Mammalian digestive tracts

Mammalian digestive tracts
Mouth: mastication, some digestive enzymes
Esophagus: simple transport tube
Stomach: most initial digestion, some physical processing
Small intestine: digestion continues, some absorption
Caecum: diverticulum at junction of small and large intestines,
digestion continues
Large intestine: absorption
Rectum: final absorption
Relative proportions and size differ according to diets

Plant material is more difficult to
digest, so digestive tract longer
and more complex in herbivores

Problem: most E in leaves/grass not stored in cell contents, but
locked up in structural carbohydrates like cellulose. No enzyme
produced by mammals can digest cellulose.
Solution: Some bacteria and protozoans can! House these in
specialized compartments and let them do the work.
Hindgut fermenters house symbionts primarily in caecum,
foregut fermenters (ruminants) house them mainly in complex
forestomach.
Hindgut: many rodents, perissodactyls, rabbits, sirenians,
hyraxes, elephants, some marsupials
Foregut: artiodactyls, sloths, kangaroos

% volume of digestive tract
Carnivore
(dog)
Hindgut F
(horse)
Foregut F
(sheep)
stomach 65 9 67*
small
intestine
20 30 20*
caecum

3 basic patterns:
Carnivores/insectivores: food (meat) is easy to process and
digest, so
1. Relatively short post-gastric tract (2-6 X HBL)
2. Caecum reduced or absent
3. Little differentiation between small and large intestines
Other 2 patterns are 2 major kinds of herbivores = hindgut and
foregut (ruminant) fermenters
Deal with housing and using microbes in different ways
(see next slide, also the many digestive tracts shown on your
handout)

post-gastric = 10-15 X body length
large caecum
long, sacculated lg. intestine
hindgut foregut
lg., compartmentalized stomach
longest post-gastric = 20-27 X body length
reduced caecum, lg intestine not sacculated

Advantages of microbial fermentation:
1. break down cellulose into volatile fatty acids
2. microbes grow and reproduce, plus can fix inorganic N (from
urea) into protein, namely their bodies, which can be digested
(yielding all essential amino acids, vitamins except A + D,
about 100-180 g protein per day from low quality food)
3. can conserve water because urea (waste product of protein
digestion) gets converted to more protein instead of excreted
(foregut benefit more from 2 and 3... some hindgut use
coprophagy to run food through a second time to better digest
and absorb the work of those microbes, YUCK)
4. microbes also can break down many plant defensive
compounds

Koalas have the largest relative
caecum of any mammal
Folivores generally have lower
metabolic rates, sometimes lower
body temperatures, and spend a
LOT of time sleeping (to
conserve E) and digesting.
Folivores: specialize on
eating leaves... very low
quality diet!

Hindgut fermenters process food about 2X faster
Foregut fermenters about 3/2 X more efficient at extracting
nutrition; longer time for fermentation allows more production
of protein and breakdown of toxic compounds by symbionts
So, when food is abundant and good quality (conditions
optimal), hindgut fermenters can obtain more energy per day.
When food becomes limiting, ruminants can extract more from
it, and therefore obtain more energy per day.
Ruminants feed in bouts, however, which is why the smallest
(like voles) and largest (like rhinos and elephants) herbivores are
foregut fermenters.

Wildebeest – 230 -275 kg, ruminant Zebra – 350 kg, hindgut
Thompson’s gazelle – 20 kg,
ruminant
3 most numerous species
during Serengeti migrations

Mammals are not just passive components of
ecosystems, simply responding to their habitat.
Mammalian activity, including feeding, can affect
successional processes and change the appearance of
the landscape.
Here are just a few examples.

Browsing by elephants, giraffes,
and impala opens up forest habitat,
slows re-growth of saplings and
shrubs, and together with altered
fire regime, maintains open
habitats like savannas

Thus, competitive advantage of
perennial grasses reduced, diversity of
annuals and forbs increases.
Wallows can create spring pools, add to
spatial heterogeneity. Urine, feces, and
carcasses create local N-rich patches.
Bison affect prairies both by
grazing and wallowing.
Grazers on grass, woody and forbs

Moose are browsers. (about 15 kg/day)
Same things in spruce that affect moose microbes also
affect soil litter microbes. Thus, litter composition
(spruce/fir opposed to aspen, etc.) affects soil N.
Feed selectively. Prefer early successional species
like aspen, paper birch, balsam poplar. Almost
never eat spruce, rarely balsam fir (when really
hungry)
Spruce has high resin, lignin content, low N. (Bad
for moose digestion)
Moose-browsed species over-topped by
spruce/fir...affects forest tree composition (shown by
exclosure experiments). But not only tree composition,
changes in soil microbes and nutrients affect rates of
plant growth and other species of forest plants.
Note: wolves good!
hungry moose!

• Exclosures for 40+ yrs
• Selective browsing by deer
affect tree growth, alter species
composition
• Critical concern: rare and
sensitive understorey plants,
esp. springtime plants,
hammered by deer
White-tailed deer in Wisconsin:
because of forestry and wildlife
management practices, way
higher numbers of deer now that
in pre-settlement period

Densities can be locally high, but
patchy, spatially variable.
Prefer forbs over grass, esp. species
with succulent, below-ground storage
organs.
Flora on mounds differs from other
veg. (light availability, subsurface
soil lower N), opens patches for good
colonizers but poor competitors.
Pocket gophers
burrow and tunnel
extensively.
Move lots of soil, lots
deposited on surface
in mounds.
No gophers: Perennial grasses take over
and diversity decreases.
Gopher foraging on tree seedlings slows
succession, invasion of fields by trees.
Latrines and storage rooms create small
N-rich patches (increases spatial
heterogeneity).
Grasshoppers like to lay eggs in softer
soil of mounds, increases arthropod
diversity!

Could note lots of other studies showing effects of mammals
on plant communities: Squirrels and mice selectively harvest
white and red oak acorns; voles and bog lemmings help stop
trees from invading old fields; kangaroo rats affect desert
plant communities and also inhibit invasion by grasses;
many mammals are important seed dispersers, pollinators,
dispersers of ectomycorhizal fungi, etc.
But now let’s look beyond interactions between 2 trophic
levels (herbivores and plants) and add a third trophic level…

Bottom-up effects (proportional changes)
Classic “trophic pyramid” model: add resources at the
bottom, get positively correlated changes up the trophic
chain
Top-down effects (trophic cascades)
Change in one level (say, top predator) results in
changes at next lower level in opposite direction, with
effects cascading down the trophic chain

Kelp forests are rich,
diverse marine
habitats.
Sea urchins graze on
algae, including kelp.
Sea otters eat a variety
of molluscs, including
sea urchins.
No sea otters, urchins increase
and devastate kelp... leave a
marine desert.
Sea otters present, keep urchin
numbers down, and kelp forest
thrives.
Killer whales eat otters, kelp
declines!

Other mammalian top-down examples include:
wolves, moose, and fir trees on Isle Royale, and
wolves, elk, and aspen and willows in Yellowstone
An interesting example of bottom-up effects is
shown by the many consequences of mast years in
eastern deciduous forests (refer to handout)

OK, didn’t quite finish this one in
time, you’ll have to refer to the
handout for the oak forest story!