Combining precision and power to maximize performance: a case study of the woodpecker's neck

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In a tensegrity structure, all components are subject
to only compression (bars) and tensions (cables and
springs). This feature conveys many interesting prop-
erties such as, lightness, reconfigurability, energyeffi-
ciency, among others. As shown recently by several
biologists like (Levin 2006), the concept of tensegrity
is present in many biological systems like cells and
musculo-skeletal systems. Recently, a human spine
model using a tensegrity structure was proposed
(Sabelhaus 2015). Tensegrity mechanisms are better
candidates to model bird necks as compared to other
potential solutions like multi-backbone continuum
robots (Wang 2014) because tensegrity lends itself
well to a realistic modeling of bones, muscles and ten-
dons. Furthermore, the modularity of the tensegrity
systems has a heuristic value in the explanation of the
morpho-functional modularity debated in evolution-
ary biology.
Funding
ANR -10-CE33-0025
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KEYWORDS Bird neck; evolution; bio-inspired robot; tensegrity
anick.abourachid@mnhn.fr
Combining precision and power to
maximize performance: a case study
of the woodpecker’s neck
C. B€
ohmer
a,
, M. Furet
b
, B. Fasquelle
b
, P. Wenger
b
,
D. Chablat
b
, C. Chevallereau
b
and A. Abourachid
a
a
UMR 7179 CNRS/MNHN, D
epartement Adaptations du Vivant,
Mus
eum National d’Histoire Naturelle, 55 rue Buffon, 75005 Paris,
France;
b
Laboratoire des Sciences du Num
erique de Nantes
(LS2N), CNRS, Ecole centrale de Nantes, 44321 Nantes, France
1. Introduction
The neck in birds has been characterized as a highly
complex system that positions the head during all
kinds of behavior (B€
ohmer et al. 2018; Zweers et al.
1994). This involves a variety of highly demanding
tasks such as feeding, manipulation, preening, sexual
display, nest building, and combat behavior (B€
ohmer
et al. 2019). This versatility is an opportunity for bio-
engineering, but prior to designing a technological
model, the biological system must be understood; in
particular regarding the form-function relationships.
One behavioral pattern in birds that appears to be
relatively constrained is pecking in woodpeckers.
There is evidence that the drilling trajectory is essen-
tially planar (Spring 1965). From a morphofunctional
point of view, woodpeckers are highly interesting
because they are very lightweight (mean body
mass ¼300 g), but are capable to dig into dense tree
trunks (Puverel et al. 2019). They strike their beak
against a tree repeatedly with high accelerations, high
velocity and large force transmissions (May et al.
1979; Wang et al. 2011). We took this bird species as
a first example for a biologically inspired robot arm
that combines precision and power to maximize per-
formance. Adding biological knowledge into the
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design process requires a simplification of the com-
plexity of the biological entity (Whitesides 2015). In
this context, we (1) analyzed the neck musculature
which supplies force for movement; (2) established a
planar robotic model using several stacked tensegrity
crossed bar mechanisms and (3) integrated all data
into an actuated model of the bird neck.
2. Methods
2.1. Dissection
The neck musculature of one generalist bird (Corvus
monedula) and three species of woodpeckers was
comparatively investigated by quantitative dissection:
the green woodpecker (Picus viridis), the great spotted
woodpecker (Dendrocopos major) and the black
woodpecker (Dryocopus martius). They differ in peck-
ing performance (Jenni 1981) which allows identifica-
tion of muscular adaptations specific for high power.
After removing the skin and fascia, each muscle was
identified and systematically detached from both the
origin and insertion. Muscles were assigned to main
functional categories indicating the direction and ver-
tebral region of action. The craniocervical muscular
system was illustrated in a schematic diagram.
2.2. Kinematic and dynamic modelling
The woodpecker neck is modelled in the sagittal plane
with a serial stack of Snelson’s X-shape tensegrity mech-
anisms (Wenger and Chablat 2017), which is a class 2
tensegrity mechanism composed of an antiparallelo-
gram made of 4 rigid links. The sides of the mechanism
are attached with two springs that ensure a stable rest
position, and cables run through the springs. By modi-
fying the cable tension with motors, different positions
can be reached. The kinematic model links the output
of the modelled stacked modules (i.e., the position and
orientation of the tip of the beak) to the input of the
robot (the orientation of each X-shape mechanism).
The dynamic modelling takes into account the influ-
ence of the rigid links masses and inertia, spring stiff-
nesses and cable tensions and links the motion of the
model (position, velocity, acceleration) to the forces
needed to move it (Furet et al. 2018).
3. Results and discussion
3.1. Muscular adaptations
The overall muscular system of the neck in the ana-
lyzed woodpeckers corresponds to an earlier
anatomical description of Dendrocopus provided by
Jenni (1981). Our quantitative analysis revealed that
the adaptation to pecking is particularly evident in
the craniocervical and ventral muscle system.
As opposed to generalist birds, the muscles of the
craniocervical system in woodpeckers show adapta-
tions specific for precision of force control. The
highly specialized tendinous architecture of the ven-
tral muscle system in woodpeckers enables forceful
and rapid movements. Compared to the other species
of woodpeckers, the tendons are most prominently
developed in the black woodpecker.
In summary, the anatomical arrangement of muscle
and tendon in the neck of woodpeckers facilitates effi-
cient pecking by combining precision and power.
3.2. Neck movement
Videos of alive woodpeckers do not allow us to precisely
detect the neck movement because of the feathers. As a
consequence, a film of the corpse of a green wood-
pecker without its feathers and moved by hand was
recorded and used to capture a neck movement, with
the aim of reproducing it with our robot in simulation.
To convert the video into a robot movement, the
frames are analyzed independently. For each of them,
the middle line of the neck is detected, then the
mechanisms of the robots are placed one by one from
the bottom of the neck. The obtained positions give
us the trajectory.
3.3. Simulation
The manipulator is made of 11 identical mechanisms
that is the number of vertebrae in the green woodpecker.
The dimensions were chosen to have a manipulator with
the same proportions than the woodpecker neck.
Simulations are performed with Matlab Simulink
and Simscape Multibody. The use of the dynamic
model in the control law allows obtaining an accurate
tracking with a full-actuated robot. In this case, the
forces applied have similar shapes for each mechan-
ism, which is promising for under-actuation. Simple
under-actuation strategies also succeed at tracking the
movement with a good precision, and the neck mus-
culature of the woodpeckers will be used to create
related under-actuation strategies.
4. Conclusions
The aim of the present work was to extract the
anatomical parameters that are essential to perform
COMPUTER METHODS IN BIOMECHANICS AND BIOMEDICAL ENGINEERING S3

precise and powerful movements. The conceptualiza-
tion of the extremely complex musculature of the
neck in birds revealed such a specialization.
Eventually, the prototype of the woodpecker’s neck
will serve as a starting point for future invention and
application such as industrial soft-robots that execute
their task in collaboration with humans in an efficient
and safe way.
Acknowledgements
We thank the Chene Association and the Goupil
Association for providing bird specimens for dissec-
tion. This study was financially supported by the
Agence National de la Recherche (ANR): Project ID
#ANR-16-CE33-0025 (AVINECK), Project coordin-
ator: A. Abourachid.
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ohmer C, Plateau O, Cornette R, Abourachid A. 2018.
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KEYWORDS Neck; bird; tensegrity; bio-inspired; robotics
boehmer@vertevo.de
Look into a fish mouth: 3D X-ray
particle tracking adapted to intra-oral
hydrodynamics in fish feeding
P. Provini
a,
, A. Brunet
a
and S. Van Wassenbergh
a,b
a
D
epartement Adaptations du Vivant, UMR M
ecanismes
adaptatifs et 
evolution (MECADEV) Mus
eum National d’Histoire
Naturelle/CNRS, 57 rue Cuvier, Case Postale 55, 75231 Paris
Cedex 05, France;
b
Department of Biology, University of Antwerp,
Universiteitsplein 1, 2610 Antwerp, Belgium
1. Introduction
Most of ray-finned fish (Actinopterygii) use suction
feeding to capture their prey. The rapid expansion of
their mouth generates a high velocity water flow (e.g.
Muller and Osse 1984), which leads the prey inside
the buccal cavity. In spite of the prominent role of
hydrodynamics to catch food successfully in suction
feeding (Day et al. 2015) difficult optical access to the
buccal cavity makes it still poorly understood. To go
beyond computational simulations (Provini and Van
Wassenbergh 2018) and overcome the technical con-
straints faced in situations without optical access, we
combined two X-ray based methods to study suction
feeding in Carp (Cyprinus carpio).
We used a biplanar X-ray system to obtain a 3D
animation of the head bones, using X-Ray
Reconstruction of Moving Morphology (XROMM)
(Brainerd et al. 2010). Simultaneously, we optimised
the technique of 3D X-ray particle tracking (Seeger
et al. 2001), to visualize intra-oral flow dynamics
using neutrally-buoyant radio-opaque particles. The
combination of these two techniques allowed us to
quantify the water flow involved during suction feed-
ing and to correlate it to the 3D kinematics of the
head bones in vivo.
2. Methods
2.1. X-Ray reconstruction of moving morphology
We performed the experiment at the University of
Antwerp, where two synchronized and calibrated Xray
sources were associated with high-speed video cameras
to film two individuals of Carps during suction feeding
(N ¼4 sequences for each individual). Prior to the
experiment, the carps were implanted with 0.35 mm
diameter radio-opaque markers on the upper jaw, lower
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