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I
NATURAL RESISTANCE OF PLANTS TO MAMMALS
M. A. Radwan
Principal Plant Physiologist Forestry Sciences Labora tory Pac. N.W. Forest and Range Expt. Station Forest Service, U.S. Dept. of Agriculture Olympia, Washington A REPRINT FROM
WILDLIFE AND
FOREST MANAGEMENT IN THE PACIFIC NORTHWEST Compiled and Edited by
Hugh C. Black ASIOciate Professor of Forest Wildlife &oIOIY December 1914 School of Forestry Oregon State University· NATURAL RESISTANCE OF PLANTS TO MAMMALS
M. A. Radwan
, Principal Plant Physiologist
Forestry Sciences Laboratory
Pac. N.W. Forest and Range Expt. Station
Forest Service, U.S. Dept. of Agriculture
Olympia, Washington
ABSTRACT
I review natural resistance of plants to mammals and assess influencing factors, especially those related
to plant chemistry. An extensive literature describes many examples of resistance to mammals, among and
within species, and suggests numerous chemical factors to explain differences in feeding preferences. Causes
of inconsistencies in the published results and suggestions for fu ure researcl\ into factors of resistance are
given. Studies' of specific, naturally occurring chemiCals may lead to new methods aimed at alleviating
damage by mammaIs to forest tree species.
INTRODUCTION
natural resistance with emphasis on forest species,
and then review and assess the influencing factors,
especially those related to physiology and bio­
chemistry of plants.
Herbivorous mammals always have exploited
plants to satisfy their need for food and shelter.
Starving herbivores probably will feed upon any
available plant material. Occasionally, various her­
bivores even consume animal food (25). When
stress for food is not great, however, these animals
show definite selectivity in their feeding upon
available vegetation. Thus, different species of
mammals have been observed to prefer different
parts of plants and to discriminate among and
within species. In these responses, however, differ­
ences occur among strains and even individuals of
the same animal species. In addition, preferences
usually change with age and physiology of the
animal, and may vary by location and even on the
same area during different seasons and after habitat
modifications.
The literature shows that terms that describe
the degree to whiCh forest plants and se,eds are
used by herbivores include: utilization, preference,
selection, acceptability, palatability, resistance, sus­
ceptibility, consumption, rejection, dislike, relish,
favor, destruction, removal, attack, and so on.
Some of these terms have been defined, but various
workers have used them carelessly. .All of these
terms are relative, however; they are also meaning­
less unless the animal and the prevailing conditions
are described. In this paper, I will give examples of
EXAMPLES OF NATURAL RESISTANCE
Resistance to feeding by domestic and wild
animals was first observed in agricultural crops.
Later, as interest in wildlife increased and forest
lands were managed more intensively. resistance
was recognized among browse plants and forest
trees. Now. extensive literature on feeding behavior
contains many contlicting reports. because re­
sistance is intluenced by many factors. including
availability of other plant species. The following
are representative examples of plant resistance to
some of the important forest mammals.
Resistance of Se eds to Rodents
Seeds vary greatly in their resistance to small
rodents. In the Northeast. Abbott (I) reported that
seed-eating rodents rejected balsam fir seed when
seeds of white pine, red pine, eastern hemlock, and
white spruce were available. Similar observations in
the West showed that white fir seed was less
preferred than that of Douglas-fir (38). and seeds
of Port-Orford-cedar. the true firs. and red alder
were less accepted than those of western hemlock
and Douglas-fir (49).
85
86
Kesistance of Trees to Gnawing Rodents
Resistance of trees to gnawing by rodents is
well known. Compared with other species, white
spruce seedlings were found resistant to the
meadow vole in New York (41), in Wisconsin (66),
and in Canada (14, 75). Scotch pine, on the other
hand, was reported most resistant in Massachusetts
(65) in contrast to earlier findings in New York
and Canada where the species was found highly
susceptible (14, 41). In addition, Rudolf (63)
found Scotch pine highly preferred by porcupines
in the Lake States and he also observed that
damage varied with the seed origin of the species.
Resistance of Trees to Hares and Rabbits
Many instances have been reported in which
hares and rabbits showed selectivity in their clip­
ping of plants. Resistant tree species include
Port-Orford-cedar and western hemlock in Oregon
and Washington (49, 69), white spruce in the Lake
States (3), balsam fir and white cedar in New York
(16), and black spruce in Minnesota (17). Differ­
ences in browsing preference by these animals also
were reported among different sources of pon­
derosa pine in Washington (68) and Nebraska (59),
among individuals of white pine and eastern
redcedar in Massachusetts (70), and among cuttings
from different grafted Douglas-fir genotypes in
Washington (E. J. Dimock, unpublished).
Resistance of Trees to Deer
The browsing habits of deer have indicated
resistance by many plants. Field observations in
western Washington and British Columbia showed
that western hemlock was less preferred than
Douglas-fir (18, 47), although the opposite was
found in feeding trials with captive black-tailed
deer in Washington (11). In Montana, Douglas-fir
was reported less preferred by white-tailed deer
than was ponderosa pine (2)', and in Ontario, white
spruce was found less susceptible than jack pine
and white pine (34). Differences in preference also
were observed among deciduous species on the
Tillamook Burn in Oregon by Crouch (19), who
reported that during the winter twigs of red alder
and vine maple were less preferred than those of
cascara and red huckleberry. In addition, differ­
ences in resistance among individuals of the same
species have been shown with western yellow pine
Wildlife and Forest Management in the Pacific Northwest
(probably ponderosa pine) in Colorado (7), red and
Utah juniper in Utah (67), ponderosa pine in
Washington (68), and big sagebrush in Utah (28).
Furthermore, experiments with penned black­
tailed deer in Washington indicated widely varying
resistance among cuttings of different grafted
Douglas-fir genotypes (E. J. Dimock, unpublished).
FACTORS OF RESISTANCE
All plants are resistant to some extent to the
mammals in their environment. This natural de­
fense by plants may be secured through mor*
phological or physiological factors or both.
Morphological factors usually associated with re­
sistance are limited; they include presence of
specialized plant structures such as thorns on stems
and harsh-textured leaves, which affect the ease of
harvesting and swallowing of plants by the animal.
Physiological factors, on the other hand, are much
more varied . They include various phytochemicals.
because physiological processes of plants ulti­
mately are manifested in their chemical com­
position. They also have been investigated and
emphasized more than other factors, mainly be­
cause of the strong belief that all animals are
sensitive to the chemistry in their environment.
This belief was reemphasized recently by Arnold
and Hill (4):
Animals, and ruminants are no ex­
ception, live in worlds linked by chemi­
cal communication systems. Their social, reproductive, and feeding behaviour are pri m a r i l y d etermined by chemical stimuli. The chemical signals which mainly influence food selection are those receiwd at reception sites for taste and smell. During the past 10 years, several reviews have
been written on the subject of selection (or
preference) of foods by animals (4, 25, 31). These
reviews and other pertinent publications show that
a multitude of chemical factors have been studied,
especially in agricultural crops, in relation to
feeding by sheep and cattle. Both nutritional and
nonnutritional chemicals have been evaluated, but
the literature shows many conflicting results.
Resistance
of P/on/s to Animals
Examples of different chemicals suspected to
influence resistance (or susceptibility) of plants to
a variety of mammals are included below.
Nutritional Chemicals
One hypothesis is that animals possess the
"nutritional wisdom" necessary to enable them to
select plants of high nutritional value and to make
the right choice to meet specific nutrient needs.
Nutritional components of plants, especially pro­
teins, have been emphasized in much of the
literature, and positive correlations between these
components and feeding preferences have been
reported. Thus, plants with high content of crude
protein were preferred by cattle, sheep, and deer in
many investigations (29,33,52,62,64). Similarly,
various mammals have been found to select plants
with high contents of sugars (13, 24,48, 53, 58),
soluble carbohydrate (61), crude fat (29, 43),
organic acids (35), phosphorus (53, 60, 71),
potassium (32, 39), calcium (71), or carotene (21).
Results, however, have not been consistent.
Several studies have shown that many plants were
not selected by mammals in spite of high levels of
important nutrients such as proteins (53, 54, 60,
71), sugars (9, 52,55),soluble carbohydrates (23),
fats (53; M. A. Radwan and G. L. Crouch,
unpublished), carotene (53), and calcium (53). In
addition, penned sheep in one experiment failed to
select an adequate diet from a variety of available
foods (26); in another test, sheep and cattle
suffering from phosphorus deficiency consumed
little of the phosphate-rich mixture offered to
them (25).
Rather than the absolute amount of a particu­
lar chemical, som
' e investigators have suggested
r a t i o s o f s p e c ific nutrients, such as the
calcium/phosphorus ratio (27), the total nutritive
value of plants (15), or the balance of nutrients
against inhibitors (42), as better indicators of
preference. Unfortunately, some of these indi­
cators are impossible to measure, and others have
not explained preference satisfactorily (20, 72).
Nonnutritional Chemicals
Unlike nutritional chemicals. which are gen­
erally believed to increase palatability of plants.
nonnutritiol1al chemicals commonly are regarded
as d eterrents, which increase plant resistance to
157
animals. These chemicals include structural ma­
terials, toxic compounds, aromatics. and phenolic
substances.
Struct ural chemicals. Fiber and lignin are
associated with the maturity of plants; as they
increase, plant succulence and digestibility by
mammals decrease. Negative correlation between
these materials (especially lignin) ahd preference,
therefore, have been shown in many studies (24,
62, 74). On the other hand, other published
reports (71) and unpublished results (M. A.
Radwan and G. L. Crouch) have failed to show
such correlations with some plants. In addition,
mammals are known to select roughages oc­
casionallY.
Toxicants. Many toxic compounds occur in
plants and these toxic plants usually are believed to
be unpalatable, because animals somehow can
detect their toxicity. Among toxic chemicals found
in poisonous plants are alkaloids, polypeptides,
amines, glycosides, oxalates, nitrates, nitrites,
hydrocyanic acids, selenium, lead, cadmium, and
copper (37). Most of the work with these
compounds concerns identification and toxicity
determinations, which are beyond the scope of this
paper. Some studies were made, however, in which
'
low concentrations of these compounds were
i n v e s t ig a ted i n relation to resistance and
preference. For example, nitrates in forage were
associated negatively with preference by livestock
(53). Similarly, grazing preference of cattle and
sheep for sudal1grass varieties and sorghum x
sudangrass hybrids was correlated negativdy with
hydrocyanic acid content (55). Unpalatable clones
of reed canarygrass contained higher concentra­
tions of indole alkaloid derivatives than did the
palatable clones (6). High levels of the alkaloid
salacin were associated with resistance of willows
and poplars to the opossum in New Zealand (45).
Essential oils. Many plants produce essential
oils containing various terpenes and non terpene
compounds. These plants have been characterized
as "aroma tic" and have been observed to possess
various degrees of resistance to animal feeding
because of their oil content. For example. ill
feeding trials in Colorado. Dietz et al. (22) found
that deer consumed only small amounts of big
sagebrush . which is known to contain from I to :2
percent of essential oils (50). Likewise. Smith (67)
88
observed that penned deer in Utah avoided junipers
that had high concentrations or oils. In addition,
Santamour, Cunningham, and Peterson (65) in
Massachusetts hypothesized that variation in re­
sistance of several exotic pines to damage by
meadow voles was associated with presence of the
terpene delta-3-carene in their oleoresins.
Many volatile oils have been known to possess
antibacterial action (46), and, recently, oils from
shrubs and trees and their effect upon ruminants
and rumen microorganisms have been investigated
in attempts to explain the role of oils in plant
resistance. Thus, Nagy, Steinhoff, and Ward (50)
reported in Colorado that big sagebrush in the diet
decreased food intake of a fistulated steer, and the
oils inhibited growth of deer microorganisms and
cellulose digestion in vitro. Also, more recently,
detailed studies of essential oils were carried out in
California with deer and sheep (42,44,5 1), and in
Washington with deer (56). Results of these studies
show that palatability and in vitro digestibility of
some oil-containing plants were correlated pos­
itively. Oils of these plants probably affected
p a l a tability through their effect on rumen
microbial activity. And, although effects varied
with concentration and composition, oils of low­
palatability plants were mostly inhibitory but
those from plants of higher palatability were
chiefly stimulatory. Unfortunately, palatability
and digestibility have not always been correlated
positively (73). We have found recently that
differential effects of oils isolated fromeifferent
browse species failed to rate accurately the plants'
preference by deer (M. A. Radwan and G. L.
Crouch, unpublished).
The characteristic odor of essential oils also
may play an important role in determining re­
sistance (or susceptibility) of plants to feeding by
mammals that rely to some extent on sense of
smell in their selection of food. Clearly, differences
,in amounts, composition, or both of volatiles
emitted by plants could be useful in explaining
animals' response. In a recent study (M. A. Radwan
and W. D. Ellis, unpublished), we used gas
chromatography to compare volatiles emanating,
under laboratory conditions, from mature foliage
of different Douglas-fir clones that had been rated
for their resistance to deer and hare. Although no
qualitative' differences appeared among these
Wildlife and Forest Management in the Pacific Northwest
dones, we found that foliage from the resistant
clones emitted llluch more volatiles than those
from the preferred trees. Possibly, the mechanism
of resistance, at least in Douglas-fir, is based on
emission of large amounts of terpenes. Because the
technique we followed was new, however, we
believe it should be explored further before a
definite conclusion is reached.
Phenolic compounds. In the plant kingdom,
phenolic substances constitute a large class of
compounds, second only to carbohydrates in abun­
dance. They display a great structural hetero­
geneity, ranging from simple phenols to complex
polymeric structures such as tannins. They also are
among the most stable plant products, and some
are inherited simply. Further, some phenolic com­
p o u n d s s uch as the flavonols and leuco­
anthocyanins cause astringency in plants (36), and
others may form complexes with protein and
render plant material resistant to microbial decom­
position (8). That some phenols have been used
extensively in chemotaxonomic studies and that
the compounds have been characterized generally
as defense chemicals against predation (40) is not
surprising, therefore.
In studies of phenolic compounds affecting
mammal preferences, tannins were investigated
more than other phenols. These compounds have
been correlated positively (53) and negatively (76)
and also have been uncorrelated (30) with pref­
erence. This inconsistency probably was caused by
the complex nature of tannins and by the different
analytical methods followed in their determina­
tion.
Coumarin is known for its bitter taste (to
man) and its wide distribution in plants. Thus,low
palatability commonly has been attributed to
presence of the compound in plants (10), and
lowered intake of food with added coumarin has
been demonstrated with sheep (4). In another
study, however, the adverse effect on palatability
of coumarin sprayed directly on plants was shown
to be only temporary (5). This temporary effect
probably was caused by rapid adjustment of the
animals to the compound, reduction in con­
centration of the chemical by volatilization, or
both.
Isoflavones and their glycosides also were
mong the phenols suspected to affect resistance of
89
Resistance of Plants to Animals
plants to feeding, because the compounds are
known to cause serious reproductive disorders in
some animals. Arnold and Hill (4), however,
reported lack of correlation between these com­
pounds and preference by sheep for different
strains of clover.
Phenols also have been shown to enhance
palatability. Positive correlation between pref­
erence by sheep for different genotypes of reed
canarygrass and the water-soluble phenolic content
has been reported (12). More recently, Hanks et al.
(28) separated phenols of big sagebrush belonging
to different palatability classes. They found a
strong association between chromatographic pat­
terns of the separated phenols and preference by
deer and livestock. Extracts of the preferred plants
contained much more of an unidentified phenol,
with blue-green fluorescence under ultraviolet,
than unpalatable plants.
My work with different clones of Douglas-fir
showed both negative and positive associations of
phenols with preference, depending upon the type
of phenol studied. Thus, in one study, I found that
foliage from Douglas-fir resistant to feeding by
deer and hare contained higher concentrations of
total phenols (tannins), flavonols, and leuco­
anthocyanins, than did susceptible foliage (56). In
other studies, I discovered the presence of
chlorogenic acid in Douglas-fir and found that
levels of this phenolic acid were consistently higher
in foliage of preferred clones than in that of more
resistant clones (56, 57). Further, my unpublished
data with additional clones and progeny of the
same genotypes support this finding; they also
show that potential for content of chlorogenic acid
was transmittable through breeding. These results
are encouraging. I believe that we soon may be able
to use low chlorogenic acid content as an indicator
for selecting trees in breeding Douglas-fir for
resistance to deer and hare.
CONCLUSlbNS
Much evidence shows that resistance to mam­
mals occurs among and within plant species.
Determinations of relative resistance of plants,
however, have given many contlicting results.
Obviously, one or more of the many differences
among the various published studies could explain
these conflicts. Such differences usually are found
in techniques followed, comparisons made, loca­
tion of study, animal and plant varieties studied,
and season of the year.
Investigations of chemical factors affecting
feeding preferences have long been emphasized.
Yet, in spite of extensive chemical analyses, about
the only agreement among investigators of chemi­
cals influencing preference is the importance of
plant chemistry. This is not surprising. Direct and
indirect effects of chemicals on animals are easily
recognized. But obtaining consistent results in
attempts to relate chemical composition of plants
to their resistance or susceptibility to animals
always has been difficult. This inconsistency may
be ascribed to many factors, especially differences
in plants and plant parts analyzed, location and
season of collection, kind and form of chemical
components determined, and methods of analysis.
Mammal preferences and aversions probably
are most affected by specific chemical components
in the form in which they occur in plants, rather
than by chemical elements or groups of com­
pounds that are measured frequently in standard
chemical analyses. Arnold and Hill (4) stated:
It is not possible for the animal to
recognize such things as nitrogen. "crude
fibre," "energy," silica, or "a sh" as these
fractions do not exist in this form at the
molecular level in the plant.
In studies of resistance factors, there fore,
emphasis should be on specific, naturally occurring
chemicals that can be determined accurately. I
doubt that results of proximate analysis and
factors suggested in the litera ture such as "total
nutritive value" and "ratio of nutrients to in­
hibitors" can be of much significance.
Range and wildlife workers have long been
interested in palatability and feeding preference by
herbivores and the factors involved, especially
those related to nutrition. Their primary I interest
has been to gain information ne ded to help
develop management programs to improve pastures
a nd
ra ng la nd to reach maximum animal
production. Inwstigation of rl'si tan\.'<..' in fl r<..' t
tre species, on th other hand, h;l rl',,'l'i,ed littk
attention. T h us, we still do not haw any inform:.1-
90
Wildlife and Forest Management in the Pacific Northwest
tion on deterrents or stimulants that may deter­
mine the selective consumption of forest tree seeds
by small rodents. Furthermore, information now
available on palatability of trees and research on
the subject of tree resistance to herbivores are
totally inadequate. Much information still is
needed, therefore, for a better understanding of
relations between mammals and trees and their
seeds. Such information would lead to methods
aimed at alleviating damage by mammals, based on
naturally occurring repellent chemicals, or utilizing
chemicals as indicators of resistance in tree­
breeding programs.
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Wildlife and Forest Management in the Pacific Northwest
94
CHECKLIST OF PLANTS AND ANIMALS
Common Name
Scientific Name
PLANTS
Alder, red
Canary grass, reed
Rhamnus pllrshiana DC. Cascara
Cedar, northern white
Douglas-fir
Fir
rir, balsam
Fir, white
Huckleberry, red
Hemlock, eastern
Hemlock, western
Juniper, red
Juniper,
Alnus rubra Bong. Phalaris arttlldinacea L. Utah
Maple, vine
Pine, eastern white
Pine, jack
Pine, ponderosa
Pine, red
Pine, Scotch
Poplar
Port-Orford-cedar
Redcedar, eastern
Sagebrush, big
Sorghum
Spruce, white
Spruce, black
Sudangrass
Willow
Thuja occidentalis L . Pseudotsuga menziesii (Mirb.) Franco Abies spp. A. balsamea (L.) Mill.
A. can color (Gord. & Glend.) Lindl.
Vaccinium parvifoliulll J .E. Sm.
Tsuga canadensis (L.) Carr.
T. heterophylla (Raf.) Sarg.
Juniperus scopulorum Sarg.
J. octeosperma (Torr.) Little
AceI' circinalllm Pursh
Pinus strobus L.
P. banksiana Lamb.
P. ponderosa Laws.
P. resinosa Ait.
P. sylvestris L. Populus spp. Chamaecyparis lawsoniana (A. Murr.) ParI. Juniperus vir!{iniana L. Artemisia tridentata Nutl. Sorghum bicolor (L.) Moench
Picea glauca (Moench) Voss
P. mariana (MilL) B.S.P.
Sorghum sudanense (Piper) Stapf.
Salix spp.
ANIMALS
Deer, black-tailed Deer, white-tailed Hare Opossum Porcupine Rabbit Vole, meadow Odocoileus hemionus colul1lbianus
O. vil'xinianus
Lepus spp.
Didelphis marsupialis
Eret/1izun dorsatulll
Sylvilagus spp.
Microtus spp.
PURCHASED BY THE FOREST SERVICE,
U.S. DEPARTMENT OF AGRICULTURE,
FOR OFFICIAL USE.
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