Effects of Condensed Tannins on Browsers and Grazers: Qualitative or Quantitative Defense?

Effects of Condensed Tannins on Browsers and Grazers: Qualitative or Quantitative 
Defense?

Jan Alldredge

Introduction
	Much recent work in the area of plant-herbivore interactions has 
focused on the inhibitory effects of condensed tannins on protein and 
fiber digestion in mammals.  Tannins have previously been classified as 
a quantitative plant defense which reduce the digestibility of nutrients 
following their ingestion by herbivores (Feeny 1976, Rhodes and Cates 
1976).  There is currently some debate, however, over whether these 
plant secondary metabolites are of greater importance to mammals as 
digestion inhibitors or as toxins (Bryant et al. 1992).  Robbins et al. 
(1991) suggested that the defensive nature of tannins as digestion 
inhibitors or toxins is dependent on the molecular characteristics of 
the tannin as it interacts with the physiological capability of the 
animal.
	Digestion inhibitors, such as tannins, act within an animal's 
digestive tract by binding with the substrate to be digested (usually 
proteins, but also carbohydrates and even lipids), inhibiting digestive 
enzymes, or being antimicrobial (Scalbert 1991).  Toxins, on the other 
hand, act by interfering with fundamental biochemical processes in cells 
(McArthur et al. 1992).
	Tannins have been defined as polyphenols of high molecular weight 
which are water soluble and capable of precipitating proteins (Bryant et 
al. 1992).  Tannins have previously been grouped into 2 classes, 
hydrolyzable and condensed.  New evidence, however, suggests that 
further classification may be warranted.  Clausen et al. (1990), for 
instance, demonstrated that structurally different condensed tannins 
vary in their effectiveness to deter browsing.  Mole et al. (1993) also 
found differences in digestibility and growth rates in rats that were 
due to different polar fractions of the same kind of condensed tannin.
	One effective strategy for dealing with the digestion inhibiting 
properties of condensed tannins has recently been elucidated in browsing 
mammals.  These herbivores, which generally consume a wide variety of 
tannin-containing trees and shrubs, are known to produce proline-rich 
proteins (PRPs) in their saliva.  These proteins complex with tannins in 
the mouth of the herbivore and then pass through the animal's gut 
intact, thereby effectively neutralizing some of the tannins' adverse 
effects.
	Grazing mammals, on the other hand, typically ingest a relatively 
tannin-free diet of grasses and forbs, and appear to be incapable of 
synthesizing PRPs.  It is commonly thought that because these animals 
have evolved with virtually tannin-free diets, they therefore have 
little reason for production of PRPs.  But, what happens when grazers 
are subjected to an increase in dietary tannin levels, as might be the 
case when an area becomes overgrazed and animals are forced to include 
more tannin-containing plants in their diets?  If they are incapable of 
PRP synthesis, are there other strategies available to them for dealing 
not only with the digestion-inhibiting properties of tannins, but their 
toxicity levels as well?
	In this paper, I will take an in depth look at several  defensive 
strategies that mammalian herbivores might use against the deleterious 
effects of condensed tannins.  In addition, I will also investigate the 
importance of tannins as digestion inhibitors in comparison to their 
importance as toxins.

Effects of Tannins on Digestion and Metabolism
	There are several possible explanations for the antinutritional 
effects of condensed tannins.  As outlined by Fahey and Jung (1989) they 
include the following:
	1)tannins depress food intake
	2)tannins complex with dietary proteins or other dietary 	 
		components
	3)tannins complex with digestive enzymes, thus interfering 	 
		with normal digestion
	4)tannins complex with endogenous protein, resulting in a 	 
		drain on the nitrogen supply, and on the amino acid 	 
		supply in particular
	5)tannins complex with or injure parts of the alimentary 	 
		tract
	6)tannins or their hydrolysis products are absorbed and have 
		a toxic effect elsewhere in the body.

	Tannins, because of their protein-binding properties, are known to 
be strongly astringent.  This astringency appears to be the major cause 
of reduced food intake in mammalian herbivores.  There is some 
controversy, however, over whether reduced food intake is a result of 
the toxic nature of tannins.  Singleton (1981) considers it unfair to 
consider the effects of tannins on feed intake as toxicity, since the 
result is due to a failure to consume, rather than to consumption 
itself.  On the other hand, Provenza et al. (1991) suggest that mammals 
may reject tannin-containing plants because they cause internal malaise.  
Severe growth depression can be a consequence of reduced feed intake, 
and has been shown to occur in rats and chicks when fed tannin-
containing diets (Fahey and Jung 1989).
	When tannins complex with protein in an animal's gut, they are 
believed to be responsible not only for growth depression, but also for 
low protein digestibility and increased fecal nitrogen concentrations.  
Thus, once they have been consumed, their adverse effects, once again, 
seem to be related to their binding of dietary protein.
	There is evidence to suggest that enzymatic proteins, as well as 
other endogenous proteins, comprise a considerable portion of excreted 
nitrogen when animals are fed tannins (Fahey and Jung 1989).  When 
endogenous proteins are lost in this manner, the animal may incur a 
deficiency in one or more essential amino acids.
	Condensed tannins are known to inhibit several digestive enzymes, 
including proteases, pectinases, amylases, cellulases, and lipases.  
Enzyme inhibition is believed to be caused mainly by nonspecific binding 
of tannins with the enzyme protein, but may also occur when tannins bind 
with the substrate (Fahey and Jung 1989).  There are many factors which 
may influence the extent of digestive enzyme inhibition by tannins.  
Included among them are the following: 1) amount of protein in the diet, 
2) relative amounts of various enzymes in the diet and the order in 
which they are encountered, 3) formation of tannin-protein complexes 
prior to and following ingestion, and 4) how various enzymes are 
affected by pH, type of tannin, and species and age of the animal 
(McArthur et al. 1992, Fahey and Jung 1989).

Defensive Strategies of Browsers and Grazers
	Because tannins have such a wide array of effects on herbivores, 
it is difficult to predict with any certainty how a particular tannin-
containing forage will affect an animal without first understanding 
something about the characteristics of the tannin (e.g. molecular size 
and configuration) and the adaptations that different animals possess 
for neutralizing or metabolizing them (Robbins et al. 1991, Clausen et 
al. 1990).
	Which strategy an herbivore uses to cope with plant secondary 
compounds depends to a great extent on the animal's behavior and its 
physiological capability (McArthur et al. 1992).
An herbivore can always just simply avoid plants containing chemical 
defenses, or it may eat a wide variety of plants to maintain a limited 
consumption of secondary chemicals.  Mice, for instance, have been shown 
to be capable of selecting the proper combinations of 2 toxin-containing 
foods (in this case, tannins and saponins) which eliminated the symptoms 
of toxicity associated with the consumption of either toxin alone 
(Freeland et al. 1985).
	In addition to behavioral adaptations, several physiological 
mechanisms are available to browsers and grazers for reducing the 
activity of plant secondary compounds (McArthur et al. 1992).  They 
include the following: 1)formation of a less reactive complex, 2) 
modification of the environment to inhibit reactions, 3) degradation, 4) 
addition of functional groups, 5) conjugation to change solubility, and 
6) alteration of metabolic rate.  I will limit my discussion of 
physiological mechanisms to the first three listed above, which McArthur 
et al. (1992) consider as the "first line of defense".
	According to McArthur et al. (1992), the first line of defense 
takes place in the gut and includes Mechanisms 1, 2 and/or 3.  If 
tannins can be inactivated or degraded to harmless compounds, then the 
toxic effects that occur after absorption are avoided.  If the first 
line of defense fails and the tannins are absorbed across the gut wall, 
then Mechanisms 4 and 5 must be utilized.  Absorbed compounds are 
transported to the liver, which is the major site for conjugation and 
addition of functional groups.

First Line Defensive Strategies
Complex Formation
	The toxic effects of tannins and other plant secondary compounds 
may be inactivated by the formation of noncovalent complexes with other 
compounds in the gut.  The resulting complex must be less reactive or 
less readily absorbed across the gut wall than the uncomplexed secondary 
compound for this mechanism to be effective.  Because of the reduced 
absorption, the complex may then be excreted in the feces.  Although 
complex formation is effective in the prevention of tannin absorption, 
thus, reducing toxicity effects, it can also be considered 
antinutritional when tannins bind with dietary protein, forming 
precipitates which then make dietary protein unavailable to the 
herbivore.
	Tannins, as previously stated, are known to form such complexes 
(both soluble and insoluble) by binding with dietary and endogenous 
proteins.  The salivary proline-rich proteins produced by many browsers 
are considered one such defensive strategy against these secondary 
metabolites.  The tannin-protein complex (at least in deer) is thought 
to be stable throughout the entire digestive tract (Austin et al. 1989).
	Browsers that possess the capability to synthesize PRPs include 
deer, moose, beavers, and bears.  Other browsing mammals, such as 
ringtail possums and koalas, which feed heavily on the tannin-rich 
leaves of Eucalyptus trees, do not produce PRPs and must rely instead on 
other lines of defense.  
	Whether soluble or insoluble tannin-protein complexes form appears 
to be a function of the tannin:protein ratio (Hagerman and Robbins 
1987).  When a mixture of tannin and protein contains excess protein, 
soluble complexes are formed.  Little is known about these complexes, 
but some authors suggest that they may enhance protein digestibility 
(Mole and Waterman 1985), while others state that absorption and 
metabolism of tannins may be increased (Butler et al. 1986).

Environment Modification
	Environmental factors, such as pH, temperature, and solution 
polarity are known to affect chemical reactivity.  If an herbivore can 
alter the reactivity of tannins by modifying conditions in the gut, then 
the toxic or antinutritional properties of these secondary plant 
compounds may be inactivated (McArthur et al. 1992).
	pH is important in governing the formation of tannin-protein 
complexes.  Binding is particularly high at the isoelectric pH of the 
protein, and is much less strong at high pH, where the phenolic groups 
of the tannins are ionized (McArthur et al.  1992).  However, acidity 
may also play an important role in breaking down some tannins in the gut 
(Mueller-Harvey and McAllan 1992).  Carbon-carbon linkages of 4->6 
apparently are more vulnerable to acid cleavage than 4->8 linkages.  
Therefore, tannins having 4->8 links may be potentially more toxic to 
animals because they will release more phenolic monomers which can be 
absorbed and will ultimately have to be detoxified (Mueller-Harvey and 
McAllan 1992).
	It is well known that the pH of the gut contents of some larval 
insects is high enough to dissociate tannin from protein (Martin et al. 
1985).  It is also understood that alkalinity is not a property of 
mammalian digestive tracts (McArthur et al. 1992).  However, not all 
tannin-protein complexes have the same pH optima for maximal binding.  
Thus, it is at least possible that tannins could actually benefit rather 
than harm some ruminants, by protecting dietary protein from microbial 
attack in the rumen and then later dissociating in the intestine upon 
appropriate change in pH, thus making the protein available for 
absorption.  One drawback to this scenario, however, is the potential 
for the newly liberated tannin to cause damage to the intestinal tract 
or to form new complexes with endogenous proteins (Fahey and Jung 1989).

Degradation of Tannin
	Animal enzymes and gut microorganisms potentially play an 
important role in deactivating tannins in an animal's digestive tract.  
Bacteria are capable of modifying a broader range of compounds than are 
mammals, and can thus be significant in determining the fate and 
toxicity of these plant defensive compounds.  However, time of exposure 
can be a key factor in how well this defense works, and it can take up 
to several months of exposure to secondary compounds before gut microbes 
have adapted to utilizing them (McArthur et al 1992).
	Osawa (1992) has identified a new strain of enterobacteria 
that degrade the tannin-protein complex and are present along caecal 
walls in the alimentary tract of koalas.  The bacteria apparently 
benefit from this arrangement by being able to satisfy their own 
nitrogen requirements.  It remains unclear, however, whether the tannins 
are absorbed across the gut wall or are excreted in the feces.  Tannins 
have been shown to reduce the permeability of the gut wall by reacting 
with the outer layer so that passage of nutrients is reduced (Mitjavila 
et al. 1977).  Thus, these bacteria may be viewed as an asset, or 
perhaps as a potential liability to the animal.

Which Strategy?
	Consumption of plant secondary compounds by mammalian herbivores 
has resulted in the evolution of several different behavioral and 
physiological responses.  According to McArthur et al. (1992), the 
primary determinants of which mechanisms are used are dependent on the 
feeding niche of the aninmal and features of its gut structure.  These 
authors divide herbivorous mammals into 4 groups, based on the feeding 
niche that they occupy; grazer, intermediate mixed feeder, generalist 
browser, and specialist browser.  Each group reflects differences in 
levels of consumption of plant secondary compounds and in strategies fro 
dealing with such compounds.

	This classification scheme represents a continuum of animals, with 
grazers on the one hand that are purported to consume only small 
quantities of tannins and other secondary compounds, to specialist 
browsers on the other end, which consume, almost exclusively, plants 
containing high levels of plant secondary metabolites.  Examples of 
grazers include domestic cows and sheep, which feed mainly on grasses 
and forbs, whereas, specialist browsers are herbivores such as koalas 
and greater gliders, which feed heavily on the tannin-rich leaves of 
Eucalyptus trees.  
	McArthur et al. (1992) further suggests that the occurrence of 
salivary PRPs is somewhat predictable, based on the feeding niche of the 
animal.  As mentioned earlier, generalist browsers, such as deer, bear, 
and rats are protected from tannins by PRP production.  Grazers, on the 
other hand, do not apparently possess this adaptation.  However, I know 
of only 2 species of grazing herbivores that have been examined for 
production of PRPs, domestic cows and sheep.  A much broader survey of 
PRP production in herbivorous mammals is an obvious need in this area of 
research!
	Specialist browsers, such as ringtailed possums and koalas, also 
do not produce salivary proline-rich proteins.  Instead, they may have 
more specific adaptations to accomodate tannins.  Their specialized 
caecum has previously been mentioned as possessing populations of 
enterobacteria which act to degrade the tannin-protein complex (Osawa 
1992).
	Gut structure influences the pathways and sites of tannin 
metabolism (McArthur et al. 1992).  For example, animals which possess a 
foregut, such as ruminants, maintain microflora in their rumens which 
degrade some secondary compounds to water-soluble, readily excreted 
products.  This degradation of secondary compounds by microbes in the 
foregut could also potentially be harmful, as discussed earlier, where 
after degradation, soluble tannins could then perhaps be absorbed across 
the gut wall.

Conclusions
	Understanding how tannins function in plant-herbivore interactions 
depends to a great extent on our knowledge not only of the chemistry of 
these polyphenolic compounds, but also of the strategies that herbivores 
possess fro dealing with these substances.  In mammals which are capable 
of PRP synthesis, ingested condensed tannin is complexed in the gut and 
then excreted in the feces.  Tannins, in this case, may somewhat reduce 
feed intake and digestibility, but can be considered as more of a 
quantitative defense according to the definitions of Feeny (1976) and 
Rhodes and Cates (1976).  On the other hand, in animals which do not 
produce PRPs, at least some part of the tannin may be absorbed, 
potentially yielding toxic effects.  Thus, in mammals such as grazers, 
and even specialist browsers, condensed tannins may be thought of as 
more of a qualitative defense.  
	Much remains to be learned about tannins and their interactions 
with mammalian herbivores.  Indeed, it may not even be possible to 
classify tannins as a qualitative or quantitative defense.  Instead, 
what type of defense they act as may depend more on what type of 
herbivore is ingesting them.

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