Why are the living primates so different from one another?
Start with body size.
Living primates vary from the 2 oz. gray mouse lemur (below) to the 350 pounds of the average male gorilla (above).
Why such a difference?
Three factors seem to predict body size:
time of day the species is active — All the nocturnal primates are small
where it is active (trees, ground) — of the primates active during the day the arboreal ones tend to be smaller than the ground dwellers
kinds of food that are eaten — species that eat leaves tend to be larger than species that eat mostly fruits and seeds.
Why do these factors predict size?
the relationship in mammals between body weight and energy needs — larger animals require more absolute energy, smaller animals require much more energy-rich food — insects, fruits, tree gum and sap — all high calories/high energy. Leaves are lower in calories, so leaf eaters have to consume a lot of food in order to get enough energy for their needs, and they have larger stomachs and intestines proportionally to extract the nutrients they need. Larger organs mean a larger skeleton and body is required.
Smaller primates which eat insects and other rich foods probably would compete with birds for food, except most are nocturnal — and birds are mainly diurnal. Still their smaller size would let them negotiate similar arboreal areas as the small birds occupy, albeit it in a different time of day.
Energy requirements may also explain why arboreal primates are usually smaller — in order that they are able to travel through small tree branches where fruit can be found.
Larger animals would need enormous energy to climb but can move effectively on the ground. Size of ground dwellers may also be a factor in protection against predators since they can’t take to the trees as easily as the little guys can.
REFLECTING ON BRAIN SIZE — Not all primates have the same sorts of brains
Since larger animals have larger brains primatologists focus on relative brain size, the ratio of brain size to body size.
Human primates have the largest brain relative to body size of any primate.
We tend to see this as a good thing, generally. But a large brain requires a great deal of metabolic energy to function and would not be favored by natural selection unless the benefits outweighed the costs of maintenance and feeding.
Fruit eating primates have larger brains relative to body size than leaf eaters.
Why might this be? Fruit eaters may need more memory in order to locate and identify ripe and appropriate foods, as opposed to leaf eaters whose food maybe more readily and easily available. Leaf eaters may not require the same memory capacity to be successful in their hunt for food.
Since fruit is a glucose heavy food, and since the brain requires large amounts of oxygen and glucose, fruit eaters may have more fuel for a larger brain. Leaf eaters may lack the energy to support a larger brain size because of a lower glucose content in their leaf diet.
LIVING IN GROUPS — hanging with your homies vs. the solitary life
Primate group sizes vary — from orangutans with solitary male/female pairs (with occasional youngsters), to a few individuals hanging together (gibbons), to 100 or more individuals as with some of the catarrhines of the old world. What is the reason for this variation in association?
Nocturnal activity predicts small body size and small group size. Since nocturnal predators hunt by sound nocturnal primates feed alone or in pairs. A larger group would be more obvious to predators, harder to keep still, and would be less able to move successfully through their feeding area.
Diurnal activity may favor large groups — the more eyes, ears, and noses in play and on the alert the quicker danger is identified. Safety in numbers! Predators may be less likely to attach a mob, and may prefer to wait for an opportunity to pounce on an individual or a smaller group. Day active ground dwelling primates also tend to be larger (think chimps, bonobos, gorillas, orangs) — another deterrent to predators.
Food supply will also affect group size — plenty of food means a larger population can be supported — scarce food supply may mean a population needs to spread out to survive. In an area with substantial food resources there may be a demonstration of territorial behavior by the occupying group to defend that spot from competitors.
Sound familiar? It should. How might this food/resource management by primate groups parallel the behavior of early humans? How about societies today?
“Who dat?”