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Habitat use, fruit consumption, and population density of the black-headed night monkey, Aotus nigriceps, in Southeastern Peru


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The study of wild black-headed night monkey ( Aotus nigriceps ) ecology is limited to a single field station, despite the species being found across a vast distributional range in the Amazon. We studied several aspects of their ecology, specifically habitat use, group size, population density, and diet. All sampled groups were found in secondary tropical rainforest, often dominated by either native bamboo or cane species. Sleeping sites were often in bamboo stands, though groups were also found in cane thickets and lianas. This is in contrast to other Aotus studies which have found groups living in tree cavities and lianas. Population density estimates varied between field sites (19 and 50 individuals per km ² ), but both were consistent with other Aotus studies (31-40 individuals per km ² ). And, twelve seed species were recovered from fecal samples over the course of two field seasons, dominated primarily by Cecropiaceae, Piperaceae and Moraceae. Our results suggest that the black-headed night monkey can survive and even thrive in secondary forest, feeding extensively on pioneer species, occupying a range of forest types, all while living in proximity to people (<1km). RESUMO El estudio de la ecología del mono nocturno salvaje ( Aotus nigriceps ) se limita a una única estación de campo, a pesar de que la especie se encuentra en un vasto rango de distribución en el Amazonas. Estudiamos varios aspectos de su ecología, específicamente el uso del hábitat, el tamaño del grupo, la densidad de población y la dieta. Todos los grupos muestreados se encontraron en la selva tropical secundaria, a menudo dominada por bambú nativo o especies de caña. Los sitios donde dormían a menudo se encontraban en puestos de bambú, aunque también se encontraron grupos en matorrales de caña y lianas. Esto contrasta con otros estudios de Aotus que han encontrado grupos que viven en cavidades de árboles y lianas. Las estimaciones de densidad de población variaron entre los sitios de campo (19 y 50 individuos por km ² ), pero ambos fueron consistentes con otros estudios de Aotus (31-40 individuos por km ² ). Y, doce especies de semillas fueron recuperadas de muestras fecales, dominadas principalmente por Cecropiaceae, Piperaceae y Moraceae. Nuestros resultados sugieren que el mono nocturno de cabeza negra puede sobrevivir e incluso prosperar en bosques secundarios, alimentándose ampliamente de especies pioneras, ocupando una variedad de tipos de bosques, mientras viven cerca de personas (<1 km).
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1 Ahead of Print 2020: 01  07
Habitat use, fruit consumption, and population
density of the black-headed night monkey, Aotus
nigriceps, in southeastern Peru
William. D. HELENBROOK1,2 * , Madison L. WILKINSON3, Jessica A. SUAREZ1
1 Tropical Conservation Fund, Marietta, GA 30064, USA
2 State University of New York , College of Environmental Science and Forestry, Syracuse, NY, USA
3 Colorado College, Colorado Springs, CO, USA
* Corresponding author:;
e black-headed night monkey, Aotus nigriceps, has one of the largest distribution ranges of the 11 night monkey species found
across Central and South America. Yet, only three studies have focused on their ecology, describing considerable variation
in habitat, group composition, and population density. erefore, we analyzed habitat use, group composition, population
density, and diet of 14 groups at two eld sites in southeastern Peru. All sampled groups were found in secondary tropical
rainforest, often dominated by native bamboo species. Half of the observed sleeping sites were in bamboo stands, though
groups also emerged from cane thickets and lianas. is contrasts with other Aotus studies which have found groups living
in tree cavities and lianas. Population density estimates for both sites were 19 and 50 individuals per km2, outside the range
previously reported for A. nigriceps (31−34 individuals per km2). We recovered seeds of 12 species from fecal samples over
the course of two eld seasons, belonging mainly to Cecropiaceae, Piperaceae and Moraceae. Our results suggest that the
black-headed night monkey in Peru can survive and even thrive in secondary forest, feeding extensively on pioneer species,
occupying a range of forest types, all while living near human settlements.
KEYWORDS: Amazon, primates, Aotidae, conservation, owl monkey, nocturnal, habitat
Uso de habitat, consumo de frutos, y densidad poblacional del mono
nocturno de cabeza negra, Aotus nigriceps, en el sureste de Perú
El mono nocturno de cabeza negra, Aotus nigriceps, tiene uno de los rangos de distribución más grandes de las 11 especies
de monos nocturnos que se encuentran en América Central y del Sur. Sin embargo, solo tres estudios se han centrado en su
ecología, describiendo una variación considerable en el hábitat, la composición del grupo y la densidad poblacional. Por lo
tanto, analisamos el uso de hábitat, la composición del grupo, la densidad poblacional y la dieta de 14 grupos en dos sitios de
campo en el sureste de Perú. Todos los grupos muestreados se encontraron en selva tropical secundaria, a menudo dominada
por especies nativas de bambú. La mitad de los dormideros observados estaban en bambú, aunque también surgieron grupos
de matorrales de caña y lianas. Esto contrasta con otros estudios, que encontraron grupos de Aotus viviendo en cavidades de
árboles y lianas. Las estimaciones de densidad poblacional para los dos sitios fueron de 19 y 50 individuos por km2, fuera del
rango previamente reportado para A. nigriceps (3134 individuos por km2). Recuperamos semillas de 12 especies de muestras
fecales en el transcurso de dos temporadas de campo, pertenecientes principalmente a Cecropiaceae, Piperaceae y Moraceae.
Nuestros resultados sugieren que el mono nocturno de cabeza negra puede sobrevivir, e incluso prosperar, en el bosque
secundario, alimentándose ampliamente de especies pioneras, ocupando una variedad de tipos de bosque, todo mientras vive
a menos de 1 km de asentamientos humanos.
PALABRAS-CLAVE: Amazonía, primatas, Aotidae, conservación, mono nocturno, hábito nocturno
CITE AS: Helenbrook, W.D.; Wilkinson, M.L.; Suarez, J.A. 2020. Habitat use, fruit consumption, and population density of the black-headed night
monkey, Aotus nigriceps, in southeastern Peru. Acta Amazonica (Epub Ahead of Print).
HELENBROOK et al. Black-headed night monkeys in southeastern Peru
2 Ahead of Print 2020: 01  07
ere are currently 11 recognized species of night monkey,
Aotus Illiger 1811 (Deer and Bueno 2007; Rylands and
Mittermeier 2009), though taxonomy of the genus is still
disputed (Ruiz-Garcia et al. 2011). These wide-ranging
nocturnal primates are found in various tropical and
subtropical habitat types ranging from Panama to Argentina
(previously outlined in Aquino and Encarnacion 1998). Aotus
habitat varies considerably, encompassing a broad altitudinal
gradient ranging from lowland rainforest to cloud forest,
primary and secondary forest as described by Chokkalingam
and De Jong (2001), and fragmented or selectively logged
habitat. ey also use lower and upper canopy, and are
tolerant of seasonal rainfall and temperature variation, along
with various levels of habitat disturbance, including bamboo
thickets, mangroves, palm trees, and gallery forest (e.g., Wright
1994; Aquino and Encarnación 1994; Fernandez-Duque et
al. 2008). Night monkey groups can also be found living
near human settlements (Wright 1989; Fernandez-Duque
et al. 2007).
Aotus groups may contain up to six individuals, including
a monogamous pair with infant, juveniles, and one subadult.
Consistent evidence of solitary behavior has also been reported
in A. azarae (Huck and Fernandez-Duque 2017). ey are
primarily frugivorous and supplement their diet with leaves,
nectar, owers, and insects (Wright 1989; Wright 1994). Wright
(1985) describes as many as 70 fruit species consumed by Aotus
nigriceps (Dollman, 1909), including Moracae, Annonaceae,
Leguminosae, and Sapindaceae species. Crepuscular and
nocturnal behavior likely provides Aotus an opportune time
for capturing a wide array of insects available at dusk and in
the night (Wolovich 2010; Wright 2011). Evidence of diurnal
behavior does exist in Aotus azarae (Fernandez-Duque 2003),
though activity in Aotus nigriceps Dollman 1909 is reported to
be minimal (Khimji and Donati 2014).
e black-headed night monkey, A. nigriceps, occurs
throughout a large part of the central and upper Amazon, and
yet the majority of relevant information on habitat, population
density, and group size for the species are centered on three
nearby sites. One study in the central-eastern Peruvian
Amazon recorded 26 groups on transect survey walks (Aquino
et al. 2013), another focused on a single group over the course
of 65 days (Khimji and Donati 2014) and another study
extensively followed two groups (Wright 1985; Wright 1994).
Habitat has been described as both primary and secondary
forest, seasonally ooded, terra rme, and lowland rainforest,
as well as forest patches with bamboo and palm species (Wright
1985; Aquino et al. 2013; Khimji and Donati 2014). Group
composition largely consists of a mating pair with an infant, a
juvenile and one subadult (Wright 1994); however, Aquino et
al. (2013) describe an average of 2.8 individuals per group with
a maximum of 4, while Khimji and Donati (2014) followed
a single pair with a subadult. A single study has reported
A. nigriceps density (31.1 individuals per km) based on the
number of sightings per 10 km (Aquino et al. 2013). Density
estimates are more widely available for other Aotus species,
ranging from 3.2 to 64 individuals per km2 (e.g., Aquino
and Encarnacion 1988; Fernandez et al. 2001; Svensson et
al. 2010; Maldanado and Peck 2014). We therefore aimed at
expanding our understanding of the ecology of A. nigriceps,
by studying their habitat use, group size, seasonal dietary
shifts, and population density two sites in Amazonian Peru.
Field work was carried out at two sites in the Manu Biosphere
Reserve in southeastern Peru: Villa Carmen Biological Station
(12°53’39»S, 71°24›16»W), a conservation area managed by
the Asociación para la Conservación de la Cuenca Amazónica
(ACCA) in the Cuzco region, and the Manú Learning
Centre (MLC) (12°4722S, 71°2332W) in the Madre de
Dios region (Figure 1). Both Amazonian sites occur within
Figure 1. (A) Geographic location of the two study areas in southeastern Peru: Manu Learning Center (B) and Villa Carmen Biological Station (C). Night monkey groups
(black stars) were located while conducting line transect surveys or during routine monitoring of trail systems (gray dots).
HELENBROOK et al. Black-headed night monkeys in southeastern Peru
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tropical premontane forest, exhibiting similarly stable average
temperatures throughout the year (22°C; maximum 32°C and
minimum 10°C). Average annual rainfall ranges from 3200-
5000 mm yr-1. e highest rainfall occurs from December to
March and the driest months occur from May to September.
Villa Carmen contains 30.65 km2 of protected primary forest
within the Manu Biosphere Reserve, though our surveys were
conducted almost entirely in secondary forest (16.8 km of
trails). MLC has a mixed history of plantations and cattle
grazing, coupled with selectively logged and undisturbed,
primary forest. We sampled both secondary (11 km of trails)
and primary forest (2.5 km) at MLC. In the secondary
forest, 7 km were in a previous timber harvest zone, 0.75
km in secondary bamboo forest, and 3.25 km in a selectively
logged area.
We collected data from 14 black-headed night monkey
groups during April 2016, November 2016 and April
2017 at Villa Carmen, and during April 2017 at MLC.
Population density was estimated using three dierent
methods previously applied in other Aotus studies: encounter
rates per distance traveled (Pontes et al. 2012), line transects
utilizing perpendicular methods using detection function
models (Marhsall et al. 2008), and direct counts (Fernandez
et al. 2001). In the rst method, we calculated the number
of groups encountered per kilometer walked. Established
trails were walked from 18:00 to 21:00 h at 1−2 km per
hour, scanning vegetation continuously as a group of two
to ve eld assistants spaced out evenly. is method was
used as a baseline for comparison between our sites and
earlier Aotus studies. For encounter rates and line transects,
surveys followed pre-existing trails so as not to disturb
habitat, which as a result sometimes included nonlinear
routes. However, most trails at Villa Carmen run parallel to
one another and those at MLC are largely linear (Figure 1).
e closest distance between trails was 250 m, and detection
of groups was not feasible beyond 25 m. ere was little
evidence of avoidance behavior as the trails are routinely
walked and night monkeys have been observed near trails.
Every 100−200 m, eld assistants would pause and listen for
calls. Once a monkey was spotted, we would observe them
in order to obtain demographic information, group size,
GPS location, record time, and to measure the perpendicular
distance from transect to group (Marshall et al. 2008). In
the second method, we calculated population density from
survey data using the program DISTANCE (version 6.2).
Lastly, we simply calculated the known number of groups
and individuals within a well-studied area at Villa Carmen
over the course of two years. We determined nesting sites
during the day and conrmed general travel patterns for
each group in the area both at dusk and dawn.
We later revisited the area where night monkey groups
were found during transect walks to perform habitat analysis,
classifying forests based on qualitative factors, i.e., secondary
versus primary forest, and presence of bamboo or cane, or
lianas (Ganzhorn 2003). We took canopy cover photos at
each group location and used ImageJ software (
to calculate canopy cover percentages. GPS points taken at
each location were paired with satellite imagery to determine
proximity of monkey groups to known human settlements.
A quantitative estimate of forest complexity for each group
was made using 50 m × 2 m plots (Gentry 1982), starting
1 m o the trail and measuring 1m beyond, for both sides
from where the group was located. All woody trees ≥ 10 cm
diameter at breast height (approximately 1.3 m above ground)
were recorded within the plot (Ganzhorn 2003).
We collected 48 fecal samples (33 from Villa Carmen and
15 from MLC; 26 in spring and 22 in the fall) (Table 1) for
a preliminary characterization of diet from identication of
deposited seeds. Fecal collection was conducted by covering
forest oor with plastic mesh or cloth nets below areas where
night monkeys had previously been observed or where there
was evidence of their feeding. e nets were used to catch
any falling fecal samples and monitored daily throughout the
study. e fecal samples were preserved in formalin and were
later analyzed in the laboratory. We then strained samples
using cheese cloth to recover seeds and then examined them
under a BSZ405 stereo microscope (Boeco, Germany). Seeds
were then identied to the lowest possible taxonomic level
using available literature and categorized based on association
with primary or secondary-forest types (e.g., Cornejo and
Janovec 2010).
Table 1. Plant taxa identied from seeds recovered from Aotus nigriceps fecal
samples. Values are the frequency of occurrence of seeds in recovered samples
from each site and sampling occasion. VC = Villa Carmen; MLC = Manu Learning
Center; N = fecal sample size.
Genus (Family)
Philodendron Schott (Araceae) 0 0 11.1 50
Annonaceae Juss. 0 0 0 33.3
Aniba Aubl. (Lauraceae) 0 13.6 0 0
Physocalymma Pohl (Lythraceae) 54.5 0 0 33.3
Bellucia Neck. (Melastomataceae) 27.3 0 22.2 16.7
Henriettella Naudin (Melastomataceae) 0 0 11.1 33.3
Ficus L. (Moraceae)* 0 27.3 55.5 0
Psidium L. (Myrtaceae) 0 31.8 0 0
Piper L. (Piperaceae) 45.5 4.5 66.7 66.7
Solanum L. (Solanaceae) 0 0 0 16.7
Urera Gaudich. (Urticaceae) 27.3 0 0 0
Cecropia Loe (Urticaceae)* 100 77.3 100 66.7
*Previously described in Aotus nigriceps (Wright 1981 and Wright 1985)
HELENBROOK et al. Black-headed night monkeys in southeastern Peru
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Nine night-monkey groups were found at Villa Carmen and
ve at MLC, ranging in size from two to ve individuals,
though a solitary individual was also encountered (Table
2). ese groups were revisited over the course of the two
years. All groups were in areas with some level of historical
logging and habitat degradation (Table 2). Basal area estimates
were consistent with previously described degraded habitat
(Brown and Lugo 1990). All but one group at Villa Carmen
was found in forest dominated by native bamboo species,
Guadua weberbaueri Pilg. and Guadua sarcocarpa Londoño
and P. M. Peterson. A single group was found in degraded
cane (Gynerium Willd ex P. Beauv) forest with bamboo. Only
two groups at MLC were found in areas with bamboo, while
the others were found in secondary forests devoid of bamboo.
We were unable to locate specic nesting sites for all
groups; however, of the five that were located at Villa
Carmen, three were in bamboo stands, one in a cane thicket,
and another in lianas. ough we could not locate an exact
nesting site for the four remaining groups (44%), we were
often present when they rst started moving in the evenings.
For example, we could often triangulate the general area
where they would begin moving at dusk but could not locate
them during the day to pinpoint their sleeping site. ese
four groups were also found in areas dominated by bamboo.
At MLC, only one nesting site was conrmed, and this was
found in lianas lacking any bamboo. All groups were found
within 1 km of each associated eld station. Two groups at
Villa Carmen and one group at MLC were routinely found
on the perimeter of the station in secondary forest (< 100 m).
Estimated DISTANCE densities were higher at Villa
Carmen (50.0 individuals per km2) than at MLC (19.2
individuals per km2). Both estimates had a large condence
interval, ranging from 17.0−147.0 individuals per km2 at
Villa Carmen, and 4.9−75.3 individuals per km2 at MLC.
e known number of individuals in our sampling area at
Villa Carmen was 17.5 individuals per km2 and the encounter
rate was 5.8 groups per 10 km. We could not calculate these
parameters for MLC since group composition there is not as
well-known as at Villa Carmen, and our sample size was too
small to calculate the encounter rate.
We recovered seeds belonging to 12 genera and ten families
(Table 1). e number of taxa recovered from Villa Carmen
groups in bamboo forest was similar (N = 10) to that from
MLC (N = 8). We recovered as many as seven genera from a
single MLC group, and just four genera from a group living
in disturbed mixed bamboo forest at Villa Carmen. Cecropia
was found in all sampled groups, and three genera were found
in both sampled seasons: Cecropia sp., Piper sp., and Ficus sp.
Groups at MLC consumed all the same fruits identied at
Villa Carmen in Spring 2016 except for Urera sp. However, we
found that groups at MLC consumed two other fruit species,
including an unidentied Annonaceae and Solanum sp.
All black-headed night monkey groups in our study were
found in secondary forest, though there is considerable
evidence to suggest that they are also found in primary habitat
based on camera trap data provided by MLC (unpublished
data) and from previous Aotus studies (Wright 1978; Aquino
and Encarnacion 1994; Cornejo et al. 2008; Aquino et al.
2013; Shanee et al. 2013). Limited sampling in primary forest
is presumably the main reason we did not encounter Aotus in
these areas. Dense canopy in primary forest also likely limited
our ability to detect auditory or visual cues of night monkeys.
Table 2. Habitat structure of Aotus nigriceps groups found at Villa Carmen (VC) and Manu Learning Centre (MLC) in southeastern Peru. N = number of individuals in group.
Field site Group N Habitat type Nest type Canopy cover (%) Total basal area (m2 ha-1) Elevation (m a.s.l.)
VC A 4 Secondary forest, bamboo Bamboo 74.7 12.66 530
VC B 3 Secondary forest, bamboo Bamboo 76.9 11.91 530
VC C 3 Secondary forest, bamboo Unknown 65.5 2.11 610
VC D 2 Secondary forest, bamboo Bamboo 69.3 10.34 529
VC E 3 Secondary forest, cane Cane 68.3 22.1 516
VC F 3 Secondary forest, Bamboo Liana 69.8 9.93 529
VC G 3 Secondary forest, Bamboo Unknown 70.2 14.15 530
VC H 3 Secondary forest, Bamboo Unknown 68.2 4.85 529
VC I 2* Secondary forest, Bamboo Unknown 72.3 19.75 529
MLC T2A 5 Secondary forest Unknown 80.8 15.62 516
MLC T2B 4 Secondary forest Liana 71.3 14.11 512
MLC T9A 3 Secondary forest Unknown 84.4 13.11 489
MLC T9B 2* Secondary forest, Bamboo Unknown 77.2 38.12 487
MLC Camp 4 Secondary forest, Bamboo Unknown 76.0 NA 456
*In two cases, groups were observed but exact numbers could not be ascertained. We include the minimum number of individuals observed (N), though the total
group size could be higher. The qualitative assessment was based on visual assessment and unpublished historical records provided by each eld station. In a single
case, basal area was not available (NA).
HELENBROOK et al. Black-headed night monkeys in southeastern Peru
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e majority of groups were found in forest dominated by
native bamboo, and all groups were found in proximity to
each eld station (< 1 km). Our results suggest that night
monkeys are likely to be found, and even potentially thrive,
in these types of secondary forest dominated by bamboo and
cane species. Aotus nigriceps were also found to attain relatively
high densities (31.1 individuals per km2) living in mixed
bamboo forests in an area ~150 km northeast from our study
sites, suggesting that this species may be able to survive in an
array of forest types (Aquino et al. 2013).
Estimated population density and encounter rates at Villa
Carmen were both higher than the only other A. nigriceps
report (Aquino et al. 2013); however, the known number of
individuals per square kilometer was actually lower. MLC
density estimates were similar to encounter rates found at Villa
Carmen. A number of factors could contribute to dierences
in density estimates, most notably habitat type. Aquino et
al. (2013) pooled results from three habitat types so we are
unable to ascertain whether there was a signicant dierence
compared to our study. However, they found primary forest
and semi-dense primary forest with some bamboo had a
higher percentage of sightings compared to open primary
forest with bamboo. Considering most of our sampled area
was secondary forest with bamboo, it is therefore dicult to
make a reasonable comparison (Nelson 1994). Svensson et
al. (2010) also surveyed three dierent types of Aotus zonalis
habitat in Panama. To our knowledge, none of these contained
bamboo but still had similar densities compared to our study
(i.e., 0− 19.7 individuals per km2; mean = 13.2).
Our DISTANCE estimate at Villa Carmen was higher
than the known number of individuals; however, the
condence interval was fairly large, likely due to a relatively
small sample size. It is conceivable that we counted some
groups twice on trails that were only 250 m apart, since night
monkeys have been reported to travel as far as 708 m in 24
hours (Wright 1989). However, duplicate sampling is not
likely since we established sleeping sites in most cases, and
monitored travel patterns for each group.
Night monkeys have been reported to use tree holes or
vine tangles as nesting sites, which are normally restrictive as
they limit their ability to travel particularly far since they need
to return to the same site the following morning (Aquino and
Encarnacion 1986; Fernandez-Duque et al. 2008). However,
night monkeys at Villa Carmen routinely returned to the same
bamboo patches despite there being other dense patches all
around and no competing groups vying for sleeping sites.
ere was no evidence that they used tree holes, and we only
observed two nesting sites composed of tangled vines. e
absence of nesting sites in tree holes may be related to the
scarcity of such resource in the secondary forest. Additionally,
the return to the same sleeping sites in consecutive days may
be explained by the familiarity in established routes and
accessibility to food sources.
Diet analysis of defecated seeds showed that Aotus relied
heavily on Cecropia spp. However, no species dominated
sample composition. Consumption of Cecropia spp. might
have little to do with preference, but rather with the availability
of this species in secondary forests. Most groups did not have
access to primary habitat, though they could have traveled to
nearby areas with reduced habitat degradation. Two of our
sampled groups also made use of abandoned guava (Psidium)
orchards as part of their home range, areas frequented by
workers at the eld station during the day. ere was no
evidence that other sympatric diurnal primates used these
guava orchards during the day, which makes them potentially
valuable food sources to the night monkeys. Psidium seeds
were recovered from one of these nearby groups along with
two others which were not in proximity to these orchards.
In total, we recovered twelve seed types from fecal samples,
and only two of these genera were previously described in
Aotus (Table 1: Wright 1981 and Wright 1985). No other
known studies have reported on fruits consumed in other
Aotus species. Given our small sample size, the diversity
of fruits consumed is likely much larger, though Wright
(1978) similarly described the use of just nine fruiting tree
species by a group over the course of one month. Of course,
analysis of fecal samples limits our results to seeds that remain
recognizable after passing the digestive tract and does not
give a full picture of other types of food consumed, such
as leaves, nectar, owers, and insects (Wright 1985; Wright
1989; Wright 1994).
e black-headed night monkey - and likely other Aotus
species – are quite adaptable based on their use of degraded
habitat, an omnivorous diet including the consumption of
several pioneer species, and an ability to persist in areas close
to human settlements. For example, there are nine other
monkey species in nearby primary habitat at Villa Carmen;
however, only A. nigriceps and Sapajus apella (Linnaeus 1758)
were found in the sampled secondary forest with bamboo,
an area which also has evidence of hunting based on camera
trap footage (unpublished data). Likewise, night monkeys
use multiple habitat types and forests with varying levels of
degradation which suggests that they can traverse ecological
matrices that might be difficult for other species. Their
consumption of fruit from a dozen tree species suggests that
they may be able to disperse an array of seeds throughout
the year, though over short distances, as their ability to
move seeds over large distances is presumably limited. We
previously conducted a limited analysis of seed germination
success and time to germination in Aotus nigriceps and found
both measurements were positively associated with digestion
(unpublished data). However, expanded sampling is necessary
HELENBROOK et al. Black-headed night monkeys in southeastern Peru
6 Ahead of Print 2020: 01  07
to determine to what degree night monkeys contribute to seed
dispersal, predation, and germination success.
We report that Aotus nigriceps is able to occupy degraded
habitat, inhabiting bamboo forests and relying on a varied
diet of fruits from pioneer species. e presence of this
species in heavily disturbed habitat suggests some level of
behavioral and dietary plasticity. Expanded sampling into
more diverse habitat types and across a larger geographical
range would likely contribute to our understanding of their
ecological niche.
We would like to thank Asociación para la Conservación de
la Cuenca Amazónica, and sta at both the Villa Carmen
Biological Station and Manu Learning Centre (CREES) for
hosting us, clearing trails, and providing valuable insight into
location and behavior of groups. We are indebted to the students
and sta from e School for Field Studies (Peru) who assisted
with data collection and logistics, specically Isabelle Berman,
Noah Linck, Audrey Nelson, Ben Sharaf, Caroline Rzucidlo,
Katlin Gott, Leigh Preston, and Sheridan Plummer. Special
thanks to Brooke Zale and two anonymous reviewers for
their valuable feedback on the manuscript. Logistical support
provided by the Tropical Conservation Fund.
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ASSOCIATE EDITOR: Paulo D. Bobrowiec
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The fate and behaviour of animals that leave their natal group (‘floaters’) is usually poorly understood, which can limit the understanding of a species' population dynamics. Attempted immigrations can have serious negative effects on residents who therefore may forcibly reject intruders. Consequently, floaters face a dilemma: they need to leave their natal range to find a breeding territory while trying to avoid potentially lethal rejections from established groups. To examine the hypothesis that floating Azara's owl monkeys avoid established groups temporally, we compared time-matched locations of floaters and groups with randomly selected distances. To examine the hypothesis that floaters avoid established groups spatially, we compared the utilization distribution overlap indices (UDOIs) for core areas of floaters and groups with randomly expected UDOIs. Based on average home range sizes and areas of overlap between floaters, we estimated the floater density in the study area to be 0.2–0.5 per group. The temporal avoidance hypothesis was not supported, since time-matched distances were smaller than distances of random locations, and not larger as predicted under this hypothesis. The spatial avoidance hypothesis, in contrast, was supported, with smaller UDOIs for core ranges than predicted. In conclusion, solitary owl monkeys seem to solve the floater's dilemma by trying to stay in relatively close proximity to groups while still avoiding their core ranges. Floaters thus maximize the number of groups with which they have contact, while being able to leave a group's territory quickly if detected by residents. While no marked sex differences in patterns were detected, there was a strong stochastic element to the number of floaters of a particular sex, thus resulting in a locally uneven operational sex ratio. This, in turn, can have important consequences for various aspects of the population dynamics.
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We investigated the role of Alouatta guariba clamitans as a seed dispersal agent at Itapuã State Park, at the southern limit of the Brazilian Atlantic Forest. During observations of a group of brown howlers, feces were collected whenever possible. Undamaged seeds were planted in forest soil inside a greenhouse, and control seeds were planted in the same soil. Germination rates of ten species were compared using Chi-square. The germination rates of only three species increased significantly after ingestion by howlers. In particular, 80.0% of the ingested seeds of Syagrus romanzoffiana (the fruit of which represented 25.4% of the group's diet) germinated, against only 3.3% of the control. Resumo. Este trabalho avalia o papel de Alouatta guariba clamitans como dispersor de sementes no Parque Estadual de Itapuã, no limite meridional da Mata Atlântica. Durante observações de um grupo de sete bugios-ruivos, coletaram-se fezes sempre que possível. Sementes não danificadas foram plantadas em solo da floresta, em uma estufa, e sementes controle foram plantadas no mesmo solo. As taxas de germinação de dez espécies comparadas com o Qui-quadrado. As taxas de germinação de apenas três espécies aumentaram significativamente após ingestão pelos animais. Em especial, 80,0% das sementes ingeridas de Syagrus romanzoffiana (cujos frutos representaram 25,4% da dieta do grupo) germinaram, contra somente 3,3% do controle.
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Habitat characteristics represent the matrix for the evolution of morphological, physiological and behavioural adaptations and life history traits of animals. These include bottom-up factors (such as distribution and abundance of food) and top-down constraints (such as protection from predators). The role of different habitat components in the survival and reproduction of individuals is of prime interest and is relevant for evolutionary understanding as well as for primate conservation. So far, this has been studied by linking habitat characteristics with phenotypic traits of animals, assuming that these traits represent adaptation to a given habitat constraint. However, we should bear in mind that the characteristics we observe today may represent adaptations to constraints that acted in the past (the Epaminondas effect, named after the little boy who always did the right thing for the previous situation; or, in evolutionary terminology: animals are tracking fitness optima by trying to climb the sides of shifting adaptive peaks, Cody, 1974). However, testing hypotheses based on these ‘ghosts of past constraints’ has so far been impossible.
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Primate seed dispersal has been increasingly recognized as having a potentially profound impact on tropical forest regeneration and plant species composition. Confirming and quantifying this impact, however, has proven to be an important challenge. We review the literature on seed dispersal by howler monkeys (Alouatta spp.) throughout their geographical range and assess the impact for plant populations and communities. Howler monkeys spend ca. 50 % of their feeding time eating fruits from many plant species and different life forms. For some plant species they are probably the only or primary dispersal agent (e.g., fruits with large seeds and/or with hard indehiscent husks), especially in anthropogenically disturbed forests, where other large-bodied frugivores tend to be absent or are locally extinct. In this regard, howlers contribute to secondary succession, aiding in the restoration of degraded habitats. Alouatta spp. swallow most (>90 %) of the seeds they handle, defecating them undamaged. Studies indicate that passage through the howler gut has a positive effect on rates of seed germination. In addition, studies of secondary dispersal indicate that seeds voided in howler dung attract secondary seed dispersers such as rodents and dung beetles. Also, nutrients in the dung may enhance seedling establishment. Although certain components of howler seed dispersal such as seed shadows, post-dispersal seed fate, and seeds/seedlings survival need to be better studied, current information suggests that howler monkeys are effective seed dispersers for many plant species, with important consequences for plant communities and forest regeneration.
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The Peruvian night monkey (Aotus miconax) is endemic to the eastern slopes of the Andes in northern Peru. We present preliminary behavioral data on A. miconax collected during 12 months of surveys on a single group living in a 1.4 ha forest fragment near the Centro Poblado La Esperanza, Amazonas Department. Follows were conducted for five nights each month around full moon. The group used 1.23 ha as their home range. Night ranges were between 0.16 and 0.63 ha. Activity budgets were 32 % feeding, 53 % travelling and 13 % resting. Average night path length was 823 m and average travel speed was 117 m/h. The study group has one of the smallest home ranges recorded for a night monkey group, probably the result of its isolated habitat. These results represent the first behavioral data on this species but results are limited by small sample sizes. A. miconax remains one of the least studied of all primates and is threatened by continued expansion of human populations and hunting.
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Guadua sarcocarpa, a new species with two subspecies from Amazonian Peru, Brazil, and Bolivia is described and illustrated. It is the first species of Guadua known to have fleshy fruits and the first report of an edible bamboo fruit in the New World. The new species shares 10 distinctive morphological traits with Guadua weberbaueri, an apparent close relative. Guadua sarcocarpa subsp. sarcocarpa and G. sarcocarpa subsp. purpuracea differ primarily in the color and length of the pseudospikelets (stramineous, 3-7 cm long in the former and purplish, 1-3 cm long in the latter), and shape and length of the caryopsis (oblong, apex obtuse to ovate, 4-6 cm long vs. ovate, apex abruptly apiculate, 1.5-2.5 cm long).
Biotic seed dispersal is a key process maintaining biodiversity in tropical forests where most trees produce vertebrate-dispersed seeds. Existing meta-analyses suggest an overall positive effect of vertebrate gut passage on seed germination, but no significant effects for non-flying mammals. However, previous meta-analyses combined rodents (seed predators) and primates (seed dispersers) into the non-flying mammals category, which may confound specific effects of each group on seed germination. However positive effects of monkeys on germination had previously been found in some studies. Here we disentangle the role of Neotropical primates as contributors to seed dispersal in tropical forests by running a meta-analysis to determine the overall magnitude of gut passage effects on seed germination percentage and mean time to germination. We also compare effect sizes as a function of different feeding guilds, gut complexities, and seed size. Our results show a strong, positive effect of primates on seed germination percentage and on the number of days to first germination. Strictly frugivorous monkeys, the group most threatened by extinction, showed the highest dispersal quality, increasing germination percentage by 75% and decreasing average germination time by 15%. Primates that include insects in their diets had no average effect on germination percentage or time. Gut passage had different outcomes on seeds with different sizes; both large and small seeds showed similar increases in germination percentages after gut passage, but only large seeds germinated faster than control seeds after gut passage. Our results show a relevant role for primates in providing high seed dispersal quality and as drivers of forest regeneration. The combined effects of defaunation and forest fragmentation may result in decreased regeneration of trees, which has the potential to affect negatively both forest structure and ecosystem processes. Finally, we provide general guidelines for standardizing research on seed dispersal by primates.