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. 2010 May 6;7(46):773-85.
doi: 10.1098/rsif.2009.0385. Epub 2009 Oct 14.

Hydrodynamic constraints on prey-capture performance in forward-striking snakes

Sam Van Wassenbergh  1 , Jonathan Brecko, Peter Aerts, Ilona Stouten, Gwen Vanheusden, Andy Camps, Raoul Van Damme, Anthony Herrel
Affiliations

Hydrodynamic constraints on prey-capture performance in forward-striking snakes

Sam Van Wassenbergh et al. J R Soc Interface. .

Abstract

Some specialized aquatic snakes such as Natrix tessellata strike at fish by rapidly accelerating their head towards the prey with their mouth opened widely. This strategy is believed to be suboptimal as relatively high drag forces act on the open jaws and, therefore, probably limit strike speed. Moreover, the bow wave in front of the snake's jaws could push prey away from the mouth, thus potentially explaining the relatively low capture success observed in these animals (<20%). Here, we used laser-scan based computational fluid dynamics to test these potential constraints on prey-capture performance for N. tessellata. Our simulations showed that drag force indeed increases drastically for striking at a high gape angle. However, we estimated the overall cost in slowing down strike speed to be less pronounced due to the instationary dynamics of the system. In contrast to the expectations, forward displacement of prey was relatively limited (<13% of head length), and forceful collisions between prey and the leading edge of the jaw regularly occurred. However, our models showed that precise aiming by the snake was needed to reduce the chance of deviating the prey to a path bypassing the mouth. Our study also indicated several hydrodynamic advantages for snakes to strike at relatively large prey.

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Figures

Figure 1.
Figure 1.
Outline of the shape of the modelled snake and spheroid prey with a midsagittal plane section through the three-dimensional unstructured tetrahedral mesh at the start of one of the CFD simulations (a). An illustration of the head surface performing jaw closing is given in (b). The latter also shows rotation centres for the lower jaw and the head, the bending zones (bordered by dotted lines) and the regions rotating as a rigid body (indicated by dashed lines and arrows). Scale bar, 10 mm.
Figure 2.
Figure 2.
Flow velocity in the Earth-bound frame of reference along the posterior to anterior axis for the midsagittal plane of N. tessellata (a), and for a series of frontal view planes (4 mm interval) at one time instant showing anterior–posterior flow velocity (b) and right–left flow velocity (c). In this CFD simulation, the snake model translated with a forward velocity of 1 m s−1 and started to close its mouth at time = 40 ms. The prey touched the lower jaw at time = 53 ms. The velocity scale is equal for (a) and (b).
Figure 3.
Figure 3.
Streamlines starting from the snake's surface, illustrating the three-dimensional flow directions (always directed towards the open ends of the lines; relative to the Earth-bound frame of reference) during a frontal strike at 1 m s−1 of N. tessellata just prior to jaw closing (simulation time = 40 ms) from a (a) lateral, (b) frontal, (c) dorsal and (d) laterofrontal view.
Figure 4.
Figure 4.
Flow patterns in the midsagittal plane ((a)(i)–(c)(i)) and a frontal plane ((a)(ii)–(c)(ii)) for (a) forward translation at 1 m s−1, (b) after 10 ms of stationary jaw closing reaching an angular velocity of approximately 6000° s−1 and (c) the combination of forward translation and jaw closing. Unidirectional flow velocities are colour coded (see colour bar on the left), and streamlines projected on the view plane are shown.
Figure 5.
Figure 5.
Effects of gape angle on drag force, drag coefficient and projected surface of the snake at two velocities (1.0 and 0.5 m s−1). Total force: filled circle, 1 m s−1; filled triangle, 0.5 m s−1. Pressure force: grey circle, 1 m s−1; grey triangle, 0.5 m s−1. Viscous force: open circle, 1 m s−1; open triangle, 0.5 m s−1.
Figure 6.
Figure 6.
Hydrodynamic forward pushing force exerted on the prey as a function of gape angle for an approach at a constant velocity of 1 m s−1 and at a constant acceleration of 12.5 m s−2. Note that the gape angle in vivo employed by N. tessellata (approx. 70°) is not optimal to avoid prey pushing. Note also that the apparent hydrodynamic advantage of wide gapes shown in this graph may trade off with the duration of mouth closure. Open circle, constant acceleration reaching 1 m s−1; closed circle, constant velocity of 1 m s−1.
Figure 7.
Figure 7.
Static pressure field in front of the snake's jaws for steady forward translation at 1 m s−1 (a) and for a constant acceleration of 12.5 m s−2 at the moment of reaching 1 m s−1 (b). Note the increased positive pressure bow wave resulting from forward acceleration.
Figure 8.
Figure 8.
Forward prey displacement kinematics for a steady 1 m s−1 strike at three different constant gape angles (green, blue and black), two simulations including jaw closing (yellow and red) and one simulation with jaw opening (pink). Note that jaw closing resulted in significantly augmented pushing of the prey, whereas jaw opening had the opposite effect.
Figure 9.
Figure 9.
Consecutive positions of the centre of mass of the prey relative to the snake during five CFD simulations where the prey started at, from right to left, 0, 1, 2, 3 and 4 mm from the midsagittal plane of the snake. Note that the two prey that are located most distantly away from the midsagittal axis of the snake were unlikely to be caught due to sideways pushing effects of the bow wave. Scale bar, 5 mm.
Figure 10.
Figure 10.
Kinematics of hydrodynamic pushing of differently sized prey, initially at the same distance from the jaws (upper graph to lower graph, respectively, 1×, 2×, 4×, 8× and 16× the mass of the standard prey). The times when the prey touched the snake's jaw during the CFD simulations are indicated by filled circles.
Figure 11.
Figure 11.
The effect of a 10% increase in head width on the bow wave, as shown by forward directed flow velocities along the midsagittal plane in the original simulation (dashed lines) and increased head width simulation (continuous lines). Note that the increase in head width resulted in a bow wave extending approximately 10% further in front of the jaws. Scale bar, 10 mm.
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