Download Sociality and asociality in white-nosed coatis

Behavioral Ecology Vol. 7 No 3: 254-263
Sociality and asociality in white-nosed coatis
{Nasua narica): foraging costs and benefits
Matthew E. Gompper
Department of Zoology and Program in Life Sciences (Ethology), University of Tennessee, Knoxville,
TN 37996-0810, USA
White-nosed coatis maintain a social structure of female-bonded groups (called bands) and solitary males. I examined the
foraging success of social and solitary individuals and the possible importance of intraspecific foraging competition in maintaining the social system, particularly in the associated context of sexual dimorphism. The study population was almost entirely
frugivorous-insectivorous. Invertebrate foraging success did not differ between solitary males and band members, although
solitary adult females were more successful than those in bands. Fruit foraging success of solitary adult males was generally
greater than that of band members, although this result varied with patch size and depended on the age class of examined
band members. Small food patches showed the greatest differential between the foraging success of solitary males and band
members. Agonistic interactions between males and bands often occurred at fruiting trees, and foraging group size was important
in determining the outcome of these events. Larger males were able to displace solitary females and small foraging groups from
fruit patches. In turn, larger groups of smaller females displaced solitary males. Male-male agonism at fruit patches was also
common, with larger, older males usually winning agonistic interactions. These findings suggest that coati social structure directly
influences foraging, and I therefore hypothesize that the coati social system is maintained (in part) by body size sexual dimorphism interacting with reliance on patchy defendable foods. Female group living allows increased access to patchy resources
that are otherwise unavailable due to small body size relative to competing males. In contrast, larger males are able to access
food patches without living in groups that might increase foraging competition. Key words: coati, Dipteryx, foraging, group living,
Nasua narica, Panama, Procyonidae, Scheelea, social structure. [Behav Ecol 7:254-263 (1996)]
G
roup living and asociality represent social structure extremes maintained by a variety of ecological costs (e.g.,
increased competition and disease transmission) and benefits
(e.g., avoidance of predators or enhanced success in resource
acquisition) (Alexander, 1974; Clark and Mangel, 1986; Hamilton, 1971; Pulliam and Caraco, 1984). For species in which
the social system varies within populations, ecological factors
selecting for or against group living may act on different individuals in different ways. For example, male African lions
(Panlhera leo) form coalitions to gain access to female prides,
while females form prides to gain the benefits of cooperative
hunting and decreased infanticide (Packer et al., 1990). In
lions, ecological factors primarily operate in the same direction: toward increased sociality. The case where ecological factors select in different directions for different segments of the
same population is more unusual. For example, male cheetahs
(Adnonyx jubatus) form coalitions to gain access to females,
while females remain solitary due to the costs of prey sharing
(Caro, 1989, 1994). Here I examine the role of a single ecological factor, food availability, in maintaining the social structure of the white-nosed coati (Nasua narica: Procyonidae).
The coati is unique within the order Carnivora in maintaining a dichotomous social structure of group-living females and
solitary males (Gittleman, 1989; Gompper, 1995; Kaufmann,
1962). Groups (called bands) contain from 6 to over 30 related and unrelated females and their immature offspring
(Gompper, 1994; Gompper and Wayne, 1996). Coati bands
are not harem groups; all adult males remain solitary with the
M. Gompper is now at the Department of Environmental and Resource Sciences and Program in Ecology, Evolution, and Conservation
Biology, University of Nevada, 1000 Valley Road, Reno, NV 895120013, USA.
Received 28 December 1994; revised 30 August 1995; accepted 30
August 1995.
1045-2249/96/S5.00 O 1996 International Society for Behavioral Ecology
exception of a brief (approximately 2 week) synchronous
breeding season. Coatis are the only truly social procyonids,
and bands display a variety of cooperative behaviors not found
in the solitary raccoons, ringtails, olingos, and kinkajous.
These behaviors (e.g., coalition formation) may influence the
ability of the individual to gain access to limited resources
such as food (Gompper, 1994).
Several researchers have postulated various costs and benefits of coati sociality and mechanisms acting in the maintenance of the coati social structure. Hypotheses for coati band
gregariousness that are not resource-based include decreased
parasite loads, increased protection from predation and infanticide, and population regulation (Burger and Gochfeld,
1992; Kaufmann, 1962; Russell, 1979, 1981, 1983). With regards to foraging, researchers working on Barro Colorado Island, Panama (BCI), have come to varied conclusions. Kaufmann (1962, p. 161) suggested that while cooperative hunting
does not occur, foraging for vertebrates such as lizards is more
effective in a group since each individual "benefits from food
chased its way by other members of the band, and more such
prey is obtained by the coati population as a whole than would
be the case if each coati hunted separately." Smythe (1970a)
suggested that female sociality and male asociality act to reduce feeding niche overlap. When fruit is rare, Smythe observed bands to concentrate foraging on leaf-litter fauna,
while males foraged more on larger prey such as small and
mid-sized vertebrates. Russell (1981, 1982, 1983) indicated
that coati sociality was primarily due to predator and infanticide avoidance. However, Russell also suggested that fluctuations in food availability (leaf-litter fauna and fruit) combined
with the predation-induced social structure strongly influence
reproduction and population structure. Thus, most previous
studies did not explicitly invoke foraging costs and benefits as
underlying coati social structure, although Smythe did view
the maintenance of coati social organization as resulting from
differentiation of feeding niches.
Gompper • Foraging costs and benefits of coati social structure
Several unstudied aspects of the relationship between coati
social structure and coati foraging costs and benefits, as well
as conflicts between the above mentioned studies, suggest that
additional analyses are needed. First, foraging by solitary adult
males was only generally described by Smythe; the actual use
of fruit, leaf-litter fauna, and vertebrates by males was not
quantified. Smythe's suggestion that males on BCI are vertebrate predators contrasts with Kaufmann's and Russell's more
detailed, longer-term observations suggesting that predation
on vertebrates is very rare. Thus, whether differentiation of
feeding niches (Smythe, 1970a) does exist has not been shown
definitively. Second, observations of the importance of antipredator cooperation within coati bands do not explain why
all males are solitary. Third, BCI coatis depend heavily on
fruits of only a few plant species, and especially on two plant
species [Scheelea (= AUaiea, Henderson, 1995) zonensis and
Dipteryx panamensis] during an extended portion of the year
(Gompper, 1994; Kaufmann, 1962; Russell, 1982). The patchiness of fruit distribution from these trees may influence
grouping patterns by limiting access to food and increasing
interference competition, as found for other species (S. Altmann, 1974; Leighton and Leighton, 1982; Terborgh, 1983).
Fourth, field observations of often intense agonism among
individuals over fruit, led me to hypothesize that the influence
of body size on direct foraging competition may be important
in maintaining coati social structure, independent of additional selective mechanisms.
In this article I examine the impact on foraging of sociality
and asociality in the BCI coati population, the same population studied by Kaufmann, Russell, and Smythe. First, I indirecdy examine the role of foraging competition by comparing
band members and solitary males foraging on leaf-litter invertebrates and on fruit I predict that the quasi-random distribution of leaf-litter invertebrates will lead to no difference
in the foraging success of solitary and group-living individuals.
In contrast, I predict that the clumped, limited, and defendable distribution of fruit patches will lead to solitary males
having increased foraging success relative to group members.
Second, I examine foraging success of females that temporarily leave bands and forage solitarily during the birthing season. In this case, it is predicted that foraging success of solitary
males and females will not differ. Finally, observations of direct agonistic interactions under fruiting trees between band
members and solitary males, and among solitary males are
reported. Specifically, I examine the hypothesis that body size
and the number of band members are important predictors
of access to fruit patches.
METHODS
Study population
Barro Colorado, Panama (9°9' N, 79°51' W) is a 1500-ha island in Gatun Lake, an artificial body of water created in 1912
by die damming of the Chagres River to create the Panama
Canal. The island is separated from the mainland by 200 to
1000 m. Many mammals including coatis have been observed
swimming in the lake, such that population isolation from the
mainland is incomplete. Wet and dry seasons produce contrasting seasons of food availability, which may limit BCI's
mammal populations (Foster, 1982; Leigh and Windsor, 1982;
Smythe, 1970b; Smythe et al., 1982). BCI is covered by tropical
moist forest and has two major habitats that are distinguished
by forest age. Older forest has escaped human intervention
for 400 to 600 years and was never cleared for agriculture
(Piperno, 1990). Younger forest was cleared 100 to 200 years
ago (Foster and Brokaw, 1982). Both habitats are included in
this study, with individual coatis and bands ranging in both
255
forest types. Reviews of die ecology, history, and mammal fauna of BCI are given by Croat (1978), Enders (1935), Gentry
(1990), Leigh et al. (1982), and Wright et al. (1994).
Field work occurred on BCI for 23 months between August
1989 and November 1993 (8/89-12/89; 6/90-8/90; 9/91-7/
92; 9/92-10/92; 7/93; 11/93). During this study the population density of coatis on BCI was 48.2 to 55.6 individuals/km2,
mean (± SD) group size was 15.3 ±6.1 individuals, and mean
foraging group size was 7.3 individuals (Gompper, 1994;
Wright et al., 1994). While the population density was high
relative to most other neotropical sites studied, group size and
foraging group size were similar to values found at other sites
(Gompper, 1994, 1995; Wright et al., 1994). The population
was sexually dimorphic in body size; adult female body weight
(kg) (mean ± SD = 3.7 ± 0.3; range = 3.1-4.8; n = 37) was
approximately 73% that of adult males (mean = 5.1 ± 0.8;
range = 3.7-6.8; n = 51). Total length (cm; head-body + tail)
of adult females (mean = 103.7 ± 9.8; range = 66.0-113.7;
n = 32) was 91% that of adult males (mean = 114.2 ± 4.6;
range = 106.0-127.0; n = 42), and mean tail length of adult
females (mean = 46.7 ± 8.0; range = 12.2-53.8; n = 33) was
87% diat of adult males (mean = 53.4 ± 2.6; range = 45.860.1; n = 43). These comparisons probably underestimate dimorphism of adults of breeding status due to the inclusion of
males that were recent emigrants from natal bands and that
had not attained full body size (Gompper, 1994). Additional
details on the population biology of the BCI coati during this
study as well as methods of trapping and marking are given
by Gompper (1994).
Behavioral sampling
Behavioral data were collected on known (usually marked)
individuals that were habituated to my presence. Five bands
(6 to 26 individuals per band) were habituated, allowing me
to follow at a distance of a few meters. Several of these bands
inhabited regions studied by Russell (1976-1978; Russell,
1979, 1982, 1983) and Kaufmann (1959-1960; Kaufmann,
1962). Behavioral data were occasionally collected from a
sixth, partially habituated band when band members could be
observed from a distance. A total of 34 solitary adult males
were habituated to my presence; this included 18 males initially habituated as juvenile band members who remained habituated following transition to a solitary lifestyle.
Identities, ages, and diets of males and band members were
recorded when first encountered. Habituated animals were
followed, and behaviors and food sources were recorded using
focal animal and ad libitum methods (J. Altmann, 1974). Focal animal samples lasted 10 min, during which all behaviors
were recorded. The resulting continuous data could then be
analyzed as both event information (i.e., the number of occurrences of a behavior) and state information (i.e., die time
spent performing a behavior). Following the 10 min samples,
no data were collected for 5 min to minimize observer fatigue
and allow individuals to be relocated, after which the next
individual in the band was sampled or the solitary male was
resampled. Due to die density of the forest, focal animals were
often lost or obscured from sight before completion of a 10
min focal sample. If the animal could not be immediately relocated, samples of <5 min were not analyzed. Samples >5
min but <10 min were used only if major activity patterns did
not change during the sample (e.g., continued foraging or
continued resting, but no change from foraging to resting
behavior), and only for behaviors analyzed as rates. Separate
analyses were performed for adult females (>24 months of
age) and subadult (12 to 24 months of age) band members.
No data were collected on foraging by juveniles (<12 months
of age).
Behavioral Ecology Vol. 7 No. 3
256
Definitions for feeding and searching for food are as follows:
• Feeding—the act of handling, chewing, and swallowing a
food item. A feeding event is the handling, chewing, and swallowing of a single food item (a fruit, an invertebrate). The
state of feeding is the time spent handling, chewing, and swallowing a single food item.
• Searching—the act of seeking food items when the food
items are in the immediate vicinity (within several meters).
While this definition is somewhat arbitrary, behaviors indicative of searching are distinct and include smelling of leaf litter
or fruit with a head-down posture; digging and scraping soil
and leaf litter; and movement within a fruiting tree's seed
shadow with a head-down posture. Field and captive studies
have suggested that coatis find food primarily via odor cues
(Chapman, 1938; Kaufmann, 1962) or by color contrast
(Chausseil, 1992). Head-down posture allows searching to be
discerned from the act of traveling toward a frequently used
food patch, during which the head is usually oriented forward.
Data on agonistic interactions were collected ad libitum (J.
Altmann, 1974) and included identification of the individuals
involved, behaviors displayed, outcome, and context (e.g.,
fruit foraging, leaf-litter foraging, resting). The following definitions pertaining to agonistic conflicts are based partly or in
whole on ethograms of Kaufmann(1962) and Krinsley (1989):
• Agonism—conflict involving directed aggression (e.g.,
fighting, chasing, vocalizing) by one or more individuals toward one or more individuals. Agonistic individuals are those
involved in the conflict, without reference to whether they
were an aggressor, recipient, winner, or loser.
• Aggressor—the individual(s) commencing the agonistic
interaction.
• Recipient—the individual(s) receiving the directed aggression from the aggressor. Identifying the aggressor and the
recipient was usually straightforward since (1) the contexts of
behaviors and vocalizations are distinct (Kaufmann, 1962;
Krinsley, 1989) and (2) a charge by one or more individuals
often began the agonistic encounter.
• Winner—the individual (s) that during or after an observed agonistic encounter (1) gained or maintained resource
access, (2) chased other agonistic individuals away from the
conflict site, or (3) displayed dominant vocal and visual behaviors (Kaufmann, 1962; Krinsley, 1989).
• Loser—the individual (s) that during or after the conflict
(1) lost or did not gain resource access, (2) was chased from
the conflict site, or (3) displayed submissive vocal and visual
behaviors (Kaufmann, 1962; Krinsley, 1989).
Diet
Although foraging on invertebrates could be directly observed, it was not generally possible to identify particular species being consumed. Foraging on fruit was readily' observable, and fruits eaten by coatis were identified using a key to
BCI's flora (Croat, 1978) or through consultation with botanists working on BCI. Scats of known individuals were examined for evidence (seeds, undigested fruit, exoskeletons,
bones, hair) of recent foraging on fruits, invertebrates, and
vertebrate prey.
BCI coatis have been observed to eat fruit from at least 48
plant species (Gompper, 1994; Kaufmann, 1962; Russell,
1982). Nonetheless, these studies have all stressed the importance of only a few tree species. Two of these species, Scheelea
zonensis and Dipteryx panamensis, were selected for detailed
analysis of the impact of fruit availabilit)' on coati foraging
patterns. These two species were selected due to (1) the contrast in their relative patch sizes (small and large as measured
in the quantity of fruits available to coatis; see below), (2) the
similar sizes (approximately 6 cm long and 3 cm wide; Croat,
1978) of the fruits, and, (3) the fact that they form the principle diet of coatis on BCI from approximately May to September and December to February, respectively. The seasonal
and daily availability and predictability of fruits of diese species may thus represent extreme but common situations that
may influence foraging success and play roles in the maintenance of the coati social system.
Scheelea zonensis is a common palm on BCI, with a density
of approximately 1.8 trees/ha (Croat, 1978; Milton, 1980). Individual plants bear fruit for an average of 6 weeks and may
produce several large infructescences per year, each of which
has approximately 100 to 1000 fruits (De Steven et al., 1987;
Wright 1990). Most foraging by coatis on Scheeleafruitis terrestrial. While coatis occasionally forage arboreally on Scheelea, die tree is difficult for diem to climb and thus canopy
access must be gained through neighboring trees, which is
often not possible (Gompper ME, personal observation). In
addition, perch space in Scheelea is limited, allowing only a
single individual access to fruit clusters (Gompper ME, personal observation; Klein and Klein, 1973). When a band member does climb a tree, the remaining band members forage
under the tree on dropped fruits. Dipteryx panamensis is a
common BCI canopy tree with a density of approximately 1
tree/ha (Croat, 1978; Milton, 1980). Dipteryx differs from
Scheelea in its abundance of fruits; an individual may drop 104
to 105 fruits/year (Bonaccorso et al., 1980). Although coatis
will enter the Dipteryx canopy to feed, most foraging is terrestrial.
To assess daily variation in the availability of fruits from individual trees, I examined the quantity of fallen fruits. Several
trees known to be used by coatis were arbitrarily chosen for
study. Trees were censused twice daily during the fruiting season. Fruits were counted, then removed (>50 m) from die
fruit shadow of die tree. I assessed the rate of fruit fall by
direct observation. Since feeding by arboreal frugivores (primates, toucans, guans) influences rates of fruitfall, observation periods were subdivided based on their presence or absence in the tree.
Data analyses
All data used in statistical analyses of foraging were corrected
per unit time. Nonparametric Kruskal-Wallis ANOVAs (corrected for ties; Siegel, 1956) were used to identify significant
differences in die foraging of different age, sex, and social
classes. Where significant differences were found, post-hoc
Mann-Whitney {/tests (corrected for ties; Siegel, 1956) were
used to compare the foraging of solitary adult males relative
to social adult females, solitary adult males relative to social
subadults, and social adult females relative to social subadults.
A Mann-Whitney {/test was also used to compare foraging on
invertebrates by solitary and social adult females.
Logistic regression was used to examine the relationship
between band size and the results of agonistic interactions
between males and band members. Dependent variables (results of encounters expressed as percentages) were transformed using die formula logit (y) = log |(y/100)/[l - (y/
100)]}. Because logistic regression requires values to fall between 0 and 1 prior to transformation, values of 0% were set
at 0.1%. A linear regression line was fitted between logit (y)
and group size to identify correlation coefficients and significance values.
Gompper • Foraging costs and benefits of coati social structure
257
on invertebrates (Kruskal-Wallis: df = 2; tied H = 1.45; tied
p = .485; Table 3). When adult females temporarily left the
band and foraged solitarily, their consumption of invertebrates increased significandy (tied z = —2.361; tied p = .018;
Foraging
Table 3). Nonetheless, foraging of solitary females did not
on
Foraging
differ significandy (tied z = 0.092; tied p = .926; Table 3)
Foraging
leaf-litter
on
Sample
from the foraging of solitary males.
on
invertevertebrates size
A total of 41.4 h of focal animal samples were collected
Month and year
fruit (%) brates (%) (%)
(n)
while individuals were foraging on fruit. Foraging could be
subdivided into actual eating and manipulation of fruit, and
September 1991
50.0
0
16
50.0
time spent in the seed shadow of the tree searching for fruit.
0
12
October 1991
16.7
83.3
My ability to define searching time-allows foraging success to
0
9
November 1991
44.4
55.6
be quantified in terms of (1) die number of fruit eaten per
36
December 1991
61.1
33.3
5.6*
0
34
January 1992
64.7
35.3
minute, (2) die time spent eating per minute, (3) the time
0
37
February 1992
73.0
27.0
spent searching per minute, and (4) die time spent eating
0
14
March 1992
14.3
85.7
fruit per minute relative to the time spent searching for fruit
—
—
—
—
April 1992
per minute. ANOVAs revealed significant differences in the
0
29
May 1992
20.7
79.3
foraging success of solitary adult males, adult female band
0
32
June 1992
53.1
46.9
members, and subadult band members for all four measures
0
43
July 1992
74.4
25.6
(Table 4). For all species of fruiting trees combined, foraging
—
—
—
—
August 1992
success
of males was significandy greater than diat of band
—
—
—
—
September 1992
members (adult females and subadults; column 2 in Table 4).
0
31
October 1992
51.6
48.4
I also examined subsets of the data for foraging only on ScheeJuly 1993
28.1
71.9
0
32
lea or only on Dipteryx. For the smaller patches (fruits not
November 1993
0
21.9
78.1
32
abundant under trees) of Scheelea fruit, ANOVAs were again
significant for all four measures. In each case, foraging success
1
Includes possible scavenging on an agouti (Dasyprocta pundaia)
of solitary males feeding on Scheelea was greater dian diat of
carcass.
band members. For die larger patches (fruits are abundant
under trees) of Dipteryx fruits, ANOVAs were significant for
RESULTS
all measures except time searching per minute (p = .366; Table 4).
Diet and fruit availability
Post-hoc analyses of differences among adults and subadults
The coatis of BCI are predominately insectivorous and frugivrevealed that, for all fruit foraging data combined, solitary
orous. I rarely observed coatis to forage on vertebrates. Of 86
adult males and adult females differed significandy only in
scats (24 from solitary males, 66 from band members) exthe number of fruits eaten per minute [p < .0001), although
amined throughout the year, 27.9% contained solely invertedie time spent searching per minute approached significance
brate matter, 58.1% contained solely fruit matter, and 14.0%
(p = .073). For all four measures, both adult males and adult
contained both fruit and insect matter. Vertebrate remains
females
had greater foraging success dian subadults (Table 4).
were not found in scats. Direct observations of foraging coatis
Thus results of die ANOVAs were primarily due to poor for(Table 1) revealed similar patterns; on average 44.2% were
aging success by subadults relative to adults.
foraging on leaf-litter invertebrates, 55.4% on fruit, and 0.4%
For analyses of only Scheelea fruits, solitary adult males alon vertebrate matter. These proportions varied greatly beways had significandy greater foraging success than both adult
tween months (Table 1). No successful predation on mamfemales and subadults. Comparisons of adult females and submals or birds was observed. Both solitary males and band
members occasionally ate caecilians (OscaecUia ochrocephala, adults indicated that Scheelea foraging success was greater for
n = 3 occasions), snakes (small Spilotes pullalus, n = 2), and adult females, widi the exception of die number of fruits
gained per minute, for which tiiere was no significant differfrogs (Bufo marinvs, n = 2). Males also feed on iguana eggs
ence (p = .074; Table 4). Thus solitary males feeding on Schee{Iguana iguana, n = 3 clutches). On several occasions band
members chased agoutis (Dasyprocta punclata) briefly, but lea have greater foraging success dian adult females, who in
turn have greater foraging success than subadults.
these presumed predation attempts were unsuccessful.
Like die patterns of foraging success on Scheeleafruits,analObservations of individual Scheelea trees showed that on a
yses of foraging on Dipteryx indicate that solitary adult males
given day the number of ripe fruits available to a terrestrially
have significandy greater foraging success dian adult females
foraging coati was small, typically less than 100 (Figure 1).
for all measures except time searching per minute. However,
Peak periods of fruit availability occurred for each tree. Howdifferences between adult males and subadults were only sigever, there was considerable variation and asynchrony in the
nificant for die number of fruit eaten per minute (Table 4).
period of fruitfall and variation in the quantity of fruit availIn addition, subadults did not differ significandy from adult
able among trees during these peak periods (Figure 1). For
Dipteryx, I did not collect data on daily variation in terrestrial females in die number of fruits eaten per minute or in die
time spent searching per minute, aldiough the ratio of time
fruit availability. However, variance similar to that seen in
Scheelea (Figure 1) was expected due to similar patterns of eating to time searching and the quantity of time eating per
minute were significant (Table 4).
animal-induced fruitfall (Table 2). That is, for both tree species, fruit did not fall as it ripened; rather, it was directly or
indirectly dropped as a result of the activity of arboreal fruAgonistic interactions
givores (Table 2; see also Bonaccorso et al., 1980).
Forty agonistic interactions between solitary males and band
members were observed. Of these, 55% occurred during forForaging behavior
aging on fruit and 12.5% occurred during foraging on invertebrates. Agonistic interactions often occurred when foraging
An ANOVA revealed no significant differences in the foraging
groups approached and started feeding at fruiting trees, and
success of adult males, adult females, and subadults feeding
Table 1
Sightings of Nasua narica foraging by food type and by month of
observation
258
Behavioral Ecology Vol. 7 No. 3
15
Figure 1
Daily availability of fruits under
six ScheeUa zonensis trees dur-
ing the period 27 July 1990 to
21 August 1990. Trees were
checked twice daily for ripe
fruits in the tree shadow that
were removed from the area so
as not to bias subsequent
checks.
2222
2 2 A 2 2 523 55532
?22522£2555553
DATE
then continued when participants chased one another away
from these trees. Therefore, the actual proportion of agonistic
interactions related to foraging on fruit may be higher, since
some of these interactions may have been misclassified. Only
two interactions (5%) were observed when a band was not
foraging or traveling; these occurred during resting and nursing activity. Of the 22 interactions at fruiting trees, 10 occurred at ScheeUa trees and five at Dipteryx trees, with the
remaining interactions occurring under Faramea, Ficus, Eugenia, and Quararibea trees.
Foraging group size was important in determining the outcome of agonistic interactions between solitary males and
bands (Table 5). Males were more likely to leave an area when
foraging group size was larger (logistic regression: r = .89; p
= .044). If the male did not immediately leave the area, it was
also more likely to be chased from the area by one or more
group members as foraging group size increased. Foraging
groups of seven or more individuals always chased males that
did not leave (Table 5). The probability of a male remaining
in the area, or continuing to feed at the fruiting tree, declined
with increasing size of the foraging group (r = —.91; p = .032;
Table 5). This may be due to the increased chances of a male
receiving aggression from band members, and to the possibility of less fruit remaining at the tree due to an increased
number of foraging individuals. The latter possibility is suggested by the decreasing relationship between foraging group
size and the sum (expressed proportionally) of the number
of males remaining plus the number of males returning to
the tree after the foraging group has departed (r = —.88; p
= .055; Table 5).
Males chased band members (Table 6). The chance of diis
occurring decreased slighdy with increasing foraging group
size, although the regression is not significant (r = —.76; p =
.162). However, this relationship may be misleading due to
categorization of foraging group size. Of die five chases diat
occurred under fruiting trees, four involved only a single for-
Gompper • Foraging costs and benefits of coati social structure
Table 2
Mean number of fruits dropped per minute observation before,
during, and after the presence of large arboreal frugivores (e.g.,
primates, toucans) for two BCI tree species
Table 3
Leaf-litter invertebrate foraging of Nasua narica subdivided by age
class and sociality during time of data collection
Age and social category
Mean
Number number
focal
of items
samples eaten/min
3.5 ± 3.5 (0-13)' 0.04 ± 0.2 (0-1)
Adult males (solitary)
Adult females (social)
Adult females (solitary)
Subadults (social)
29
32
23
25
57
101
Subadults include individuals 12 to 24 mondis old.
0.6 ± 0.7 (0-2)
0.0
36
9
may be related, at least in part, to body size sexual dimorphism interacting with a reliance on patchy and defendable
food resources.
Adult male coatis are significantly larger than adult females
(Gompper, 1994). Several lines of evidence suggest that this
dimorphism acts direcdy and indirectly in the maintenance
of the coati social system. First, larger and older individuals
generally were winners of conflicts which usually occurred
over food patches. Larger males were able to displace solitary
females and small foraging groups. In turn, larger groups of
smaller individuals (females) displaced large solitary males.
Second, smaller individuals may be more vulnerable to predation pressure than large individuals, who may be able to
aggressively defend against predators such as ocelots, tayras,
Cebus monkeys, raptors, and large snakes (all reported to feed
on juvenile and subadult coatis). While no detailed study has
been made of body size-based selection of coatis by predators
(but see relevant discussion by Karanth and Sunquist, 1995),
this supposition does imply that smaller individuals are under
increased selection to minimize predau'on pressure via group
living which would increase the number of vigilant, detecting
eyes and dilute the chances of being chosen for attack (Gitdeman, 1989; Hamilton, 1971; Treisman, 1975). Third, if larger individuals require more food than small individuals, they
may also be more detrimentally influenced by living with individuals that are simultaneously competing at food patches.
My qualitative observations of the ontogeny of male solitary
foraging support this third line of evidence. Subadult males
of age 20 to 24 months increase foraging time away from the
band, arrive at fruiting trees earlier than other band members, and leave later than other band members, often running
to catch up to the traveling band. Yet band members do not
appear to increase aggression toward these males as they age.
This suggests that males become solitary not because they are
driven from the band, but rather for some other reason, such
as to increase foraging success.
Before
vertebrate
arrives During vertebrate After vertebrate
at tree arboreal feeding leaves tree
Dipteryx panamensis
Fruit dropped/min 0.0
Observation nme
(min)
90
259
0.833
0.565
0.743
0.592
±SE
0.118
0.047
0.057
0.076
Scheelea zonensis
Fruit dropped/min 0.0
Observation nme
(min)
60
* Values are means ± SD with range in parentheses.
aging group member that remained at the food patch after
the remainder of the group departed (Table 6). This suggests
that a solitary female may have difficulty gaining access to
food patches when faced with competition from larger solitary
males.
Of 41 agonistic interactions between solitary adult males,
22 occurred in the context of foraging on fruit. In two cases
individuals sustained injuries during the conflict; a winner suffered a cut shoulder and a loser a cut ear. I may have missed
recording additional injuries, as conflicts were often obscured
from observation by forest undergrowth. In 18 conflicts the
age or relative age of both males was known. In all cases with
an age differential between the males (n = 15), the older
male was the winner.
To examine the characteristics associated with the outcome
of agonistic encounters further, I compared the morphometrics of winners and losers (Table 7). The average winner had
a marginally higher (p = .055) mean weight (measured within
6 months of the encounter) than the average loser. A paired
comparison of nine interactions in which the weights of both
the winner and the loser were known confirmed this pattern
(p = .035). However, body weight of an individual may vary
significantly within a year(Gompper, 1994; Russell, 1982), and
there was no significant difference in body weights of winners
and losers when weights were measured within 6 weeks of the
conflict For less variable measures, winners were significantly
larger than losers for head-body length, but not for testes
length (Table 7).
DISCUSSION
These results suggest that (1) coati social structure directly
influences foraging success, and (2) that intraspecific foraging
competition may play an important role in maintaining this
sexually dimorphic social system. Solitary adult males have
greater foraging success than band members when feeding on
fruit (Table 4), suggesting that males may remain solitary for
foraging benefits rather than due to an inability to join bands.
In contrast, although adult females had higher foraging success when feeding solitarily, they were occasionally unable to
gain access to valuable food patches without assistance due to
direct competition and aggression from solitary males. These
observations of agonism also emphasize the importance of
body size in structuring the outcome of contested food patch
acquisition. In essence, the coati sociality-asociality dichotomy
Variance in foraging relating to sociality and direct conflict
over food sources suggests that food access plays a greater role
in coati social structure than previously stated (Kaufrnann,
1962; Russell, 1982) and differendy than implied (Smythe,
1970a). The paucity of my observations of coati predation on
vertebrates is very similar to findings of Kaufrnann (1962) and
Russell (1982) who reported only rare predation on mice and
lizards. Thus Smythe's hypothesis (1970a) that solitary adult
males are more likely to hunt vertebrates than are females or
band members and that the hunting of vertebrates is more
common in the dry season when fruit availability is low is unsupported. Nonetheless, all studies of the coati on BCI stress
the importance of only a few plant species, including Dipleryx
and Scheelea.
The variance in foraging success between males and band
members, and within band age classes, is especially great when
Behavioral Ecology Vol. 7 No. 3
260
Table 4
Fruit foraging of solitary males relative to band members
Tied x, tied p
Bands
Males
All fruiting tree species (41.43 h)
A
1.059 ± 0.479; 132
0.777
0.717; 148
B
32.493 ± 12.835; 128
23.743
13.807; 124
28.251 ± 14.601; 52
C
23.144 ± 12.238; 128
30.313
11.830; 120
27.508 ± 12.846; 52
D
2.690 ± 3.407; 128
1.196
1.296; 120
1.635 ± 1.619; 52
Scheelea (24.07 h)
A
1.058 ± 0.467; 88
0.652 ± 0.415; 80
H.p
Subadults
Adult females
0.764 ± 0.379; 68
1
0.591 ± 0.493; 68
50.603;
<.0O01
20.228 ± 12.988; 64 33.105;
<.0001
32.612 ± 11.261; 60 24.850;
<.0O01
0.879 ± 0.904; 60
30.962;
<.0O01
0.681 ± 0.328; 52
0.600 ± 0.544; 28
B
38.915 ± 8.742; 84
29.519
12.856; 64
34.279 ± 10.323; 40
21.586 ± 12.921; 24
C
17.151 ± 8.952; 84
24.737
10.213; 60
22.588 ± 9.195; 40
29.036 ± 11.007; 20
D
3.733 ± 3.799; 84
1.717
1.523; 60
2.064 ± 1.614; 40
1.022 ± 1.047; 20
0.673 ± 0.136; 8
0.594 ± 0.484; 32
25.585;
<.0001
31.279;
<.0001
18.910;
<.0001
26.158;
•C.0001
-4.354;
<.0001
-1.515;
.1297
-1.793;
.0730
-1.566;
.1174
-6.427;
<.0001
-5.687;
<.0001
-4.694;
<.0001
-5.430;
<.0001
2.9697
.0030
-2.844;
.0045
-2.195;
.0281
-2.755;
.0059
-4.426;
<.0001
-2.225;
.0261
-2.696;
.0070
-2.396;
.0166
-3.515;
.0004
-5.442;
•C.OOOl
-3.896;
<.0001
-4.820;
•C.OOOl
-1.788;
.0737
-3.557;
.0004
-2.011;
.0443
-3.269;
.0011
-2.215;
.0268
-2.933;
.0034
-1.466;
.1426
-2.933;
.0034
-3.950;
<.0001
-0.591;
.5547
-0.394;
.6937
-0.591;
.5547
-1.631;
.1030
-2.174;
.0297
-1.087;
.2770
-2.174;
.0297
Dipteryx (13.69 h)
0.633 ± 0.414; 48
A
0.942 ± 0.350; 36
B
22.627 ± 9.472; 36
C
32.039 ± 7.706; 36
D
0.806 ± 0.491; 36
19.693
33.044
12.281; 48
11.041 ± 5.622; 8
8.990; 48
37.764 ± 1.536; 8
0.761; 48
0.287 ± 0.137; 8
0.787
17.718;
.0001
21.141 ± 13.966; 32 7.780;
.0204
32.301 ± 10.685; 32 2.010;
.3660
0.927 ± 0.884; 32
7.780;
.0204
Values for sex and social classes are mean ± SD; sample size. Four measures of foraging are given: (A) number of fruits/min foraging time;
(B) time eating/min; (C) time searching/min; (D) time eating per min/time searching per min. H and Rvalues are for a Kruskal-Wallis
ANOVA comparing solitary adult males, adult female band members, and subadult band members. Tied z and tied p values are for MannWhitney (/tests comparing (1) solitary adult males and adult female band members, (2) solitary adult males and subadult band members,
and (3) adult female and subadult band members.
Table 5
Results of encounters between solitary adult males and bands or
foraging groups of varying numbers of individuals
Group
size
n
1-3
4-6
7-9
10-12
Mean
± SD
Male
leaves'
Male
chased6
Male
remains'
Male
returns'1
0 (0/3)
10.0 (1/10)
27.3 (3/11)
22.2 (2/9)
66.7 (2/3)
33.3 (1/3)
77.8 (7/9)
100 (8/8)
100 (7/7)
100 (1/1)
66.7
50.0
27.3
11.1
(2/3)
(5/10)
(3/11)
(1/9)
0
(0/3)
66.7
60.0
54.5
11.1
0
25.2 ± 25.5
82.2 ± 29.0
31.0 ± 27.4
(2/3)
(6/10)
(6/11)
(1/9)
(0/3)
the population depends on Scheelea zonensis fruits. While this
species is common on BCI, each tree has only a small number
of ripe fruits available at any given momenL Solitary individuals are able to monopolize and eventually eat virtually all
fruits under a given tree, with foraging success limited primarily by patch size. In contrast, band members are limited
Table 6
Characteristics of aggression directed by solitary adult males toward
band members during male-band encounters, subdivided by
foraging group size
38.5 ± 30.6
Values are percentages, with actual counts in parendieses.
" Male leaves area without being chased by band member(s).
b
Male chased from area by band member (s). Excludes males that left
without being chased.
' Male remains in area or is not chased away despite band presence
and chase attempts by band members.
d
Male returns to the site after the band leaves the area. Includes
individuals in column five.
Foraging
group size
1-3
4-6
7-9
10-12
>12
Mean ± SD
Time males
chased band
members
during
encounters
33.3
20.0
18.2
22.2
0
18.7
(1/3)
(2/10)
(2/11)
(2/9)
(0/3)
± 12.0
Band
members
remaining
during
aggression
Foraging context
1
5; 6
1; 1
1; 12
n/a
Dipteryx
Leaf litter, Scheelea
Ficus, Ficus
Scheelea; Leaf litter
n/a
Gompper • Foraging costs and benefits of coati social structure
261
Table 7
Morphometric measures of males involved in male-male conflicts
Winner
Loser
Unpaired t; p
Paired difference
Paired t, p
Weight' ± 6 weeks
Weight ± 6 months
Head-body length
± 6 months
Testes length ± 6 mondis
5.5 ± 0.33; 15"
5.41 ± 0.67; 9
0.256; .8003
0.38 ± 0.67; 5
1.27; .2738
5.7 ± 0.6; 21
5.2 ± 0.7; 11
1.993; .0554
0.45 ± 0.53; 9
2.54; .0349
119.72 ± 4.42; 16
114.27 ± 3.10; 7
2.942; .0078
4.8 ± 2.77; 4
3.469; .0404
14.39 ± 0.72; 15
13.66 ± 1.21; 7
1.802; .0866
0.70 ± 1.61; 4
0.869; .4486
Analyses exclude measurements made when the individual was a subadult Paired difference is the size of the winner minus the size of the
loser.
* Weight measured in kilograms; length measured in centimeters.
b
Values given are means ± SD; sample size.
in fruit access not only by patch size, but also by direct competition (group size). For example, if a Scheelea has the mean
number of fruits available at any given moment (25.94 ±
34.21 fruits when the number of fruits is >0; 6 trees, n = 68
days; Figure 1) and the mean foraging group size is 7.3 individuals (Gompper, 1994), then the average individual will gain
only 3.6 fruits per tree visit However, the variance in this value
is likely to be high, especially when the number of fruits is
low, since individuals do not arrive at a tree simultaneously,
and individuals vary in their ability to secure food. Thus, some
individuals may gain few or no fruits. Clark and Mangel
(1986) have noted that, in general, the benefits of group foraging for ephemeral patches increase with patch size and with
the search time required to locate patches. Foraging on Scheelea and Dipteryx represent opposite ends of the continuum
suggested by this relationship.
Increased patch size (as seen for Dipteryx relative to Scheelea) leads to decreased foraging differentials between solitary
males and band members (Table 4). Nonetheless, some band
members continue to forage poorly relative to solitary individuals (Table 4), suggesting one or more nonexclusive phenomena. First, some band members may have increased foraging
success relative to other band members due to increased competitive ability (or decreased feeding rates due to satiation or
feeding efficiency, see below). For example, adult females
have greater fruit foraging success than subadults for three of
four Scheelea foraging measures (Table 4). Second, foraging
success of some band members may vary due to an additional
cost or benefit of group living unrelated to foraging competition. For example, social adult females foraging on invertebrates have foraging success equal to subadults (Table 3). This
is contrary to the expectation that older, more experienced
individuals should have increased foraging success relative to
younger, less experienced individuals. The foraging success of
females foraging solitarily is higher dian that of females foraging socially, and not significantly different from that of solitary adult males. Similarly, the foraging success (two of four
measures; Table 4) of subadults is greater than that of adult
females when feeding on Dipleryxfruits.These results suggest
that the lower foraging success of group-foraging adult females is not due to competition from other group members
or due to some factor intrinsic to adult females, such as poorer health. Excluding foraging competition, costs of sociality
extrinsic to the adult females but intrinsic to the band include
increased time for care of juveniles (e.g., vigilance, nursing,
grooming) and increased time devoted to social functions
(e.g., allogrooming). Russell (1979, 1982) attributed higher
dry season foraging success of juveniles (<12 months of age;
2.1 items/min) relative to adult females (1.2 items/min) to
less frequent vigilance behavior by juveniles.
I have interpreted the differences between sex and age
classes as being due to competitive disadvantages. However,
an alternative explanation is that the apparent competitive
disadvantage is an artifact of some individuals foraging less
intensively than other individuals. Thus, adult females may
have poorer Scheelea foraging success than solitary males simply because they are able to gain the resources they require
with less effort, or because they are more selective in fruit
choice. This might also explain patterns of foraging success
when feeding on Dipteryx, where differences in the feeding
of males and subadults are less extensive than differences between solitary males and adult females. In fact, foraging on
Dipteryx by subadults is more efficient than foraging by adult
females (Table 4). Dipteryx fruiting overlaps the coati mating
period when some solitary adult males join bands and interact
nonagonistically (or less aggressively) with band members.
They often feed with band members during this time. Female
foraging during this time may be more efficient, or females
may spend more time foraging solitarily (an observation that
may be important in understanding the coati mating system;
Gompper, 1994; Gompper and Wayne, 1996). Unfortunately
data do not currently exist to allow a comparison of the foraging success of adult males and band members foraging on
Dipteryx in the presence of one another.
Similarly, solitary males may be more motivated to feed at
a patch than females for reasons such as greater body size
requiring greater food intake, or perhaps due to lower feeding success between fruit patches. Because invertebrate foraging often occurs while individuals are traveling between
fruit patches, and solitary males do not differ from band
members in feeding on invertebrates, it is tempting to rule
out solitary males having lower feeding success between fruit
patches than members. However, information on differences
in the daily activity budget of males and females, especially
quantification of food obtained within versus between patches, as well as metabolic rates of males and females, is needed.
When available, this information would allow assessment of
differences in the daily feeding success of solitary and social
individuals relative to caloric requirements.
I conclude that the size of fruit patches plays a role in structuring the BCI coati societies. Are coati populations at other
sites greatly dependent during some portion of the year on
species with relatively small fruit crops which might foster intersexual foraging competition? Unfortunately, detailed dietary studies of coatis at other sites have yet to be carried out.
However, among primate societies the importance of keystone
plant species is well established, and the limited feeding site
availability in these plants has been shown to influence social
structure (Andelman, 1986; Cant, 1990; Ghiglieri, 1984; Terborgh, 1983).
An intriguing extension of these findings is that where reliance on patchy food sources is relaxed, social structure may
262
shift. Observations in Arizona and Panama suggest males may
occasionally join bands for brief periods (a few days to a
month) outside the breeding season and may also occasionally
delay dispersal by up to a year (Gilbert, 1973; Gompper and
Krinsley, 1992; Kaufmann et al., 1976; Wallmo and Gallizioli,
1954). These observations suggest that die coati social system
is extremely flexible and shaped by an array of ecological and
evolutionary factors.
I thank J. Gitdeman, J. Kaufmann, M. Mangel, G. Burghardt, C.
Boake, A. Hoylman, R. Kays, G. Anderson, and an anonymous reviewer for comments on the manuscript. I also thank S Rand, N. Smythe,
and the Smithsonian Tropical Research Institute for generous logistic
support in the field. Support for this research came from a National
Science Foundation dissertation improvement grant (DEB 9212747),
Smithsonian Institution fellowships, a Fulbright Foundation fellowship, Sigma Xi grants in aid of research, an Explorers Club exploration grant, the Theodore Roosevelt Fund of the American Museum
of Natural History, and an American Society of Mammalogists grant
in aid of research. Additional support came from the Science Alliance,
the Department of Zoology of the University of Tennessee, and a
predoctoral training grant fellowship from the National Institute of
Child Health and Development awarded to the University of Tennessee Graduate Program in Ethology (HD 07303).
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