Length of digestive tract

The morphology of mammals’ digestive tracts reflects their evolutionary adaption to different diets (Fig. 1). The digestive tract of herbivorous mammals is generally much longer than that of carnivores [1]. The increased length – especially of the small intestine – allows for more time for the cellulose of plant cell walls to be broken down by microorganisms.

Fig. 1: Digestive systems of various mammals with different diets. Note how much longer the digestive tracts (especially the small intestine) of herbivores are compared to that of carnivores [1].

Fig. 2: The human digestive tract. Its length is around seven times that of the body length [2].

Table 1 gives some examples of digestive tract length as a percentage of body length (distance from mouth to anus).

What about humans? Although the human gastrointestinal tract is not as long as that of herbivores such as horses, cattle, or sheep, it is certainly not as short as that of carnivores such as lions or dogs (Tab. 1). In fact, humans and elephants have the same ratio of digestive tract length to body length.

It is important to note that there are exceptions to every rule. This is especially true when it comes to highly specialised feeders. The sloth and the panda, for example, both herbivorous, have digestive tracts that are only 3.3 and 4.5 times as long as the body length, respectively [2]. On the other hand, seals and dolphins, both carnivores, have digestive tracts that are up to 30 times as long as their body length [2] – longer than that of ruminant herbivores such cattle and sheep (Tab. 1).

Treeshrew (Ptilocercus) 125 Quoll (Dasyrus) 200 Lion (Panthera leo) 300 Tarsier (Tarsius) 470 Guenon (Cercopithecus) 500 Dog/Wolf (Canis) 500 Platypus (Ornithorhynchus) 520 Human (Homo sapiens) 700 African elephant (Loxodonta) 700 Bear (Ursus) 800 Rock hyrax (Procavia) 900 Kangaroo/Wallaby (Macropus) 1000 Orangutan (Pongo) 1000 Horse (Equus) 1000 Cattle (Bos) 2000 Sheep (Ovis) 2300

Tab. 1: Length of digestive tract as percentage of body length (measured from mouth to anus) for some carnivores (red), omnivores (orange), herbivores (green), and humans (black); data from [2] and references therein.

REFERENCES:

[1] Linzey, Donald W. Vertebrate Biology. McGraw-Hill Science/Engineering/Math, 2000.

[2] Starck, Dietrich. Vergleichende Anatomie der Wirbeltiere auf evolutionsbiologischer Grundlage. Band 3: Organe des aktiven Bewegungsapparates, der Koordination, der Umweltbeziehung, des Stoffwechsels und der Fortpflanzung, 1982.