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What is the Relationship Between Brain Size and Digestion?

Updated on June 4, 2008

An Analysis of Katharine Milton's "Diet and Primate Evolution"

Katharine Milton’s “Diet and Primate Evolution” indicates a relationship between brain size and gut size by examining two species of primates with a one common ancestor. Spider monkeys seem to be very intelligent animals and possess large brains. Howler monkeys have a similar body size, but the spider monkey’s brain weighs twice that of the howlers. A large brain is advantageous in a tree dwellers life because it allows for memory and recognition. Fruit-bearing trees are sparse in the specific Panama forest Milton was studying in and spider monkeys have an advantage over other species if they are able to remember where fruit-bearing trees are located. Fruit-dependent species must also know when the fruit is ripe. Spider monkeys also show an ability to recognize members of their particular social unit and communicate through food-related calls to other members in their region. These food-related calls allow the spider monkeys to take advantage of all fruit-bearing trees over a large area. In general, primates who eat high quality and dispersed foods like the spider monkeys also exhibit larger brains than those who rely primarily on an abundant source of leaves.

Enlarged brains are only favored when the species will greatly benefit from the adaptation due to the energy cost of maintaining a large brain. The brain extracts a disproportionate amount of glucose from food, which makes large-brained animals dependent on high-quality, glucose-rich foods, such as ripe fruit. Spider monkeys experienced pressure to obtain sparse, high-quality foods which encouraged greater brain power to identify edible foods, remember their location, and know what foods are ripe. This pressure is necessary for the natural selection of larger brains due to the energy costs of maintaining them.

The evolution of a larger brain is accompanied by digestive tract adaptations, which is shown by comparing spider and howler monkeys in typical diet and digestive features. Spider monkeys consume fruits as 72% of their diet, leaves as 22%, and flowers as 6%. Forty-two percent of the howler monkey’s diet is fruit, 48% leaves, and 10% of their nutrients are derived from flowers. To accommodate for diet differences, spider monkeys pass food quickly through a small colon. This speed allows for spider monkeys to move masses of food through their GI tract and extract all of their required nutrients. Fruit is easily digested and calorically dense, so fruit is a good choice for this digestive strategy. On the other hand, howler monkeys have a slow passage of food through a large colon. This is an adaptation to leaf dependence, which requires a long gut residence time for efficient fermentation and nutrient extraction. A large gut also offers more surface area for calorie uptake, which would be a necessity due to the low calorie content in leaves. A smaller gut with less surface area can extract enough nutrients to sustain an individual when foods contain a high amount of readily accessible calories, such as in fruit.

The progression from Australopithecus to H. Sapien is typified by increases in brain size and reduction in gut size and complexity. Energy from food must be allocated to either a large brain or complex digestive tract which causes differences in anatomy based on food pressures. Our hominid ancestors used brain power to solve dietary problems which has led to humans’ current dependence on high quality foods that require advanced food acquisition methods. Milton’s article also points out the function of our ancestor’s short gut residence time for foods. More fiber in the modern human diet facilitates quick food passage through the GI tract and adhering to our ancestor’s digestive cycles, characterized by short gut residence time, may positively impact human digestive health.


Milton, Katharine. “Diet and Primate Evolution.” Nutritional Anthropology: Biocultural Perspectives on Food and Nutrition. Ed. Alan Goodman, Darna Dufour, & Gretel Pelto. Mountain View, CA: Mayfield Publishing Company, 2000. 46-54.


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