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2013 SOUTHEASTERN NATURALIST 12(2):283–296
Variation Across Years in Rumen-Reticulum Capacity and
Digesta Load in White-tailed Deer (Odocoileus virginianus)
Ryan S. Luna1,* and Floyd W. Weckerly1
Abstract - How gut capacity and digesta loads vary with unpredictable forage quality and
abundance has not been examined in ruminants. Odocoileus virginianus (White-tailed
Deer) should have greater rumen-reticulum capacity during drought years to accommodate
heavier digesta loads due to diets that contain a greater fraction of indigestible
material. In contrast, in years with above-average precipitation, digesta loads should be
lighter due to a greater fraction of digestible material in the diet which would result in
less need for a large rumen-reticulum capacity. Data were collected from White-tailed
Deer obtained during October, 2006–2008, from a 214-ha enclosure at the Mason Mountain
Wildlife Management Area, Mason County, TX. Digesta load, liver weights (used
as a proxy to indicate metabolic workload), empty rumen-reticulum organ weights, and
rumen-reticulum volume were measured. Findings, adjusted for body weight, indicated
that in the year with above-average precipitation, liver weights and rumen-reticulum capacity
were less than in drought years. Although the influence of year on rumen-reticulum
organ weight, adjusted for body weight, was not statistically significant, graphical representation
did show a trend that followed yearly precipitation. Digesta loads, adjusted for
body weight, progressively increased over the study, which did not coincide with changes
in precipitation. Overall, this study provided information on how rumen-reticulum attributes
change with environmental heterogeneity across years.
Environmental changes across years can affect the nutritional content and
availability of forage for large herbivores. Variation across years in forage
quality and quantity may be coupled to changes in the capacity of the gastrointestinal
organ of herbivores. Capacity of the gastrointestinal organ should
increase during periods of hyperphagia and decrease during hypophagia.
However, variation in gut capacity within a season across years has not been
explicitly examined in long-lived herbivores. Flexibility in gastrointestinal
organs may be needed to accommodate erratic climatic conditions that influence
temporal availability of forage (Canale and Henry 2010). In semiarid
environments, abundance and quality of ruminant forages such as browse and
forbs are positively correlated with precipitation, which is capricious in both
space and time, and the predictability of precipitation from one year to the
next is poor (Beatley 1969; Marshal et al. 2002, 2005; Noy-Meir 1973; Polis
et al. 1997; Robertson 1987; Teer et al. 1965). Variation among years in forage
quality and quantity should be coupled to the capacity of gastrointestinal organs
so that ruminants can adjust to density-independent factors that influence
survival and reproduction (Canale and Henry 2010).
1Department of Biology, Texas State University – San Marcos, San Marcos, TX 78666.
*Corresponding author - firstname.lastname@example.org.
284 Southeastern Naturalist Vol. 12, No. 2
The capacity of gastrointestinal organs can change in predictable ways
across seasons for several species of ruminants (Jenks et al. 1994, Jiang et al.
2009, Weckerly 1989). These changes in capacity are largely due to changes in
diet quality, which will have direct effects on the amount of food a ruminant
consumes (Barboza et al. 2009). Ovibos moschatus Zimmermann (Muskoxen),
for example, experience changes in digesta loads of over 150 percent between
seasons when hypo- and hyperphagia occur (Barboza et al. 2006). During hyperphagia,
digesta load increases and tissue is added to the gastrointestinal
tract to enlarge capacity (Jenks et al. 1994, Jiang et al. 2009, Weckerly 1989).
The increased capacity occurs when nutritious forage is abundant and predictable
from one year to the next. The animal then has the capability to ingest
enough nutrients to meet life-history demands and replenish fat stores lost during
seasons when hypophagia occurs. An investment in gastrointestinal tissue,
which has high energy demands, is returned by the animals’ ability to elevate
forage intake and retain digesta long enough so that plant material can be fermented
(Barboza et al. 2006, Holand 1994).
In contrast to Barboza et al. (2006), numerous studies have shown that as
diet quality decreases, forage intake increases (Barboza et al. 2009, Karasov
and Martinez del Rio 2007). An increase in forage intake attributed to a decrease
in forage quality was termed the instantaneous response strategy by
Meyer et al. (2010). The instantaneous response strategy has been noted in
numerous trials and on a variety of mammals including rodents, marsupials,
pigs, primates, horses, and ruminants (Baer et al. 1985; Dinius and Baumgardt
1970; Edwards and Ullrey 1999a, 1999b; Laut et al. 1985; Owen and Ridgman
1968; Peterson and Baumgardt 1971; Plowman 2002; Schwartz et al.1988; Wellard
and Hume 1981). Artificial feeds were used in the trials documenting the
instantaneous response; even though it is assumed that the same food intake
strategies would apply in herbivores consuming natural forage (Karasov and
Martinez del Rio 2007), the strategy has not been directly tested.
When forage quality and quantity become unpredictable, it seems unlikely
that variation in gut capacity will follow the pattern noted in ruminants at high
latitudes where forage availability changes predictability from winter to summer.
Animals inhabiting semi-arid environments might not have a marked
change in forage quality and quantity between seasons. Because changes in
forage quality and quantity are less predictable in semi-arid environments,
animals inhabiting these environments might have gut capacities that respond
differently than observed in animals that inhabit high latitudes. Variation in
gut capacity in less predictable environments may be influenced by the quality
of the recent diet (Barboza and Hume 2006). However, the metabolic expense
of adding gut tissue may be too risky in environments with unpredictable forage
resources (Naya et al. 2007), which may play a role in semi-arid environments.
When the quality of the diet is high, excess gut capacity may not be critical
because the fraction of the diet that is indigestible is low resulting in increased
rates of digesta passage. Conversely, when animals consume a low-quality diet,
a greater amount of time may be required to extract nutrients from forage. For
2013 R.S. Luna and F.W. Weckerly 285
this reason, gut capacity may increase by stretching of the existing gut tissue,
thereby not requiring any additional tissue to increase capacity.
In addition to understanding variation in gut capacity, liver weight should
also be noted. Liver weight can be used as an indicator of metabolic workload;
therefore, liver weights should be obtained when considering the influence
of forage quality. Liver weights of Cervus elaphus L. (Red Deer) have been
shown to decrease in conjunction with a reduced forage intake (Wolkers et al.
1994). Numerous studies have noted co-variation of liver weights with nutrition
and animal demands (Gerhart et al. 1996, McLeod and Baldwin 2000,
Reynolds et al. 2004, Swanson et al. 1999, Verme and Ozoga 1980, Wolkers et
al. 1994). Therefore, variations in liver weight should be coupled with variations
in rumen-reticulum morphology.
We measured rumen-reticulum capacity, digesta load, and liver weight in male
and female Odocoileus virginianus Zimmermann (White-tailed Deer) in central
Texas across three consecutive Octobers. In this semi-arid environment, primary
productivity often differs from one autumn to the next as a result of fluctuations
during summer and early autumn in precipitation. We set out to determine how
rumen-reticulum capacity and digesta loads fluctuate in a semi-arid environment
across years within a season. We tested predictions about how rumen-reticulum
capacity and digesta load should vary across years of drought and above-average
precipitation. During drought years, nutritious forage in summer and early autumn
should be scarce. Therefore, we predicted that White-tailed Deer would
exhibit greater liver weights and rumen-reticulum capacity during a drought year
as a result of consuming forage with a greater fraction of indigestible material,
which thereby should also result in increased digesta loads. We hypothesized
that in years with above-average precipitation, rumen-reticulum capacity would
be decreased because less rumen-reticulum capacity is needed to accommodate
lighter digesta loads. The lighter digesta loads in above-average precipitation
years would presumably result from the consumption of forage that has a greater
fraction of digestible material.
Animals were obtained from a 214-ha enclosure at Mason Mountain Wildlife
Management Area (WMA), Mason County, TX. Daytime temperatures are hot
(35–40 °C) in summer and mild (15–20 °C) in winter. Mason Mountain WMA
has an average annual precipitation of approximately 66 cm with a coefficient
of variation = 21% and first order serial r = 0.09. Precipitation varies from year
to year, and it is also unpredictable across years. The vegetation in the enclosure
is primarily comprised of various Quercus spp. (oaks), Aloysia gratissima
Gillies and Hook. (Whitebrush), and grassland with Bouteloua spp. (grama
species), Eragrostis spp. (love grasses), Schizachyrium scoparium Michx. (Little
Bluestem), and Bothriochloa saccharoides Steud. (Silver Bluestem). The entire
enclosure was fenced with a 2.46-m-tall game fence.
286 Southeastern Naturalist Vol. 12, No. 2
We measured liver weight and three rumen-reticulum variables: organ weight,
digesta load, and capacity. Variation in liver weight was used to assess whether
changes in precipitation resulted in changes in nutrition of deer. We measured
rumen-reticulum organ weights and capacity. Distension of the rumen-reticulum
is needed to accommodate digesta load changes (Weckerly 2010). Rumen-reticulum
volume was used to measure distension of these organs and concomitantly
capacity, because rumen-reticulum organ weights do not necessarily measure the
extent to which these organs distend (Sibbald and Milne 1993). The remaining
rumen-reticulum variable measured was digesta load.
In 2006, 2007, and 2008, deer were trapped from sites in central Texas
and translocated to the enclosure from February through April (Weckerly
2010). The deer were provided supplemental corn for two weeks after introduction
to the enclosure. Supplemental corn was dispensed from 5 feeders
programmed to release 1.13 kg of corn 0.5 hrs before sunrise and 1 hr before
sunset. From May through August, deer were not provided supplemental feed,
and consumed forage available within the enclosure. Supplemental feeding
recommenced for 2 weeks in September as part of a population study and again
when deer were harvested in October (Weckerly and Foster 2010). Numbers of
deer in the enclosure were 58 in 2006, 63 in 2007, and 48 in 2008. There were
6–7 males for every 10 females in each year of the study. Precipitation was recorded
at the wildlife management area by Texas Parks and Wildlife personnel.
To show how precipitation was distributed across the nine months that the deer
were in the enclosure, the data were combined into seasons of winter (January–
March), spring (April–June), and summer (July–September) for each year
of the study.
The deer were harvested by Texas Parks and Wildlife personnel with highpowered
rifles in October within three hours of dawn or dusk (Weckerly 2010).
Respective sample sizes were 26 (12 females, 14 males), 29 (19 f, 10 m), and
31 (21 f, 10 m) for 2006, 2007, and 2008. The carcasses were necropsied within
three hours of the time of death. The whole weight of each animal minus blood
loss was recorded before the animal was eviscerated. The liver was excised from
the entrails and weighed. The mesentery was then removed to expose the rumenreticulum.
The rumen-reticulum was separated from the rest of the entrails by
ligating the esophagus approximately five cm above its junction with the reticulum
and making a second incision at the reticulo-omasal sphincter (Ramzinski
and Weckerly 2007, Weckerly et al. 2003). The rumen-reticulum along with its
contents was then weighed to the nearest 0.1 kg. The contents in the rumenreticulum
were then removed through the reticulum. The rumen-reticulum was
inverted (by slightly elongating the ligation at the reticulo-omasal sphincter) and
rinsed thoroughly to ensure that the rumen-reticulum was void of all particulate
matter. After rinsing, the rumen-reticulum was reverted and rumen-reticulum organ
weight was recorded. Digesta load was determined by the difference between
2013 R.S. Luna and F.W. Weckerly 287
the weight of rumen-reticulum organ with its contents and rumen-reticulum organ
weight without contents.
To measure rumen-reticulum capacity, the rumen-reticulum organ was
placed in a plastic drum that contained 208 L of tap water. Keeping the opening
of the reticulum at water level for hydrostatic support, water was poured
into the rumen-reticulum, and the amount of water the organ held was recorded
to the nearest 0.1 L. The capacity measurement was taken in triplicate. Collection
of animals followed an Institutional Animal Care and Use protocol from
Texas State University.
Only adult animals (≥2.5 yr) were included in the analyses because gastrointestinal
mucosal and submucosal morphology probably differ between yearling
and adult deer (Knott et al. 2004, Pacha 2000). We conducted a multivariate
analysis of covariance (MANCOVA) to account for correlations among response
variables and examine overall multivariate significance of interactions between
body weight and year, sex, and diel period. We used Pillai’s trace as the multivariate
test statistic for our MANCOVA. Body weight was used as a covariate (Sokal
and Rohlf 1995) in the analysis because liver weight, rumen-reticulum organ
weight, rumen-reticulum capacity, and digesta load covary with body weight
(Jenks et al. 1994, Jiang et al. 2009, Weckerly 2010). Year, sex, and diel period of
collection (AM, PM) have been shown to influence response variables and were
main effects (Short et al. 1969, Weckerly 2010). If no interactions were detected,
a reduced model MANCOVA was conducted with only main effects. A univariate
ANCOVA was then performed for response variables to interpret the influence
of main effects. Also, we used univariate ANCOVA models to estimate means of
response variables adjusted for body weight. We also analyzed a linear contrast
to assess if annual variation in response variables coincided with patterns of precipitation
among years (Sokal and Rohlf 1995). Assumptions of normality and
homoscedasticity were met because no patterns were observed in scatter plots
between fitted values and residuals. All statistical analyses were conducted in
program R (R Core Development Team 2009).
Mean body weights of females and males in 2006 were 34 (range = 28–42) kg
and 50 (range = 40–62) kg, respectively. In 2007, mean body weights of females
were 39 (range = 32–47) kg and 59 (range = 42–73) kg for males. Mean body
weights in 2008 were 37 (range = 32–41) kg for females and 50 (range = 39–61)
kg for males. Precipitation for the past 20 years averaged 54 cm from January–
September. Precipitation occurred in every season in each year, with seasonal
precipitation in 2007 at least double the precipitation in 2006 and 2008 (Table 1).
From January–September, precipitation was 76% of the 20-year average in 2006,
164% of the 20-year average in 2007, and 70% of that average in 2008.
The MANCOVA with second-order interactions of body weight, main effects,
and response variables of rumen-reticulum weight, rumen-reticulum
288 Southeastern Naturalist Vol. 12, No. 2
volume, digesta load, and liver weight indicated that none of the interactions
were substantial (body weight:sex-TS = 0.031, P = 0.664; body weight:diel
period-TS = 0.049, P = 0.436; and body weight:year-TS = 0.163, P = 0.105).
Therefore, a reduced model MANCOVA was conducted which excluded
the interaction terms. For the reduced MANCOVA, we detected influences
from year (TS = 0.663, P < 0.001), body weight (TS = 0.664, P < 0.001),
and sex (TS = 0.124, P = 0.031). There was a marginal effect of diel period
(TS = 0.107, P = 0.060). The reduced MANCOVA was followed by univariate
ANCOVAs on each response variable.
Every response variable in the univariate ANCOVA was influenced by body
weight. Each response variable was also influenced by year, with the exception of
rumen-reticulum weight. Also, rumen-reticulum organ weights and capacity varied
between diel periods, and digesta loads differed between females and males
The variation across years in liver weight (F1,82 = 6.09, P = 0.003) and rumenreticulum
capacity (F1,82 = 28.96, P < 0.001) indicated differences across years,
and matched patterns in precipitation. Means of liver weight were 0.85, 0.73, and
0.81 for 2006, 2007, and 2008, respectively. Rumen-reticulum capacity means
were least in 2007 and similar in 2006 and 2008 (Fig. 1). Rumen-reticulum organ
weight graphically followed the precipitation trend; however, year was not statistically
significant (F1,82 = 1.23, P = 0.297). In contrast, variation in digesta loads
did not match variation in precipitation among years (F1,82 = 11.55, P < 0.001).
Mean digesta loads, adjusted for body weight, increased progressively from 2006
to 2008, with females having heavier digesta loads than males (Fig. 2).
Table 2. Findings from analyses of variance examining the influence of whole weight, year, sex,
and diel period (AM, PM) at time of death on response variables of liver weight, rumen-reticulum
(RR) organ weight, digesta load, and RR volume of Odocoileus virginianus (White-tailed Deer)
sampled in a 214-ha enclosure, Mason Mountain Wildlife Management Area, TX. The error degrees
of freedom for each analysis was 82.
Source of Liver weight RR organ weight RR volume Digesta load
variation df F P F P F P F P
Body weight 1 288.08 less than 0.001 100.59 less than 0.001 14.14 less than 0.001 19.72 less than 0.001
Year 2 6.09 0.003 1.23 0.297 28.96 less than 0.001 11.55 less than 0.001
Sex 1 0.85 0.359 2.75 0.101 0.12 0.730 7.97 0.006
Diel period 1 0.48 0.489 5.94 0.170 4.75 0.032 2.26 0.136
Table 1. Seasonal and annual precipitation (cm) at Mason Mountain Wildlife Management Area,
TX across 3 study years.
Season 2006 2007 2008
January–March 8.5 16.9 5.8
April–June 17.5 49.3 22.2
July–September 10.3 31.3 11.5
Total 36.3 97.5 39.5
2013 R.S. Luna and F.W. Weckerly 289
The findings of this study, with the exception of digesta loads, supported our
predictions. In 2007, when precipitation was greatest and diets were presumably
higher in digestible materials, the rumen-reticulum capacity was the lowest recorded
during our study. Rumen-reticulum organ weights were also low, but we
did not detect a statistically significant effect. In 2006 and 2008, when there was
less precipitation, rumen-reticulum capacity was greater. Greater precipitation
Figure 1. Rumen-reticulum organ weight (kg) and volume (L) adjusted for body weights
for each diel period during 3 years at Mason Mountain Wildlife Management Area, TX.
Error bars represent 1 SE.
290 Southeastern Naturalist Vol. 12, No. 2
during 2007 presumably caused an increase in the availability and nutritional
content of forage within the enclosure. Changes in the quality and quantity of
forage are coupled with changes in appetite and digestive function (Barboza et
al. 2006). These changes in appetite and digestive function permit physiological
adjustments that allow ungulates to respond to alterations in forage supplies that
coincide with fluctuating environmental conditions (Barboza and Hume 2006).
Numerous studies have demonstrated how the rumen-reticulum changes between
seasons when forage supplies fluctuate predictably such as from winter to summer
(Jenks et al. 1994, Jiang et al. 2009, Sibbald and Milne 1993). However,
previous studies have not indicated how changing environmental conditions
influence rumen-reticulum capacity across years. Our study is one of the first to
show how rumen-reticulum capacity responds to unpredictable changes in environmental
factors that influence forage resources.
Annual changes in forage quality and quantity should be coupled to forage intake
and metabolic demands. Therefore, liver weights should also change among
years. Our study indicated that liver weight variation coincided with precipitation
patterns. Liver weights, adjusted for body weight, were lighter in the year
with the greatest precipitation, which presumably corresponded with a decrease
in metabolic workload as a result of increased dietary nutrition. These results
differed from previous studies. Verme and Ozoga (1980) found that liver weights
were heavier in White-tailed Deer fawns consuming pelleted diets high in protein
(16.2%) and with more digestible energy (3030 vs. 2668 kcal/kg dry matter). In
studies of cattle, goats, and sheep, liver weights increased in animals consuming
Figure 2. Digesta load weights (kg) adjusted for body weight for male and female Whitetailed
Deer for 3 years at Mason Mountain Wildlife Management Area, TX. Error bars
represent 1 SE.
2013 R.S. Luna and F.W. Weckerly 291
forage higher in metabolizable energy and protein (Haddad 2005, McLeod and
Baldwin 2000, Swanson et al. 1999). In regards to the adult deer in this study,
animals in drought conditions showed enhanced rumen-reticulum volume and
slightly heavier organ weights. Hersom et al.(2004) indicated that adult castrated
cattle exposed to a lower plane of nutrition had enhanced growth of the rumenreticulum
and liver, presumably to provide greater capacity to hold digesta and
Diel changes in the capacity and weight of the rumen-reticulum were also
detected. Changes in the distention of the rumen-reticulum can occur over a
short period of time as a result of periods of foraging, which typically occur in
the morning, or ruminating, which typically occur in the afternoon (Tulloh and
Hughes 1965). Changes in rumen-reticulum capacity were 13%, 22%, and 12%
between the morning and evening in 2006, 2007, and 2008, respectively. Moreover,
rumen-reticulum organ weight varied by 6% between the diel periods for
each respective year. The increase in rumen-reticulum organ weight associated
with diel period is likely linked with rumination. During rumination, there is
likely an increase in blood flow through the rumen to aid in absorption of volatile
fatty acids. Exchange of metabolites, water, and minerals between the rumen
and vascular system can also help explain the changes between morning and
evening in rumen-reticulum organ weights (see Remond et al. 1996). The rumenreticulum
walls probably contain greater amounts of blood during periods of high
nutrient exchange. Therefore the rumen-reticulum organ will have fluctuations in
weight during periods of foraging compared to times of inactivity. This finding
is not surprising due to reports of previous studies that have indicated that time
of kill can have an influence on weights and capacities of the rumen-reticulum
(Short et al. 1969, Tulloh 1966, Weckerly 2010).
Throughout the study, after adjusting for body mass, female White-tailed Deer
had greater digesta loads than the males sampled. These differences in digesta
load are likely the result of differing metabolic demands between the sexes.
Additionally, digesta loads increased 26% from the first to the third year of the
study. The observed changes in digesta load may be due to the amount of fibrous
material within the forage and changes in forage availability across years. It is
possible that a slight shift in age structure accounted for the observed increase
in digesta loads throughout the study. Veiberg et al. (2009) detected heavier
wet-rumen-reticulum fills and lower body conditions in older Rangifer tarandus
platyrhynchus L. (Svalbard Reindeer). Veiberg et al. (2009) also noted that
there might be an increase in digesta load within the rumen-reticulum due to a
decrease in mastication efficiency. The mastication efficiency decreases as teeth
wear, which can result in larger particles within the rumen-reticulum. As particle
size and retention time of the digesta increase, there is a concomitant increase in
retention time of fluid in the rumen-reticulum (Lechner et al. 2010). Therefore,
larger digesta particles may also lead to more fluid in the rumen-reticulum. The
resulting increase of fluid within the rumen-reticulum associated with age could
have contributed to the increase in digesta load weights observed in our study.
Digesta load may also vary in response to changes in the fraction of water within
292 Southeastern Naturalist Vol. 12, No. 2
the forage due to diet quality and the type of diet; as a result, forage intake and
fermentation rates can change as will the amount of fluid within the rumen (Barboza
et al. 2006).
White-tailed Deer did not increase rumen-reticulum organ weight in response
to the presumed higher-quality diet in 2007. A possible explanation for this is the
high passage rates associated with high-quality forage. Also, due to the unpredictability
of forage quality and quantity in semi-arid environments, it might be
more beneficial for ruminants to have rumens that are elastic, and therefore do
not require additional metabolically expensive tissue in order to accommodate
increased digesta loads.
Slight changes in the rumen-reticulum organ weight were associated with
greater changes in rumen-reticulum capacity. Therefore small changes in rumenreticulum
tissue could greatly influence capacity without incurring high energetic
demands. McLeod and Baldwin (2000) reported increased intakes of metabolizable
energy associated with rumen-reticulum and liver weights. During drought
years, animals might increase their forage intakes to obtain more metabolizable
energy. When forage is of low quality, there might be a decrease in passage rates,
which would allow for longer exposure of the digesta to rumen microorganisms,
thereby increasing the amount of nutrients extracted from the forage. In order
to increase metabolic energy intake, rumen-reticulum capacity as well as liver
weight might increase in order to accommodate the greater forage intake, digesta
load, and digestive workload.
Changes in the rumen-reticulum should be directly correlated with changes
in liver weights because forage that is consumed must be metabolized in order
to extract nutrients. In our study, the correlation between rumen-reticulum and
liver weight was present, but not very strong (r = 0.64). The low correlation
might be the result of differences in the quality of forage between the sampling
years. The liver metabolizes many nutrients (Van Soest 1994). Rumen-reticulum
organ weight and capacity reflect, to some degree, the nutritional and physiological
state of the animals over a long period of time, whereas digesta load may
be indicative of current forage availability and life-history demands (gestation,
lactation, or breeding) of the animal.
Our findings support previous studies that indicate variation in the rumenreticulum
capacity and organ weight is correlated with the quality of the forage
(Kouakou et al. 1997, McLeod and Baldwin 2000). The quality of forage can
be unpredictable across years in autumn due to the irregularity of precipitation.
These unpredictable changes in precipitation affect forage quality, which, in turn,
affects gut capacity. Previous research has shown variation in gut capacity among
seasons in which there are predictable changes in seasonal forage (Barboza et al.
2006, Jenks et al. 1994, Jiang et al. 2009, Sibbald and Milne 1993). During these
predictable seasonal changes, ungulates can increase dry matter intake due to
an increase in gut tissue. An increase in gut capacity is needed to accommodate
metabolic demands from lactation, gestation, and mating activities. In turn, the
increase in gut capacity aids digestive efficiency. However, differences in forage
quality can be unpredictable across years; therefore gut morphology is altered
2013 R.S. Luna and F.W. Weckerly 293
to limit energetic demands. Our findings indicated that when there is enough
precipitation to yield high forage quality, there is a less pronounced increase in
gut tissue. The minute increase in gut tissue results in lower rumen-reticulum
organ weight and capacity compared to years with low precipitation that consequently
yields low-quality forage and increased plasticity and rumen-reticulum
organ weight to accommodate diets which might have high amounts of structural
carbohydrates. These digestive adjustments, however, are probably energetically
efficient to the animal, and allow their gut morphology to adjust to environmental
changes that affect forage quality and abundance.
We thank M. Mitchell, J. Carroll, K. Behrens, J. Foreman, and R. Reitz of Mason
Mountain Wildlife Management Area for assistance. We are grateful to J. Bell, R. Jones,
M. O’Day, R. Keleher, J. Korn, R. Simpson, G. Street, A. Ferguson, A. Connolly, C. Faas,
J. Jackson, A. Jonker, K. McDermid, S. Stephenson, and R. Swanson for assistance in
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