The digestive system in humans is dominated by intestines that are relatively larger than in other primates, and a colon that is relatively smaller. Overall, the human digestive system is also a less significant portion of the body when compared to other primates. According to Milton(7) the human small intestine makes up greater than 56% of the total gut, whereas the colon makes up only 17 to 23%. However, in all other apes the colon makes up greater than 45% of the total, and the intestines from 14 to 29%. This corroborates Chivers findings, and demonstrates that the human digestive anatomy is in a class distinct from the other apes. This being the case is logical to look outside of the apes for a species that might better match our digestive anatomy, perhaps at monkeys, birds or bats.
In general, larger primates including all of the great apes are foli-frugivoresbut eat some animal matter, and the smaller are usually faunivores (Tarsius sp.) that may also eat fruit (e.g. Galagoides demidoff). Amongst the primates only Callithrix humeralifer (tassel-eared marmoset) and Ateles paniscus (black spider monkey) eat more than 80% of their diet as fruit (11), with the remainder coming mainly from gum or foliage respectively and then a small percentage from animal matter. The tassel-eared marmoset is almost totally frugivorous, in that the gums that make up 17% of its diet are also chemically similar to fruits in being primarily a source of carbohydrate. The remaining 0.5% of feeding time is spent on ingesting small insects. Strong frugivory is therefore found in only a couple of species out of the 234 known primate species.
A study of the literature on functional anatomy reveals that foliage is digested mainly in highly sacculated stomachs or haustrated colons. These adaptations dramatically increase the gut volume for a given length, thus slowing digestion down so that bacterial fermentation can occur. Humans also have haustrated colons, but the degree is not as great as in the great apes. The foods which digest mainly in the intestines are animal matter and fruits, which can be broken down speedily compared to leaves, due to lack of the indigestible cell walls found in foliage.
Chivers work omits birds and bats, so also omits any highly frugivorous species. It is only amongst birds and bats that we encounter animals that live exclusively on fruits such as the totally frugivorous pteropodid bats. It's worth noting that fruits are often infested with insects, so that frugivores are incidental insect eaters. Jordano mentions in his chapter 'Fruits and Frugivory'(5), that a gut dominated by the intestines is also characteristic of strong frugivores (p.145). For example frugivorous bats such as Wahlberg's fruit bat are reported to have small intestines that makes up 94% of the total digestive system(16), although frugivorous bats may spit the fruit fibres out, ingesting only the juices. Jordanoalso points out (p.138) that frugivores require no special adaptations or special digestive processes for processing fruit, the same claim usually made for "omnivores".
In contrast to Chivers findings(1), Hladik, Chivers and Pasquet(12) plotted the area of functional mucosa vs. functional body size for folivores, frugivores and faunivores, and found that humans fitted the frugivore trend. Each trend line was completely separate in this study. This technique therefore seems to be somewhat more accurate at prediction than Chivers methods, yet both researchers basically confirm that human gut anatomy is effective for speedily digested foods.
In summary, digestive anatomy research shows features of the human digestive system consistent with a diet of foods digested more rapidly than tough plant fibres. The surface area to functional body size ratio is consistent with that of frugivores. In terms of ratio of intestine to colon, humans fall between the figures found for the foli-frugivorous apes, and the extreme condition of soft fruit and juice eating bats (see table below). The digestive system in humans is dominated by the small intestines, a feature common to frugivores, but also to faunivores and omnivores.
Proportion of Intestine and Colon in Apes, Humans and a Frugivorous BatGreat Ape Human Frugivorous Bat Intestines 14% - 29% 56% 94%
Colon 45% 17% 23% 4%
Diet Foli-frugivore Intermediate? soft fruit/fruit juices
Fruit ~64%* ? ~100
Fibres ~27%* ? ~0 (ejected)
Animal Matter ~4%* ? ~0
* for chimpanzees (The Feeding Ecology Of Apes, Nancy L. Conklin-Brittain,1 Cheryl D. Knott,1 and Richard W. Wrangham)
Reducing the highly complex digestive system to a few simple measurements and in the absence of consideration of the chemistry and physiology is over-reductive. Digestive system anatomy can tend to reflect the physical properties of the food, rather than the source of food, and as such cannot determine the fine details of the diet, or may be misleading and is certainly inconclusive. Even so, there is no reason to exclude humans from being classed as highly frugivorous based on their digestive system anatomy.