Other Morphological Traits of Primates
The characteristics that separate primate from other animals fall into four major categories: locomotor, neural and sensory, feeding, and life histories. According to Jolly & White (1995), each group influences various features of survival and reproduction. For example, the sorts of teeth most suitable for processing various food or the types of visual systems best suited for seeing daylight versus at night are the results of ecological selection pressures that influenced primates throughout their evolutionary histories. Ecological and social selection pressures might have been accountable for the evolution of primate neural and sensory systems that influence the size of primate brains as highlighted above.
To comprehend morphological (anatomical characteristics) variations that separate primates from other animals, it is essential to differentiate characteristic that have functional benefits, which might consequently have been designated for, from traits that might differ only because they evolved in association with other functional traits. Plenty of physical characteristics, like tooth size, or limb length, are mounted with body size amongst primate with comparable feeding or locomotor systems. This basically means that larger primates have bigger teeth and longer limbs than smaller primates, although there are no other functional variations between them. When a trait strays from what is relatively constant relationship with another trait; nevertheless, the digression might be suggestive of a functional difference connected with selection on that trait (Ross et al, 2002). For example, muriquis and their spider monkey relatives have longer arms than other monkeys similar to them in body size - a trait that has been associated with their remarkable style of suspensory locomotion. An alternative to travelling on four limbs like quadrupedal primates, miriquis, along with a few other primates that have long arms compared to body size, travel by swinging from a suspended, hand-over-hand position too. Suspensory locomotion is an energetically expensive mode of travel compared to quadrupedalism, but it has the advantage of being a fast way to travel through the trees, particularly when the canopy is closed or uneven. The ability to traverse long distances quickly cuts down on time when energy rich fruit resources are widely dispersed. Therefore, the long arms of muriquis, relative to their body size, can be seen as an adaptation.
Allometric Scaling of Brain & Body Size
Two traits might be isometrically, which means one to one variations between two traits, or allometrically connected, which means proposional changes in one trait as a function of another, frequently due to developmental processes. Allometric scaling, or allometry, happens when two variables increase or decrease at speeds that differ. Allometric comparisons let us to differentiate between absolute and relative effects, and to recognise which is possibly specialised adaptation and not the products of natural laws, and influence all animals. For example, heavier animals categorically have smaller brain sizes than smaller mammals. Hence, an elephant's brain is approximately 4 times bigger than human beings (Martin, 1998). Nevertheless, brain size usually grows at a slower pace compared to body size; hence, even though the human brain is definitely smaller than an elephant's in size, it is actually larger, compared to our body weight, than the elephant's brain is to its body weight.
Human brain size is obviously different. When we compare humans along the mammalian brain-to-body size curve, our brains stand out as being much larger compared to our body size than what we would imagine from alllometric relationship that explains other mammals. Mysteriously, the next largest brain-to-body size ratio is discovered in dolphins, not another primate. However, as a group, primates are inclined to having bigger brains compared to their body sizes than other mammals (Martin, 1996). The brain sizes of early human ancestors, known as hominids or hominins subject to one's standards, also fall along the brain-to-body size curve of other primates until approximately 2-3 million years ago, when hominid brain sizes started to diverge from the primate pattern by becoming larger, relative to their body size, than those of other primates (Marks 2005).
Identifying allometric relationships is a crucial task because these relationships offer a foundation for understanding whether a distinctive characteristic like brain size signifies evolutionary selection pressures, or whether it is simply a result of selection pressures functioning on a connected trait, like body size (Martin, 1996). The big size of an elephant's brain is a result of their large body size. Whilst evolutionary selection pressures might have directed elephant's large body size, no such selection is needed to clarify its brain. In contrast, the comparatively large size primate brains imply that they are not merely a result of body size. Something else might have happened in their evolving past that awarded individuals with larger brains a discerning advantage.
References
- Cartmill, M (1990) Human uniqueness and theoretical content in palaeoanthropology. International Journal of Primatology 11:173-192
- Marks, J (2005) Phylogenetic trees and evolutionary forests. Evolutionary Anthropology 14:49-53
- Martin, R.D (1998) Comparative aspects of human brain evolution: Scaling, energy costs and confounding variables. In Jablonsiki, N.G & Aiello, L.C (eds). The Origin and Diversification of Language, Memoirs of the California Academy of Sciences, Number 24, Pasadena pp.35-68