Wapiti Foraging Linked to Energy Gain

Behavioral Ecology Vol. 6 No. 2: 209-217
Forage quality and patch choice by wapiti
{Cervus elaphus)
John F. Wilmshurst,1
John M. Fryxell," and Robert J. Hudsonb
'Department of Zoology, University of Guelph, Guelph, Ontario NIG 2W1, Canada, and
b
Department of Animal Science, University of Alberta, Edmonton, Alberta T6G 2P5, Canada
Recent models suggest that herbivores might optimize energy gain by selecting patches of intermediate vegetation biomass. We
tested this hypothesis in wapiti (Cervus elaphus) by estimating daily rates of energy gain in relation to grass biomass and by
measuring patch choice in experimental pastures in which grass biomass was manipulated by mowing. The digestible energy
content of grasses declined with increasing biomass due to maturational changes in fiber and lignin content. Daily rates of dry
matter intake by wapiti increased with grass biomass at a decelerating rate, implying a Type II functional response. Linking
these values to published ad libitum energy intake and energy expenditure parameters, Fryxell's (1991) model predicted that
the daily rate of energy gain should be highest when wapiti feed in grasslands with 1000-1100 kg/ha. In trials in which grass
biomass within a mosaic of patches was manipulated experimentally between 800-2900 kg/ha, wapiti preferred patches of 1200
kg/ha, close to the value predicted by the energy gain model. Our results suggest that the rate of energy gain by wapiti is
constrained by both grass biomass and grass fiber content, the latter of which varies inversely with grass biomass. Behavioral
preference for grass patches of intermediate biomass and fiber content could help explain patterns of aggregation and seasonal
migration reported previously for wapiti. Key words: Cervidae, Cervus elaphus, digestible energy, forage maturation, functional
response, grasses, herbivory, patch choice, wapiti. [Behav Ecot 6:209-217 (1995)]
Optimal foraging theory assumes that foraging decisions
by herbivores should be strongly influenced by physi-
ological and environmental constraints on rates of nutrient
uptake. Two such constraints commonly invoked for verte-
brate grazers are the effect of plant density on the short-term
rate of food intake (availability constraint) and the effect of
digestive capacity on the long-term rate of energy assimilation
(processing constraint) (Belovsky, 1978, 1986; Fryxell, 1991;
Owen-Smith and Novellie, 1982).
The short-term rate of food intake (i.e., the functional re-
sponse) of herbivores should be related positively to plant
size, bite size, and plant density (Gross et al., 199Sa,b; Shipley
et al., 1994; Shipley and Spalinger, 1992; Spalinger and
Hobbs, 1992). Accordingly, several studies have shown that
herbivore functional responses are often positive but decel-
erating functions of plant density or biomass (Fryxell et al.,
1994; Fryxell and Doucet, 1993; Gross et al., 1993a,b; Lund-
berg, 1988; Short, 1985; Wickstrom et al., 1984).
The digestive capacity of herbivores is primarily governed
by the interaction of energy and fiber in their diet (Broom
and Arnold, 1986). The digestibility of energy in grasses for
grazing ruminants is inversely related to cell wall (fiber) con-
tent of the forage (Mould and Robbins, 1982; Osbourn, 1989;
Van Soest, 1982). As grasses mature the proportion of cell wall
in their tissues increases, reducing digestibility (Laycock and
Price, 1970; Oelberg, 1956; Osbourn, 1989). The processing
time (digestion and passage) in the gut often increases as
plants mature (Blaxter et al., 1961; White, 1983). This suggests
that both digestibility and the rate of turnover of ingesta
should be negatively related to plant biomass, if biomass is
positively associated with plant maturation stage.
This inverse correlation between availability and processing
constraints creates a potential trade-off for grazing herbivores
(Demment and Van Soest, 1985; Fryxell, 1991; Hobbs and
Swift, 1988; McNaughton, 1984; Owen-Smith and Novellie,
Received 23 December 1993; revised 16 June 1994; accepted 24
June 1994.
1045-2249/95/S5.00 © 1995 International Society for Behavioral Ecology
1982). At low biomass, the processing rate is high but the
short-term rate of intake is low, whereas at high pasture bio-
mass, the processing rate is low but the short-term rate of
intake is high. The net rate of energy intake for grazing her-
bivores should be maximized accordingly on patches of inter-
mediate plant biomass (the forage maturation hypothesis).
Moreover, natural selection should favor grazers that choose
vegetation patches of intermediate biomass, yielding the high-
est rate of daily energy gain.
We tested the forage maturation hypothesis using wapiti
(Cervus elaphus L.) feeding in a mosaic of grass patches that
were manipulated experimentally. Wapiti are large, gregarious
ruminants of western North America. Although wapiti are
known to feed on a wide variety of plants, they are primarily
grazers (Baker and Hobbs, 1987; Boyce, 1989; Hobbs et al.,
1981; Nelson and Leege, 1982). To predict the processing
constraint, we measured changes in fiber content in relation
to grass biomass. To predict the availability constraint, we mea-
sured the functional response by wapiti to changes in grass
biomass. We used Fryxell's (1991) model and the data on en-
ergy constraints of foraging wapiti to predict daily rates of
energy gain in relation to grass biomass. We then tested
whether wapiti preferred patches of intermediate biomass, as
predicted in general by the forage maturation hypothesis.
METHODS
The model
In this section, we summarize the model used to predict the
net rate of energy intake by wapiti and report parameter val-
ues obtained from the literature. Model structure is described
in greater detail in Fryxell (1991). We defined the availability
constraint (/,, measured in kj/day) as the maximum amount
of digestible energy that an herbivore could consume given
an unlimited gut capacity. We calculated this constraint as the
product of the digestible energy content of forage (Q, in kj/
kg), the maximum time available for foraging per day (T, in
h/day) and the short-term rate of dry matter intake (C, in
kg/h).
atUniversityofManitobaonJanuary11,2011beheco.oxfordjournals.orgDownloadedfrom

210 Behavioral Ecology Vol. 6 No. 2
Recent theoretical work (Spalinger and Hobbs, 1992) sup-
ported by a growing body of experimental studies (Gross et
al., 199Sa,b; Shipley and Spalinger, 1992; Spalinger et al.,
1988) suggests that functional responses of herbivores should
vary according to plant spacing, plant apparency, and the size
and spatial distribution of bite-sized portions among individ-
ual stems. Consequently, a wide variety of functional response
forms are theoretically plausible, depending on the ecological
circumstances. Wapiti usually feed on a dense sward of closely
spaced grass stems, with each bite representing a substantial
fraction of an individual rameL Under these conditions, the
Michaelis-Menton formula should well approximate the func-
tional response (Spalinger and Hobbs, 1992), and this has
been corroborated in a previous field study (Wickstrom et al.,
1984). We therefore calculated the availability constraint as
QTaV
1
b + V
(1)
where a is the maximum rate of forage consumption per her-
bivore (in kg/h), Vis forage biomass (in kg/ha), and b is the
biomass at which the intake rate is Vi maximum (in kg/ha).
The maximum daily foraging time for wapiti has been re-
ported by both Gates and Hudson (1983) and Wairimu and
Hudson (1993) as 13 h, which we used to estimate 7! In our
results, we show how Q varies with gravs biomass.
We defined the digestive constraint (/,, in kj/day) as the
maximum amount of energy that a herbivore could digest per
day given ad libitum consumption. Previous research suggests
that ad libitum consumption by wapiti is the following positive
function of digestible energy content of the forage (Mould
and Robbins, 1982):
I? = 20.3Q - 45.5. (2)
We estimated energetic expenditure (X) as the sum of rest-
ing metabolic expenditure plus the cost of eating and loco-
motion while foraging
X = 24w + F(x + yW) (3)
where w is the resting metabolic expenditure (in kj/h), Fix
the total time spent foraging per day (in h/day), xis the cost
of standing plus eating (in kj/h), and jrV* is the cost of loco-
motion (in kj/h).
The resting metabolic expenditure for a 170-kg wapiti (the
mass of our experimental animals) was reported by Gates and
Hudson (1978) as 733 kj/h. Gates and Hudson (1978) also
give a value of 925 kj/h for the cost of resting plus standing,
which we added to the cost of eating reported by Wickstrom
et al. (1984) of 241 kj/h, yielding a total value of x = 1166
kj/h. Values of y and z for wapiti were determined from Wick-
strom et al. (1984) as 6766 kj/h and -0.6 respectively.
Net daily energy intake was estimated as / - X, where / is
the minimum of /, and Z, (note that our definition of net
energy differs from definitions found elsewhere in the physi-
ological literature; e.g., Robbins, 1983).
Study area and animals
We conducted research at the Ministik Reid Station, 45 km
southeast of Edmonton, Alberta, Canada, from May to August
1991. The experiments took place on a 2-ha, mixed-species
pasture composed of Kentucky bluegrass (Pon pralmsis L.),
smooth brome (Bromus inXrrmus Leysxer), clover (Trifotium
sp.), dandelion (Taraxacum ojficinale Weber), and Canada
thistle (drsiiim arvrnsel^).
In May, the pasture was stocked with four yearling females.
Animals had access to ad libitum water and a mineral lick, but
were not supplementalh/ fed during the study. Each animal
W.LS fitted with a numbered ear tag and colored collar to en-
able identification from a distance. Daily human activity on
the pasture ensured that the wapiti were acclimated to the
presence of an observer. We used five additional yearlings in
bite mass trials. The animals were weighed monthly on an
electronic balance (±1 kg).
Forage intake
Short-term rates of grazing intake can be calculated as the
product of bile size and cropping rate (Hudson and Watkins,
1986; Wickstrom et al., 1984). To determine bite size, individ-
ual wapiti were each presented with a pre-weighed sod mea-
suring 25 cm X 76 cm. These sods were simply samples of the
typical grass pasture cut into rectangular blocks. We cut each
sod in half and trimmed it until it weighed less than 4 kg (the
capacity of the balance), and dien placed each sod half in a
plastic container and weighed them on an electronic balance
(±0.01 g). One container (experimental) was staked to the
ground in a 15 m X 8 m feeding pen devoid of vegetation,
whereas the control container was placed outside the pen.
The number of bites taken from the experimental sod by a
captive yearling wapiti was recorded for a maximum of 15 min
(to limit desiccation) or maximum of 15 bites (to limit deple-
tion), whichever occurred first- The experimental and control
sods were then re-weighed. Bite mass (j, measured in g/bite)
was calculated as
(4)
where p, is the pre-grazing mass of the experimental sod (in
g), fa is the post-grazing mass of the experimental sod (in g),
r is the proportion of evaporative loss determined from the
control sod, g is the number of bites taken, and h is the pro-
portion of dry matter in the fresh vegetation.
Cropping rates of wapiti were measured during a foraging
trial conducted with patches of widely varying grass biomass
(described in detail below). From an elevated platform, we
continuously observed a focal animal foraging on a mosaic of
8 X 10 m patches of known biomass. Each bite, defined as a
single grasping and tearing of the grass, was entered into an
event recorder that also recorded time during each cropping
sequence. We defined a cropping sequence as beginning
when the focal animal put its head down to graze and ending
when it lifted its head. Uninterrupted cropping rate was cal-
culated as the number of bites taken during the cropping
sequence divided by time. As wapiti typically spent a small
portion of foraging bouts engaged in activities other than
cropping plants, we corrected for this bias by multiplying by
the proportion of time that the wapiti actually spent in crop-
ping sequences during foraging bouts.
The short-term rate of dry matter intake (kg/h) was cal-
culated as the product of bite mass (g/bite) multiplied by the
cropping rate (bites/min) multiplied by 60 min/h and 0.001
kg/g, to keep the measurement units consistent. Parameters
a and b from the Michaelis-Menton formula (Equation 2)
were estimated by fitting to intake data using a standard least-
squares algorithm (Wickstrom et al., 1984).
Forage growth and quality
To obtain undisturbed forage samples for the estimation of
digestible energy content in relation to grass biomass, we
erected 10 wire mesh grazing exclosures (basal area 1.5 m1
,
2-m high) at random locations on the pasture. Each exrlosure
was sampled biweekly from 10 May to 16 August 1991.
Samples were taken by securing a 0.125 m5
wire hoop to
the ground beneath the exclosure and clipping all vegetation
rooted within the hoop at ground level. A previously unsuut-

Wilmshurst et al. • Forage selection by wapiti 211
pled pan of the exclosurc was sampled each time. Vegetation
samples were stored in a cool room at approximately 2°C and
frozen within 3 days of collection. They were dried at 60°C
for 48 h for analysis and weighed (±0.01 g). Samples were
then ground through a 1-mm diameter screen in a Wiley mill
and stored at room temperature. A subsample from each sam-
ple was analyzed for dry matter content (AOAC, 1980).
Hence, biomass values are reported as kg dry matter/ha (kg-
DM/ha).
For ruminants, digestible energy content of forage depends
on fiber content. The greater die concentration of fiber, par-
ticularly lignin, the less digestible the forage (Van Soest,
1982). We analyzed forage samples for neutral detergent fiber
(NDF), acid detergent fiber (ADF), permanganate lignin con-
tent (Van Soest and Robertson, 1980; Van Soest and Wine,
1967), and Kjeldahl nitrogen. Digestibility was calculated us-
ing Mould and Robbins' (1982) regression equations for wap-
iti. The percent digestible dry matter (DDM) of each sample
is the sum of the digestible percentage of the neutral deter-
gent soluble (NDS) fraction and the digestible percentage of
the NDF fraction. Following Mould and Robbins (1982), we
calculated DDM as the sum of
1.00
and
Digestible NDF(%) = %NDF(176.92 - 40.5(ln x)) (6)
where x is the percent of the ADF diat is lignin and cutin.
Percent digestible energy (DE) was subsequently calculated
from Mould and Robbins (1982) according to
DE = -0.61 + 0.98(DDM). (7)
The relationship between DE and sample biomass was dien
determined using least squares regression. The gross energy
content of each sample was determined by bomb calorimetry
(Parr, 1977).
Patch use experiment
To test whedier wapiti selectively exploit patches of interme-
diate biomass, we conducted feeding trials on two areas of the
2-ha pasture in which a mosaic of patches at different growth
stages had been created by mowing. Animals were allowed to
move freely among the patches, and preferences were assessed
by recording the amount of time spent on each patch (Roy-
ama, 1970; Simons and Alcock, 1971; Smith and Dawkins,
1971).
On the two most level areas of die pasture, two 30 m x 40
m paddocks were enclosed with 2-m-high deer fence. Within
each paddock, 15 patches of 8 X 10 m were surveyed and
marked with a stake at each corner. We arranged patches in
three blocks of five along a slight elevational gradient in die
pasture to control statistically (Dutilleul, 1993) for possible
confounding effects due to spatial variation in soil moisture
or shading. Al 2-week intervals, one patch in each block was
randomly selected and mowed 50 mm above the ground widi
a commercial mower. When all patches had been mowed
once, the paddock was left to regrow for 2 weeks before ani-
mals were admitted. This mowing regime produced a mosaic
of grass patches widi 2, 4, 6, 8, or 10 weeks regrowth.
The patch use experiment was initiated by removing die
30-m fence from die south end of the paddock and allowing
four wapiti to enter. From an elevated platform behind the
north end of die paddock, an observer recorded time allo-
cation and cropping rates widiin patches. Observations were
made from first to last light for diree consecutive days or until
all four animals spent one bout foraging outside die paddock.
H
O.00
Digestible NDS(%) = -21.88 + 1.11 %NDS (5) £
0
UJ
IQ.
B
° 2000 4000
GRASS BIOMASS (kg/ha)
Figure 1
Wapiti bite size (a) and cropping rate (b) in relation to grass
biomasj. See text for regression equations.
At night, die animals were removed, and the paddock was
closed.
Three forage samples were taken from each patch before
and after the trial. Samples were taken, as before, by securing
a 0.125 m' wire hoop to die ground and clipping die vege-
tation rooted widiin. Samples were dried and weighed as de-
scribed earlier and analyzed to estimate forage quality in each
patch.
Where appropriate to meet the criteria for statistical anal-
yses, we have log- or arcsine-transformed variables.
RESULTS
Model parameter estimates
We estimated bite mass using sods ranging in biomass between
551-3128 kg/ha. Bite mass (in g/bite) increased linearly widi
grass biomass (Figure la; y = 0.087 + 0.0014* t = 2.379, p
= .03, r1
= .303, n = 15). Cropping rates were determined

2000 4000
Figure 2
The daily rate of dry matier intake by wapiti in relation to grass
biomass. See text for regression equation.
on forage patches ranging from 653-3557 kg/ha. Wapiti spent
10% (±1.0) of each foraging bout in activities other than
cropping. Therefore, bite rates were multiplied by 0.90 to
eliminate this bias. Cropping rates were a negative curvilinear
function of grass biomass (Figure lb; y = 438,357/(x + 7706),
r1
= .315, n = 18), consistent with previous grazing studies
(Gross et al., 1993a; Shipley and Spalinger, 1992). Dry matter
intake values, calculated from the product of bite mass and
cropping rates, suggested a Type II functional response, as
postulated (Figure 2). Parameters a and b for Equation 1 were
estimated as 1.87 kg/h and 1604 kg/ha, respectively.
Grass biomass (in kg/ha) in the ungrazed exclosures in-
creased over time (in days since 1 January 1991) at a decel-
erating rate (y = 15.467*/(196.0 + x), r1
= .636, n = 70). To
distinguish between possible effects of grass biomass (matu-
ration) and time on forage quality, we conducted multiple
regression analyses for each of percentage NDF, ADF, lignin,
and protein using sampled biomass and date as independent
variables, in each case with 70 forage samples. Percentage
NDF increased with both grass biomass (Figure 3a; y = 1.64
+ 7.75 ln(x), r* = .40, t = 6.65, p < .001) and date (y =
-123.8 + 35.2 ln(z), r3
= .34, I = 5.88, p < .001), but in a
multiple regression analysis only the coefficient for grass bio-
mass was significant (4, = 2.91, p = .05, 4 = 1.42, p = .16).
Percentage ADF increased with both grass biomass (Figure 3b;
y = -14.3 + 6.7 ln(x), r1
= .59, t = 10.0, p < .001) and date
(Figure 4a; y = -132.3 + 32.3 ln(z), r* = .56, t = 9.3, p <
.001), with both coefficients significant in the multiple re-
gression analysis (y = -77.7 + 4.14 ln(x) + 15.9 ln(z), T2
=
.64, L, = 3.9, p < .001, t, = 3.04, p < .001). Percentage lignin
also increased with both grass biomass (y = -3.75 + 1.25
ln(x), r1
= .26, t = 4.92, p < .001) and date (Figure 4b; y =
-39.0 + 8.55 n(z), r1
= .65, 4 = 10.9, p < .001), but in a
multiple regression analysis only the coefficient for date was
significant (t, = 0.8, p = .43, 4 = 5.8, p < .001). Hence, based
on variables with the strongest explanatory power, we used
univariate estimates of percentage NDF and lignin and the
multivariate relationship of percentage ADF to estimate per-
centage DE by the equation y = 210.9 - 0.67 ln(x) - 27.9
ln(z). As our patch use trials were conducted over the same
100 1000 10000
Figure 3
Percentage neutral detergent fiber (a) and acid detergent fiber (b)
from grazing exclosure samples in relation to grass biomass. See
text for regression equations.
week, the time effects are simply constants in the model of
energy gain, but they do improve the precision of our digest-
ible energy estimates.
Percentage protein in the undisturbed samples decreased
with both grass biomass (y = 36.3 — 6.3 ln(x), r1
= .50, t =
6.30, p < .001) and date (y = 101.7 - 17.3 ln(z), r1
= .59, /
= 7.52, p < .001). Multiple regression indicated that only the
effect of date was significant when date and grass biomass
were considered together (t, = 1.47, p = .15, 4 = 3.30, p =
.002).
Linking of the foraging parameters and maturational vari-
ation in DE content with estimates of energy expenditure in
the model, we calculated daily rates of net energy intake in
relation to grass biomass. Our model predicted that daily en-
ergy gains should rise steeply at low forage biomass, peak at
roughly 1000-1100 kg/ha, then slope downward with further
increases in grass biomass (Figure 5). At the optimal grass
biomass, the maximum daily intake of DE should be 53 kj/

WUmshurst et al. • Forage selection by wapiti 21S
10.0
in ••
100 150 200 250
JULIAN DATE
Figure 4
Percentage acid detergent fiber (a) and lignin (b) from grazing
exdosure samples in relation to Julian date. See text for regression
equation*.
kg body mass, or 9010 kj/day for the 170 kg animals used in
our study.
Patch use experiment
In the foraging experiment, grass biomass within patches
spanned the predicted optimum, ranging between 796 kg/ha
in the patches with 2 weeks growth and 2867 kg/ha in the
patches with 10 weeks growth (Figure 6). The patches with 4
weeks growth (mean = 1212 kg/ha) were closest to optimal,
according to the model.
There was no significant difference in DE content between
the experimental patches and the ungrazed exclosures (AN-
COVA: /",l n = 1.13, p = .29), indicating that model param-
eters based on the exclosures should be appropriate to the
experimental patches. The total time spent by the animals on
each paddock did not differ (ANOVA: FtM = 2.553, p = .132),
so the data for each paddock were pooled for subsequent
analysis.
2000 4000
Figure 5
Net energy intake predicted for wapiti in relation to gran biomass.
The optimal value is marked by an asterisk.
We used two types of statistical procedures to test whether
wapiti exploited grass patches of intermediate biomass: (1)
polynomial regression and (2) ANOVA with Tukey post-hoc
comparisons of daily foraging time and percentage foraging
time per day in relation to the number of weeks of uninter-
rupted grass growth (2, 4, 6, 8, or 10 weeks) within patches.
In die case of polynomial regression, the critical test is wheth-
er the coefficient of a second order term is significantly less
than 0, implying that animals use patches of intermediate
growth more heavily than other patches. More specifically, die
patches widi 4 weeks growth would be predicted to yield the
4000
2 A 6 8 10 12
PATCH GROWTH (weeks)
Figure 6
Grass biomass within patches in relation to the number of weeks of
grass growth within patches. Vertical bars are 1 SE-

highest rate of energy gain and therefore be preferred relative
to all other patches. We took a conservative approach in an-
alyzing patch use data to reduce the possibility of spurious
inference due to pseudoreplication (Aebischer et al., 1994).
Accordingly, we lumped data for replicates of the same patch
growth treatment. This left us with 59 degrees of freedom,
corresponding to 4 animals, 3 days of observation, and 5 cat-
egories of patches. We used conventional parametric tests be-
cause there was Iitde evidence of non-normality in die source
data or in the residuals. There was Iitde reason to suspect that
use of one patch type precluded use of other patches (Ae-
bischer et al., 1994; Johnson, 1980) because none of the study
animals foraged for more than 4.2 hours out of die maximum
12 hours possible during each observation period and diree
replicate patches of similar biomass were always available to
study animals.
Polynomial regression indicated that daily foraging time (in
s) was highest in patches of intermediate growth (in weeks)
(Figure 7a;^= -2176 + 1581x - 119**, r* = .377, ^ = 5.601,
p < .001, L^ = -5.155, p < .001). Inclusion of trial day im-
proved slightly die degree of fit (r* = .439, / = -2.496, p =
.02). Similarly, significant variation occurred among individ-
uals with respect to die quadratic relationship between daily
foraging time and patch growth stage (r1
— 0.473, I = 1.886,
p = .005).
We normalized daily foraging time data by dividing time per
patch by the total time summed over all patches. We dien
arcsine-transformed these normalized patch use data and re-
gressed die transformed values against weeks of patch growdi.
As before, percentage foraging time per patch was better pre-
dicted by a quadratic dian a linear model (Figure 7b; y = 14.4
+ 14.2JC- 1.05^,7" = .559,1^ = 7.885, p<. 001, <*, = -7.118,
p < .001), implying diat patches of intermediate growdi were
used most heavily.
Results of die regression tests were corroborated by ANO-
VA_ Daily foraging time varied significandy amongst die five
patch categories (FAM = 13.254, p < .001). When multifactor
ANOVA models were examined, dairy foraging time was also
affected by trial day (F = 5.385, p < .01) and animal (F =
8.658, p < .001). The arcsine-transformed values for percent-
age foraging time also varied amongst patches (F^M = 42.871,
p < .001). In die latter case, no direct effect of eidier trial
day or animal (p > .10) was detectable statistically, but an
interaction effect did imply significant variation among ani-
mals widi respect to die relationship between patch growth
stage and percentage patch use by wapiti (F = 7.352, p <
.001).
Wapiti spent 1.3% of dieir foraging time in patches widi 2
weeks growdi, well below die value of 20% expected from a
random model. Wapiti spent over 30% of dieir time in patch-
es widi 4 weeks growdi and lesser amounts of time in patches
widi longer growdi, as predicted by die optimal energy gain
model. Tukey's test for multiple post-hoc comparisons (a <
0.05) confirmed diat patches widi 2 weeks of growth were
used significandy less dian die others, patches widi 4 weeks
of growdi were used significandy more dian die odiers, and
patches with 6, 8, or 10 weeks growdi were indistinguishable
statistically.
DISCUSSION
Three conditions must be met for die forage maturation hy-
podiesis to apply to patch selection by grazing herbivores: (1)
short-term rates of grazing intake must increase widi plant
biomass, (2) ad libitum limits on intake must be linked to
indices of forage quality such as digestible energy or protein
content, and (3) digestible energy or protein content must
decline widi increasing plant biomass widiin patches. We have
4000
2 4 6 8 10 12
PATCH GROWTH (weeks)
Figure 7
Daily foraging time (a) and percentage foraging time per day (b) in
relation to the number of weeks of grass growth within patches.
Vertical bars ire 1 SE. Expected values under the null hypothesis
are indicated by the dotted line.
shown diat diese conditions exist for wapiti and have dem-
onstrated experimentally diat wapiti preferentially selected
patches of intermediate abundance.
Patch use patterns during die trial indicated diat wapiti pre-
ferred grass patches widi 1200 kg/ha, close to die model pre-
diction. This result is also consistent widi results recendy re-
ported by Langvatn and Hanley (1993) for red deer (Cervus
daphus). They conclude, however diat patch selectivity by red
deer optimized die rate of protein intake radier than die rate
of energy gain, as we suggest. Indeed, protein levels in our
pastures did decrease over die growing season at die same
time that digestible energy was declining. Hence, digestible
energy and protein content covaried strongly in both our
study and diat of Langvatn and Hanley (1993). We show else-
where (Wilmshurst JF and Fryxell JM, unpublished) diat die
difference between our conclusions and diose of Langvatn
and Hanley Ls largely a matter of die time period of foraging

under consideration. By incorporating a digestive constraint
in our model, we consider the implications of gut fill over an
entire foraging bout on net energy intake. Langvatn and Han-
ley (1993) predicted intake using only the availability con-
straint and model instantaneous intake. Hence, patterns of
forage selection that maximize long-term rates of energy in-
take also maximize short-term rates of protein intake, at least
in the two systems that have been studied thus far. To resolve
this issue, studies are needed from systems with weak covari-
ation, if this is indeed possible, between protein content and
energy digestibility.
Variation in forage quality is associated with a number of
factors. First and foremost, physiological processes cause the
accumulation of fiber as individual ramets grow and mature
(Kamstra, 1973; Wake, 1963). Other factors, such as temper-
ature and humidity (Wilson et al., 1976), soil moisture (Cook
et al., 1961), and light intensity (VanDyne and Heady, 1965),
may result in temporal variation in forage quality that are not
associated with increasing biomass per se (Van Soest et al.,
1978). These factors played a relatively large role in forage
quality variation measured here, particularly with respect to
lignin, which continued to accumulate even after growth had
slowed late in the growing season. Like maturational variation
in forage quality, these additional sources represent poten-
tially important variation in forage quality for herbivores in
their natural ranges.
Strictly speaking, our optimality model suggests that wapiti
should have spent all of their time in the most profitable
patches, whereas they spent considerable amounts of time for-
aging in other patches. A common feature of most foraging
experiments, such partial preferences could be due to (1) in-
dividual differences in optima among animals, (2) discrimi-
nation errors in selecting optimal patches, or (3) sampling to
update information (Krebs and McQeery, 1984). Any or all
of these factors could contribute to an animal spending time
in less profitable patches.
We would caution against the indiscriminant use of the op-
timality model for ungulates of sizes differing from that of the
yearling wapiti examined here. Both bite size and cropping
rate parameters vary allometrically among grazers (Gross et
al., 1993b; Shipley et al., 1994; Shipley and Spalinger, 1992),
which could change the constraint curves markedly. More-
over, energetic value and processing time can vary consider-
ably among various food items available to herbivores (Baker
and Hobbs, 1987; Belovsky, 1978, 1986; Fryxell et al., 1994;
Spalinger et al., 1986), which could influence patterns of
patch use strongly.
The pattern of patch use exhibited by wapiti could help
explain seasonal movement patterns of wapiti. In montane
habitat, wapiti migrate along an altitudinal gradient biannu-
ally, moving to higher altitudes in the spring and to lower
altitudes in the fall (Boyce, 1989). As has been suggested for
bighorn sheep (Ouis canadensis) (Festa-Bianchet, 1988; He-
bert, 1973) and red deer (Qrruus daphus L.) (Albon and La-
ngvatn, 1992), wapiti may time their migration to exploit high
quality emergent forage as the snow cover melts in the spring.
The forage selection patterns we observed suggest that if mi-
gration is timed to maximize net energy intake, then wapiti
should lag several weeks (depending on vegetation growth
rates) behind the melt Several studies report that wapiti move
to higher elevations a short time after snow melt in the spring
(Brazda, 1953; Murie, 1951; Ward et al., 1973). Brazda (1953:
Table 3) suggested that wapiti do not occupy the summer
range until the forage has reached a "succulent . .. develop-
ing" stage. However, Brazda did not measure seasonal
changes in plant nutritional quality.
What evidence is there that the three conditions necessary
for the application of the forage maturation hypothesis found
to hold here may also exist in other systems? Gross et al.
(1993b) have recendy demonstrated that functional responses
of 12 species of herbivores ranging in body size from 0.05 to
548 kg increase monotonically. Previous studies have shown
similar trends with other taxa (Fryxell et al., 1994; Fryxell and
Doucet, 1993; Lundberg, 1988; Renecker and Hudson, 1986;
Short, 1985), suggesting that Type II functional responses are
common among grazing herbivores. Variation in DE intake
with respect to forage DE content has not been widely studied.
However, data for other ungulates have shown this relation-
ship to be positive and decelerating (Ammann et al., 1973;
Clancy et al., 1976). The mechanism behind this relationship
should apply to most ruminants and possibly even hind-gut
fermenters (Duncan et al., 1990). Finally, maturational de-
clines in forage quality have been extensively documented,
varying in degree among forage species (Armstrong et al.,
1986; Fonnesbeck et al., 1981; Lewis et al., 1975) and among
climatic zones (Barton et al., 1976; Kamstra, 1973). Hence,
conditions favoring selection for patches of intermediate
plant abundance could apply to many large grazing herbi-
vores.
The trade-off between forage quality and abundance has
interesting implications for the application of optimal patch
use models to herbivores (Charnov, 1976; Stephens and
Krebs, 1986). One might predict from such models diat vari-
ation in resource abundance among patches should decline
over time as patches of high resource abundance become de-
pleted, provided that rates of resource renewal are minor rel-
ative to rates of consumption. Unlike classic patch choice
models, our work suggests that herbivores should concentrate
in patches of intermediate resource abundance. Accordingly,
patches of high resource abundance should tend to be ig-
nored and accumulate biomass, if by chance they have es-
caped attack at earlier stages of growth, whereas patches of
lower abundance should experience periodic depletion and
regrowth. Hence, maturational effects for herbivores should
lead to increased spatial variation in resource abundance over
time, contrary to classic patch use models.
A number of factors are thought to contribute to aggregat-
ed spatial distributions of large herbivores, including predator
avoidance (Estes, 1974;Jarman, 1974) and exploitation of spa-
tially concentrated resources (Kreulen, 1975; McNaughton,
1988, 1990; Sinclair, 1977; Western, 1975). Hence, habitat se-
lection for patches of intermediate resource abundance could
lead to aggregation provided that resources are patchy in
space or time (Fryxell and Sinclair, 1988; Gordon, 1988; Mad-
dock, 1979). Indeed, energy-maximizing decisions by herbi-
vores themselves could lead to aggregations that maintain
patches at a preferred biomass level (Fryxell, 1991; Hobbs and
Swift, 1988; McNaughton, 1984). Hence, understanding the
adaptive basis of patch selection (Gordon and Illius, 1989),
such as that demonstrated in our study, may lead to improved
understanding of migratory and grouping behavior by wapiti
or other generalist grazers.
Understanding patterns of forage selection by herbivores in
relation to energetic constraints is an important step to pre-
dicting habitat use and the resulting long-term dynamics of
plant-herbivore communities (Hobbs and Swift, 1985; Illius et
al., 1992). Determination of these constraints should better
enable us to model herbivore population dynamics effectively
and also gain insight into the processes that ultimately limit
and regulate population growth.
We would like to thank C Olsen, S. Poluch, and C Doucet for tech-
nical assistance. We also thank S. Albon, J. Atkinson, N. T. Hobbs, D.
Kramer, E. Izm D. Lavigne, R. Walton, and an anonymous reviewer
for comments on an earlier version of this article. Research was sup-
ported by a Natural Sciences and Engineering Research Council

(NSERC) operating grant to J.M.F., Agricultural Research Council of
Alberta grant to R.J.H., and a Sigma Xi research grant and Ontario
Graduate and NSERC scholarships to J.F.W.
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