Table Of Content^
GreatBasinNaturalist55(1),© 1995,pp.29-36
FACTORS AFFECTING SELECTION OF WINTER FOOD AND
ROOSTING RESOURCES BY PORCUPINES IN UTAH
Dave Strickland-2,JerranT. Flinders1'3^ and RexG. Gates
—
Abstract. Ecological and phytochemical factors potentially affectingwinterdietar)' discrimination by porcupines
{Erethizondorsatiim)intliemountainbrushzoneofUtahwerestudied. Porcupinesutilizedgaml^elOiik{Quercitsgamhelii)
as theirprimarywinterfood and roostingresource. Big-tooth maple [Acergrandidentatum)was themost conmion tree
species in the study area but was rarely utilized by porcupines. Conifer species were used as a food and roosting
resourcesignificantlylessoften thantheyoccurredinthestudyarea,despitethemialadvantagesprovidedbytheirrela-
tivelydensecanopies. Oakfeedtreesweresuccessfullyseparatedfromconiferfeedtreesbydiscriminantanalysis 100%
ofthetime. Oaktreeswerecorrectlyclassifiedasfeedandnonfeedtrees71%ofthetime. Gambeloakcontainedhigher
amountsofcrudeprotein,fiber,andtannins,butwaslowerinetherextractfractionsandfattyacidcontentthanconifers.
Alayerofadiposetissueusedasanenergyreservebyporcupines mayhaverelaxedenergyintakedemands sufficiendy
topermitthemtoconcentrateonadietofoaktissue,whichishighinprotein, ratherthanahigh-fatconiferdiet.Adiet
relativelyhighinproteinmayhavefacilitateddigestionoffoodmaterialhigh in fiber.Temperature didnotaffectselec-
tionoftreespecies usedforroosting. Rockand snowcaveswereutilized infrequentlyandthe studypopulationranged
widely.Threeof15studyanimalswereeatenbypredators.
Keij words:porcupine, Erethizon dorsatum,gambeloak, Quercus gambelii, dietaryselection, mountain brush '.one.
predation.
Porcupines {Erethizon dorsatum) roost and StudyArea
feed in canopies ofdeciduous trees and shrubs
forextended periods duringwinter in much of The study was conducted in the mountain
western North America (Oveson 1983, Craig brush zone near the mouth of Spanish Fork
and Keller 1986, Sweitzer and Berger 1992). Canyon in north central Utah. Elevations at
Apparent localized interspecific and intra- the study site range from 1650 to 2075 m. The
specific preferences for food and shelter general exposure is northern, and terrain is
resources by porcupines imply that chemical steep. Overstorywoodyvegetation is dominated
and/or physical advantages are available to by gambel oak {Quercus gambelii) and big-
them. Further, since snow caves, rock dens, toodi maple {Acergrandidentatum). Aspen {Pop-
and cover in canopies of coniferous tree ulus tremidoides), chokecherry {Primus virgini-
species likely offer increased thermal advan- ana), Douglas fir {Pseudostuga menziesii), white
tages in the form ofenergy savings to porcu- fir {Abies concolor), and mountain maple {Acer
pines (Clarke and Brander 1973, Roze 1987, glabrum) are also represented in the woody
1989), their dependence on a deciduous food flora. The climate in Spanish Fork Canyon
and roosting resource (which does not offer during the winter of1984-85 was not atypical.
those advantages) further strengthens the Datafrom the Spanish Fork U.S. Climatological
implication that chemical and/or physical Station, located approximately 5.5 km from
selective advantages are realized by dietary the study site, indicate that temperatures were
selection. Predator avoidance may also be an slightly colder and precipitation was slightly
important force in food and roost tree selec- higher than average (U.S. Climatological Data
tion. The objective of this research was to for Utah 1984-85). Coyote {Canis latrans) and
investigate physical, phytochemical, and eco- mountain lion {Felis concolor) tracks were fre-
logical agents involved in selection of gambel quently encountered in the study area. Private
oakbyporcupines in south central Utah. access into the study area allowed observation
^DepartmentofBotanyandRangeScience,BrighamYoungUniversity,Provo,UT84602.
^Presentaddress:USDAForestService,PleasantGrove,UT84062.
'Addresscorrespondenceandreprintrequeststothisauthor
29
30 Great Basin Naturalist [Volume 55
ofa porcupine population relatively free from low temperature readings were taken dail>' at
human disturbance. an elevation of 1597 m, as well as from the
Spanish Fork climatological station.
Methods
Laboratory and Statistical Methods
Fieldwork Tissues from feed and nonfeed trees were
We conducted fieldwork from late Decem- analyzed forprotein and phosphorus using the
ber 1984 through April 1985, at which time auto analyzer semiautomated method #12 for
the study population had shifted from a diet of feeds (Horwitz 1980). Calcium, magnesium,
inner bark (phloem and cambium) of woody potassium, and sodium content were deter-
vegetation to herbaceous vegetation. The study mined by the atomic absorption method #2
areawas systematicallysearchedby researchers for plants (Horwitz 1980). Sulphur content
on snowshoes. Study animals were captured was determined by a wet-ash process using
by hand, usually while they were still in tree nitric and perchloric acid. Crude fiber was
canopies. This was accomplished by grasping determined by the acid detergent fiber and
distal guard hairs at the posterior end of the lignin #21 method (Hoi-witz 1980). An evalua-
tail between thumb and forefinger and pulling tion of crude fat was made using the direct
the tail taut. The captured animal was then method (Hoi-witz 1980) on a Lab Con soxlet
securedby grasping the tail with the free hand extractor. A limited number of tissue samples
using a backward stroking motion to flatten were analyzed on a Hewlett Packard model
the quills. Fifteen porcupines, 10 females and 5 5995 gas chromatograph/mass spectrometer
males, were instrumentedwith radio transmit- (GCMS) for fatty acids and terpenes. Tannin
tercollars (Telonics, Inc.). Animals werelocated content was measured by the radial diffusion
dailyby triangulation, andvisual sightings were method (Hagerman 1987) with quebracho tan-
made on each animal approximatelyweekly. nin being the standard, and by astringency
Percent occurrence ofwoody species was (Gambliel et al. 1985). Soluble carbohydrates
calculated from point-quarter measurements were determined according to daSilveira
using the feed/roost tree as the center point (1978). Urine samples of captive porcupines
(Cottam and Curtis 1956). Percent occurrence on a strict diet of gambel oak were analyzed
ofwoody species vs. percent utilization ofeach forcalcium and phosphorus content when lab-
feed tree species was compared using chi- oratory results indicated the Ca/P ratio in the
square analysis to testwhetherfeed tree selec- tissue of food materials was greater than ex-
tion was random. Diameter at breast height pected. Eight oak tissue samples were chosen
(dbh), species, and distance from the feed tree at random and retested for calcium and phos-
center point were recorded for the nearest phorus content according to Horwitz (1980)
woody stem in each quadrant. Point-quarter on a Beckman DU-30 spectrophotometer
measurements were repeated using the near- Differences between oak, white fir, and
est neighbor nonfeed tree ofthe same species Douglas firfeed and nonfeed trees were statis-
as the center point. Tissues from feed and tically analyzed to help discern foraging pat-
nonfeed trees were collected to investigate terns used by instrumented porcupines.
possible differences in chemical makeup. Chemical and ecological factors were evaluat-
Tissue samples from feed trees were collected ed for between-species differences using two-
where fresh bark removal indicated the roost- sample t tests, and for within-species differ-
ing animal had foraged. Samples from nearest ences with paired t tests (Minitab 1982).
neighbor nonfeed trees were taken from Statistical results are reported at the p < .05
branches at the same height and with adiame- and p < .1 levels. Chi-square analysis was
ter similar to those from corresponding feed used to determine if utilization of feed tree
trees. Bark samples were frozen and analyzed species by porcupines differed from the ex-
for dietary components. Results from those pected. Discriminant analysis using backward
analyses reasonably approximated values elimination and forward selection (SAS 1985)
reported for gambel oak (Smith 1957, Kufeld was used to determine chemical and ecologi-
et al. 1981, Welch 1989). Location, slope, cal factors that best discriminate between tree
aspect, snow depth, and climatic conditions species, and between feed and nonfeed trees
were recordedat each feed tree site. High and ofthe same species.
1995] Wintering Porcupines in Gambel Oak 31
Table 1. Meanvaluesforfactorstestedforpossibleeffectsonporcupineherbivory.
32 Great Basin Naturalist [Volume 55
(Underwood 1966). High calcium-phosphorus locating porcupines in station trees compared
ratios have also been reported by Masslich to locatingporcupines in rockorsnowdens.
(1985) for aspen {Popidus tremuloides) tissue There were approximately 7.0 porcupines/
utilized by beaver. After an independent test km^ in the study area. Radio-collared animals
of feed tree tissue confirmed the high ratio, were far ranging and did not utilize a single
we tested the mineral content of feces and den or station tree as a base from which to
urine from captive porcupines on an oak diet. launch foraging expeditions. Rather, they
Calcium-phosphorus ratios from fecal material roosted and fed in a single tree for one to sev-
were 10:1, while ratios from urine were eral days and then moved to another roost and
approximately221:1. feeding tree. Death loss due to predation and
Tissue samples from feed trees were ana- other causes left only 3 of5 male and 6 of 10
lyzed by GCMS primarily as a check on ether female porcupines instrumented with radio
extract fractions. The small sample size did transmitting devices for the entire winter This
not permit statistical analysis, but trends sample size made statistical analysis of home
showinglowerfattyacidcontent in oak than in ranges unreliable. Several animals spent the
conifers concurred with our observation of winter in relatively small areas, but most had
lower ether extract fractions in oak. The relatively large, overlapping home ranges.
amount offatty acids was lower in oak than in Male home range extremes were 6.8 and 47.5
eitherconifer species. ha. Extremes forfemales were 9.2 and 61.8 ha.
Discriminant analysis correctly classified One female's home range overlapped those of
feed trees as either conifer or oak 100% ofthe three males and at least four other females.
time (Table 2). Six factors were important con- Movements ofup to 400-500 m between relo-
tributors to the model. Conifer feed trees had cations of some of the larger, mature animals
higher amounts of phosphorus and a greater were not uncommon. Some juvenile animals
ether extract fraction than oak feed trees. had reduced home ranges and movements,
Alternatively, oak feed trees were higher in which generally agrees with observations by
protein, calcium, tannins, and magnesium. Roze (1989). Mean distance from oak feed
Although tannins entered into the model, they trees to a potential conifer feed tree was sig-
were not a significant contributor These dif- nificantly less {p < .05) than the distance ofan
ferences between oak and conifer feed trees average move by aporcupine from an oak feed
generally are in agreement with differences in tree to any other feed tree (Table 1).
Table 1. The classification ofoak feed and non- Three of 15 porcupines (20%) were eaten
feed trees was less successful (71%, Table 2). by predators in a 4-mo period. Tracks in the
Oak feed trees were significantly higher in snow indicated that one porcupine was pur-
sodium and fiber than nonfeed trees, while sued, worried, and killed by two coyotes. The
nonfeed trees were higherinwatercontent. other two porcupines eaten by predators died
Porcupines used gambel oakas afood source late in the season on south-facing slopes bare
more often than it occurred in the study site ofsnow; neither the cause ofdeath nor carni-
{p values listed in Table 3). Six of 15 animals vore species could be positively determined.
were found roosting and feedingexclusively in Carcasses of two other porcupines that died
oak, while 9 roosted and fed in conifer species presumably ofstarvation and/or exposure dur-
at least once. Snow depths and temperatures ing the course of the study were not scav-
were analyzed for thewinterperiodbefore the engedbycoyotes.
main snowmelt (judged to be 18 March).
Average snow depths at porcupine location Discussion
sites for that time period were 0.60 m.
Maximum snow depth was 1.20 m (median Chemical Factors
0.65 m). Mean minimum temperature for the Dietary alternatives in the form ofdifferent
night previous to locating study animals was feed tree species, with significantly different
-10°C; the extreme low was -27°C. Mean chemical makeup, were available to the study
temperature for the night previous to locating population. In winter, vegetative oils have the
animals in rock or snow caves was -12°C. potential to be the most important source of
There was no statistical difference between energy forporcupines. Datafrom etherextract
the minimum nightly temperature previous to fi-actions derived fi-oni feed tree tissues indicate
1995] Wintering Porcupines in GambelOak 33
Table 2. Standardized canonical discriminant function coefficients for factors that discriminated between oak and
coniferfeedtrees (100%correctclassification), andbetweenoakfeedtrees andoaknonfeedtrees (71%correctclassifi-
cation).
Oak(+)vs.conifer(-)feedtreesn
34 Great Basin Natufuijst [Volume 55
Table3.Chi
1995] Wintering Porcupines in GambelOak 35
dependent on gambel oak for a roosting and periods in the winter or that more subtle cues
feeding resource. Considering that this re- were used to transferthe information.
hance was during a season ofenergetic stress, Long movements between feed trees in
it is hkely that remaining motionless in the dense oak cover by some study animals sug-
canopy ofoak trees to consei-ve energy while gest that predator-prey relationships may have
exploiting a high-protein food source is an influenced movements. Sweitzer and Berger
adaptive strategy. (1992) found that habitat use was related to
the age or size class of porcupines, presum-
Movements and Predation
ably in response to increased risk ofpredation
The availability ofconiferfeed trees was not to smaller porcupines. Our observations gen-
limiting since the average distance between erally agree with their findings. Mountain lion
locations of study animals was significantly and coyote tracks were seen regularly in the
greaterthan the mean distance ofa move from study area. Both species are known to prey on
any roost tree to a conifer roost tree (Table 1). porcupines (Keller 1935, Robinette et al. 1959,
It does not appear that spatial relationships of Toweilland Meslow 1977, MaserandRohweder
the various feed tree species played a role in 1983). The strongurine scentat station trees or
feed tree selection by our study population. dens makes porcupines readily detectable.
The relatively large overlapping winter home Mountain lions are capable ofknocking porcu-
ranges of animals in this study differ from pines from the canopies oftrees (Taylor 1935).
reports ofother researchers. Home ranges for If long moves decreased the predictability of
porcupines in northwestern Minnesota were mountain lions locating porcupines in station
small enough to be reported in square meters trees, it would be an adaptive strategy. How-
(Tenneson and Oring 1985). Curtis (1941), ever, long moves expose porcupines to terres-
Dodge (1967), Brander (1973), Roze (1987, trial predation by mountain lions, coyotes, and
1989), and others have documented that por- wolves {Canis lupis, which are now extirpated
cupines move short distances from dens to fi-om the study area) andwould presumablybe
feed trees, sometimes along permanent trails nonadaptive. Since ample forage exists
in the snow. Craig and Keller (1986) and throughout the study site and long moves to
Smith (1979) also reported reduced ranges in locate food resources do not appear to be a
the winter. However, Dodge and Barnes dietary necessity, long movements may be an
adaptive strategy to avoid arboreal predation
(1975) did not indicate a similar restriction in
winter movements. Roze (1987) suggested the by mountain lions. This hypothesis deserves
reason may be crusted snows that bear the furtherexamination.
weight ofthe animals. Porcupines in our study Acknowledgments
did adeptly toboggan on crusted snows down
eHxotwreevmeer,sloopnees ifnemaanlaettmeomvpetdtooavveorid45c0aptmurei.n mouWseretvhiaenwkerSs. fHo.r hJeelnpkfuilnssuagngedsttiwoonsatnoonthyi-s
fresh snow. Trails in powdery snowwere often manuscript.
direct and suggested that a destination may
have been predetermined. Literature Cited
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485-494. Accepted30September1994
