Int J Primatol (2007) 28:997–1023
DOI 10.1007/s10764-007-9166-0
Subsistence Technology of Nigerian Chimpanzees
Andrew Fowler & Volker Sommer
Received: 21 June 2005 / Revised: 14 February 2006 / Accepted: 10 May 2006 /
Published online: 11 October 2007
# Springer Science + Business Media, LLC 2007
Abstract A trademark of Homo sapiens is the enormous variation in behavioral
patterns across populations. Insight into the development of human cultures can be
aided by studies on communities of Pan across Africa that display unique
combinations of social behavior and elementary technology. Only cross-population
comparisons can reveal whether the diversity reflects differential genetics, environmental constraints, or is a cultural variant. However, the recently recognized and most
endangered subspecies, Pan troglodytes vellerosus, remains completely unstudied in
this respect. We report first evidence from a new long-term study of Nigerian
chimpanzees at Gashaka. Their dietary composition is highly varied and they have to
cope with high concentrations of antifeedant defenses of plants against consumption.
Gashaka chimpanzees use a varied tool kit for extractive foraging. For example, they
harvest insects throughout the year, via digging sticks and probes, to obtain honey
from stingless-bee and honeybee nests, dipping wands to prey on army ants, and
fishing rods to eat arboreal ants. Tools appeared to be custom-made with a
considerable degree of standardization in length, diameter, and preferential use of
distal ends. Moreover, compared to the rainy season, tools were longer during the dry
season when insects retreat further into their nests. Many of the expressions of
subsistence technology seem to be environmentally constrained. Most notably, the
absence of termite-eating could reflect a low abundance of mounds. Other traits may
represent cultural variation. For example, the chimpanzees did not hammer open 2
types of hard-shelled nuts with tools, unlike what occurs elsewhere in West Africa. The
prevalence of elementary technology may indicate that the material culture of Gashaka
chimpanzees is most related to core cultural tendencies of Central African populations.
A. Fowler (*) : V. Sommer
Department of Anthropology, University College London, London WC1E 6BT, UK
e-mail: a.fowler@ucl.ac.uk
A. Fowler : V. Sommer
Gashaka Primate Project, PMB 08, Serti, Taraba, Nigeria
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A. Fowler, V. Sommer
Keywords cultural primatology . material culture . Nigerian chimpanzee . tool use
Introduction
A trademark of Homo sapiens is the enormous variation in behavioral patterns between
populations. The variation constitutes the basis of what researchers commonly refer
to as cultural diversity (McGrew 1992). While a consensus about what constitutes
culture does not exist (McGrew 1998, 2004), social anthropologists tend to resort
to a “humanist” stance, reserving the label culture exclusively for our own species,
whereas biological anthropologists tend to be “universalists” and assume an
evolutionary continuum of traits that constitute culture (cf. McGrew 2004).
The biological paradigm maintains that studies of nonhuman primates, being our
closest phylogenetic relatives, can help us understand how the capacity for intraspecific diversity (Lott 1984), including cultural variety in humans, might have
evolved. The assumption is supported by observations of species that display
considerable flexibility in ecological and social arenas. For example, Indian langur
monkeys thrive in habitats of very different quality, ranging from semi-arid
conditions, the Himalayan mountains, and urban settings to moist evergreen forests.
They are also socially flexible, in that they form reproductive units of a multimalemultifemale type in some places, and strict 1-male-multifemale units in others
(Sommer 1996). A similar social flexibility is displayed by Thailand’s white-handed
gibbons that often deviate from a monogamous family type and lives in polyandrous,
polygynous or polygynandrous groups (Sommer and Reichard 2000).
The greatest degree of behavioral diversity among nonhuman primates is probably
exhibited by chimpanzees (Pan troglodytes) and bonobos (P. paniscus; Boesch et al.
2002). Studies across Africa strongly suggest for each community a unique
combination of the presence or absence of traits related to social customs, communication, territorial aggression, war-like raiding, hunting strategies, tool-kits, foodprocessing and consumption, and ingestion of plant matter for self-medication
(Hohmann and Fruth 2003; McGrew et al. 1996; Wrangham et al. 1994). The degree
of plasticity in behavioral patterns common to both Pan and Homo is perhaps not
surprising, given that they last shared a common ancestor only about 7 million years ago.
Potential sources of the diversity include different genetic makeup, behavior
molded by certain ecological conditions, and behavioral variants not brought about
by specific environmental constraints. A compilation of behavioral patterns at 9
long-term chimpanzee study sites discounted ones with ecological explanations and
revealed dozens of pattern to be customary or habitual among some groups but
absent in others (Whiten et al. 1999, 2001). A well-known example is nut-cracking
with stone or wooden hammers against an anvil, which occurs only in West Africa,
despite an abundance of nuts and potential tools elsewhere (McGrew 1992). The
practice is in all likelihood neither genetically determined nor a reflection of particular environmental conditions because some communities exhibit the behavior
while others within a closely related population, separated by the banks of a river but
exposed to virtually identical environmental conditions, do not (Boesch et al. 1994).
Many primatologists therefore consider the diversity of chimpanzee behavior to be
a reflection of culture (de Waal 1999; McGrew 1992; Sommer 2003). The argument
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999
is based on such criteria as the behavior is learned and not instinctive, socially
acquired, normative, and not random or idiosyncratic in its execution, and a
collective characteristic of a group (McGrew 2004). It is important to note that
ecologically determined behavior can also be socially transmitted and would thus be
part of the particular cultural profile of a population, but such behavioral pattern
would not count as a cultural variant.
Pan includes bonobos, restricted to the Congo basin, and chimpanzees with
subspecies in Central Africa (Pan troglodytes), West Africa (P.t. verus), and East
Africa (P.t. schweinfurthii). Primatologists have only recently recognized another
subspecies, vellerosus from Eastern Nigeria and Western Cameroon (Fig. 1). While
genetically the most distinct (Gagneux et al. 2001; Gonder et al. 1997), they are also
the most endangered (Hughes 2003; Kormos et al. 2003). Field studies of the
socioecology of Pan trolodytes vellerosus are currently restricted to the Gashaka
Gumti National Park in Nigeria (Sommer et al. 2004). Thus, we know very little
about one-fifth of the subgenera of Pan, which also limits a comprehensive
comparison within Pan.
We attempt to fill this gap by developing an ethnography of Pan troglodytes
vellerosus, i.e., a descriptive account of behavioral patterns within this particular
population of wild chimpanzees. This will also be a contribution to ethnology of
Pan, as we compare the traits of the study population to those at long-term study
sites across Africa (Whiten et al. 1999, 2001).
Our study is limited because we are typically not able to observe the chimpanzees
directly (Sommer et al. 2004). Therefore, our records do not refer to potential
cultural variants in social behavior such as different techniques of grooming,
courtship, or playing. Instead, most of our evidence is indirect, i.e., based on traces,
objects, and tools the apes left behind while they make use of elementary
technology. The “knowledgeable use of [...] physical objects as a means to achieve
an end” (McGrew 2004: 103) is an expression of chimpanzee material culture
(McGrew 1992), which researchers can study even when the users are absent or with
unhabituated apes (McGrew et al. 2003). The situation is thus somewhat similar to
the challenges archaeologists or palaeoanthropologists face who do not watch their
subjects and who have to infer their likely actions.
Fig. 1 Historic range of Pan
troglodytes vellerosus (shaded
area) in Nigeria and Cameroon,
and site of study in the GashakaGumti National Park, Nigeria.
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A. Fowler, V. Sommer
Only through studies of “as many groups of chimpanzees in as many parts of
Africa as possible” (Goodall 1994: 397) will we be able to discern which behavioral
patterns are universals, which are variants, and whether they are due to differences in
genes, environment, or a reflection of different cultures. Many primatologists echo
such a call for cross-population comparison (McGrew 2004; Whiten et al. 2001;
Wrangham et al. 1994). Nigerian chimpanzees are clearly a “missing link” in that
respect.
Methods
Chimpanzee Socioecology
Chimpanzee populations survive in ≥18 nations from Tanzania and Uganda in the
East to Mali and Senegal in the West. They inhabit a variety of biotopes such as
evergreen and semideciduous rain forests, open woodland-savannah, gallery forests,
and mosaic habitats that may include plantations and grassland (Boesch and BoeschAchermann 2000; Goodall 1986; Heltne and Marquardt 1989; Reynolds 2005;
Wrangham et al. 1994).
Chimpanzees live in communities or unit groups of 20–100 members that range
over 5–38 km2 in forests, and 25–560 km2 in open habitats. Patchy distribution of
food causes communities to forage in small parties of 6 members on average (range
3–10). Different members may join these parties (fusion) or split from them (fission).
Males are philopatric, whereas females tend to leave their natal community on sexual
maturity. Males are generally closely related and cooperate to defend their ranges
against neighboring communities with which they may engage in violent conflicts
(lethal raiding).
Each night, and often also during the day, they build a new nest (sleeping
platform) from leafy twigs, typically in trees. Chimpanzees feed on ripe fruit for 56–
71% of foraging time; on leaves, 18–21%; and 11–23% on other plant parts, in
particular terrestrial herbs. Faunivory constitutes 0.1–4%, comprising ≥25 vertebrate
species (80% colobus monkeys, 20% mammals such as duikers, bush pigs, baboons,
and rodents) that may be hunted cooperatively. Leopards, lions, humans, and
occasionally conspecifics prey on chimpanzees.
Wild chimpanzees manufacture and/or use a variety of tools from materials such
as bark, leaves, sticks and rocks, as sponges, wipes, probes, hooks, drills, missiles,
hammers, and toys, to extract resources (water, insect prey, honey, seeds) and in
social contacts (sexual invitations, conflicts, play).
Many local populations of chimpanzee have disappeared over the last few decades
or are in danger of extinction as a result of hunting, deforestation, and other forms
of human encroachment (Ammann et al. 2003).
Study Site
Gashaka Gumti National Park (GGNP) lies in Eastern Nigeria on the border with
Cameroon (06°55′–08°13′ N and 011°13′–012°11′ E; Fig. 1). GGNP is, at ca.
6500 km2, Nigeria’s largest National Park (Dunn 1999). The Southern (Gashaka)
Subsistence Technology of Nigerian Chimpanzees
1001
sector has rugged terrain from ca. 300 m up to 2419 m. Abundant rivers flow
continuously, even throughout the distinct dry season. The park area around the
village of Gashaka in Southern Taraba state harbors a great diversity of wildlife,
including 5 species of diurnal monkeys—olive baboon, tantalus monkey, mona
monkey, putty-nosed monkey, black-and-white colobus—as well as a population of
Pan troglodytes vellerosus of at least several hundred individuals. During the first
2 yr of study, we frequently encountered chimpanzees directly (average length of
sighting: 27 min; range 1–190 min). Party size averaged 3.7 (range 1–17) and average
nest group size 5.7 (range 1–23). We estimate that the Gashaka-Kwano community
comprises ≥35 members occupying a home range of ≥26 km2 (Sommer et al. 2004).
Our major study site is inside the park at Kwano (583 m; 07°19′ N–011°35′ E), an
abandoned settlement, 11 aerial km from the nearest village of Gashaka. An ancient
footpath connects Gashaka with Kwano, leading to the highlands and on to Cameroon
(traffic ca. 1 person/h). The Gashaka-Kwano area has experienced anthropogenic
influences for centuries. Most notable is the deliberate yearly burning of grass
(December–January), which has probably turned large parts of previously
semideciduous forest into grassy woodland (Louppe et al. 1995). Large-scale
logging does not occur, but some timber and nontimber forest products are extracted.
Sporadically, Fulani pastoralists graze cattle. All hunting is prohibited in GGNP
though ungulates such as buffalo, duiker, and pigs are sometimes poached. Primates,
with rare exceptions, are not hunted in the Gashaka-Kwano region, not least because
of Islamic religious taboos.
There is an annual wet and dry season, with associated fluctuations in temperature
and humidity (Fig. 2). From 2000 to 2003, the average number of rainy d/mo
during the heavy downpours from mid-April to mid-October is 18 (range 11–24).
The wettest day (Aug. 2001) had 144 mm rain. The yearly average reached 1864 mm
(2000: 1875 mm during 127 d; 2001: 1777 mm during 124 d, 2002: 1856 mm during
132 d, 2003: 1946 mm during 140 d). No rain usually falls for 4 mo/yr (December–
March). Mean monthly humidity (at 1600 h) is a minimum of 15% (Feb. 2001) and a
Fig. 2 Climate at Gashaka (2000–2003), indicating pronounced monthly seasonality.
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A. Fowler, V. Sommer
maximum of 82% (Oct. 2002). The dry season coincides with the Harmattan period
(November–March), when a dry dusty wind blows from the Sahara. The mean
minimum temperature is 21°C, the coolest recorded temperature 13°C (Jan. 2001,
Jan. 2002, Jan. 2003, Dec. 2001, Dec. 2003), the mean maximum 33°C, and the
hottest day on record 41°C (Mar. 2000).
The Gashaka-Kwano area is located in the sub-Saharan Guinea zone and borders
on the Cameroonian highland forests, representing a mosaic of habitats. Montane
forests, open (montane) grassland, and swamps occur outside the study area; the
Kwano region itself is characterized by savannah-woodland, lowland, and gallery
forest (Chapman and Chapman 2002; Dunn 1999).
The habitat supports a wide variety of large animals, including, apart from primates,
carnivores (African civet, golden cat, leopard), ungulates (red river hog, giant forest
hog, African buffalo, bushbuck, red-flanked duiker, yellow-backed duiker, waterbuck), rodents (crested porcupine), and aardvark.
Candidate Behavioral Patterns Reflecting Subsistence Technology
We analyzed the data set accumulated for the Gashaka chimpanzees for evidence of
elementary technology, which included tools the chimpanzees manufactured and used
and then left behind, i.e., artefacts (“the end-product of modification of an object to
fulfil a useful purpose”; McGrew 2004: 104, citing the definition given by Oswalt
1976) as well as naturefacts (“a natural form, used without prior modification”; ibid.).
We scrutinized the 65 behavioral patterns described in a benchmark paper on
charting cultural variation in chimpanzees (Whiten et al. 2001) to seek evidence as
to whether our accumulated field data established reasonable details about the
presence of a certain technology trait at our field site, or if we had good enough
reasons to assume their absence. The resulting list of candidate patterns (Table I)
includes subsistence technology related to insectivory and herbivory (sensu McGrew
2004). The particular activity associated with the categories is not always
synonymous with a name provided in Whiten et al. (2001: Table I) because any
given pattern may encompass elements of several related patterns; e.g., dig is similar
to perforate or expel/stir. A brief description of the behavior therefore concludes the
definition of candidate pattern provided in our tabulation.
Our scrutiny produced 6 candidate behavioral patterns, which encompass 18
related patterns. We are thus able to make a comparison with about one quarter of
the original list of Whiten et al. (2001) of potential cultural variants of behavioral
patterns in wild chimpanzees.
Data Collection
Researchers, field assistants, volunteers, and students of the Gashaka Primate Project
conducted field work on the chimpanzees of Gashaka-Kwano over 5 yr (Jan. 2000–
Apr. 2005). We gathered data on chimpanzee material culture ad libitum in
conjunction with socioecological research (nest-building behavior, plant food ecology,
party size variation; cf. Sommer et al. 2004). Any team member who came across a
tool-site (atelier) informed Fowler, who consequently investigated almost all the
ateliers and measured all tools.
Subsistence Technology of Nigerian Chimpanzees
1003
Table I Candidate behavioral patterns involving subsistence technology in Nigerian chimpanzees at
Gashaka
Artefact (A)/ Descriptiona
naturefact (N)
Subsistence Definition
category
of
behavioral
patterna
Definition may encompass
elements of the following
related behavioral patternb
Insectivory Dig
Dig (14), perforate (13), open- A: Digging
and-probe (22), lever open
stick
(44), expel/stir (45)
Brush-stick (15), fluid-dip (41), A: Probing
bee-probe (42), expel/stir (45) stick
Bee-probe
Ant-dip
Ant-fish
Termitefish
Herbivory
a
b
Nuthammer
Ant-dip-wipe (39), ant-dip (40) A: Dipping
stick
Ant-fish (38) probes
A: Fishing
rod
Termite-fish using leaf midrib A: Probing
(36) or nonleaf materials (37) leaf midrib
or other
probe
Nut-hammer: wood or stone
A/N:
hammer on wood or stone
Hammer on
anvil or hard ground (29–33)
anvil
Stout stick used as lever or
spade to enlarge bee nest
entrance
Stick used to probe bee nest
entrance or to obtain honey,
or both
Slender wand used to
harvest army ants
Fine probe used to extract
arboreal (wood-boring)
ants from tunnels
Leaf midrib or other probing
instrument used to extract
termites from tunnels
Stone or wood used to crack
nuts
Modified after McGrew (2004); Whiten et al. (2001).
Descriptors as named and numbered in Whiten et al. (2001: Table I).
The local environment may strongly influence the likelihood that certain behavioral
patterns occur, e.g., termite-eating or nut-hammering. We therefore measured the
distribution of suitable material in the habitat. We documented termite-mound abundance by walking an 8 km straight line transect, which covers both forests (76%) and
woodland-savannah areas (24%). We recorded each termite mound of ≥20 cm height
within a 20 m wide strip along the transect (total survey area=16 ha). We visited trees
that produced hard-shelled nuts to assess the abundance of hammers and anvils that
chimpanzees could reasonably use for nut-hammering (for weight and size criteria, see
Humle and Matsuzawa 2004; McGrew et al. 1997). Within a 5 m radius of the nutproducing tree, we tried to find 5 stones or pieces of wood that seemed suitable as
hammers, and a further 5 as anvils. The latter could also be embedded in the ground.
We pounded stones or pieces of wood that fulfilled certain dimensions (hammers:
weight >100 g-2.5 kg, hammering surface ≥5×10 cm; anvils: embedded in the
ground or mass >400 g-10 kg and with a surface area of ≥7 cm×10 cm) against a
rock to test whether they would crack or fragment. We repeated the selection of
stones until we identified 5 suitable hammers and anvils, or until no more suitable
material was available within a 5-m radius.
Results
Five years of field work around the field station at Gashaka-Kwano produced ample
evidence that wild Nigerian chimpanzees use a varied tool kit in elementary
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A. Fowler, V. Sommer
technology. At the same time, some forms of subsistence technology reported from
other chimpanzee study sites are missing.
Subsistence: Insectivory
Dig We found relatively short and sturdy sticks, typically soiled and with frayed
ends (Fig. 3a), at entrances to the subterranean nests of stingless bees, at times
indicated by the presence of dead insects. Subjects used the sticks to enlarge nest
entrances and to extract honey or insects or both of species such as Hypotrigona
gribodoi (Magretti) and Meliponula erythra (Schletterer). (We discovered evidence
for similar tool use during a brief Feb. 2002 survey of a montane forest at Ngel
Nyaki, ca. 25 aerial km from Kwano, consisting of recently disturbed earth and
sticks with frayed ends around a nest entrance. Local guides at Ngel Nyaki were
familiar with digging tools, and also reported ant dipping as well as probing for the
honey of both honeybees and stingless bees).
Fig. 3 Material culture kit of Gashaka chimpanzees. (a) Stingless-bee digging sticks. (b) Stingless-bee
probing sticks. (c) Ant-dipping wands. (d) Ant-fishing rods including leaf midribs.
Subsistence Technology of Nigerian Chimpanzees
1005
Bee-probe Relatively long sticks, slender or sturdy (Fig. 3b) were left behind at sites
where chimpanzees obtain the honey of African honeybees (Apis mellifera) or
stingless bees. The type of insect was again discernible by the presence of dead
imagos. We regularly found sticks with the bark stripped and exhibiting traces or
the odor of honey, with frayed ends indicating manipulation, biting, or chewing
beneath trees, sometimes with honeycomb fragments and wads of chewed honeycomb
and bees.
Ant-dip We discovered long thin sticks from which leaves and side twigs had been
removed, often with frayed or bitten off ends (Fig. 3c), at disturbed nests of army
ants (driver ants; safari ants; Dorylus [Anomma]). Subjects probably used the wands
to ferry ants to the mouth at a safe distance from the nest, thereby reducing the
painful bites of the larger workers that swarm out to defend the nest entrance.
Remains of army ants were present in 42% of n=381 feces of individual chimpanzees collected from beneath night nests between 2001 and 2005. We recovered
ant remains in each month of the year (Schöning et al. 2007)
Ant-fish Chimpanzees use thin elastic twiglets, grass, pieces of stripped bark, and the
midribs of large leaves (Fig. 3d) to fish for arboreal ants such as Camponotus
chrysurus (Gerst.).
Episode 1 (Apr. 19, 2001. 67 min of direct observations by Fowler): One adult male
chimpanzee, 3 adult females, and an infant sat on the ground and fished
for ants from inside the trunk of a large tree, first selecting the twigs,
often stripping them of leaves and then placing them in the nest hole for
5–10 s. They removed the tools and picked off ants with the lips.
Episode 2 (June 11, 2001. Observers: Hammaunde Guruza, Yakubu Wakirwa):
Three adult chimpanzees are observed at the base of a tree. They flee
when the observers approach. The stripped midribs of 4 leaves are
discarded on the ground. Ants are seen on the tree’s surface.
Episode 3 (Feb. 23, 2005. 14 min of direct observation by Hammaunde Guruza,
Klaus Meister, Fowler): One adult female chimpanzee made and used
stick tools to probe into the nest of wood-boring ants in a newly fallen
tree. She held the end of a stick in her mouth and stripped leaves off in a
sweeping motion, using a foot. She leaned down from above the nest
and probed, withdrawing and eating ants from the stick, hand, and tree
trunk. Nests of both ants and stingless bees are discovered in the tree.
Termite-fish Wild chimpanzees use thin probes as instruments to extract termites
from tunnels. Mounds built by Macrotermes such as M. bellicosus, which farm
underground fungus gardens, exist in the Gashaka area, particularly in the savannah
woodland, often with dimensions of 2 m height and a base diameter of 2 m. During
the dry season, the mounds are baked from the sun, and Macrotermes retreats
underground. During the rains, the imagos are generally closer to the surface, and
winged forms swarm. We determined the abundance of mounds in a 16-ha area,
along an 8 km straight line transect. However, we recorded only 2 small mounds in a
forested section. Nevertheless, in savannah-woodland off transect, inhabited mounds
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A. Fowler, V. Sommer
regularly occur in clumped distribution with, e.g., a density of 6/50 m2, a maximum
height of 1.8 m, and a base diameter of 2.4–2.9 m. Mounds also occur at the forest
edge. Field workers were unable to extract a single termite from a mound, using
plant probes. Moreover, even when mounds were partly broken and the termites
directly provoked with the plant part, not a single one, including the largest workers,
would attach itself to any blade or stem, despite the fact that soldiers readily bite into
human fingers and inflict blood-drawing injuries. In any case, we never noticed
discarded tools on or near termite mounds. Fecal samples covering each month of
the year (n=381) were likewise completely devoid of termite remains.
We recovered 313 tools from 41 tool-site ateliers (Table II). Nearly half of all
ateliers had ant-dipping wands (41%), followed by ateliers of stingless-bee probing
sticks (37%), whereas ateliers with stingless-bee digging sticks (10%), ant-fishing
rods (7%), and honeybee probing sticks (5%) were much less common (Fig. 4). Half
of all ateliers had 1–4 tools (Fig. 5), but some had >12. Half of all tools are from 6
ateliers that contained 16–48 tools.
The greatest average number of tools was associated with ateliers of stingless-bee
probing sticks (n=12) and ant-fishing rods (n=13), whereas other types of ateliers
contained an average of 4–5 tools only. Thus, stingless-bee probing sticks made up
more than half (57%) of all recovered tools, and ant-dipping wands about a quarter
(23%). Ant-fishing rods were less common (12%) and only a fraction of tools are
stingless-bee digging sticks (3%) or honeybee probing sticks (3%).
Almost half (46%) of the ateliers were new, i.e., <1 d old. Another 41% were
fresh, i.e., a few days old. Just 8% had been used about a week before discovery, and
5% were >1 wk (Fig. 6). We found tools throughout the year (Fig. 7) but with fewer
recoveries during the heavy rains (May–November).
There are differences in the dimensions of tools (Table II). Wands for ant-dipping
are by far the longest (84 cm) and relatively thin (diameter at mid-point 6 mm)
whereas stingless-bee digging sticks are shortest (30 cm) and thicker (8 mm). Antfishing rods are the shortest (20 cm) and thinnest (1.2 mm). Unlike most other tools,
they are often not woody, as the 38 rods consist of stripped twigs (40%), midribs of
large leaves (24%), grass blades or vines (18%), twigs with bark scraped off (13%),
twig (3%), and bark fiber (3%).
Only 12% of all tools still contained projections such as small twigs or leaves,
indicating that individuals removed protruding parts before use. The bark of the most
tools is stripped (mean across types: 79%), typically partially (mean: 81%) and
sometimes completely (mean: 7%).
We measured lengths of tools and plotted their cumulative distribution across
classes of 10 cm, which followed the rationale that the greater the degree of standardization, the steeper the curve to the asymptote. It became obvious that tool types
exhibited varying degrees of standardization: Stingless-bee digging sticks show the
least variation in length, whereas the length of stingless-bee probing sticks and antdipping wands fluctuate considerably (Fig. 8).
There are noticeable differences between a tool’s proximal end (the end that was
closer to the stem of the originating plant before the tool was removed) and the distal
end (the end that was closer to the terminal end of the twig, stem, or leave). For
example, average diameter of tools decreased by 20% from the proximal end
(8.4 mm) toward the distal end (6.6 mm; Mann-Whitney U test, n [proximal]=300,
Subsistence Technology of Nigerian Chimpanzees
1007
Table II Dimensions of tools Nigerian chimpanzees use for insectivory
Tool
Stingless-bee Stingless-bee Honeybee
digging stick probing stick probing
stick
Antdipping
wand
Antfishing
rod
Ateliers
Tools
Tools/Atelier
4
15
3.8
1
5
9
29.6
30
6.4
20
37
9
8.1
9
2.0
5
11
9
8.5
9
2.1
5
11
9
7.7
8
1.9
5
11
2
67.5
68
38.9
40
95
7
31.7
23
18.4
14
58
17
73
4.3
1
16
72
83.8
81
27.4
28
160
71
6.4
6
2.1
3
12
72
7.7
7
2.1
4
13
71
5.1
5
2.0
2
11
15
43.9
29
39.9
15
150
30
28.2
20
23.7
3
95
3
38
12.7
4
20
38
19.5
17
8.4
5
44
38
1.2
1
0.9
0
6
38
1.5
1.4
0.9
0.5
5.6
38
1.5
1
0.9
1
6
8
19.2
6
23.4
3
61
6
4.9
6
3.2
0
8
n
n
Mean
Min
Max
Tool length (cm) n
Mean
Median
SD
Min
Max
Tool diameter at n
mid-point (mm) Mean
Median
SD
Min
Max
Proximal end:
n
diameter (mm) Mean
Median
SD
Min
Max
Distal end:
n
diameter (mm) Mean
Median
SD
Min
Max
Proximal end:
n
length of fray
Mean
(mm)
Median
SD
Min
Max
Distal end: length n
of fray (mm)
Mean
Median
SD
Min
Max
15
178
11.9
1
48
171
35.3
33
19.3
7
105
172
6.3
6
4.0
2
50
172
7.3
7
4.2
3
50
172
5.8
5
4.0
2
50
38
30.7
26
16.8
3
86
81
32.3
27
26.9
3
160
2
9
4.5
2
7
9
46.1
47
13.6
19
69
9
8.4
9
2.4
5
12
9
10.2
10
2.9
5
14
9
8.0
9
2.4
4
12
0
4
14.8
12
8.7
9
27
Average
7.4
42.8
41.8
6.1
6.0
7.0
6.7
5.6
5.4
40.3
31.9
22.4
17.4
Proximal = end of a tool that was closest to stem before its removal from mother plant; distal = the other
end.
n [distal]=299; z=−2.526, p=0.010). The decrease held true for all types of tools
(stingless-bee digging stick proximal end vs. distal end 8.5 vs. 7.7 mm; stingless-bee
probing stick 7.3 vs. 5.8 mm, honeybee probing stick 10.2 vs. 8.0 mm; ant-dipping
stick 7.7 vs. 5.1 mm) except for the very thin ant-fishing rods (1.5 mm at both ends).
1008
A. Fowler, V. Sommer
Fig. 4 Proportion of ateliers and tools of different categories recovered at study site.
It is easier to insert the thinner end of an object into an opening. Indeed, judging
from the condition of tools, chimpanzees more often preferred to manipulate (poke,
probe, stir) with the distal end of twigs (86%; Fig. 9), whereas the proximal end was
used in only about a third of cases (34%; total >100%, because both ends were used
for one-fifth of tools). The preference for distal ends held true for all types of tools.
Fig. 5 Number of tools per atelier.
Subsistence Technology of Nigerian Chimpanzees
1009
Fig. 6 Age of recovered ateliers and tools.
Subjects thus exerted pressure preferentially with the thinner ends. As a further
reflection of the positioning, distal ends tended to be more often compacted (40%)
than proximal ends (23%), and distal ends were more often stripped (69%) than
proximal ends (47%). Likewise, traces occurred twice as often on distal ends (59%)
Fig. 7 Tool ateliers recovered during different months of the year.
1010
A. Fowler, V. Sommer
100
80
60
%
40
STINGLESS-BEE DIGGING STICK
STINGLESS-BEE PROBING STICK
HONEYBEE PROBING STICK
20
ANT-DIPPING WAND
ANT-FISHING ROD
0
1
2
3
4
5
6
7
8
9
10
11
12
NUMBER OF 10-CM LENGTH CLASSES
Fig. 8 Degree of standardization of tool lengths of different tool types, as measured by the cumulative
distribution across classes of 10 cm (1–10 cm, 11–20 cm, ..., 81–90 cm, 91–100 cm, 101+ cm).
than on proximal ends (31%). The type of trace reflected the purpose or technique
for which subjects used the tool: 63% of stingless-bee probing sticks exhibited traces
of honey and 3% traces of beeswax; we found soil on 34% of the sticks. All traces
on honeybee probing sticks (100%) were honey, whereas we noted no soil trace at
all, because the preyed-upon honeybee nests were exclusively arboreal. Conversely,
all of the identified traces on stingless-bee digging sticks and ant-dipping sticks were
soil. Traces on ant-fishing rods were always soil, possibly because grass blades
subjects used as ant-fishing tools were pulled directly from the earth.
Distal ends were also more often frayed (45%) than proximal ends (17%). The
length of frays (Table II) at proximal ends was almost double (40 mm) the ones at
distal tool ends (22 mm). The greater extent of proximal fraying held true for all tool
types except stingless-bee probing sticks (proximal 31 mm vs. 32 mm) and
honeybee probing sticks, which lacked proximal frays.
Finally, tools used during the dry season were one-fifth longer (average across
tool categories=52 cm) than those used during the wet season (41 cm; MannWhitney U test, n [dry]=131, n [wet]=140; U=10129, z=−1.49, p=0.0681). This
statistical trend held true for most tool types for which sample size was sufficient to
conduct separate tests (Fig. 10; stingless-bee probing stick, n [dry]=94, n [wet]=67,
U=2426, z=2.48, p=0.0066; ant-dipping wand, n [dry]=10, n [wet]=53, U=153.5,
z=2.09, p=0.0183) except ant-fishing rods (n [dry]=20, n [wet]=18, U=218.5, z=
−1.11, p=0.1335).
Subsistence Technology of Nigerian Chimpanzees
1011
Fig. 9 Differences between distal (D) and proximal (P) ends of tools.
Subsistence: Plant Matter
Nut-hammer In this percussive activity (described for West-African populations by
Boesch and Boesch-Achermann 2000), individuals use hammers and anvils of stone
or wood to crack hard-shelled nuts. At Gashaka-Kwano nuts are limited to oil palms
Fig. 10 Mean length of tools during wet season and dry season.
1012
A. Fowler, V. Sommer
(Elaeis guineensis) and Detarium microcarpum. Both species occur at low density,
as records from a permanent 8-km straight line transect suggest, where Detarium
does not appear at all and Elaeis only 4 times. However, oil palms occur also in
clumped distribution at sites of abandoned farms and human settlements. Nuts of
Detarium are a sought-after product sold in local markets.
Humans within the chimpanzee home range crack both types of nuts with stone
hammers and anvils. Olive baboons at Gashaka open the nuts of Detarium and
Elaeis with their teeth and eat the seeds (Warren 2004). Judging from feeding
remains, other animals, e.g., wild pigs, also bite open both types of nuts, but it is not
known if chimpanzees are among them. Chimpanzees eat at least the fruit of
Detarium. The outer skin and flesh is normally removed, exposing the fibrous husk
that covers the nut shell. These husks have been found freshly discarded at
chimpanzee feeding sites.
We found no evidence that chimpanzees crack open either Detarium or Elaei
using tools. Hammers or anvils, or both, of suitable dimensions occur in the vicinity
of nut-bearing trees, particularly at the banks of small and often dry riverbeds, but
the materials are not common. Within a 5 m radius around 3 Detarium and 3 Elaeis
trees, we measured the abundance of stone hammers (SH), wooden hammers (WH),
stone anvils (SA), and wooden anvils (WA). Many stones of suitable size were brittle
and fragmented on impact. Of 48 potential hammer stones, 67% broke on usage, and
of 17 potential anvil stones 18% broke. The procedure yielded the following results:
Detarium
Detarium
Detarium
Elaeis 1:
Elaeis 2:
Elaeis 3:
1: SH 5, WH 0; SA 5, WA 0
2: SH 0, WH 0; SA 0, WA 0
3: SH 1, WH 0; SA 0, WA 0
SH 3, WH 0; SA 3, WA 0 but trunk portions suitable
SH 2, WH 0; SA 1, WA 0 but trunk portions suitable
SH 5, WH 0; SA 5, WA 0 but trunk portions suitable.
Thus, by our definition, nut-cracking would have been possible under two-thirds
(4/6) of nut-bearing trees.
We conducted a brief survey in another habitat of Pan troglodytes vellerosus,
400 km SW of Gashaka in Korup National Park/Cameroon (Feb. 5–9, 2001, VS).
The forest harbors trees of Poga oleosa (Pierre) (Anisophylleacea), which produce
very hard-shelled poga (or inui) nuts. At least some suitable stones for cracking were
present. Local humans indeed crack the nuts in the forest with stone hammers and
anvils of stone or wood. However, evidence for nut-cracking by chimpanzees was
absent.
Discussion
Our study contains the first to report on the material culture of Nigerian
chimpanzees, thus broadening our understanding of chimpanzee behavioral diversity.
Moreover, the data allow us to draw limited conclusions about whether or not certain
behavioral patterns are the result of particular genotypes or environmental
conditions, or reflect cultural variation arising from social transmission. Potential
cultural variants are absent without ecological explanation in at least one chimpanzee
Subsistence Technology of Nigerian Chimpanzees
1013
population, but regularly observed in at least one other (Whiten et al. 2001).
However, it is difficult to identify one or the other cause of behavioral variation and
vice versa, to exclude ecological explanations and weed out observational bias.
Potential Biases in Data Collection
We are probably underreporting the variety of the tool kit of Gashaka chimpanzees
because positive observations and the accuracy of records tend to increase with study
length and how well we were able to observe the individuals (Boesch and BoeschAchermann 2000; McGrew 2004).
For example, that most tool ateliers were new (Fig. 5) was probably because older
sites had deteriorated more than fresher sites. The latter were more conspicuous, with
freshly moved earth or unwilted vegetation that had been broken off or scattered
about. Moreover, we were more likely to survey areas where chimpanzees had
currently or recently called, increasing the chances of encountering new tool sites.
Ateliers with ant-fishing rods were invariably new and recovered only after direct
observations of ant-fishing. Given that many ant-fishing tools were fragile leaf
midribs and grass blades, it is unlikely that tools would have been found after the
chimpanzees had left the site. The relative paucity of ateliers and tools used for
honeybee-probing and ant-fishing is probably also influenced by the fact that they
are arboreal activities that leave fewer traces for terrestrial human observers than the
more terrestrial ant-dipping and stingless-bee-probing.
Similarly, though studies at other sites indicate seasonality in tool use (Goodall
1986; McGrew et al. 1979), we did not control for time spent in the field in the
annual distribution of tool finds (Fig. 7). Instead, we collected artifacts used in
extractive foraging opportunistically while searching for or tracking chimpanzees (cf.
McGrew et al. 2005). The relative rarity of finds during the wet season might
therefore be misleading, as chimpanzee tracking is less easy during the period and
because heavy rains quickly disintegrate tool ateliers.
A detailed taxonomy of insect prey, an often confused matter at primatological
research sites, is currently being developed for our study area (Schöning et al. 2007).
Meanwhile, and in the absence of sufficient direct observations, we may be ignorant
about fine-tuned differences and inadvertently lump tools of different functions into
a single category. Our finding of varying degrees of standardization with respect to
certain tool categories (Fig. 8) is therefore problematic. The fluctuating length of
tools used to dip for ants and probe for stingless bees might in fact reflect an adaptation to specific situations or species. For example, an enormous number of army
ants might swarm out from larger nests attacked by the chimpanzees, in which case
they might retreat further from the nest entrance and use longer dipping wands,
whereas smaller nests could be exploited with shorter sticks. Humle and Matsuzawa
(2002) suggested that chimpanzees at Bossou use different length tools to prey on 2
different types of Dorylus species. Moreover, shorter wands might be needed when
dipping at foraging or migratory ant trails (Humle and Matsuzawa 2002; though
at Gashaka, tools occured only at nests, Schöning et al. 2007). Similarly, different
tool lengths could be optimal to harvest different types of stingless bees. Tutin et al.
(1995) compared tools used to obtain honey from Meliponula and Trigona, finding
that the former were longer, with a greater range of lengths. Finally, tools may form
1014
A. Fowler, V. Sommer
a set, in which individuals use ≥2 types sequentially (Brewer and McGrew 1990) but
limited direct observations may prevent us from noticing that.
Lack of Behavioral Variation (Universals)
Certain behavioral patterns, such as buttress-beat, in which chimpanzees drum on a
tree base, occur during all long-term studies and thus lack obvious behavioral
variation (cf. Whiten et al. 2001). They may still represent “cultural” traits: “Just that
if chimpanzees do acquire them by social learning, they do so in all communities
studied” (Whiten et al. 2001, p. 1496). In any case, none of the exact subsistence
technology patterns that are present or absent at Gashaka is a putative chimpanzee
universal, as they may or may not occur at certain study sites (See below, Fig. 11).
To assume that cultural variation is the cause of the diversity requires us to exclude
genetic differences as well as environmental constraints.
Genetic Explanations for Behavioral Variation
It seems reasonable to reject genetic explanations if the variation occurs between
gene-exchanging neighboring communities, such as when the Tanzanian Mahale K
group fishes for termites, and the Mahale M group does not (Whiten et al. 2001).
Unfortunately, behavioral studies of Pan troglodytes vellerosus are restricted to
Gashaka, with only anecdotal comparative data for neighboring communities or
populations such as Ngel Nyaki, where bee-probing likewise occurs, and Korup,
where nut-hammering seems likewise absent. However, both captive and wild studies
have demonstrated that chimpanzees easily learn to use objects (Whiten et al. 2001),
rendering tool use as “poor candidates for merely instinctual variations” (Whiten
et al. 2001: 1511).
Environmental Explanations for Behavioral Variation
Ecological factors do not normally influence the evolution of behavior unrelated
to subsistence. For example, some populations of wild chimpanzees place
ectoparasites removed during grooming on leaves to inspect them (Assersohn et
al. 2004). The behavior is probably a cultural variant, because vegetation is available
everywhere.
To exclude environmental influences for different patterns of tool use is more
difficult, as many variables are involved (McGrew and Tutin 1978). For example,
the likelihood that one or both ends of tools are used differs between sites. At
Gashaka, individuals use the distal end in 86% of all cases, the proximal end in 34%,
and thus use about one-fifth of tools at both ends. The preference for one end is
intermediary between Gombe, where individuals usually use both ends, and Assirik,
where almost always only one end is used (McGrew 1992). A potential
environmental explanation for the 2-ended use could be that tools are made from
vines, which are roughly uniformly cylindrical throughout their length and thus
suitable for use from both ends. In reality, chimpanzees use vines to the same degree
in Gombe and Assirik, so a cultural explanation is the most likely (McGrew 1992).
However, the preference for distal ends at Gashaka seems to have a plausible
Subsistence Technology of Nigerian Chimpanzees
1015
environmental explanation, because most tools are made from twigs that become
thinner toward the distal end (Table II) and are thus simply better suited for use as a
tool point.
The greater length of dry-season tools (Fig. 10) also has a likely ecological
explanation. Insects retreat further into their nests when the air and the nesting
substrate (earth, logs, trunks) are less humid to prevent desiccation, necessitating the
use of longer tools to gain access to them (cf. Goodall 1986, for equivalent observations on termite fishing). The chimpanzees seem to respond flexibly to the
situation. Potential seasonal adjustment to the exploitation of honeycomb might be
based on another mechanism, as combs are not moveable, forcing bees to regulate
actively their temperature and humidity. However, it could be that combs are annually
depleted and that bees refill them starting far from or near to the hive entrance, thus
inducing a seasonal variation in chimpanzee tool length.
Similarly, whether or not chimpanzees use the 1-handed “mouthing-off”
technique or the 2-handed “pull-through” technique for army-ant dipping may seem
like a straightforward cultural trait. However, differences in gregariousness and
aggressivity of Dorylus may determine how chimpanzees prey on the insects (Humle
and Matsuzawa 2002). Environmental factors may also influence the dimensions of
tools. Ant-dipping wands at Gashaka seem to have the greatest average length
(83 cm) reported from any study. Some sites come close (Tenkere, 80 cm; Kalinzu,
79 cm; Fongoli, 79 cm; Mount Nimba, 73 cm; Assirik, 72 cm; Gombe, 66 cm), but
other chimpanzees use noticeably shorter sticks (Bossou, 45 cm; Taï, 24 cm; data
from reviews in Alp 1993; Hashimoto et al. 2000; McGrew et al. 2005; Yamakoshi
and Myowa-Yamakoshi 2004). A number of ecological factors could explain these
differences: 1) Dorylus ant nests in Gashaka may be deeper or more hidden; 2) the
ants may be more aggressive or live in larger colonies; 3) we did not recover tools
used to prey on migrating or foraging ant trails, either because our direct
observations were limited and chance recoveries from tails are less likely, or
because the chimpanzees do not fish from trails (Schöning et al. 2007). This may
result in an increased mean length, as dipping tools utilized at ant trails tend to be
shorter given that the risk to be bitten is reduced compared to nest harvests (Humle
and Matsuzawa 2002).
Based on the principle that form reflects function, McGrew et al. (2005) predicted
that researchers will, through future direct observations, find chimpanzees at Fongoli/
Senegal use the relatively long ant-dipping tools in the 2-handed pull-through
technique, i.e., while chimpanzees stand bipedally or perched on trunks, to reduce the
painful bites of driver ants. The even greater average length of ant-dipping wands at
Gashaka suggests the same.
Another reflection of environmental constraints concerns the absence of termite
eating at Gashaka. Termites are a favorite prey of chimpanzees across Africa, though
they are not eaten everywhere (McGrew 1992). At Gashaka, we found no evidence
that chimpanzees use tools to harvest termites. Termite-fishing tools are typically
made from rather perishable material, such as grass and leaf midribs (Goodall 1986;
McGrew et al. 1979), which renders discoveries post hoc difficult. Nevertheless, we
never identified termite heads in feces despite the fact that chitin is indigestible for
apes, which clearly contrasts with findings at other sites, where termites might be
present in as much as 27% of feces (McGrew 1992). Various environmental factors
1016
A. Fowler, V. Sommer
could explain the absence of eating termites. First, the overall abundance of mounds
might be low as only 2 were located in a random 16-ha plot. Still, mounds exist,
sometimes in clumped distribution in the woodland-savannah, and large mounds
also occur at forest edges and in the forest itself. Second, suitably flexible and
resilient vegetation for the manufacture of tools might not be easily available,
because the yearly burning of the savannah-woodland favors coarse grasses in the
vicinity of mounds. However, termite-fishing is common in Assirik/Senegal where
intense dry-season burning is likewise practiced (Hunt and McGrew 2002). Third,
termites did not bite into probes of various materials with which we provoked them,
despite the fact that soldiers will immediately bite into skin. Thus, on balance we
tentatively conclude that the absence of termite-fishing reflects environmental
constraints, while at the same time suggesting that the extremely high preponderance
of fecal samples with ant remains (42%, more than at any other chimpanzee site;
Schöning et al. 2007) might be related to the absence of termite-eating.
Cultural Explanations for Behavioral Variation
The degree of modification of the substrate from which tools are manufactured is a first
potential cultural variation. e.g., the raw materials used for tools are, similar to other
sites, typically altered through reduction, i.e., stripping leaves, breaking off twigs,
peeling off bark, and clipping ends. About 4 out of 5 Gashaka tools were at least
partially stripped. A similar proportion of peeling (86%) occured at Assirik, whereas
tools at Gombe are virtually never peeled (McGrew et al. 1979, cit. in McGrew 1992).
It is hard to imagine that intrinsic characteristics of vegetation differ so much between
West, Central, and East Africa as to predetermine the degree to which tools are peeled.
Thus, the varying proportions could well reflect cultural variation.
A second potential example concerns the lack of evidence for nut-hammering at
Gashaka, despite the presence of 2 large hard-shelled and edible nuts (Elaeis guineensis,
Detarium microcarpum) along with suitable hammers and anvils under two-thirds
of nut-bearing trees. The Gashaka chimpanzees appear not to crack either species of
nuts with tools. Chimpanzees crack a similar species, Detarium senegalense, via
hammers at the West African sites of Tiwai/Sierra Leone (Whitesides 1985; cit. in
McGrew 1992), and Taï/Ivory Coast (Boesch and Boesch-Achermann 2000).
Individuals crack open oil-palm nuts with tools at Kanton/Liberia (Kortlandt and
Holzhaus 1987), Nimba/Guinea (Yamakoshi and Matsuzawa 1993) as well as
Bossou/Guinea (Sugiyama and Koman 1979) where no other hard-shelled nut suitable for cracking occurs (Matsuzawa 1994).
The nearby population of Pan troglodytes vellerosus in Korup/Cameroon seems
likewise to ignore hard nuts, despite occasional presence of suitable hammer stones.
Again, environmental constraints might be fine-tuned so as to remain unrecognized.
For example, nuts predisposed for cracking might be so rare at Gashaka as to render
their exploitation uneconomical, particularly as suitable hammers and anvils occur in
relatively low abundance. Nevertheless, on balance, the scenario at Gashaka, and
Korup, seems to be similar to that at Lopé/Gabon. There, nut-producing trees,
including Detarium microcarpum, are also present at low density, but none of the
available species of nuts is cracked with any of the readily available tools (McGrew
Subsistence Technology of Nigerian Chimpanzees
1017
et al. 1997). The finding supports the hypothesis that nut-hammering is restricted to
the most northern populations of West Africa (Boesch et al. 1994), and that the
absence of nut-hammering reflects cultural variation.
Finally, it might be possible to recognize a likely cultural variant in the degree and
purpose to which individuals use finer tools and tools with brush ends.
Frayed ends are an altered form of digging and probing stick. Sugiyama (1985)
termed the ends, if >30 mm, brush-sticks. The average fray of stingless-bee digging
sticks and stingless-bee probing-sticks at Gashaka exceeded this criterion. Overall,
17% of proximal tool ends had frays, compared to 45% of distal ends. Frays at
proximal ends had almost double the length of distal end frays (Table II), probably
because individuals used the distal ends more often as tool points, which reduces the
length of frays as they become compacted or are slivered or bitten off. In addition,
proximal ends might fray inadvertently when removed from the stem, and the frays
would then be a byproduct of tool manufacture (Takemoto et al. 2005). Lacking
sufficient direct observations, we do not know if frayed ends are also deliberately
produced by pulling the tool sideways through partially closed teeth (Sanz et al.
2004) or if they are simply byproducts of wear through repeated use (McGrew and
Collins 1985).
While frays reduce the ease of insertion, they also increase the working surface of
the tool, making it more likely to retrieve greater quantities of honey or insects that
bite them. Individuals also use brush ends at Lopé to harvest honey of both stingless
bees and African honeybees. Longer frays may allow for a greater quantity of honey
to be gathered per dip, as experiments suggest that a volume of up to 6 times more
can be obtained if tools have frayed rather than compacted ends (Tutin et al. 1995).
However, important differences exist between Gashaka and other sites with respect
to brush ends. Gashaka chimpanzees use them only to obtain honey, whereas Lopé
chimpanzees use brush ends not only to probe for honey but also to harvest arboreal
ants (Camponotus brutus; Tutin et al. 1995). At other African sites, brush ends are
likewise used to obtain termites (Cameroon, Equatorial Guinea, Central African
Republic, Congo; Sanz et al. 2004; Tutin et al. 1995).
Gashaka chimpanzees do not seem to harvest termites with tools, and when they
fish for arboreal ants, they use grass and leaf midribs, a technique so far reported only
from Mahale, where Camponotus vividus and C. brutus are preyed upon (Nishida
1973; Nishida and Hiraiwa 1982).
Given the propensity for chimpanzees to adapt to situations, it seems reasonable
to expect that a tool that is used in one context could be applied to another.
Fishing for wood-boring arboreal ants or for termites from their mounds are
intrinsically similar activities. In both cases, the insects are invisible and the
probing tools require pliability and strength. However, neither at Mahale (Nishida
1973) nor at Gashaka do chimpanzees use the fine ant-fishing tools for termitefishing. Similarly, Gashaka chimpanzees do not recontextualize the brush-end tools
by using them unlike Lopé chimpanzees to harvest arboreal ants. Future researchers
should investigate whether or not species like Camponotus chrysurus present at
Gashaka are amenable to such a method of extraction. Currently, plausible ecological
explanations are absent and one should therefore consider the trait variation to be
cultural.
1018
A. Fowler, V. Sommer
Core Cultural Tendencies in Behavioral Variations?
Whiten et al. (2001) pondered the question of whether chimpanzee cultures, similar
to the situation in humans, are characterized by a “central cultural core or theme.”
One suggestion is that some populations, like those at Taï/Ivory Coast, might show a
greater inclination toward technology, whereas others, such as the ones at Budongo /
Uganda, might be more nontechnological, because they employ a much smaller tool
kit (Whiten et al. 2001).
Still, it is difficult to rule out that tool use is not considerably influenced by the
local environment, e.g., by different climatic, floral, and faunal characteristics of the
habitat as well as food availability and quality. It could be argued that if a forest is
rich in fruit, as at Budongo, chimpanzees might simply not need tools for extractive
foraging. A similar argument can be made for bonobos in the fruit bowl of the
central Congo cuvette, which also do not use tools in conjunction with foraging
(Hohmann and Fruth 2003). Conversely, Gashaka chimpanzees use a rather extensive
tool kit that might reflect needs stimulated by a lower quality habitat. Indeed, at least
compared to bonobos, Gashaka chimpanzees have access to fewer plant macronutrients and cope with more plant-produced antifeedants such as tannins and phenols
(Hohmann et al. 2006).
Faunivory seems an obvious way to supplement a meagre diet. Evidence for meateating at Gashaka is limited (Bauer 2006), but apes that are not fully habituated tend to
cease hunting activity when observers approach (cf. Boesch and Boesch-Achermann
2000; Goodall 1986). In any case, mammalian meat makes up <5% of the chimpanzee
diet (Stanford 1998), suggesting that the benefits of carnivory may lie in the gain of
specific micronutrients or as a commodity to exchange for sex with females.
Entomophagy, in terms of macronutrients and calories, might be more important
for chimpanzees than meat-eating (McGrew 1992). Chimpanzees at Gashaka used
most tools to prey on social insects and their products, and insectivory, as elsewhere,
is likely to be a daily activity (Schöning et al. 2007). In any case, a cross-site review
indicates that, while only few chimpanzee fecal samples contain traces of vertebrate
prey (1–6%), many more contain ants (22–24%), termites (2–27%), or bees (1–23%;
McGrew 1992). Thus, the variables of frequency and volume of harvesting class
insects as a chimpanzee staple food. Conversely honey should be considered a treat,
given that it is of high quality, but less often consumed (McGrew 1992).
Raiding apian nests not only provides energy through the consumption of honey,
but the honeycombs also yield fat and protein from larvae, pupae, pollen, and
imagos (McGrew 1992). Some authors doubt that the procurement of protein is a
major function of ant-eating, assuming instead that specific nutrients, e.g., essential
amino acids or vitamins of the B-group, might be important (Hladik 1977; Nishida
and Hiraiwa 1982). Researchers have also proposed non-nutritional interpretations,
e.g., that the formic acid of ants is simply a spicy snack (Nishida and Hiraiwa 1982).
In Southeast Asia, weaver ants are an integral component of curry powder
(Bodenheimer 1951, cit. in Nishida and Hiraiwa 1982). Termites might be a
different matter, because the winged reproductive forms are particularly rich sources
of fat and protein (Nishida and Hiraiwa 1982). Nevertheless, the absence of termite
eating at Gashaka is puzzling because it is hard to reconcile with the assumption that
food constraints drive tool use.
Subsistence Technology of Nigerian Chimpanzees
1019
We cannot yet assess to what degree the relatively diverse tool kit of Gashaka
chimpanzees and their inclination toward elementary technology is shaped by the
environment, instead of reflecting social customs. In any case, one should not
measure cultural tendencies in terms of all-or-nothing, but rather via the frequency or
degree of the occurrence.
The Gashaka data are confusing with respect to the second potential cultural
tendency Whiten et al. (2001) raised, i.e., the use of brush tools in Central Africa to
harvest ants and termites (e.g., Southwest Cameroon: Sugiyama 1985; Congo: Fay
and Carroll 1992; Sanz et al. 2004) vs. the use of finer tools elsewhere. The Gashaka
chimpanzees seem to defy either classification, because they do not use brush tools
to harvest ants or termites, but only honey, whereas they use fine tools such as the
midribs of leaves to harvest ants. The hypothesis of a geographic dichotomy between
the employment of brush tools vs. finer tools to harvest ants and termites is therefore
not supported by our data.
The use of leaf midribs should be investigated further. First, given that the
material is perishable, unlike sticks, there is no possibility of a significant time lag
between extraction and use, which makes the use of midribs a clear case of
intentional manufacture (sensu Beck 1980). Second, it has implications for the debate
on culture. Of all the tools used for ant-fishing midribs are constructed from the least
obvious material, i.e., a whole leaf stripped to its middle part. Chimpanzees have to
visualize, or at least be aware of, the tool embedded within the leaf structure, rather
than use another more obvious material, such as a grass blade, for tool manufacture.
Whether it can be argued that the complexity makes it a less likely candidate for
independent development and a more likely candidate for cultural transmission is an
appealing question.
Finally, we compared the presence or absence of behavioral patterns associated
with material culture of Gashaka chimpanzees with data (Whiten et al. 2001) for 9
long-term study sites (Fig. 11), though the categories are not always identical.
BEHAVIOUR
DEFINITIONS OF WHITEN et al. (2001)
Dig
Bee-probe
Lever open (stick used to enlarge entrance)
Bee-probe (disable bees, flick with probe)
Brush-stick (probing stick with brush end)
Ant-dip (dip stick on ants to harvest)
Ant-dip single (one handed dip stick on ants)
Ant-dip-wipe (manually wipe ants off wand)
Ant-fish (probe used to extract ants)
Termite-fish using leaf midrib
Termite-fish using non-leaf materials
Ant-dip
Ant-fish
Termite-fish
Nut-hammer
Nut hammer, stone hammer on stone anvil
Nut-hammer, stone hammer on wood anvil
Nut-hammer, wood hammer on stone anvil
Nut-hammer, wood hammer on wood anvil
Nut-hammer, other (e.g. on ground)
WEST AFRICA
As
Bs Tai
U
-H
U
-C
U
--U
C
C
-C
C
H
+
-H
+
-H
+
e
H
-e
e
e
e
e
e
C
+
----
C
C
C
C
H
WEST-CENTRAL
Ga
Lo
x
C
x
U
x
-x
-U
-x
-x
C
e?
e?
e?
e?
------
------
EAST AFRICA
Ma Mk Go Kib
-- -- C --+
---- -- ---- -- C --- -+
--- -- C -C C +
--C -e
-C C
e
-e
-e
--
-e
-e
--
------
e?
e?
e?
e?
e?
Bd
-------e?
e?
e
e
e
e
e
Fig. 11 Presence or absence of behavioral patterns associated with subsistence technology of Nigerian
chimpanzees compared to 9 long-term study sites across Africa (Whiten et al. 2001). Study sites: West
Africa: As, Assirik, Senegal; Bs, Bossou, Guinea; Taï, Ivory Coast; West-Central Africa: Lo, Lopé,
Gabon; Ga, Gashaka, Nigeria; East Africa: Ma, Mahale M community, Tanzania; Mk, Mahale K
community, Tanzania; Go, Gombe, Tanzania; Kib, Kibale Kanyawara community, Uganda; Bd, Budongo,
Uganda. Occurrence codes of behavioral pattern (after Whiten et al. 2001, except code X): C = customary
(occurs in all or most able-bodied individuals), H = habitual (occurs repeatedly in several individuals), + =
present (occurs but not customary or habitual), X = occurs (individual frequencies unknown; can be C, H,
or P) , − = absent (absent with no ecological explanation), e = absence ecological (explained by local
environmental or ecological constraint), U = unknown (not recorded, but inadequate observations).
1020
A. Fowler, V. Sommer
Intuitively, one would assume that Nigerian chimpanzees are culturally more related
to Central African chimpanzee populations than to those in West or East Africa.
Indeed, the Gashaka profile has obvious similarities with that of Lopé. However,
Gashaka could share as many as 5 of 7 ecologically possible non-hammer patterns
with Gombe. Thus, more data, particularly from Central Africa, are needed, to reconstruct potential regional cultural clusters.
We cannot currently speculate about transmission mechanisms of cultural
variants, e.g., whether they are based on imitation, emulation, or more individualistic
processes such as social or local enhancement (Byrne 1995; Whiten and Ham 1992),
because sensible conclusions would require more direct, close-quarter observations
than those available for the Gashaka chimpanzees.
Cultural Primatology and Conservation
With respect to chimpanzees, Whiten et al. (2003) coined the term “cultural
panthropology”, reflecting the fact that the emerging research area is at the interface
between social sciences (with subdisciplines such as social anthropology, material
culture, psychology, ethnobotany, archaeology) and biological sciences (with
subdisciplines such as paleoanthropology, primatology, physiology, genetics, and
evolutionary medicine).
However, little time remains to execute the ambitious research agenda of cultural
panthropology, given that many populations of wild-living apes are in imminent
danger of extinction as a result of habitat destruction, diseases, and the trade in bush
meat (Ammann et al. 2003; Caldecott and Miles 2005; Sommer and Ammann 1998).
Correspondingly, not only genetic diversity but also cultural diversity is lost, similarly to the way in which globalization has destroyed many traditional human ways
of life (McGrew 2004). It is already clear that “we can never know the true extent of
cultural diversity in chimpanzees because so many communities, along with their
cultures, are already gone” (Goodall 1994: 397).
The situation is particularly dire for the Nigerian chimpanzee. Our survey work in
southeastern Nigeria from 2003 onward indicates that Pan troglodytes vellerosus has
a realistic chance to survive only in Gashaka because adjoining regions are already
devoid of wildlife (Chapman et al. 2004). Population viability models based on the
intake rates of ape sanctuaries in Nigeria and Cameroon predict extinction in as little
as 18 yr (Hughes 2003).
Studies like ours not only shed light on the pathways of human evolution, but are
also meant to shine a spotlight on a little known ape population because publicity
may help to preserve their biological and cultural identity: “all of the above will be
academic [...] if the chimpanzee becomes extinct” (McGrew 2004: 194).
Acknowledgments The Nigeria National Park Service kindly granted a research permit to the Gashaka
Primate Project (GPP). Gashaka-Gumti National Park and the Nigerian Conservation Foundation (NCF)
provided logistical support. Jeremiah Adanu, Judith Bovensiepen, Umaru Buba, Aylin McNamara,
Jennifer Rogan, Asako Saegusa, and Yakubu Wakirwa contributed to the data collection. For identification
of insects or plants, we thank J. D. Chapman (Canterbury), Britta Kunz (Wuerzburg), Judith Korb
(Regensburg), David W. Roubik (Washington), Barry Bolton (Isle of Wight), Caspar Schöning (Berlin and
Copenhagen). William McGrew, Caspar Schöning, and 2 anonymous reviewers provided useful comments
on an earlier draft. The work would have been impossible without local field assistants, in particular
Subsistence Technology of Nigerian Chimpanzees
1021
Hammaunde Guruza, Buba Bello, Ali Tappare, and Sam Yusufu. The Leakey Foundation (USA) and
generous grants from the North of England Zoological Society/Chester Zoo Nigeria Biodiversity
Programme supported the field work. This is Gashaka publication no. 59.
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