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Population trends of the mantled howler groups of La Pacifica Guanacaste Costa Rica.

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American Journal of Primatology 11:79-88 (1986)
Population Trends of the Mantled Howler Groups of La
Pacifica, Guanacaste, Costa Rica
MARGARET R. CLARKE,l EVAN L. ZUCKER,l AND NORMAN J. SCOlT, JRZ
'Department of Psychology, Loyola Uniuersity, New Orleans, Louisiana; 'U.S. Fish and
Wildlife Seruice, Denver Wildlife Research Center, Museum of Southwestern Biology, Uniuersity
of New Mexico, Albuquerque, New Mexico
A complete survey of La Pacifica in Guanacaste Province, Costa Rica was
conducted in July 1984 in order to determine whether the howler (Alouatta
palliata) population had declined since 1972 as a result of deforestation.
During the 6-day survey, 257 howlers were located, representing 16 different social groupings and nine solitary animals. The total number of howlers,
the number and location of groups, and the age-sex composition were very
similar to a 1972-1976 survey of the same population. Age-sex composition
of La Pacifica howler groups was similar to those of another population of
mantled howlers in Costa Rica and of populations in Mexico and in Panama,
although La Pacifica had a higher mean number of adult females per group.
There was no evidence to support the hypothesis that the La Pacifica howler
population has declined in recent times.
Key words: Alouatta palliata, deforestation, demography, survey
INTRODUCTION
Diminishing nonhuman primate populations are of concern to conservationists
and field biologists alike. As human populations increase, a greater number of
nonhuman primates are hunted for food, killed as cropraiding pests, or have had
their habitats reduced by deforestation for agricultural expansion. The export of
nonhuman primates for medical purposes, while reduced in recent years, also continues [Heltne & Thorington, 1976; Southwick & Smith, 1986; Wolfheim, 19831. Although some New World monkey populations are being threatened by hunting for
export trade as well as for food, mantled howling monkeys (Alouatta palliata) in
Guanacaste Province, Costa Rica appear to be threatened primarily by the conversion of forest habitats to grassland for raising cattle [Boucher et al, 19831. Although
the Costa Rican government is attempting to slow deforestation by establishing
national parks and strictly regulating deforestation of private property, habitat
destruction continues [Gomez & Savage, 19831.
Received September 16, 1985; revision accepted March 23, 1986.
Please address reprint requests to Margaret R. Clarke, Delta Regional Primate Research Center of Tulane
University, Three Rivers Road, Covington, LA 70433.
0 1986 Alan R. Liss, Inc.
80 I Clark, Zucker, and Scott
Surveys of howler populations were carried out a t La Pacifica and at Taboga in
Guanacaste Province between 1966 and 1971 and were compared to similar surveys
of the howler population at Barro Colorado Island in Panama [Heltne et al, 19761.
The conclusion was reached that the Costa Rican populations represented “a distressed and declining population” [Heltne et al, 1976:17] while the Barro Colorado
Island population was considered relatively stable. La Pacifica and Taboga have
been deforested, while Barro Colorado Island has been protected for over 60 years
[Carpenter, 1934; Milton, 19821. Since a relatively continuous population decline
would be expected to affect group size and structure as well as patterns of migration,
competition, and group formation [Altmann et al, 19851, it also might affect the
results of studies of social behavior, which have been carried out on howlers at La
Pacifica since 1970 [eg, Clarke, 1982; Clarke & Glander, 1984; Glander, 1980, 1981;
Jones, 1978,19801. In order to estimate what changes actually have occurred in this
howler population, Clarke and Zucker conducted a complete survey of La Pacifica in
1984 in order to compare the results with the population data collected by Scott
between 1972 and 1976 [Scott et al, 1978, unpublished data]. The total number of
howlers, as well as the number, location, and age-sex composition of howling monkey
groups, could be compared over this 10-year period in order to determine if the
population decline predicted by Heltne et a1 [1976], based on 1966-1971 estimates,
actually had occurred. In addition, the results of the La Pacifica survey allow
comparisons with data and census results from three other populations of A. palliata.
METHODS
Study Site
The study site is located 5 km northwest of Canas in Guanacaste Province,
Costa Rica. This region has been classified as dry, deciduous tropical forest [Holdridge, 19671 with little or no rainfall during 5 months of the year. Rainfall amounts
vary between 1,300 and 1,600 mm during the wet season, which extends from midMay to mid-December. La Pacifica is a working cattle ranch that was converted to
pastureland in four stages between 1960 and 1973. The ranch is presently over 1,330
hectares in size, and about one-quarter of the original forest remains. La Pacifica
has been deforested in a conservative manner: riparian forests were retained, and
windbreaks and patches of upland forest remain interconnected. In addition, areas
used for short-term agriculture are rotated, and the unused areas are allowed to
revert to natural growth. Some of the ranch is irrigated, which may stabilize some
of the forest area during the dry season, and cattle are methodically rotated from
pasture to pasture to maximize natural regeneration of vegetation. Howlers are
found in both the riparian and the upland forest habitats.
Survey Plan
The forests of La Pacifica were surveyed for howling monkeys between 0800
and 1300 h on 6 consecutive days in July 1984. The survey plan was a modified
quadrat approach [National Research Council, 19811, and the 14 fieldworkers were
divided into survey parties of three to five individuals. Approximately one-sixth of
the total forest was surveyed per day, and each forest patch, river side, or windbreak
section was assigned to a different survey party. The forest canopy ranges between
15 and 50 m in height, which is low enough for howler groups to be spotted relatively
easily. Surveying began on the southern side of the ranch and systematically progressed northward to minimize chances of counting the same animals more than
once. Survey party members slowly walked their designated forest area at evenly
spaced intervals, which ranged between 10 and 30 meters. When howlers were
Howler Demography / 81
sighted, all survey party members left their positions on their transects, and a
minimum of one-half hour was spent counting and identifying howlers once a
sighting occurred. The count was considered complete when all survey party members agreed on the sex, age, and total number of animals in that area. When the
count was complete, the location of the animals was marked on a map, and the
observers returned to their locations on the transects to resume the survey. During
the actual survey, animals 75 m or more apart were considered as separate sightings. At the end of each day, the counts and locations of howlers were entered into a
permanent log.
Data Analysis
Groups were defined as bisexual associations of three or more animals, wit,h a t
least one fully adult male. For the purpose of analysis, groups counted as separate
sightings but located within 150 m of each other were considered part of the same
social group. Group size, distribution, and composition from both the 1984 survey
and the 1972-1976 counts were analyzed using descriptive statistics and were
compared to each other. The previous count was the result of a series of capture
sessions in which all animals received tattoos and/or were ear notched [Scott et al,
1976,1978, unpublished data]. One adjustment was made during the present study
for comparative purposes. When a female loses a n infant, she ceases lactation within
1to 3 days [Clarke, 19821; it can be assumed that a lactating female actually does
have a n infant, whether it was captured with her or not. Therefore, one infant
(immature) was added to the 1972-1976 count whenever a lactating female was
captured without a n infant. Survey data from other mantled howler populations
located in Costa Rica, Mexico, and Panama also were analyzed using the same
descriptive statistics in order to allow for a direct comparison with the La Pacifica
population.
RESULTS
A total of 248 animals were located in 16 distinct social groupings (Table I).
Each group included one to four adult males (2 = 2.56), 2 to 15 adult females (x =
7.63), one to four juveniles ( j t = 2.19), and zero to seven infants (% = 2.13). Total
group size ranged from 4 to 29 (% = 15.5). Nine solitary howlers were also located
five males, three females, and one juvenile (unknown sex). Thus, 257 animals were
sighted during the 6-day survey. “Density” of howlers was estimated a t 77.3 per
km2. This was calculated by dividing the number of howlers counted by the area of
remaining forest (3.3 km2, estimated from aerial photographs). The overall ratio of
adult males to adult females was 1:3.2, and that of adults to immatures was 1:0.4.
Comparisons of the results of this survey with the results of the survey conducted during 1972-1976 showed no difference in totals and age-sex distributions
(Table ID. The only notable difference was in the adult ma1e:adult female sex ratio.
Even if the unidentified adults from the present survey were included as females,
the 1984 adult ma1e:adult female ratio would be 1:3.2 as compared to 1:4.2 recorded
in 1972-1976. This difference, however, was not statistically significant (x21
1.56, n.s.). Group locations were also remarkably similar: eight groups were found
in the same locations as 10 years ago, and most groups were sighted within the
same home ranges estimated a decade ago (Fig. 1).
A comparison of La Pacifica survey data with data from 1972 and 1983184
surveys a t Santa Rosa National Park in Costa Rica [Fedigan et al, 1985; Freese,
19761, from a long-term study at Los Tuxtlas in Mexico [Estrada, 19821, and from
estimates and counts at Barro Colorado Island in Panama [Milton, 1982; Mittermeier, 19731 disclosed differences in sex ratios and densities. Ranges for age-sex
*=
82 I Clark, Zucker, and Scott
TABLE 1. Survey Results-Howlers in Groups (La Pacifica, July 1984)
Group”
2
3
5
6
7
8
9
11
12
13
14
16
Cabina
Roea
Corral
Tiki
Total
Adult
males
4
4
3
2
1
4
1
2
Subadult
males
Adult
females
Juveniles
Infants
Total
3
8
2
6
0
1
2
0
0
1
0
0
1
0
0
0
0
4
0
0
3
2
4
2
2
1
1
2
2
1
3
2
2
2
2
4
3
7
1
2
3
2
0
1
7
3
2
1
0
0
0
2
22
27
16
15
11
23
4
15
29
15
17
10
6
18
6
14
122
9
35
34
248
10
13
6
8
3
3
3
1
3
1
2
2
0
0
1
1
0
0
1
0
0
0
0
0
1
1
0
41
7
4
Unidentified
adults
4
15
2
9
15
8
9
4
aGroup number based on Scott records plus four groups named during the 1984 survey.
TABLE 11. Ten Year Comparison of Howler Survey Data (La Pacifica 1972/76 and 1984)
Adult
males
Totals
Scott
1972176
32
ClarkelZucker
1984
41
Means per group
Scott
1972176
2.1
ClarkelZucker
1984
2.6
Subadult
males
Adult Unidentified
Population
Grand
females adults
Immatures in groups Solitaries total
8
134
1
60a
235
12
247
7
122
9
69
248
9
257
0.4
8.9
4.0
15.7
0.5
7.6
4.3
15.5
No. of x Adults
adults per group A u :A 9
Immatures:
adults
No. of
groups
Range of
group size “Density”
Scott
1972176
175
11.7
1:4.2
1:2.9
15
4-31
74.3 per
km2
Clarke/Zucker
1984
179
11.1
1:3.0
1:2.6
16
4-29
77.3 wer
km2
aIncludes infants on adult females plus one additional infant for each lactating female without a n infant.
Howler Demography I 83
Fig. 1. Map of Hacienda La Pacifica indicating locations of howler groups surveyed by Scott in 19721976 and by Clarke and Zucker in 1984.
classes and group sizes overlapped, but the group size and mean number of females
per group were smaller at Santa Rosa in 1972 and at Los k t l a s (Table 111).Groups
a t La Pacifica had a higher proportion of females than those a t the other sites. Other
striking differences were seen in the comparative population parameters presented
in Table IV.Population densities ranged from approximately one to about 90 howlers
per km2, and the ratios of females to males varied from 1:1.5 to 1:4.0. The ratios of
adults to immatures also varied.
It should be noted that the numbers for the 1984 survey represent the number
of animals actually located and identified during the 6-day survey and should not be
construed as the total number of howlers on La Pacifica. We estimate that we
counted 75% of the howlers, making the total population closer to 350 animals. We
arrived at this estimate for the following reasons: on Day 1of the survey, 35 animals
were counted in a n area where we know we missed 12; during July and August of
1984, we located and identified 43 additional howlers that were not seen during the
survey; and it is probable that some of the smaller animals (juveniles, young females)
also were missed. The use of this 75% estimate does not weaken the comparisons
with the population data from the 1972-1976 capture sessions, as the same sampling
limitations apply to both survey methods.
84 I Clark, Zucker, and Scott
TABLE 111. Grow ComDosition: ComDarative Howler Data*
Costa Rica
La Pacifica
Scotta
Mean
Range
Clarkeb
Mean
Range
Santa Rosa
Freese‘
Mean
Range
Fedigand
Mean
Range
Mexico
Los Tuxtlas
Estrada“
Mean
Range
Panama
Barro Colorado
Mittermeierf
Mean
Range
Miltong
Mean
Ranee
Adult cr
Adult 9
3.0
1-5
9.0
2-17
3.0
1-6
9.8
2-16
2.2
1-4
1.6
1-4
3.6
1-10
3.0
1-8
Juveniles
Infants
4.0
1-8
Group size
No. groups
15.7
4-31
15
2.2
0-7
15.5
4-29
16
2.0
0-7
0.9
0-3
7.8
3-24
8
5.9
2-18
2.8
2-9
1.9
0-9
13.7
3-40
25
3.0
1-5
4.1
2-6
1.6
0-3
1.5
0-3
9.1
5-16
17
3.2
2-5
5.8
4-8
4.3
3-6
2.3
1-4
16.2
13-23
3.5
NA
9.2
NA
2.6
NA
5.5
NA
20.8
9-32
6
13
*Adulta, includes adult males plus subadult males; Adult Q , includes adult females plus unidentified
adults; NA, data not available
‘Scott, 1972/1976 survey.
bClarke and Zucker, 1984 survey.
‘Based on Freese [1976].
dBased on Fedigan et a1 [1985].
eEstrada [1982].
fBased on Mittermeier [1973].
Wilton [1982] (1977 data).
DISCUSSION
These results do not support the hypothesis that the howling monkeys at La
Pacifica are declining in numbers. The number of howlers may have decreased
immediately following deforestation, contributing to the “stressed population” interpretation of Heltne et a1 [1976] based on brief surveys made between 1966 and 1971.
The present population, however, has remained relatively unchanged, both in size
and in group location and composition, between 1972-76 and 1984. All groups except
two were multimale, multifemale groups, and one of the one-male groups is known
to have formed in 1980 when the then solitary adult male attracted the first adult
female to his home range. Thus, the La Pacifica population of howlers continues to
exhibit the “typical” mantled howler social structure [Crockett & Eisenberg, in
press].
Howler Demography / 85
TABLE IV. Population Parameters: Comparative Howler Data*
Total
howlers
Costa Rica
La Pacifica
Scotta
Clarkeb
Santa Rosa
Freesec
Fedigand
Mexico
Los Tuxtlas
Estrada“
Panama
Barro Colorado
Mittermeierf
Miltong
Usable
habitat
(ha)
Estimated
Ratios
density
per km2 Adult o :Adult Q Adu1ts:immatures
247
257
332.5
332.5
74.3
77.3
1:4.2
1:3.0
1:0.34
1:0.38
70-100h
341
10,000
7,560
0.7-1.0h
4.9
12.2’
12.0
1:0.56’
1:0.53
155
700
22.1
1:1.4
1:0.27
8001,050h
1,352
1,351
59.277.7h
91.7
1:l.g
1:0.77J
NA
NA
1,475
*NA, data not available.
=Scott, 1972/76 survey.
bClarke and Zucker, 1984 survey.
‘Based on Freese [1976].
dBased on Fedigan et a1 [1985].
eEstrada [1982].
fBased on Mittermeier [1973].
Wilton [1982] (1977 data).
hBased on estimates rather than survey counts.
‘Based on a sample of eight groups.
’Based on a sample of six groups.
Although the comparison of the four howler populations reveals similar age-sex
compositions of groups, the size of the groups and the density of howlers in the
usable habitat are variable. Both surveys a t La Pacifica and the results from B m o
Colorado indicate high densities of howlers, while the two surveys at Santa Rosa
and the data from Los Tuxtlas reveal considerably lower numbers. This variation
could be due to different environmental stresses on the populations, to differences in
methods of estimating “density,” or could simply reflect normal variability in population parameters. Despite the fact that La Pacifica was largely deforested, the
remaining forest has been relatively stable since 1973. Barro Colorado Island is
protected and has a history of population fluctuations [Collias & Southwick, 19521,
but even at the lowest estimates Barro Colorado Island has a “density” of howlers
that is fairly close to that calculated for La Pacifica. The population at Los Tuxtlas
was described by Estrada [19821 as a “stressed” population due to habitat destruction, small groups, and low proportions of females to males and of immatures to
adults. Deforestation may account for low population density at Los Tuxtlas, but it
would not explain the low density of howlers a t Santa Rosa, a protected National
Park since 1970 [Fedigan et al, 19851. A resurgence in the population of monkeys,
such as has been documented for other species when they became protected andlor
provisioned [Malik et al, 1984; Southwick et al, 19821, has not occurred yet at Santa
Rosa.
There was an increase in density of howlers between the 1974 and 1984 surveys
at Santa Rosa, but part of the increase was due to a change in the definition of
86 I Clark, Zucker, and Scott
“usable habitat” as well as to a more thorough survey of the area. Different
estimates of “usable habitat” and errors in sampling, including the differential
observability of particular age-sex classes, may account for some of the variability
in population density estimates from site to site. “Available habitat” may not be
“usable” for a monkey [Glander et al, 19841. The actual density of animals in
“usable habitat” would be considerably higher than the density for “available
habitat.” It is possible that the smaller sites, such as Barro Colorado Island, La
Pacifica, and Los Tuxtlas, can be more thoroughly surveyed with relatively few
fieldworkers and the amount of usable habitat can be more accurately assessed.
The low ratios of immatures to adults at the Costa Rican and Mexican sites
have been interpreted as indicating stressed populations [Estrada, 1982; Heltne et
al, 19761. Models of population growth, however, assume that immatures remain in
the group until they are adult. As juvenile mantled howler males emigrate as early
as 1year of age and females as early as 2 years of age [Clarke, 19841, these solitary
juveniles would be easy to miss in a survey and yet should be considered as part of
the total population. Indeed, a n inflated estimate of a howling monkey group’s size
would be obtained if the group were surveyed during the brief period when new
infants have just been born and juveniles are in the process of emigrating. Thus,
both a n overestimate of usable habitat and a n underestimate of solitary animals
would result in artificially low density estimates.
These density estimates also may reflect normal fluctuations in mantled howling monkey population growth cycles. Crockett and Eisenberg (in press) calculated
a regression of mean group size on population density for A. palliata, and the results
of the present census are consistent with their predictions. The range of population
densities reported for the different mantled howler populations are within the range
of documented fluctuations for a baboon population (Papw cynocephalus) in response
to natural environmental changes [Altmann et al, 19851. Population changes, both
in total numbers and in group size and number, also have been documented for red
howlers (A. seniculus) in Venezuela [Crockett, 1985; Crockett & Eisenberg, in press].
Changes in density within a population could reflect immediate responses to
changes in food supply and competition with other nonhuman populations, as well
as to human intervention. This highlights the need to continue to monitor primate
populations in order to make accurate assessments of their success in a world of
diminishing habitat and to understand the specifics of population dynamics. The
howlers at La Pacifica, despite a probable immediate decline in numbers, appear to
have adjusted to the altered environment and to have maintained their numbers
over a 10-15-year period. This suggests that planned deforestation, including large
undisturbed patches of forests with interconnected forest “pathways,” may allow
howling monkey populations to persist in areas where deforestation is unavoidable.
CONCLUSIONS
1. A complete survey of La Pacifica indicated a population similar to the population surveyed there 10 years earlier.
2. A total of 257 animals were sighted during the 6-day survey. Since 43
animals were known to be missed in the survey, and it was assumed some immatures and solitaries were also missed, the total population was estimated at about
350.
3. The age-sex composition of La Pacifica howler groups was similar to those of
the Santa Rosa, Los Tuxtlas, and Barro Colorado Island groups, but the “density” of
howlers was quite variable among the sites.
4. Comparisons of population densities across sites are confounded by differing
Howler Demography / 87
estimates of usable habitats, differential observability of immature and solitary
animals, and the timing of births vis-a-vis emigration by juveniles.
ACKNOWLEDGMENTS
This research was supported by the School for Field Studies, Cambridge, MA,
and by the World Wildlife Fund U.S. We would like to thank our field workers for
their enthusiastic help: Aaron Ferster, Betsy Thompson, Romana Prokopiw, Rob
Rand, Chuck Rosenthal, Adrian Forman, Ron Phillips, Lieta Comfort, Kate Offutt,
Todd Broseghini, Jay Sears, and Albie Nash from the 1984 survey, and Linda
Malmgren, Alan Scott, Bruce Woodward, Bob Reynolds, and Ken Glander from the
1972-1976 field sessions. We would also like to thank Lillian and Werner Hagnauer
and Vreni Leigh for their gracious hospitality while we were at La Pacifica. We
would also like to thank Dr. Michael A. Bogan, Dr. Carolyn M. Crockett, Dr.
Kenneth E. Glander, Dr. Curtis H. Halvorson, Dr. Fritz L. Knopf, Dr. Paul A. Opler,
Dr. Elizabeth S. Watts, and reviewers for comments on this manuscript. The secretarial assistance of Carolyn Boudreaw and Sharon Nastasi of Delta Primate Center
is greatly appreciated. A different version of this manuscript was presented at the
54th Annual Meeting of the American Association of Physical Anthropologists,
Knoxville TN, April, 1985.
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