homo contains modern humans and their closest extinct relatives. The genus is thought to have emerged 2.3 to 2.4 MYA with the appearance of Homo habilis, while modern humans emerged approximately 250,000 years ago.

There is some debate as to where some of these species began and others ended, which fossils are specimens of species and which are merely of subspecies. This is due in part to the fact that some species (such as H. erectus and H. habilis) are well-represented in the fossil record, while other more-recently named species have only a few known specimens. Given the genetic evidence that anatomically modern humans interbred with at least H. neanderthalensis and the Denisova hominins, it seems increasingly likely that there was a continuum of interbreeding between specially and temporally adjacent hominin species. The chart to the right shows one possible model of such a continuum.



For example, the species status of Homo rudolfensis, H. ergaster, H. georgicus, H. antecessor, H. cepranensis, H. rhodesiensis and H. floresiensis are currently disputed, while the Denisova hominin has not even been given a formal taxonomic name.

Structure/Anatomy of Primate Brains
The brain is an organ shared by all vertebrates and most invertebrate animals. It has many shapes and forms which vary both between and within species. In the case of primates, all living species share similar large-scale structures, but have the most notable differences in the relative sizes of the regions. This can partially be explained by Mosaic brain evolution in primate evolution. As mentioned previously, one of the main differences among primate brains is their size, which is larger than other animals of a similar body size. Specifically, Homo sapiens have the largest of the brains among primates, about three times larger than chimpanzees of an equivalent body size (~1350 g vs. ~400 g).

It is important to note that the human brain is not simply a scaled-up version of a chimpanzee’s brain. Rather, the size difference is primarily the result of variable growth in different parts of the brain. For example, by comparing relative size (as a fraction of the whole brain) of the frontal lobe of humans and other apes, no significant difference has been found (the human frontal cortex has an increased density, but not increased volume). However, the human cerebellum is smaller than expected for an ape brain of its size. It turns out that the degree of interconnectivity between neurons and various parts of the brain may be a more important factor than overall brain size in making humans different from other primates.

One readily apparent difference when looking at primate brains is the variation in density of grooves (a combination of gyri and sulci) on the surface of the brain. Just like the villi and plicae circulares which increase the surface area of the intestines, these grooves increase the surface area of the brain, allowing for more neural connection.

Skulls




Like the brain, the skeletal structure of head is well-conserved among primate species. However, the 'degree' of growth and development of each bone is under the control of genetic and environmental regulation.

The next major variation in the primate brain is seen in the shape and features of skull, more specifically the location of the foramen magnum (the large opening in the base of the skull through which the spinal cord exits). Differences in the position and orientation of the foramen magnum could reflect differences in the habitual body posture and mode of locomotion. Studies on skulls from many living and extinct primate species have shown a positive relationship with the degree of bipedalism and anterior positioning of the foramen magnum. Some researchers hypothesise that the position of the foramen magnum in humans evolved primarily to help balance the weight of an increasingly large brain and cranium, located on top of the vertebral column. In humans, the foramen magnum is positioned anteriorly and inferiorly, which allows the spine to exit the skull at an angle of almost 90 degrees. In all other known primates, this opening is located further posteriorly. This difference in the positioning of the foramen magnum is due to variable growth at different parts of the occipital bone. In humans, the anterior-inferior portion grows far less than posterior portions of the bone, pushing the opening forward. In other primates, this is reversed, pushing the opening back.

Primate Brain Development
During primate evolution, one major change to the brain has been the volumetric expansion of the neocortex. Humans show a significant increase of the neocortex relative to other apes, that in turn show an increase relative to monkeys. The increase in volume is due to an increased quantity of both neuronal and non-neuronal cells, and to an expansion of surface area (i.e., tissue folding). The expansion of the neocortex starts prenatally through neurogenesis and postnatally through gliogenesis, axon growth and myelination.

Although there is an expansion of the neocortex, there has not been a proportional increase in the thickness of the cerebral cortex. Two hypotheses have been proposed as potential explanations for this observation of primate brain expansion. The first hypothesis is the Radial Unit Hypothesis, which purports that the expansion occurred due to the prolonged cell division of the progenitor cells found within ontogenic columns (columns of cortical neurons that are stacked on top of each other in a radial manner). The increased duration of divisions of the apical progenitor cells within these ontogenic columns increases the surface area of the columns and thus, of the neocortex. There is proof that when mutation of the regulatory genes responsible for the symmetrical and asymmetrical divisions of the aforementioned cells, the expansion of the cortex will ensue . This hypothesis is controversial as an increase would be expected in the lateral ventricle surface area, or a growth in the lateral ventricles that is in proportion to the expansion of the neocortex.

The Intermediate Progenitor Hypothesis focuses on the asymmetrical neurogenesis of apical progenitor cells. This process would result in the production of a neuron and a glial cell, which would undergo further asymmetrical divisions to produce another round of glial cells and neurons or progenitor cells within the subventricular zone (SVZ). The progenitor cells in the SVZ would further give rise to another two neurons. Thus, the abundant increase in the number of neurons would produce lateral expansion.

Although these two hypotheses are being considered as potential explanations for the expansion of the neocortex in primates, one definite and established factor that is involved in expansion is the control of the timing of the activation or suppression of cell fate determination points during development . Also, the timing of neural progenitor apoptosis has been shown to affect the abundance of neurons in the neocortex. Overall, the timing of neuronal cell production throughout development within the cortex and how the neurons are organized within the neocortex both have both been shown to be crucial to the resulting architecture of the brain.

Primate Brain Genetics
Genes and gene mutations are the underlying cause of structural changes that are seen during evolution and it is only recently that researchers have started looking at changes in genes and their level of expression in brain evolution. The first indication that genes were controlling brain development came from patients with primary microcephaly disorder. In these patients, there is a null mutation of microcephalin gene (MCPH1), which hinders the growth of brain during development. It has therefore been proposed that genes regulate brain size during development and may have the general propensity to contribute to brain evolution in primates and particularly humans. Other genes that are also involved in Microcephaly are asp (abnormal spindle)-like, microcephaly-associated (ASPM), and sonic hedgehog (SSH). These two genes plus MCPH1 have been shown to take part in adaptive evolution in the lineage leading to humans.

Primate Cognition
Cognition refers to the ability to learn, remember, and use information to modify behaviour in order to solve a problem in a given situation or environment, and at a given level of motivation. Within primates, having cognition, or cognitive ability, is an adaptive quality because it helps to improve the efficiency with which tasks or actions are carried out.

The two types of cognition that can be found in primates are: 1) Physical Cognition and 2) Social Cognition. The former refers to a primate’s ability to use foraging skills, learned information and memory, and defense mechanisms to solve problems within an environment in order to survive. The latter makes reference to the skills that are key to survival and required to interact with in-group and out-group members.

Intelligence Measures/Tools in Primates
It is often difficult to determine animal intelligence using the same methods that are used on modern humans. Thus, researchers have made use of a few tools to measure indirectly intelligence in animals, including primates, living and extinct.

Encephalization Quotient
The first and most often used method involves calculating the ratio of the observed brain size to the expected brain size which is also known as the Encephalization Quotient (EQ). The expected brain size is predicted from comparisons to the brain, based on the idea that brain size scales allometrically.

Progression Index
Another measure that has been proposed as a potential tool to measure intelligence is called a ‘progression index,’ which involves selecting a primitive baseline lineage (i.e., Family Tenrecidae and determining the presence of deviations as markers of evolutionary changes . Specifically, the index is a residual statistic (the difference in a regression analysis between the actual value and the statistical model predicted value of a dependent variable ) of a log brain mass vs. log body mass regression analysis. It is important to choose carefully the baseline lineage, as there is current debate about the choice of using the tenrecs as a baseline.

Neocortex Ratio
The last tool that is used to measure intelligence is a ‘neocortex ratio,’ which is a ratio of the size of the neocortex to the rest of the animal brain or to evolutionarily primitive or conserved brain regions (e.g., the medulla or the brainstem ).

Although all three measures are unique and used, each of them lacks the ability to be validated in its relationship to cognitive ability. This would be important to have, as it would have major implications on the conclusions that could be formed on the results. Although the absolute total brain and neocortex masses were found to be the best predictors of global intelligence and cognitive ability in primates, EQ remains the most commonly used measure of intelligence.

Primate Brain Expansion Hypotheses
With an expanding primate brain comes energetic costs (e.g., metabolic expenses, chemical and thermoregulatory conditions ) that must be counterbalanced, if not outweighed, by benefits. If the costs of having an expanded brain are too great, then evolution will favour less-expanded primate brains that are able to function equally or better than those primates with expanded brains. Given that the primate brains that are seen today have expanded, it begs the questions, ‘Why have primate brain expansion been favoured throughout evolution?’ Several hypotheses have been put forth to try to provide plausible answers to the previous question.

Epiphenomenal Hypotheses
This class of hypotheses suggests that the brain evolved and expanded in concordance with the expansion of the body, and that any increase in an individual brain region occurred as a result of the total increase in brain size. Thus, because of the increase in body size, the brain had to follow suit and increase in size. Moreover, because the brain increased in size on the whole, the individual brain areas had to increase in size, too.

Epiphenomenal hypotheses are predicated upon the idea that the brain evolved not as a result of the fundamental external selection pressure and instead, as a result of the biological processes evolving in such a way as to give rise to an expanded primate brain.

Developmental Hypotheses
Similar to epiphenomenal hypotheses, developmental hypotheses are based on the idea that brain expansion occurred as a result of an increase in body size and not as a result of selection pressures externally.

Where developmental hypotheses differ from epiphenomenal hypotheses is in the ability to provide a specific mechanism by which this brain expansion could have occurred. Specifically, these hypotheses propose that brain development is directly affected by the mother’s metabolic input. Thus, any energy that is more than that required by the mother’s basic metabolic requirement can be allocated to the fetus during fetal development which may increase brain size, according to developmental hypotheses. This idea has been supported by the observation that adult brain sizes in frugivores are larger than those of foliovores. It may be that there are more nutrients and energy that can be gained from eating fruits than from eating leaves, nutrients that may extracted by the mother and allocated to the fetus during development.

Ecological Hypotheses
There are three versions of this class of hypotheses:

I. Dietary Hypothesis
According to this first ecological hypothesis, if primates rely on extracting nutrients from a predominantly fruit-based diet (i.e., they are ‘frugivorous’), they will need an expanded, or larger, brain to be able to remember details such as the location of the food, the route or direction to the location of the fruits , and the patterns of availability of any given fruit on a temporal basis (i.e., from year to year).

II. Mental Maps Hypothesis
This hypothesis suggests that larger brains would have provided the primates with the capacity make mental maps of the range of their ecological environment. Brain size would be purported to affect how vast a home range the primate could occupy, as a larger ecological environment would require the primate to be able to make a larger, more complicated mental map of its environment. Moreover, brain size would need to increase, as it would affect how long the primate would be able to travel each day, which would also be integrated in a mental map. A larger brain size would be hypothesised as being able to allow for larger home ranges and for longer journey lengths.

III. Extractive Foraging Hypothesis
In certain cases, primates might have needed to extract their food from a matrix, or contained space, which would have called upon cognitive resources to help them figure out how to obtain their food. For example, a larger brain would be needed if a primate was trying to extract fruit pulp from within its casing, manipulate a tree to release gum or retrieve termites from their termitarium.

Social Hypotheses
These hypotheses are based on the social complexities with which primates are often confronted. At the root of these hypotheses is the idea that the larger the network the social interactions that a primate has, the greater it will be constrained by its brain size in terms of being able to remember relationships and the contents thereof, as well as in terms of being able to perfect social skills in a given social relationship.

Overall, epiphenomenal and developmental hypotheses are opposed because they do not take into account the idea that brains do not expand just because they can, given that there is a great, inherent cost that comes along with an increase in brain tissue. The evolution of a larger brain would only occur if selection factors were to overcome the expensive costs associated with brain expansion. Moreover, developmental hypotheses have been criticised because of data on precocial vs. altricial mammals. Precocial young have brain sizes that are twice as large as those of altricial young, irrespective of the fact that precocial mammals do not have higher metabolic rates than do altricial mammals. Social hypotheses are also viewed somewhat skeptically because they do not provide as robust a method of measuring social complexity, which would allow predictions of how large brains would need to grow in order to produce cognitive abilities that could help the organism survive and thrive within those social environments. Furthermore, studies that test social hypotheses do not take into consideration the idea that the total brain size may not be increasing and its parts follow suit; rather, the individual parts of the brain may be increasing which would thereby increase the total size of the brain. Although there are many arguments to take into account, these are the current classes of hypotheses that are being investigated as potential explanations for why the brain has expanded in primates.

Physical Cognition
This type of cognition is important in that it helps an organism to make sense of and interact with its environment ). Physical cognition refers specifically to skills that involve determining the location of food, obtaining food, and extracting the important nutrients. These skills may help to ensure the survival of the organism.

Object Manipulation and Tool Use
In order to use a tool, an animal has to be able to understand how it works or how to use it, as well as when to use the tool. Animals that manipulate objects have a wider diet and even the ability to decide between a variety of foraging methods. Of the primates, the ‘prosimians’ have the most restricted diet and are also the least inclined to manipulate objects. Monkeys, on the other hand, are more interactive with objects and are even capable of serially arranging objects. Great apes also show high interaction of and manipulation of objects; they show precision in their manipulations, including object stacking. The primates that are the most exploratory and manipulate objects the most are humans, being able to manipulate objects bimanually, relate and combine objects, and switch attention between objects.

Using tools is a complex feat which requires the intricate cognitive tasks of recognising and representing objects mentally. Being able to mentally represent an object is often defined by the ability to select a specific tool to carry out a task without a great deal of trial-and-error. This ability is found only within some species, which include some species of monkeys and great apes. Some monkeys that are help in captivity show signs of being able to use tools that are not typical or found within the wild.

Although one may be inclined to think that tool use is found only within primates, it might be a homoplasous characteristic that is actually found within primates, corvids, some elephants, and dolphins.

Features and Categorization
Primates are able to use features to distinguish between tools and objects, as well as to categorise them. This helps to make life easier and more organised. It is important to be able to recognise those features, remember what they look like and then, create a generalised rule system to use in a variety of novel situations.

One feature that primates use to categorise is an object’s physical features or other characteristics that are shared among the objects. For example, they would be able to categorise or group together geometrical figures that are three-sided (i.e., triangles). Moreover, primates are able to use abstract thinking and categorisation about objects more quickly and apply to a vaster array of objects than are other animals.

Lastly, primates have the fascinating ability to categorise objects based on concepts that are ecological in nature, as well as on their functionality. As an example, chimpanzees are able to sort objects, which is a type of categorisation that is often more difficult to perform and involves comparing objects to one another, as well as manipulating those objects on the basis of their function.

Delay of Gratification and Planning
Delay of gratification is an idea or term that is used to refer to the ability to postpone one outcome for one that is better at a later time. Being able to use self-control to reap the benefits of a larger reward at a later time instead of the gains of a smaller reward in the present has implications for goal-directed behaviour and planning. This ability has been shown in primates (as well as in dolphins). As an example, apes have been shown to cache or create tools to use a later point in time, showing signs of planning.

The ability to delay gratification and the degree to which this behaviour is produced is determined scientifically using either the ‘smaller-sooner/larger-later’ paradigm or the ‘accumulation paradigm .’ The smaller-sooner/larger-later paradigm is used to determine if an individual is able to abstain from taking a smaller reward sooner and to wait to receive a larger reward at a later point in time. The accumulation paradigm, on the other hand, attempts to determine if an individual is able to refrain from using the rewards in order to let them accumulate. Studies that have used such methods have demonstrated that primates in fission-fusion societies, in addition to food ecology, can favour the ability to delay gratification. It may be that fission-fusion societies promote higher cognitive ability, such as inhibition (i.e., delayed gratification), which may help to ensure their appropriate functioning in a social group that is ever-changing. All of the reasons that promote delayed gratification demonstrate that primates have foresight of events that may occur in the future.

Memory
Primates must have the ability to remember their physical environment and their interactions therein, as this will be important to survival and reproduction. They have the ability to remember where foods are found, where they are stored, the location of water, their daily journey, as well as where they are with respect to other geographical markers. This ability to create a mental map of the environment while foraging has been observed in apes, Old World monkeys and New World monkeys. Using their mental maps to guide future behaviour implies that there is the presence of episodic memory. It is thought that episodic memory is a construct of only the human brain and so, non-human primates are said to have ‘episodic-like memory.’ This episodic-like memory has been shown to occur in primate such as apes and monkeys, but also in non-primates such as mice and scrub jays, indicating that episodic-like memory is not an autapomorphy in primates.

Social Cognition
Primates live in highly social groups which makes skills that involve interpreting and predicting the behaviour of others very important, especially to their survival. This type of cognition refers to the ability to make memories and remember how others relate to oneself, as well as how others relate to each other.

The Social Intelligence hypothesis has been put forth in attempt to provide an explanation for why social cognitive ability may have arisen in primates. Specifically, it proposes that living in social groups was the pertinent factor in the outcome of primate intelligence. A related hypothesis is the Machiavellian hypothesis, which proposes that intelligence evolved to assist an organism to outsmart one’s group members. This would have been affected by factors that include: cooperation, deceit, and manipulation.

Cooperation
Observations of primates have shown that cooperation is a behaviour or set of behaviours that occurs between kin and non-kin, which implies that there must be an evolutionary explanation, especially for the latter. Cooperation is thought to occur as a result of potential gain in indirect fitness, and in situations where defection is unlikely to occur or in situations where there is reciprocity. It is a characteristic that predominates in apes and monkeys during ‘cooperative hunting,’ as well as during formation of alliances for defense and mating. However, reciprocity in non-human primates has been contested; evidence has shown that it does occur in apes and monkeys when grooming and sharing food. Being able to remember moments and occasions of reciprocity in the long-term is definitely a characteristic showing increased cognitive ability.

Deception
By definition, deception involves an intentional change in another individual’s behaviour for one’s personal gain. The goal is to elicit an effect on another’s behaviour, and may or may not be to also introduce false thoughts. Given that the idea is abstract and often difficult to inquire about verbally with non-human primates, studies have still shown that deception is a behaviour that is observed in primates.

In passive deception, primates will forego signaling or showing behaviour, as to not draw the attention of others. For example, refraining from making facial expressions or from making food calls would qualify as passive deception. Active deception, however, involves misleading others or presenting them with false information. When less dominant capuchin monkeys are in competitive situations where food is being consumed, they will use alarm calls at an inappropriate time (i.e., provide false information) to distract more dominant monkeys so that they can eat more food than usual. There is even counterdeception, which involves taking preventative measures to avoid falling into the deceiving trap that others have created. Being able to deceive others or even anticipate others’ deception is a definite sign of increased cognitive ability in primates.

Theory of Mind
When one is able to reflect upon and understand the mental states of other individuals, it can be said that one has theory of mind. This ability is important because it allows for understanding of the emotions, beliefs and intentions of other individuals, which is an important aspect of being able to survive. More specifically, having theory of mind makes one better able to generalize patterns of behaviour and to make predictions based on the potential mental state of others. This will allow one to take advantage of the situation and of others’ behaviour for one’s own benefit and for the benefit of one’s kin.

A classic characteristic of theory of mind is having self-awareness, which is often tested using the mirror self-recognition or mirror test. Studies using the mirror test have found that chimpanzees, along with other apes, elephants and dolphins do possess self-awareness (i.e., can tell that it is their own reflection in the mirror and not another chimpanzee, or ape, etc.). Monkeys, on the other hand, do not show signs of self-awareness. In scientific studies, no monkey is successful in theory of mind-related tasks or shows signs of being self-aware, yet apes do show positive signs of having theory of mind, can understand that others have mental states and can manipulate those states, can deceive, and even understand what knowledge others may possess. Even in humans, theory of mind is merely a spectrum, a continuum of a trait, rather than a characteristic that is uniquely human.

Allometry
Quantities that scale with a relationship in the form of $$y = kx^a$$ (where a, known as the "scaling factor", ≠ 1) are said to be allometric. In the context of biology, the term "allometry" deals primarily with the way that anatomy, physiology, or behaviour scales with body size, although it can also be used to compare such quantities while accounting for the confounding effects of body size. Such comparisons can be made either between species, or between members of the same species at different stages in their development.

By contrast, quantities which scale linearly (y = kx, with a scaling factor equal to 1) are said to be isometric. Isometric scaling is often used as a null hypothesis in scaling studies, with 'deviations from isometry' considered evidence of physiological factors forcing allometric growth.

One classic example of such a factor is the square-cube law. An organism that doubles in length isometrically will find its surface area increased by a factor of four and its mass by a factor of eight. This can lead to physiological problems. For example, skeleton and musculature scale with cross-sectional area, and would thus experience a doubled relative load, which may make them unable to support the increased weight. This is part of the reason why large organisms such as elephants require proportionally thicker and heavier skeletons than small organisms such as mice.

Allometrics of Brain Size
The size of an organism's brain can be used as a very rough indicator of its intelligence. However, given that humans by no means have the largest brains of any animals, and larger bodies are thought to require larger brains to control their bodies brain-to-body mass ratio is viewed as a better indicator. A more complex measurement, Encephalization Quotient, that is used commonly, as mentioned previously, takes allometric effects of body size into account and is thought to be a good indicator.

Because many of the species of interest now exist only as fossils (and thus there are no brains available to be compared to brains of modern organisms), cranial capacity is often used as a proxy for brain size. This causes problems for numerous reasons, not least of which is the failure to account for the proportion of "higher" areas of the brain (such as the frontal lobes) with those of 'lower' areas (such as the cerebellum). As such, overall brain size is a better indicator of intelligence than EQ.

Brain Size and Life Span
Within haplorhine primates (tarsiers, monkeys, and great apes including humans), a significant correlation has been demonstrated between brain weight and maximum life span, when the effect of body size is removed. It also has been found that there exists an inverse correlation between life span and metabolic rate (which was also negatively correlated with body size).