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First published online January 27, 2004
Journal of Experimental Biology 207, 827-839 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00819
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A three-dimensional kinematic analysis of tongue flicking in Python molurus

Jurriaan H. de Groot1,*, Inke van der Sluijs1, Peter Ch. Snelderwaard1 and Johan L. van Leeuwen2

1 Section Evolutionary Morphology, Institute of Biology (IBL), Leiden University, PO Box 9516, 2300 RA Leiden, The Netherlands
2 Experimental Zoology Group, Wageningen Institute of Animal Sciences (WIAS), Wageningen University, Marijkeweg 40, 6709 PG, Wageningen, The Netherlands



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  		Fig. 1. Schematic dorso-ventral representation of the tongue of Python molurus and its extrinsic muscles. (A) Abbreviations: cbr, ceratobranchial; m, mandible; dsh, distal tongue ensheathing; ggl, m. genioglossus; hgl, m. hyoglossus; psh, proximal tongue ensheathing; sh, tongue sheet. The dental bone forms the local coordinate system of the head. Although the head is highly deformable during feeding, the lower jaws do not deform during tongue flicking. The position of the ceratobranchials is assumed to be fixed to the skull during tongue flicking (e.g. Bels et al., 1994Go). The mm. genioglossi and the mm. hyoglossi are able to protract and retract the tongue relative to the mandibles and ceratobranchials. Proximally on the tongue, the mm. hyoglossi are ensheathed. This tubular tongue sheet encloses the tongue distally up to the tongue tips, inverts into itself and connects to the tongue at the distal tongue ensheathing (McDowell, 1972Go). The outer tongue sheet is connected to the muscles and connective tissue of the mouth floor. The inverted inner part is protruded while the tongue elongates. (B) Radio-opaque marker positions at rest: 1–5, local coordinate system fixed (glued) to the jaws (skin); 6, fold of the tongue sheet at maximum tongue retraction – the outer layer of the tongue sheet is fixed to the connective tissue of the mouth floor; 7, point of bifurcation (marker injected); 8, proximal tongue ensheathing (marker glued after manual protrusion of the tongue); 9, 10, tongue base (markers injected). (C) Hypothetical displacements of the markers indicating the relative translation and elongation of the soft tissues.

 


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  		Fig. 2. Schematic representation of the longitudinal deformation of the tongue body and the interaction with the tongue ensheathing. The extrinsic muscles are only partly drawn. Abbreviations: dsh, distal tongue ensheathing; ggl, m. genioglossus; hgl, m. hyoglossus; psh, proximal tongue ensheathing; sh, tongue sheet. (A) Detail from Fig. 1A. The tongue sheet is a tubular structure that inserts the tongue body at the proximal and distal ensheathing, thus forming a loose second `skin' around the tongue body. (B) In the retracted `rest' position, the tubular sheet distally folds inward, resulting in a double sheathing along the tongue tip. The outer layer of the tongue sheet is fixed to the connective tissue of the mouth floor (as indicated by the thin vertical lines). (C) While the tongue protrudes, the inner sheet unfolds outward as the posterior tongue part, i.e. between the proximal and distal ensheathing, elongates and the distal tongue part is revealed. Thus, the tongue sheet forms an almost frictionless bearing for tongue protrusion.

 


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  		Fig. 3. Camera frame of Python molurus during a tongue flick (Kodak SR-500; resolution 512x480 pixels). Four images were recorded synchronically by the use of three supplementary mirrors, resulting in: (A) frontal image (direct camera view); (B) right lateral image (mirror); (C) dorsal image (mirror); (D) left lateral image (mirror).

 


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  		Fig. 4. Schematic representation for the calculation of tongue curvature. The solid line, including seven discrete points, represents a digitised section of the tongue. A circle with radius r and centre M is stepwise estimated along the tongue through each set of three contiguous points along the tongue axis, e.g. the three black dots sn–1, sn and sn+1 [i.e. for s=(0, 0.02, 0.04,..., 1); equations 1, 2, 3]. Subsequently, at sn, the curvature C (i.e. 1/r) was determined. This was repeated for each point at the tongue axis except for the tongue base (s=0) and tongue tip (s=1).

 


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  		Fig. 5. Protrusion (A) and protrusion velocity (B) of P. molurus for three recorded tongue-flick clusters (cluster 1, black circles; cluster 2, grey circles; cluster 3, open circles). The clusters differ in duration and maximum protrusion length. The series of small panels at the top of the figure shows the tongue axis position (lateral view, tongue tip pointing to the right) of the protrusion trace with the longest duration (black circles). Each numbered panel corresponds to the same number in the position time trace. Protrusion length was calculated along the 3-D tongue axis from mouth opening to the bifurcation point of the tongue. At time t=0 s, the tips of the tongue started to become visible. Protrusion was started before the bifurcation point became externally visible, which explains the initial high protrusion velocity. The marked (grey) traces in flick clusters 1 and 2 indicate ground contact of the tongue tips.

 


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  		Fig. 6. The 3-D trajectory of the bifurcation point (thick trace) recorded at three different tongue flick clusters (rows 1–3) and for three different views: (A) lateral, (B) frontal and (C) dorso-ventral. The thin lines show the (calculated) position of the protruded part of the tongue axes for each recorded frame. The arrows indicate the motion direction of the bifurcation point. In the lateral view (A1–A3), the arrow coincides with the first flick within each of the clusters. The initial tongue flick started twice with a downward protrusion (clusters 1 and 2) and ground contact of the tongue tips (marked by light traces along the tongue tip trajectory) and once with an upward protrusion (cluster 3).

 


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  		Fig. 7. (A) Velocity and (B) acceleration traces of the point of bifurcation of the tongue along the covered trajectory for the three tongue flick clusters shown in Figs 5, 6. The range of the time axis is chosen to represent the time interval that the tongue tips are visible for each of the three tongue flick clusters. The blue curve indicates forward velocity and acceleration, the red curve indicates lateral velocity and acceleration and the green curve indicates vertical velocity and acceleration.

 


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  		Fig. 8. Time trace of the covered distance of the bifurcation point during the three tongue flick clusters (1, black circles; 2, grey circles; 3, open circles) and indicating the spatial and temporal exposure of the tongue. The mean velocity (0.25 m s–1) coincides with the overall slope of the curves. Numbering of flick clusters corresponds with Figs 5, 6, 7, 9.

 


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  		Fig. 9. Tongue length (vertical amplitude) and tongue curvature along the tongue axis (colour code) through time (horizontal axis) for each of the first (A), second (B) and third (C) flick clusters. Numbering of flick clusters corresponds to those of Figs 5, 6, 7, 8. The upper boundary of each plot represents the changing external tongue length (from the mouth to the bifurcation point; similar to Fig. 5A). Each vertical line under the curve represents the absolute curvature C (mm–1) indicated by the colour code (see colour bar) along the tongue axis (mm) at a specific time (s). The numbered series of lateral tongue shapes in the small pictograms above each curve coincides with the numbered marks along the upper boundary. For instance, in A at recording 2 (0.1 s), the anterior portion of the tongue shows the highest curvature, indicated by the green area under the upper boundary. At recording 4 (0.17 s), the tongue is fairly straight, indicated by the orange area along the vertical axis (see also the lateral view of the tongue in pictogram 4).

 

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© The Company of Biologists Ltd 2004