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New Guinea bone daggers were engineered to preserve social prestige

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Bone daggers were once widespread in New Guinea. Their purpose was both symbolic and utilitarian; they functioned as objects of artistic expression with the primary function of stabbing and killing people at close quarters. Most daggers were shaped from the tibiotarsus of cassowaries, but daggers shaped from the femora of respected men carried greater social prestige. The greater cross-sectional curvature of human bone daggers indicates superior strength, but the material properties of cassowary bone are unknown. It is, therefore, uncertain whether the macrostructure of human bone daggers exists to compensate for inferior material properties of human femora or to preserve the symbolic value of a prestigious object. To explore this question, we used computed tomography to examine the structural mechanics of 11 bone daggers, 10 of which are museum-accessioned objects of art. We found that human and cassowary bones have similar material properties and that the geometry of human bone daggers results in higher moments of inertia and a greater resistance to bending. Data from finite-element models corroborated the superior mechanical performance of human bone daggers, revealing greater resistance to larger loads with fewer failed elements. Taken together, our findings suggest that human bone daggers were engineered to preserve symbolic capital, an outcome that agrees well with the predictions of signalling theory.
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Research
Cite this article: DominyNJ,MillsST,Yakacki
CM, Roscoe PB, Carpenter RD. 2018 New Guinea
bone daggers were engineered to preserve
social prestige. R. Soc. open sci. 5: 172067.
http://dx.doi.org/10.1098/rsos.172067
Received: 2 December 2017
Accepted: 20 March 2018
Subject Category:
Biology (whole organism)
Subject Areas:
biomechanics/behaviour/evolution
Keywords:
signalling theory, symbolic capital, bone
mechanics, Casuarius unappendiculatus
Authors for correspondence:
Nathaniel J. Dominy
e-mail: nathaniel.j.dominy@dartmouth.edu
R. Dana Carpenter
e-mail:dana.carpenter@ucdenver.edu
Electronic supplementary material is available
online at https://dx.doi.org/10.6084/m9.
gshare.c.4062551.
New Guinea bone daggers
were engineered to
preserve social prestige
Nathaniel J. Dominy1,2, Samuel T. Mills3, Christopher
M. Yakacki3,PaulB.Roscoe
4and R. Dana Carpenter3
1Department of Anthropology, and 2Department of Biological Sciences,
Dartmouth College, Hanover,NH 03755, USA
3Department of Mechanical Engineering, University of Colorado, Denver, CO 80217, USA
4Department of Anthropology, University of Maine, Orono, ME 04469, USA
NJD, 0000-0001-5916-418X;RDC,0000-0003-4178-1707
Bone daggers were once widespread in New Guinea. Their
purpose was both symbolic and utilitarian; they functioned
as objects of artistic expression with the primary function of
stabbing and killing people at close quarters. Most daggers
were shaped from the tibiotarsus of cassowaries, but daggers
shaped from the femora of respected men carried greater social
prestige. The greater cross-sectional curvature of human bone
daggers indicates superior strength, but the material properties
of cassowary bone are unknown. It is, therefore, uncertain
whether the macrostructure of human bone daggers exists to
compensate for inferior material properties of human femora
or to preserve the symbolic value of a prestigious object.
To explore this question, we used computed tomography to
examine the structural mechanics of 11 bone daggers, 10 of
which are museum-accessioned objects of art. We found that
human and cassowary bones have similar material properties
and that the geometry of human bone daggers results in
higher moments of inertia and a greater resistance to bending.
Data from finite-element models corroborated the superior
mechanical performance of human bone daggers, revealing
greater resistance to larger loads with fewer failed elements.
Taken together, our findings suggest that human bone daggers
were engineered to preserve symbolic capital, an outcome that
agrees well with the predictions of signalling theory.
1. Introduction
Signalling theory is a unifying concept in the social and
biological sciences [1]. It proposes that social prestige, or symbolic
capital [2], is a mechanism for communicating underlying
traits with adaptive value. A central tenet of signalling theory
is that status-accruing signals are honest (indexical) and,
2018 The Authors. Published by the Royal Society under the terms of the Creative Commons
Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted
use, provided the original author and source are credited.
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5 cm
(b)
(a)
Figure 1. Bone daggers of the Sepik watershed, New Guinea. (a) Human bone dagger attributed to the Upper Sepik River (accession no.
990.54.28190.21, ©Hood Museum of Art, Dartmouth College). (b) Cassowary bone dagger attributed to the Abelam people (accession
no. 990.54.28190.12, ©Hood Museum of Art, Dartmouth College).
(a)
(c)(d)
(b)
Figure 2. Anthropologist Albert Buell Lewis visited the Malol lagoon, Aitape, northern New Guinea in September 1909 [5]. (a)Abone
dagger is attached to the left arm of the man in the centre of the canoe. (b) Armbands of woven rattan were used to secure daggers and
other overt adornments, such as these shell rings. (c) The Asmat also produce bone daggers [6]; a cassowary bone dagger is attached to
the left arm of an Asmat man punting a pirogue on the Seper River. (d) Posterior view reveals the removal of mediolateral cortical bone
to create a atter blade. Asmat photographs by Bruno Zanzottera, reproduced with permission.
therefore, reliable indicators of the intrinsic qualities of the signaller. An intriguing application of
signalling theory involves the decorative arts, a topic that is usually viewed as purely symbolic. Yet
virtually every culture devotes effort to the elaboration of utilitarian objects (clothing, pots, tools,
dwellings, etc.), as well as their own bodies. Universal behaviours invite an adaptive explanation [3],
and signalling theory argues that symbolic expression can serve a functional purpose if it communicates,
by proxy, attributes of the signaller, such as fine cognitive and motor skills or time available for
non-subsistence behaviours. These qualities, in turn, are expected to attract higher quality allies and
reproductive partners, thus enhancing the reproductive success of both the signaller and receiver.
The bone daggers of New Guinea were potent objects of artistic expression [4]. They were incised
with elaborate designs, both abstract and representational (figure 1), and worn as conspicuous personal
adornments (figure 2). It is a signalling tradition that invites study because, as close-combat weapons
[4,7], bone daggers were exemplars par excellence of male fighting abilities, and a highly desirable status
symbol among men [8,9]. In addition, bone itself was the embodiment of strength, both mechanically
and symbolically with powers enmeshed in the supernatural world [4]. This dual concept of strength,
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5 cm
Figure 3. Mechanical failure near the tip is evident in some bone daggers. This human bone dagger is attributed to the Kwoma people,
with anities to the Iatmul area of the Middle Sepik River (early- or mid-twentieth century; accession no. 990.54.28190.11, ©Hood
Museum of Art, Dartmouth College).
together with the dual function of bone daggers as weapons and symbols, is intriguing when one
considers the different macrostructures of bone daggers. Those made from a human femur were shaped
differently and appear to be better engineered for mechanical performance, which raises the possibility
of biomechanical trade-offs in a social signal, a topic that intersects the arts with the physical, life and
social sciences.
1.1. Bone daggers as weapons
In New Guinea, bone daggers were close-combat weapons used to kill outright or finish off victims
wounded with arrows or spears, by stabbing them in the neck ([811]; Bragge LE. n.d. (1970–1974)
Interview notes (unpublished). Koetong, Australia: Bragge Archives.) a process that Schultze Jena [12]
described vividly in 1914:
The lethal point at which one aims [the bone dagger] is the neck just above the breastbone end
of the collarbone, the area of the subclavia and carotid. The dagger serves not only to stab into
the main arteries but at the same time as a lever with which one twists the punctured neck of the
enemy in order to tear the throat and, with sufficient power, break the neck (p. 9; German–English
translation by P. Roscoe).
Landtman published a similar account based on interactions with the Kiwai from 1910 to 1912 [13].
According to Kiwai respondents, the eastern Gulf tribes used bone daggers for ‘stabbing prisoners, taken
in a fight, through their hip joints, knees or ankles’. Thus disabled, the prisoners ‘could be kept alive until
needed for a later cannibal feast’ (p. 57).
The veracity of these accounts is difficult to assess; contact-era narratives were often based on the
views of informants toward their own adversaries. However, the reports are consistent insofar as they
describe stabbing actions in various joints (cervical, hip, knee, ankle). Another similarity is implicit, and
it concerns compressive and torsional loads near the tip and the potential for mechanical failure, which is
evident in some specimens (figure 3). Such failure would empty the dagger of all symbolic strength and
potentially jeopardize the user during hand-to-hand fighting, suggesting that the strength of daggers can
hold adaptive value. It is telling that peacemaking ceremonies in the Lower Arafundi require the mutual
destruction of spears but the exchange of bone daggers [14], a distinction that highlights the practical
value of the latter weapon.
1.2. Bone daggers as objects of social prestige
The biological source of a bone dagger (human or cassowary) is readily apparent [4]. Those shaped from
human femora have distinct pommels (notched femoral condyles and a steep patellar groove; figure 1a)
and greater curvature, both longitudinally and transversely. Human bone daggers were prestigious [4]
in part because the most suitable femora were sourced from battle-proven men, usually those of a father,
once the corpse was reduced to a skeleton [4], or those of a vanquished enemy [15]. They were weapons
filled with substantial strength, i.e. they were the manifestation of spiritual power [16], allowing the
owner to lay claim to the powers of the man who surrendered the bone.
Daggers were also shaped from the tibiotarsus of cassowaries (figure 4), and these were widespread
(figure 5), especially in the Sepik region [24]. There is reason to surmise that cassowary bone daggers
were also symbols of male strength, although to lesser extent [4]. Cassowaries are described as ‘sullen,
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tibiotarsus
tarsometatarsus
(a)
(c)
(b)
Figure 4. (a) Northern or single-wattled cassowary (Casuarius unappendiculatus); females can stand 2 m tall and weigh up to 58 kg
(photograph by Holger Ehlers, reproduced with permission). (b) Cassowaries have stout legs with dense, apneumatic bones and large,
three-toed (tridactylus) feet [17]. Massive leg muscles [18] enable running speeds up to 50 km h1and standing jumps as high as
1.5 m [19]. An outstanding peculiarity of cassowaries is their medial toe (digit II), which is equipped with a prodigious spike-like claw
(photograph by Christian Hütter, reproduced with permission). The claw can be 12 cm long and 3 cm at the base [20] and used to telling
eect by quickly extending (kicking) the leg forward or to the side. A review of 221 incidents between 1926 and 1999 found that southern
cassowaries (C. casuarius) inicted serious wounds on domestic animals, with kicks resulting in lacerations, punctures, and ruptures to
internal organs [21]. At least one person, a boy aged 16 years, stumbled and fell while assailing a cassowary and later succumbed to an
exsanguinating puncture wound to the neck [22]. (c) Articulation of the tibiotarsus and tarsometatarsus at the intratarsal joint. The area
that appears to be the knee of a bird is homologous to the human ankle.
0
150° E145° E140° E135° E130° E
150° E145° E140° E135° E130° E
10° S 5° S
10° S 5° S
80 160 240 320 N
km
Geographic distributions:
Casuarius unappendiculatus
Casuarius bennetti
CORAL SEA
SOLOMON SEA
BISMARCK SEA
Casuarius casuarius
bone daggers: occurrence and
extent of survey
Figure 5. Geographical ranges of cassowary species: the northern or single-wattled cassowary (C. unappendiculatus); the Bennett’s
or dwarf cassowary (C. bennetti); the southern or double-wattled cassowary (C. casuarius)[23]. Points depict the recorded
occurrence of cassowary bone daggers; the hashed line depicts the limits of our ethnographic survey (P. Roscoe 2017, unpublished
dataset).
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treacherous and extremely pugnacious’ [25], words that speak to their size, agility and aggression
when provoked (figure 4). Hunting such an animal was an additional source of male status in New
Guinea [26,27]; indeed, some odes to deceased men recounted the number of his cassowary kills [28].
Cassowary bone daggers also featured prominently in local prestige economies [29,30], in part because
cassowaries were imbued with deep cultural significance: commonly sexed as female, they were widely
metaphorized and mythically and ritually cast as women, wives and sometimes enemies rather than as
birds [3134]. Possession of a cassowary bone dagger was thus a plausible signal of male hunting ability,
physical and ritual strength and status.
Another explanation for the prevalence of cassowary bone daggers is more utilitarian: it might have
outstanding mechanical properties. When compared to mammals, the mass of compact (cortical) bone in
birds is distributed relatively far from the long axis, leading to higher second and polar moments of area
and greater inferred resistance to bending and twisting [35], ideal properties for any tool. It is perhaps
unsurprising that the tibiotarsius and tarsometatarsus of cassowaries (figure 4c) were often fashioned
into practical tools, e.g. coconut splitters [36,37] and pandanus splitters [3841]. Indeed, the mechanical
strength and dagger-like appearance of these implements has led some authors to suggest that early
accounts misidentified bone ‘daggers’ as weapons rather than tools. However, this is not the case: the
tips of these tools are usually blunted (see electronic supplementary material, figure S2), whereas true
bone daggers were sharpened to a fine point (cf. figures 1and 2d).
In sum, cassowary bone appears superficially to have a similar mechanical utility for dagger
manufacture to human bone, yet human bone daggers have greater prestige than those of cassowary
bone. These differences focus the aims of the present study.
1.3. Study aims
Bone daggers were ornaments and armaments, and the retention of greater cross-sectional curvature
appears to be a deliberate design feature of all human-derived daggers. It is a difference that, apriori,
would suggest better mechanical performance. Yet the material properties of cassowary bone daggers
are unknown, and the widespread use of ratite leg bones—e.g. moa bone daggers in prehistoric New
Zealand [23] and emu bone daggers in Australia [42,43]—raises the possibility that cassowary bone has
ideal material properties. It is, therefore, uncertain whether the superior macrostructure of human bone
daggers exists to compensate for inferior material properties or to better preserve an object with greater
symbolic capital. Here we test between these competing possibilities.
2. Material and methods
2.1. Specimens and imaging
We examined intact bone daggers accessioned in the Hood Museum of Art, Dartmouth College.
This sample includes early- and mid-twentieth century specimens derived from human femora and
cassowary tibiotarsi (n=5each;table 1). In addition, we purchased a modern (ca. 1970s) cassowary
bone dagger from a private art dealer (see Ethics statement). The human-derived daggers in our sample
are readily distinguished by their greater curvature and richer patina. They are also rather rare. An
unpublished survey of museum collections (those of the American Museum of Natural History; the
Field Museum of Natural History; the Peabody Museum of Archaeology and Ethnology; and others)
found that 21 of 499 bone daggers (4.2%) were shaped from human femora (M. Golitko 2017, personal
communication).
We scanned each specimen in a 16-slice spiral computed tomography (CT) system (LightSpeed
16, General Electric Medical Systems, Milwaukee, WI, USA) located in the Department of Radiology,
Dartmouth-Hitchcock Medical Center. We used a voxel size of 0.2 ×0.2 ×1.25 mm to create three-
dimensional reconstructions of each dagger (figure 6) and we measured the length (L) of each dagger
on the basis of these images. We estimated dagger penetration in human joints at 20% of the overall
length (figure 7), as measured from the tip. We, therefore, used this distance to compare cross-
sectional geometries. We used BoneJ [44] to measure geometrical properties, including cross-sectional
area (CSA), minimum and maximum moments of inertia (Imin,Imax —measures of bending resistance;
see electronic supplementary material, figure S2), minimum and maximum section moduli (Zmax,
Zmin—proportional to bending strength) and polar section modulus (Zp—proportional to torsional
strength).
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10 cm cassowary tibiotarsushuman femur
Figure 6. Three-dimensional reconstructions of each dagger in the present study. This grouping of (a) human-derived daggers and (b)
cassowary-derived daggers is useful for highlighting dierences in the pommels, which stem from anatomical dierences in the human
knee joint and cassowary intratarsal joint (gure 4c). The distal condyles of the human femur are asymmetrical, and the patellar groove
is much steeper than the shallow trochlear surface of the distal tibiotarsus of cassowaries. In human bone daggers, much of the medial
and lateral epicondyles of the femur were removed to create a steeply notched, V-shaped pommel.
Tabl e 1. Museum-accessioned bone daggers examined in the present study. The daggers were acquired by Harry A. Franklin (1904–1983),
Los Angeles, California, in the 1950s; bequeathed to the Harry A. Franklin Family Collection, Los Angeles, California, in 1983; and gifted to
the Hood Museum of Art, Dartmouth College, Hanover, New Hampshire, in 1990.
accession no. bone sourceaknown provenance; cultural attributiona
990.54.27734 cassowary Ndu language family
.........................................................................................................................................................................................................................
990.54.27742 cassowar y Sepik River, Abelam people
.........................................................................................................................................................................................................................
990.54.27743 human Middle Sepik River, Kwoma people
.........................................................................................................................................................................................................................
990.54.27751 cassowary Sepik River, Abelam people
.........................................................................................................................................................................................................................
990.54.28190.7 human Kwoma people (likely)
.........................................................................................................................................................................................................................
990.54.28190.9 human Upper Sepik River
.........................................................................................................................................................................................................................
990.54.28190.10 cassowary Abelam people
.........................................................................................................................................................................................................................
990.54.28190.12 cassowary Abelam people (likely)
.........................................................................................................................................................................................................................
990.54.28190.16 human Kwoma people (likely)
.........................................................................................................................................................................................................................
990.54.28190.21 human Upper Sepik River
.........................................................................................................................................................................................................................
aArmed in 1992 by Douglas Newton, Metropolitan Museum of Art, and Kathleen Barlow, Central Washington University.
2.2. Cassowary bone material properties
To measure bone properties and dagger strength, we used a modern cassowary bone dagger for CT
imaging and destructive testing. It was first tested in cantilever bending using a uniaxial mechanical
tester with a 500 N load cell (Insight 30, MTS, Eden Prairie, MN, USA; figure 7). To simulate insertion
into a human joint, we embedded 20% of the dagger length into urethane casting material (DynaCast,
Freeman Manufacturing and Supply, Avon, OH, USA). Then we inserted a small sheet of rubber between
the compression platen and bone in order to apply load evenly to the dagger handle. We loaded the
dagger in the anteroposterior direction to failure at a displacement rate of 1mm s1. We used the
maximum force measured in this test (see electronic supplementary material, figure S3) as a benchmark
to establish a failure criterion for the finite-element (FE) modelling of each dagger.
We used excess pieces of the dagger to create three dog-bone samples for mechanical testing (figure 8).
The samples were taken from the mid-diaphysis of the bone, and were cut and sanded to a uniform
cross-section with no curvature. The samples were tested to failure in tension using a 30kN load
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Figure 7. Cantilever bending test conguration; the cassowary bone dagger was xed at 20% of the overall length from the tip.
10 mm
Figure 8. Example dog-bone preparation for mechanical testing. Scale bar, 10 mm.
cell at a displacement rate of 0.01 mm s1. We used uniaxial strain gages (L2A-06-062LW-120, Micro-
Measurements, Vishay Measurements Group, Raleigh, NC, USA) to measure strain at the narrow section
of each sample. After testing, we measured the CSA at the failure location using a flatbed scanner at a
resolution of 0.01 mm pixel1. We used the resulting area to convert force measurements to equivalent
stress. We calculated the Young’s modulus (E) of each sample using the slope of the linear portion of the
stress–strain curve, and defined the ultimate stress (σult) as the highest stress achieved during a test.
2.3. Finite-element model and analysis
We converted CT scans into FE models using ScanIP +FE software (Simpleware, Synopsys, Mountain
View, CA, USA). We used FE modelling because it converts complex structures into simpler, smaller
sections for mechanical analysis and because it is non-destructive. FE models have become a powerful
tool for evaluating the mechanical performance of osteological and fossil specimens in museum
collections [4550]; however, the focus of these ‘osteometric eyes’ [50] is seldom turned on objects of
art or material culture in the ethnographic or archaeological records (but see Thomas et al.[51] and their
FE analysis of fluting in North American Pleistocene weaponry).
We segmented images using a threshold-driven region-growing algorithm, and meshed each model
with linear four-node tetrahedral elements (average number of elements =88 110). The number of
elements was determined to be sufficient after a convergence study found no significant increase in the
accuracy of the models with more elements. We separated trabecular bone from cortical bone using the
grey-scale value of the CT scans for each voxel, where the trabecular bone could then be assigned material
properties. Trabecular bone was confined to the grip and pommel regions of each dagger; i.e. the distal
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condylar region of the human femur or cassowary tibiotarsus. Our mechanical tests suggest that the
Young’s modulus of cassowary and human cortical bone [5257] are practically equivalent (see Results
and discussion). Accordingly, we used identical material properties for all models, assigning a Young’s
modulus of 24.0 GPa to cortical bone (present results), a Young’s modulus of 0.4 GPa to trabecular bone
[58] and a Poisson’s ratio of 0.3 to all bone tissue.
We imported the FE models into ABAQUS (Simulia, Dassault Systèmes, Waltham, MA, USA) for
analysis. We fixed each model at 20% of the length measured from the tip, and applied a 225 N force to
the handle end. The tested dagger failed at 200 N, but a slightly higher force of 225 N was applied to the
FE models to ensure failure. Bending was performed in two perpendicular directions (corresponding to
Imax and Imin) in order to estimate the maximum and minimum bending strength values of each dagger.
We imported the von Mises stress distribution of each dagger into Matlab (Mathworks, Natick, MA) to
determine failure load, which we estimated based on a per cent volume failure criterion. In the model of
the cassowary dagger used for testing, the von Mises stress in 3.3% of the overall volume exceeded the
σult of bone tissue at the measured failure load. Given these results, we used a 3.3% volume criterion for
all models. We used the FE-based stress distribution for each dagger and scaled the applied load until
3.3% of the volume exceeded σult. This method to calculate the failure force allows for any force value to
be applied to the models, and the correct failure force will still be calculated. The resulting force (Fmax )is
assumed to be the force required to induce fracture. For additional comparison, we calculated the forces
required to induce failure in 1%, 3% and 5% of the volume of each dagger.
2.4. Statistical analysis
We used Mann–Whitney U tests to compare the means of all geometric properties and FE-predicted
failure loads in our sample of five human bone daggers and six cassowary bone daggers, where the sixth
cassowary bone dagger was purchased and used for mechanical testing. Statistical differences were set
at α<0.05.
3. Results and discussion
Ethnographic accounts of bone daggers report that users targeted the cervical vertebrae or the hip,
knee and ankle joints of their victims. We estimated that penetration of these joints would entail
20% of the length of dagger; therefore, this distance, as measured from the tip, was used to compare
the cross-sectional geometries of human- and cassowary-derived daggers. CT images affirmed our
subjective impressions of greater cross-sectional curvature among the human bone daggers, a shape that
predicts superior mechanical performance (see electronic supplementary material, figure S2). Indeed,
Imin differed between the two dagger types (table 2), with the mean value of human bone daggers being
290% greater than that of cassowary bone daggers despite similar cross-sectional areas (table 2). Thus, the
retention of greater cross-sectional curvature during the manufacture of human bone daggers appears
to be a design feature that results in higher moments of inertia and greater resistance to bending; i.e. a
stronger dagger.
To explore whether the superior macrostructure of human bone daggers exists to compensate for the
inferior material properties of human bone, we tested the mechanical properties of a cassowary bone
dagger to establish baseline properties for FE models. Flexural strength in cantilever bending was 200 N,
and corresponded to testing in the Imin direction. Tensile tests revealed a mean (±1 s.d.) elastic modulus
(E) of 24.01 ±1.57 GPa and an ultimate stress (σult) of 153.9 ±42.3MPa. This finding compares well with
measures from an ostrich (Struthio camelus;E: 13.90 GPa [55]) and emu (Dromaius novaehollandiae;E:
13.05 ±3.94 GPa, range: 5.62 to 19.83 GPa; σult: 146 MPa [59]), with the caveat that these authors examined
the compact bone of fresh (undried) femora. In our tests, the stress–strain behaviour of dry, tibiotarsal
compact bone was relatively linear-elastic, showing a brittle response and failure before 1% strain (see
electronic supplementary material, figure S4). Crucially, our measures of material properties fall squarely
between (E=18.0 and 27.4 GPa [53,58]) and overlap (σult =103–133 MPa [5257]) published values for
dry compact bone from human femora, which simplified the construction of our FE models.
FE models of the daggers underwent simulated cantilever-bending experiments to analyse the failure
loads of each dagger (figure 9). First, an FE model of the experimental bending test was subjected to
200 N (i.e. the flexural strength of the dagger). The simulation showed that 3.3% of the volume of the
dagger had a stress greater than the σult. Next, FE models of all daggers were subjected to simulated
bending tests. The influence of force on the percentage of volume failed in the daggers was compared
in bending with respect to Imin and Imax (figure 10). All daggers followed the same general trends. At
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Von Mises
(MPa)
1872
400
367
333
300
267
233
200
167
133
100
33
0
(a)
(b)
Figure 9. (a) Example of mesh density in the FE model. (b) Example stress distribution during a simulated cantilever bending test.
5%
5%
3.3%
3.3%
1%
1%
bending in Imax
cassowary
human
bending in Imin
cassowary
human
0
2
% volume failed
4
6
8
10
0
2
% volume failed
4
6
8
10
200 400 600
load (N)
800 1000200 400 600
load (N)
800 1000
(b)(a)
Figure 10. Per cent volume of failed elements as a function of simulated bending models in the (a)Imin and (b)Imax directions.
Table 2. Mean (±1 s.d.) geometric and mechanical parameters for the human and cassowary bone daggers in our sample. Moment of
inertia: resistance to bending (dependent on bending direction). Section modulus: directly proportional to bending strength (dependent
on bending direction). Polar section modulus: Directly proportional to torsional strength.
measured parameters human bone daggers (n=5) cassowary bone daggers (n=6)
length (mm) 358.2 ±19.6 374.9 ±16.7
.........................................................................................................................................................................................................................
cross-sectional area (mm2)87.0±25.7 78.4 ±9.3
.........................................................................................................................................................................................................................
min. moment of inertia, Imin (mm4) 607.1 ±393.0* 206.7 ±73.2*
.........................................................................................................................................................................................................................
max. moment of inertia, Imax (mm4)1762.4±945.0 1639.1 ±328.1
.........................................................................................................................................................................................................................
min. section modulus, Zmin (mm3) 98.3 ±47.6 59.5 ±15.2
.........................................................................................................................................................................................................................
max. section modulus, Zmax (mm3) 225.7 ±99.7 182.4 ±23.6
.........................................................................................................................................................................................................................
polar section modulus, Zp(mm3) 253.7 ±118.9 193.7 ±26.5
.........................................................................................................................................................................................................................
nite-element breaking force, Imin (N) 462.6 ±131.6** 159.5 ±29.3**
.........................................................................................................................................................................................................................
nite-element breaking force, Imax (N) 455.2 ±104.4* 327.8 ±54.2*
.........................................................................................................................................................................................................................
*p<0.05.
**p<0.01.
low loads (less than 50 N) the stresses in the models remained below the σult and thus no volume of the
model was considered ‘failed’.
With increasing load, stresses in the model increased and the volume of failed elements increased in
a linear fashion. When testing in the Imin direction, each cassowary model demonstrated less force was
required to induce failure for a given percentage of volume; however, the cassowary daggers showed
more similar behaviour to the human daggers when tested in the Imax direction. We compared the failure
load of the daggers at 1.0, 3.0, 3.3 and 5.0% of the failed volume of elements, where 3.3% corresponded
10
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force at 3.3%
0
200
400
600
800
failure
criterion
cassowary
human
0
200
400
600
800
force (N)
force at 1% force at 3% force at 5% force at 3.3% force at 1% force at 3% force at 5%
(b)(a)
Figure 11. Mean forces (±1 s.d.) required to reach a critical volume of failed elements in the (a)Imin and (b)Imax directions.
to the volume of failed elements for the dagger tested experimentally (figure 11). When tested in the Imin
direction, the human bone daggers were significantly stronger than the cassowary bone daggers at all
levels of failed volume (table 2). For example, the human bone daggers required 254 N to fail at 3.3%
of the total volume of the dagger, which corresponds to 31% more force, on average, compared to the
cassowary bone daggers. However, when tested in the Imax direction, the human bone daggers required
only 27% more force on average.
4. Conclusion
Our results suggest that the mechanical properties of compact bone are similar in the human femur and
cassowary tibiotarsus, although our analysis is limited by necessity to a single sample of cassowary bone.
Still, this finding suggests that systematic differences in the macrostructure (cross-sectional curvature) of
bone daggers will determine differences in mechanical strength. We affirmed this prediction with FE
models, finding that human bone daggers can support larger loads with a smaller volume of elements
failing. We conclude by suggesting that the retention of greater diaphyseal curvature is a deliberate
design feature intended to produce a stronger bone dagger.
It is, therefore, difficult to explain why dagger-makers working with a tibiotarsus would choose
to remove so much of the mediolateral wall (figure 2d). A flatter cross-sectional shape is a weaker
macrostructure and it is tempting to speculate that the disadvantages of this design are balanced by
greater comfort for the owner (when fixed to the upper arm; cf. figure 2) or perhaps reduced weight
during fighting or friction during insertion. In the event of breakage, a cassowary bone dagger is easily
replaced, whereas a human bone dagger is not. We conclude by suggesting that people in the Sepik
region of New Guinea engineered human bone daggers to withstand breakage, and that their prevailing
motivation was to preserve intact the embodiment of symbolic strength and social prestige, an outcome
that agrees well with the predictions of signalling theory.
Ethics. This project entailed material testing (destruction) of a contemporary (ca. 1970s) cassowary bone dagger. It was
purchased from an art dealer based in the USA, and any trade in cultural heritage raises potential ethical concerns.
In the present case, a handicraft object produced for sale to tourists or art dealers does not rise to the definition
of national cultural property, as regulated by the 1965 National Cultural Property (Preservation) Act or the 1970
UNESCO Convention on the Means of Prohibiting and Preventing the Illicit Import, Export and Transfer of Ownership
of Cultural Property. The 1973 Convention on International Trade in Endangered Species of Wild Fauna and Flora
(CITES) is another potential prohibition, but no species of cassowary is presently protected under CITES. On a
philosophical level, we considered the recent origin of the bone dagger and the widespread availability of comparable
daggers in museum and private collections when weighing our decision to perform destructive material testing. To
the best of our knowledge, our study adheres closely to the principles of professional responsibility.
Data accessibility. Three-dimensional meshes were deposited in the MorphoSource data archive (http://morphosource.
org; media nos. 16844–16859); corresponding data on the geometry and mechanical parameters of each specimen were
deposited in the Dryad Digital Repository (http://dx.doi.org/10.5061/dryad.f7p1n17)[60].
Authors’contributions. N.J.D. conceived and coordinated the study, and obtained the CT scans. S.T.M and R.D.C conducted
mechanical tests and analysed the data. P.B.R. supplied data on dagger use and geographical distribution in New
Guinea. All authors wrote the manuscript and gave final approval for publication.
Competing interests. The authors declare no competing interests.
11
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Funding. This work was supported by Dartmouth College (Year of Arts Faculty Grant to N.J.D.; the Hood Museum of
Art’s Andrew W. Mellon Faculty Residency to N.J.D.) and the David and Lucile Packard Foundation (Fellowship in
Science and Engineering no. 2007-31754 to N.J.D.).
Acknowledgements. We are grateful for the gift from the Harry A. Franklin Family Collection and for the technical and
practical inputs of R.N. Beaulieu, J. Benoit, M. Busse, S.R. Craig, C. Fluehr-Lobban, S.P. Gaughan, C.A. Gilliland, M.
Golitko, K.W. Hart, D.T. Haynes, W.K. Heppenstall, R. Hide, A.B. Kahl, J. Nintzel, D.J. Pluta, J.E. Terrell, D.W. van
Citters, and R.L. Welsch.
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