Diet and Activity of the Bear Cuscus, Ailurops ursinus, in North Sulawesi, Indonesia

Abstract

We studied the daily time budget and feeding activity of the bear cuscus, Ailurops ursinus, in the Tangkoko-Duasudara Nature Reserve, North Sulawesi, Indonesia. Bear cuscuses spent 63.4% of their time resting, and feeding accounted for only 5.6% of their activity. Bear cuscuses fed on 31 species of plants, including 26 identified trees and lianas from 17 families and 5 unidentified mistletoes. Dietary preference was influenced by availability of young leaves, and bear cuscuses maximized the amount of young leaves in the diet.

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American Society of Mammalogists
Diet and Activity of the Bear Cuscus, Ailurops ursinus, in North Sulawesi, Indonesia
Author(s): Asri A. Dwiyahreni, Margaret F. Kinnaird, Timothy G. O'Brien, Jatna Supriatna,
Noviar Andayani
Source:
Journal of Mammalogy,
Vol. 80, No. 3 (Aug., 1999), pp. 905-912
Published by: American Society of Mammalogists
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DIET AND ACTIVITY OF THE BEAR CUSCUS, AILUROPS
URSINUS,
IN NORTH SULAWESI,
INDONESIA
ASRI A. DWIYAHRENI, MARGARET E KINNAIRD, TIMOTHY G. O'BRIEN, JATNA SUPRIATNA, AND
NOVIAR ANDAYANI
Wildlife Conservation Society-Indonesia Program, Post Office Box 311,
Bogor 16003, Indonesia (AAD, MFK, TGO)
Department of Biology, Faculty of Mathematics and Sciences, University of Indonesia,
Depok 16424, Indonesia (JS, NA)
We studied the daily time budget and feeding activity of the bear cuscus, Ailurops ursinus,
in the Tangkoko-Duasudara Nature Reserve, North Sulawesi, Indonesia. Bear cuscuses
spent 63.4% of their time resting, and feeding accounted for only 5.6% of their activity.
Bear cuscuses fed on 31 species of plants, including 26 identified trees and lianas from 17
families and 5 unidentified mistletoes. Dietary preference was influenced by availability of
young leaves, and bear cuscuses maximized the amount of young leaves in the diet.
Key words: Ailurops ursinus, bear cuscus, Garuga floribunda, Sulawesi, time budget,
diet, feeding preferences
The marsupial family Phalangeridae is a
monophyletic group that includes ->17 liv-
ing species, all of which are partly or whol-
ly arboreal omnivores or herbivores (Flan-
nery et al., 1987). The Phalangeridae has
the most extensive range of any Austral-
asian family of marsupials and occurs in
Australia, Tasmania, Papua New Guinea,
and throughout islands east of Wallace's
Line as far as the Solomon Islands (George,
1987). The Indonesian island of Sulawesi
forms the northwestern limit of its distri-
bution (Flannery et al., 1987; George,
1987). Although known to European zool-
ogists as early as the 18th century, the Phal-
angeridae remains poorly known, especially
its ecology, with the possible exception of
the Australian brush-tailed possum, Tricho-
surus vulpecula, and the Australian moun-
tain possum, T. caninus (Flannery et al.,
1987).
The bear cuscus, Ailurops ursinus, is the
largest and most primitive species of the
Phalangeridae, and it is found only on Su-
lawesi and surrounding islands. The bear
cuscus lives in sympatry with the dwarf
cuscus, Strigocuscus celebensis, except on
Peleng Island in Central Sulawesi, where it
is sympatric with the Peleng Island cuscus,
S. pelengensis, an endemic species (Flan-
nery, 1995). The bear cuscus is an arboreal
folivore that lives in the upper canopy of
lowland forest. It is active during the day
and found mostly in pairs (Tarmudji and
MacKinnon, 1980). Unlike the bear cuscus,
both species of Strigocuscus are predomi-
nantly nocturnal and frugivorous.
There is no detailed information on the
ecology of the bear cuscus on Sulawesi, es-
pecially its feeding habits (George, 1987).
The only previous study was a survey con-
ducted by Tarmudji and MacKinnon (1980)
that provided general information about be-
havior. Detailed information is needed to
enhance conservation of this little-known
marsupial. O'Brien and Kinnaird (1996) re-
ported that the population of bear cuscuses
in the Tangkoko-Duasudara Nature Reserve
on Sulawesi decreased 95% over 15 years
as a result of hunting pressure. We describe
daily activity patterns of the bear cuscus,
focusing on feeding behavior, and provide
the first preliminary data on its dietary di-
versity and preference.
Journal of Mammalogy, 80(3):905-912, 1999 905

906 JOURNAL OF MAMMALOGY Vol. 80, No. 3
MATERIALS
AND METHODS
Study area.-We conducted our study in
April-July 1995 in Tangkoko-Duasudara Nature
Reserve. The 8,867-ha reserve lies on the north-
ernmost tip of Sulawesi (1'30'-1o34'N, 125081'-
125010'E) and contains many habitats, including
beach forest, lowland forest, montane forest, and
even moss forest on peaks of Mount Tangkoko
and Mount Duasudara. Annual rainfall varied
from ca. 1,500 to 2,400 mm and was distributed
in distinct wet (October-April) and dry seasons
(May-September). We worked within a 416-ha
area of lowland forest, located on the north
slopes of Mount Tangkoko. The area was grid-
ded by trails forming 1-ha blocks. Canopy with-
in the area was dominated by five species of
trees: Palaquium amboinense (Sapotaceae),
Cananga odorata (Annonaceae), Homalium ce-
lebicum (Flacourtiaceae), Dendrocnide micro-
stigma (Urticaceae), and Eugenia (Myrtaceae).
Collection of data.-To collect behavioral
data, an individual or a group of bear cuscus was
located within the study area between 0600 and
0900 h and followed until 1700 h. Data collected
included height of a bear cuscus in the tree, spe-
cies of tree, and behavioral data. Behavioral data
were collected using focal-animal sampling (Alt-
mann, 1974) at 30-s intervals during 10-min
sampling periods followed by 5 min of no ob-
servation. Whenever possible, the focal animal
was selected according to a fixed rotation among
individuals in a group.
Animals were categorized into four classes of
age and sex. Adult males were identified by their
large size and prominent genitalia. Adult fe-
males were identified by large size and being
paired with an adult male, or large size and pres-
ence of an independent juvenile, or presence of
a clinging infant. Subadults were smaller than
adults and usually still living in a group with
adults; infants were smaller than subadults and
generally still carried by an adult female.
Behavior was classified into five activities.
Resting was defined as a behavior during which
no locomotion occured, regardless of posture;
sleeping, sitting still, and sitting with slight
movement of head, tail, hand, or leg were in-
cluded in that category. Moving was defined as
walking or moving the whole body to change
position, and feeding was defined as taking food
into mouth, biting, chewing, and swallowing;
sniffing and investigating food on branches were
included in this category. When the focal animal
was feeding, the plant species and part of the
plant that was eaten were recorded. Grooming
was defined as picking, scratching, or removing
debris from an individual's own fur, and social
activity was defined as grooming others, nurs-
ing, or being nursed.
Leafing phenologies were studied along a
transect (5 m by 2,500 m), twice each month, to
assess availability of food. We used a score of
0-4 for each tree (Struhsaker, 1975), in which 0
indicated no leaves; 1, 1-25% coverage; 2, 26-
50% coverage; 3, 51-75% coverage; 4, 76-
100% coverage of leaf buds, young leaves, and
mature leaves.
Analysis of data.-We calculated average per-
centage of records spent in the five behavioral
categories for each individual bear cuscus for 3
periods: morning (0600-1100 h), mid-day
(1101-1400 h), and afternoon (1401-1700 h).
We summed the average value for each individ-
ual and calculated an overall mean percentage
of records for each behavior by time of day, and
then calculated the overall mean percentage of
each behavior for bear cuscuses in general. We
used analysis of variance and Duncan's multi-
ple-range tests (Sokal and Rohlf, 1981) to ex-
amine the influence of time of day, age, and sex
on each behavior. Because data were propor-
tions, we applied an arcsine-square-root trans-
formation before analysis.
For feeding activity, we calculated the aver-
age proportion of records for eating a particular
species of plant and items (leaf, flower, or fruit)
for each bear cuscus. We then summed the av-
erage value for all individuals and calculated an
overall mean percentage of activity spent feed-
ing on each species of plant and item.
We calculated a biweekly phenological index
using the midpoint of the range of percentages
for each phenological score (Kinnaird, 1992)
and considered this a measure of the availability
of leaves as dietary items. We used Jacob's in-
dex of preference (Jacob, 1974) to rank feeding
preference by bear cuscuses. Jacob's index (D)
compared the frequency of each species being
eaten to its availability in the environment. D-
values range from -1 to + 1; as the index ap-
proached +1, it indicated increasing preference,
and as index approached -1, it indicated strong
avoidance.
To determine the effect of availability of
leaves on feeding preferences and the proportion
of activity spent feeding by bear cuscuses, we

August 1999 DWIYAHRENI
ET AL.-ACTIVITY OF BEAR CUSCUS 907
used multiple regression analysis (Sokal and
Rohlf, 1981). For the analysis, we used the bi-
weekly availability of leaf buds, young leaves,
mature leaves for each dietary species and the
week of study as independent
variables.
Feeding
preferences and proportion of activity spent
feeding by bear cuscuses were dependent
vari-
ables. The partial slope (P3)
of each independent
variable was evaluated using type-III sums of
square, and all analysis were performed
using
the Statistical Analysis System (SAS Institute
Inc., 1985).
RESULTS
We collected 32,823 behavioral obser-
vations during 1,643 10-min sampling pe-
riods on individual cuscuses. Although bear
cuscuses were not individually identifiable,
those found in about the same location with
the same group composition were consid-
ered the same individual. Fifty bear cuscus-
es were found in pairs, 33 were in groups
of three or four individuals, and 17 were
solitary. We found bear cuscuses in 29 spe-
cies of tree, at an average height of 20 m
? 2.66 SD.
Daily activities.--Bear cuscuses spent
most of the time resting and sleeping (Fig.
la). Resting declined from morning to af-
ternoon, although not significantly, while
feeding showed opposite trends that were
significant. Feeding comprised only 5.6%
of the activity budget of the bear cuscus,
and proportion of activity spent eating was
significantly greater in the afternoon than in
the morning (F = 3.2; d.f. = 2.243; P =
0.04). Moving accounted for 23% of the
time budget. Movements declined from
morning to afternoon, but the difference
was not significant. Grooming accounted
for 7.5% of the daily activity budget, and
the bear cuscus spent only 0.4% of its ac-
tivity in social behavior. There were no sig-
nificant differences in the distribution of be-
haviors among classes of age and sex (Fig.
ib).
Dietary items.-Bear cuscuses fed on 31
species of plants, including 26 identified
trees and lianas from 17 families and 5 un-
identified mistletoes. However, >50% of
a)
70 C3Morning EMid-day OAfternoon
60
,D
S 5so
o 40
• 30
S 20
10
0 Resting Moving Feeding Social Grooming
Activity
b)
70 - Resting OMoving EFeeding ~ISocial EGrooming
60
F 50
44
o 40
m 30
C 20
S 10
0 Male Female Sub adult Juvenile
Age-Sex Class
FIG.
1.-Percentage of activity spent by bear
cuscuses in five behavioral
categories
according
to a) time of day and b) age-sex class.
their diet comprised only three species of
tree: Garuga floribunda (23.5%), Melia
azedarach (19.4%), and Dracontomelum
dao (17.9%, Table 1). They fed mainly on
young leaves (54.4% of their diet), fol-
lowed by mature leaves (22.9%), and leaf
buds (7.8%; Table 1). For some species,
such as mistletoes, Cananga odorata and
Palaquium amboinense, bear cuscuses
chose mature leaves over young leaves.
Bear cuscuses ate petioles (leaf stalk) of
some species, especially when leaves were
mature. We once observed a bear cuscus
removing the blade of mature leaves of C.
odorata before eating petioles, and leaves
of Pterocymbium javanicum with eaten pet-
ioles were found under a tree used by a bear
cuscus for sleeping. Fruit was a minor part
of the diet, and unripe fruits were eaten
more than ripe fruits (unripe, 4.2%; ripe,
0.4%). Bear cuscuses also occasionally fed
on flowers and flower buds (2.9%). The re-

908 JOURNAL OF MAMMALOGY Vol. 80, No. 3
TABLE
1.-Species and part of plants eaten by bear cuscuses during the sampling periods as a
percentage of activity budget spent feeding, based on 2,653 feeding records.
Leaves
Family Speciesa nb Bud Young Mature Otherc Total
Anacardiaceae Dracontomelum dao 475 3.7 12 0.1 2.1 17.9
Koodersiodendron pinnatum 25 0.2 0.6 0.1 0.9
Annonaceae Cananga odorata 26 0.3 0.4 0.3 1
Alstonia ranvolfia 19 0.7 0.7
Urceola 47 1.8 1.8
Asclepiadaceae Marsdenia 5 0.1 0.1 0.2
Burceraceae Garuga floribunda 624 2.7 14.8 3.9 2.1 23.5
Ebenaceae Diospyros rumpii 3 0.1 0.1
Euphorbiaceae Macaranga tannarius 72 2.7 2.7
Fabaceae Derris elliptica 15 0.1 0.5 0.6
Flacourtiaceae Homalium celebicum 61 2.3 2.3
Meliaceae Melia azedarach 514 0.1 10.1 8.1 1.1 19.4
Moraceae Artocarpus dadah 16 0.5 0.1 0.6
Ficus variegata 55 0.8 1.3 2.1
Myrtaceae Eugenia 3 0.1 0.1
Syzygium 68 2.3 0.2 0.1 2.6
Sapotaceae Palaquium amboinense 206 0.1 0.8 3.5 3.4 7.8
Sterculiaceae Pterospermum javanicum 15 0.3 0.3 0.6
Verbenaceae Vitex quinata 136 0.1 4.2 0.2 0.6 5.1
Mistletoe A 100 0.2 0.9 2.5 0.2 3.8
Mistletoe B 82 0.1 0.1 2.8 0.1 3.1
Total 2567d 7.8 54.4 22.9 11.8 96.9d
a Four species of plants-Dracontomelum mangiferum (Anacardiaceae; young leaves), Alstonia sumatrana (Apocynaceae; young
and mature leaves), Ficus (Moraceae; unripe fruits), and Pterocymbium javanicum (Sterculiaceae; young leaves and petioles)-
were eaten outside the sampling periods and not included in the percentage of time spent feeding.
b
The number of feeding records in which a species was eaten by bear cuscuses.
c Other category consists of flowers and fruits and unidentified parts for listed species.
d
The remaining 86 observations (3.1%) of the diet, consisted of unidentified parts, flowers and fruits, and leaves of four plants
species (Aglaia and species of mistletoe) that each comprised <0.1% of the diets.
maining 7.4% of the diet comprised un-
known parts of plants and four species of
plants (Aglaia and species of mistletoe) that
each comprised <0.1% of the diets.
Feeding preferences.-Although the val-
ues of Jacob's index do not indicate a pref-
erence for any species in the diet, the values
do indicate the ranking of dietary prefer-
ence. G. floribunda was the most preferred
species in the diet (D = 0.021), followed
by D. dao (D = -0.230) and M. azedarach
(D = -0.237; Table 2). Among all dietary
species, multiple regression analysis
showed that Jacob's index was influenced
only by biweekly availability of young
leaves (partial P = 0.03, t = 2.91, P =
0.004). Similarly, multiple regression anal-
ysis indicated that availability of young
leaves also influenced the percentage of ac-
tivity spent eating each species (partial P =
0.004, t = 2.67, P = 0.009). Although
availability of young leaves across all spe-
cies stayed relatively constant throughout
the study (Fig. 2a), it fluctuated among di-
etary species on a biweekly basis (Fig. 2b).
DIscussIoN
As the largest arboreal folivores in Su-
lawesi, bear cuscuses fill a niche occupied
by primates west of Wallace's Line and tree
kangaroos, Dendrolagus, further east. Ac-
tivity budgets of bear cuscuses are similar
to diurnal, canopy-dwelling, folivorous pri-
mates (e.g., howler monkey, Alouatta-
Crockett and Eisenberg, 1986; Milton,
1979) and presumably sloths (Bradypus

August 1999 DWIYAHRENI ET AL.-ACTIVITY OF BEAR CUSCUS 909
TABLE
2.-Index of preferences (Jacob's
D) and density for 17 dietary species of bear
cuscuses.
Density
Jacob's (plants
Species of plant D /ha)
Garuga floribunda 0.021 12
Dracontomelum dao -0.230 8
Melia azedarach -0.237 13
Vitex quinata -0.374 38
Ficus variegata -0.403 9
Palaquium amboinense -0.580 54
Koodersiodendron pinnatum -0.634 8
Macaranga tannarius -0.638 7
Barringtonia acutangula -0.666 3
Cananga odorata -0.677 27
Syzygium -0.678 16
Cryptocarya bicolor -0.729 14
Alstonia ranvolfia -0.739 17
Diospyros rumpii -0.822 12
Pterospermum javanicum -0.841 12
Homalium celebicum -0.867 40
Eugenia -0.964 23
griseus and Choloepus hofinanni-Dawson
and Degabriele, 1973) in that they spent the
majority of their activity budget inactive,
usually resting and sleeping. Such skewed
activity patterns may reflect spatial distri-
bution, and nutritive characteristics of di-
etary species. In many primates that obtain
the majority of their protein from leaves,
there is a negative relationship between
proportion of time spent moving and pro-
portion of foliage in the diet (Clutton-Brock
and Harvey, 1977; Oates, 1986). The need
to move is reduced, in part, because leaves
tend to occur in large patches, which only
require short intrapatch movements to sat-
isfy feeding requirements. In addition, bear
cuscuses presumably require relatively
large amounts of food to compensate for the
low nutritional levels of their diet. Feeding
on leaves necessitates a defined post-feed-
ing, processing period to use a bulky diet
that is comprised of highly fibrous items of
low digestibility (Chapman and Chapman,
1991; Robbins, 1993). The long resting pe-
riods of bear cuscuses may be necessary be-
fore another meal can be consumed (Oates,
a)
60 T 3OLeaf buds MYoung leaves lMature leaves
S o
o 40
-__
S 30
4-l 20
20
o
1 2 3 4 5 6 7
Biweekly Periods
b)
40 EGaruga floribunda OMelia azedarach LDracontomelum dao
0 30
(do
0 20
0 0
1 2 3 4 5 6 7
Biweekly Periods
Biweekly
Periods
FIG. 2.-The phenological index of a) leaf
buds and young and mature leaves in consecu-
tive biweekly periods and b) young leaves for
the three most common species in the diet of
bear cuscuses.
1986; Oates et al., 1980). Extended periods
of rest also may be critical for effective use
of bacterial fermentation to breakdown cel-
lulose in the hindgut (Tyndale-Biscoe,
1973).
The significant increase in feeding activ-
ity of bear cuscuses in the afternoon may
allow for more efficient use of time if di-
gestion proceeds during the night. Chapman
and Chapman (1991) suggest that, for spi-
der monkeys, Ateles geoffroyi, late after-
noon feeding may provide calories prior to
an overnight fast, minimize travel costs as-
sociated with full stomachs, and minimize
the potential of encountering patches of
food that cannot be exploited. We are not
certain, however, that bear cuscuses restrict
their feeding activities to daylight hours. On
several occasions, we observed animals
moving within a tree after dark, and this
nocturnal activity usually followed periods
of inactivity in the late afternoon. Local in-

910 JOURNAL OF MAMMALOGY Vol. 80, No. 3
formants also report that cuscuses move be-
tween trees and travel on the ground during
the night. Only a few marsupials are strictly
diurnal and the amount of diurnal activity
varies with season and species (Walker,
1975). The larger kangaroos (Macropus gi-
ganteus, M. parryi, M. robustus, and M. ru-
fus), for example, are active to some extent
throughout the 24-h cycle, and in most spe-
cies, more nocturnal activity occurs in sum-
mer than winter (Kaufmann, 1974). It is
possible that activities of bear cuscuses also
are spread throughout the 24-h cycle, but
this remains to be investigated.
Bear cuscuses fed on a variety of plants,
but they showed selectivity in both species
chosen and items eaten. Dietary selectivity
may result from the need to optimize the
mix of nutrients and total bulk of the diet
(Westoby, 1974) and choose species that
contain low levels of toxic and digestion-
inhibiting chemicals (Edwards and Wratten,
1980; Freeland and Janzen, 1974; Milton,
1979; Walker, 1975). Herbivores with spe-
cialized digestive features, such as hindgut
fermentation of the bear cuscus (Tyndale-
Biscoe, 1973), are more efficient at utilizing
fibrous foods than animals without such
specializations, and they should, therefore,
be less selective in feeding. A small herbi-
vore like the bear cuscus (ca. 5 kg), how-
ever, has a proportionately smaller capacity
of its digestive tract and a higher mass-spe-
cific maintenance requirement relative to
larger herbivores, which put limits on pas-
sage rates of food. Faced with this limita-
tion, the bear cuscus may be under more
pressure to be selective in its feeding and
may not be able to tolerate as much fiber
as larger herbivores (Milton, 1979).
During our study, bear cuscuses showed
a weak preference for Garuga floribunda, a
species of canopy tree. The emergence of
G. floribunda as the most preferred species,
however, may be due more to temporal
availability of young leaves than to a strong
preference for the species. Values of Jacob's
index (Table 2) show that cuscuses display
little or no preference for most dietary spe-
cies. Rather, cuscuses display a preference
for young leaves regardless of species.
Availability of young leaves also influenced
the time spent eating a given dietary species
during any biweekly period. We expect that
preference will change for various species
throughout the year as the phenological pat-
terns of leaf emergence change. Similar
changes in foraging behavior related to sea-
sonal changes in leafing phenologies have
been shown for greater gliders, Petaurides
volans, in Australia (Kavanagh, 1984). Bear
cuscuses probably choose young leaves be-
cause they tend to be higher in protein, low-
er in condensed tannins and lignins, and
more easily digested than mature leaves
(Feeny, 1970; McKey et al., 1981; Milton,
1979; Oates et al., 1980; Waterman, 1984).
Although infrequent, bear cuscuses did
consume mature leaves. For a few species,
such as mistletoes, Cananga odorata and
Palaquium amboinense, bear cuscuses
chose mature leaves over young leaves.
Oates et al. (1980) note that Palaquium is
unusual in that mature leaves contain higher
protein than young leaves. When eating the
mature leaves of C. odorata and Pterocym-
bium javanicum, bear cuscuses were ob-
served consuming petioles and rejecting the
leaf blade. Struhsaker and Leland (1986)
observed similar behaviors by the red col-
obus monkey, Colobus badius, in Uganda
and showed that petioles of mature leaves
have fewer secondary compounds and
greater digestibility than leaf blades.
Bear cuscuses also selected flowers and
fruits of a few species when available. Bear
cuscuses consumed flower and mature
leaves of unidentified mistletoes and P. am-
boinense. Waterman (1984) reported that
flowers and mature leaves of the same spe-
cies frequently contain similar chemical
compounds. When choosing fruits, bear
cuscuses consumed greater amounts of un-
ripe fruits relative to ripe fruits. Ripe fruits
tend to have lower concentrations of nutri-
ents, especially protein (McKey, 1975; Wa-
terman, 1984), and bear cuscuses would
have to consume large quantities that pass

August 1999 DWIYAHRENI ET AL.-ACTIVITY OF BEAR CUSCUS 911
quickly through the gut to obtain sufficient
nutrients (Davies, 1984; Davies et al.,
1988). A fast rate of passage, however is
probably incompatible with the fermenta-
tive digestive system of the bear cuscus
(Davies, 1984; Janis, 1976; Kool, 1993;
McKey, 1975). Additionally, like foregut
fermenters, a large intake of simple sugars
may cause hyperacidity and be detrimental
to intestinal microflora (Goltenboth, 1976;
Kool, 1993).
Activity and feeding preferences of bear
cuscuses over 4 months appear influenced
by availability and nutritional composition
of foods. Choices of young leaves, petioles,
and poorly protected mature leaves are con-
sistent with choices made by folivorous pri-
mates. Presumably, folivorous primates and
cuscuses apply similar criteria for dietary
selection, despite differences in morpholo-
gy of their allimentary tract, and these
choices affect the time allocated to various
behaviors. While choice of dietary species
by bear cuscuses on Sulawesi may change
between seasons, the criteria for selection
likely remain the same.
ACKNOWLEDGMENTS
This study was funded by the Wildlife Con-
servation Society. We thank the Indonesian In-
stitute of Sciences and the Directorate
General
for Nature Preservation and Forest Protection
for their assistance and permission to work in
North Sulawesi. We especially acknowledge
E.
Mashudi, Sub Balai Konservasi Sumber Daya
Alam in Manado,
and give special thanks
to N.
Kakauhe
and Y. Manderos for assistance
in col-
lection of data. Finally, we thank J. Eisenberg
for reviewing an earlier
draft of the manuscript.
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Submitted 5 January 1998. Accepted 26 October 1998.
Associate Editor was Allen Kurta.