Anthropology P380: Primates and guts
All else being equal, larger animals have relatively slower metabolic rates than smaller animals. In fact, the metabolic rate scales to body size raised to 2/3-3/4 power: = "Kleiber rule"
This is important for primates, because smaller animals need more concentrated sources of energy .... (only has so many mouthfuls per day in which to cram all calories) = energy/volume (or sugars/cellulose)
And also because it COSTS more for a big animal to move in search of food.
Note that this difference is most pronounced in source of protein which different primates choose. Smaller primates tend to eat more insects, whereas larger primates eat more leaves. (GRAPH)
Example: Gorillas, largest of living primates is a specialist in "THV" = terrestrial herbaceous vegetation... selects tender shoots, rather than mature leaves (less cell wall material), but still eating a very bulky diet.. has a big gut to hold it all and a relatively slow metabolism.
But what about Colobus monkey -- almost exclusively a leaf-eater -- and not even too choosey about mature leaves... how does it manage?!
Digestive Strategies: FERMENTATION = extracting energy from fiber
Bacteria can degrade polysaccharides into Volatile Fatty Acids (VFA"s) such as Acetic acid (acetyl Co-A= transition molecule into Krebs cycle & precursor to AA's ), easily absorbed directly into blood stream
- Fore-gut fermenters = "Ruminants" (cows), kangaroo, Colobines & Indrii
- stomachs are normally acid, but these guys have specialized fore-stomach that is anaerobic & pH neutral, allowing a gut flora (bacteria) to munch away... their guts also contain lysozyme enzymes can "lyse" or digest bacteria too
- Hind-gut fermenters = most mammals... fermentation in caecum and colon (after digestion = fewer nutrients to animal... coprophagy in rabbits and other animals improves this...)
Proportion of Energy from VFA"s (Volatile Fatty Acids = fermentation):
Ruminants: Hind-Gutters:
Cattle = 75% Pig = 15%
Sheep = 68% Human = 1-10%
Digesting Fiber in hind gut is proportional to retention time and length of gut... bigger body sizes digest fiber more effectively... (e.g. gorillas!)
(Note: an organism can gradually improve fermentation capabilities... pick up bacteria, etc)
Katie Milton: Digestion of food represents a compromise between 2 conflicting demands:
1) efficiency of nutritent extraction
retention of food maximizes extraction
but reduces volume/unit time
2) food passage rate & food volume
rapid gut passage increases potential volume of food
but reduces efficiency of nutrient extraction
A Tale of Two Caecae...Howler Monkeys & Spider Monkeys
2 sympatric species (Barro Colorado Island, Panama)... both 7-9 kg... most of year they both eat a mix of leaves and fruit, but during food shortages, Howlers depend heavily on leaves (85% of feeding time/day, vs 50% other times of year), while spider monkeys are very frugivorous (typically 83% of time eating fruit, but during transition season, switch to only 60% fruit & more diverse diet)
Gut passage rates: average first appearance of markers...
- Howler monkeys = 20 hours... max of 72 hours!
- Spider monkeys = 4 1/5 hours... most passed in 8 hours
Milton's study of ape/human gut passage rates also show humans very fast
Chivers and Hladik plot the proportions of potential fermentation surface area (stomach + caecum + colon) vs absorbtion (small intestine surface area) and find this is a good predictor of whether animals adapted as folivores, frugivores or faunivores...
GUTS:
- While some enzymes in saliva begin breaking down carbos, most simple carbohydrate digestion / energy absorbtion takes place in small intestine
- Most mechanical breakdown of food into small particles occurs with tools, teeth or in the stomach
- Stomach acids can break down some muscle tissues and enzymes break down peptide bonds in proteins (more about this next time). Sacculated stomachs isolate the bacterial fermentation from the normal stomach function
- Bile acids and salts (e.g. detergents) help break down lipids in the colon and fatty acids are absorbed through cells lining the colon. The caecum is a chamber attached to the colon where hind-gut fermentation can take place, and is very large in many hind-gut fermenters. Note that the human appendix is essentially a vestige of the primate caecum
Animal Gut Capacity Stomach Small Intestine Caecum + colon Pig 28 litres 29% 33% 38% Dog 7 litres 62% 24% 14% Spider Monkey 28% 31% 40% Howler Monkey 26% 35% 39% Human 6 litres 17% 66% 17% Implications?
The basic pattern: the larger you are, the lower your metabolic rate is likely to be, and the more likely it is that you can survive on relatively high-fiber, low-energy foods by processing them in large quantities (big gut). Alternatively, you can develop specialized digestive adaptations like the Colobus and partially ferment the cellulose.
Question: Where do humans fit in? We are one of the largest primates -- are we adapted to eating low quality foods? Some clues in our gut morphology: modern humans are selecting for nutrient-rich items and absorbing like mad!
Syllabus | Readings | Lectures | Assignments | Quiz Site
Human Origins and Evolution in Africa Home Page
Jeanne Sept's Personal Home Page
Last updated: 9/9/99
URL: http://www.indiana.edu/~origins/teach/P380syll.html
Comments: sept@indiana.edu
Copyright 1998 Jeanne Sept
