Primates as a group is an example of ecephalization, they all have relatively large brains for their body size compare to other mammals (Dunber, 1998). Brains are extremely costly to evolve and to preserve. A human adult brain weights about 2% of body weight but used up about 20% of total energy intake (Aiello and Wheeler, 1995), making the brain the most expensive organ to maintain. Natural selection would hardly evolve something that is totally functionless and yet so costly and still maintain it because species are able to evolve the character. Therefore, even with the energetic considerations constrain brain size, there must be a valid adaptive explanation to why for evolution, they grow to the size they are now.
There are four different hypotheses that have been put forward to explain primate brain evolution.
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Epiphenomenal, large brains are an inevitable outcome of evolving a larger body.
Developmental, maternal energy constraints decide the rate for foetal brain development.
Ecological; large brain confer improved ability to integrate complex and interact with environmental information.
Social brain hypothesis; brains are constrained by size of social group size.
The epiphenomenal hypothesis
This hypothesis was brought out by Finlay and Darlington (1995), assuming that evolutionary of brain is not a consequence but a by-product of external selection pressure and have nothing to do with the way biological growth processes are organized. It argues the brain evolution is just a simple by-product of body size evolution and the increase of brain size is a positive outcome of total brain evolution.
The developmental hypothesis
This hypothesis assumes that the development of brain size was determined in early life stage. It is influenced by the amount of maternal resources invested. This is based on the fact that higher taxa mammal, had a longer period to completion of brain development. Therefore, a volume of brain growth during early birth stage, with little postnatal brain growth occurs being done by the time the infant is weaned. This conclusion is drawn that brain evolution must be constrained by the spare energy, exceeding maternal basal metabolic requests, which the mother has to pass down to foetal growth (Hofman, 1983). Some support of this thought by the fact that frugivorous primates have larger brains compare to body size than folivorous primates (Clutton-Brock, Harvey, 1980). This has been explained as that frugivores have a better nutrition diet than folivores do and as result have more additional energy to invest in foetal development. Large brains are seen as a kind of emergent epigenetic effect of spare capacity in the system.
Less support for developmental hypothesis
Both hypothesis suffer from lacking of scientific support, the problem is that they ignore the primary principle of evolutionary theory, the middle point of expenses
and benefits. As mentioned before, maintaining a large brain is extremely costly; basically not wise to evolve large brains just because they can. Large brains shall evolve only when the selection factors (such as size) are in favour and animals are able to overcome the high expenditure. Development constraints are unquestionably important, but rather than being a constraint, it’s something that must be conquering if larger brain is needed to develop. And this theory offers no clarification to why the brain should always develop to certain limit and not bigger. But this can help to understand if big brains need to develop; a larger body also need to co-evolve to facilitate the energetic costs to maintain the normal function of the brain or a diet that ensures sufficient energy can be supply for brain growth (Fig1).
Ecological hypothesis
The ecological hypothesis predicts that the increased cognitive ability allows individuals to solve more difficult ecological problem. Basically, big brains allow more behavioural flexibility and extract useful resource from novel environment. The ecological hypothesis are a combinations of three elements; Dietary, mental maps and extractive foraging. Each argues that primate will need larger brains if they want to achieve more in the predicament.
Some experiments have indicate frugivorous primates have larger brain for their body size because they have a better ability in remembering and put together information on the spatial-temporal distribution of patchy food supply (Milton,1988). Scientists suggested colour vision development and frugivory are intertwine in primates (Jacobs, 1995).A better developed brain enables animals to discriminate between fruits of different colour, better sensitivity of fruits against a background of green leaves (Jacob, 1995). Therefore, frugivores’ large brain can be selected basic on the frequent use of colour vision. The non-visual cortex is responsible for this function and therefore predicts frugivores should have larger brains (Barton, 2011)
The second component of this theory is; when food is patchy and spaced out, animal need to hold a mental map inside its head in order to navigate its way from one food resource to another within a huge home range. Tests had proven primates do have mental maps or spatial cognition (Olton, 1985). Therefore we would expect the area in the brain which linked with spatial awareness will increase. Hippocampus is mostly associated with spatial memory (O’Keefe & Nadle, 1978) and would expect an increase in that part of the area.
And the last one is extractive foraging, such as cracking nuts. This hypothesis predicts that primate species should develop bigger brain if they need to extract food resources from a medium which they are embedded in such as fruit pulp from case; gum flow from tree, termites(ref). Cognitive activities are needed more than those who are not extractive foragers (Dunbar, 1998).
So far, there is not enough significant evidence to support the correlation between neocortex and the diet, mental map and extractive foraging but that is a slight positive trend.
Social brain hypothesis
Small animals live in groups to avoid predations and enhance reproductive success; the predation rate from predators on an individual animal is a negative correlation of social group size (Shultz et al 2004). As a social system began to build up, a more complex relationship is formed between each individual in the group. Larger brains may develop due to the demands of complex social systems such as tactical deception and coalition- formation (Dunbar1998). Allowing individuals to keep track of more than one relationship, respond appropriately during interactions with other individuals. Cognitive demands should therefore increase with group size because the number of individuals in the group should be proportional to the minimum number of relationships each member might have to build (Baron 1996). Still, the complexity and nature of these relationships may vary in different types of group style. For example, large aggregations which are often unstable due to the fact that they don’t perform cooperative foraging; then individuals in these groups will find inevitably hard to form stable social bonds or frequent encounter with others of the crowd. Quite the opposite, a stable, cohesive social group interact frequently with other members within the group (Figure 2). Ability to recognize group members appearance will help to keep track of the dominance or affinitive relationships can appear between group members and in long term formed a stable cohesive pack. Once relationships are established, animals need to constantly maintain those status; either by grooming cliques or tactical deceptions (deliberately mislead other members) to interact with others. It has shown that lower ranking species will come up with ways that take advantage of social strategies like alliances or female choice to get round what would be consider as the higher rank males’ domination over matings. Both activities showed a high cognitive level (being able to keep track of other’s ranking in the group) and significantly correlated with the increase of neocortex ratio (Neocortex is associated with thinking ability) (Byrne &Corp, 2004; Kudo& Dunbar, 2001). Also social play was found positively correlated with neocortex ratio; social play provides members a chance to learn from others in the group, avoiding individual to go through the long trial-error learning period. Familiarity of socialization are built up through watching, mimicking other members. Evidence showed that between the period of weaning and first reproduction, there is a huge increase of development in volume of neocortex frontal to the primary visual. Growing up in a socially stable environment is one important factor in developing a larger brain.
